U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2005-5293

Recovery of the Black-footed Ferret:

Progress and Continuing Challenges

Proceedings of the Symposium on 

the Status of the Black-footed Ferret 

and Its Habitat, Fort Collins, Colorado,

January 28-29, 2004
Cover photo of black-footed ferret by Dean Biggins taken on 

East Core Prairie Dog Colony, Meeteetse, Wyo., February 1983

Recovery of the Black-footed Ferret: 

Progress and Continuing Challenges

Proceedings of the Symposium on the Status of the 

Black-footed Ferret and Its Habitat, Fort Collins, Colorado, 

January 28-29, 2004

Edited by James E. Roelle, Brian J. Miller, Jerry L. Godbey, and Dean E. Biggins

Scientific Investigations Report 2005–5293

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006

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Suggested citation:

Roelle, J.E., Miller, B.J., Godbey, J.L., and Biggins, D.E., eds., 2006, Recovery of the black-footed ferret—progress and 

continuing challenges: U.S. Geological Survey Scientific Investigations Report 2005–5293, 288 p. 
iii

Contents

Acknowledgments.........................................................................................................................................v

Dedication.....................................................................................................................................................vii

Section I. Background.................................................................................................................................1

The Symposium in Context (Dean E. Biggins) ...........................................................................................3

A Historical Perspective on Recovery of the Black-footed Ferret and the Biological and 

Political Challenges Affecting Its Future (J. Michael Lockhart, E. Tom Thorne, 

and Donald R. Gober).......................................................................................................................6

Section II. Managing Captive Populations............................................................................................21

An Adaptive Management Approach for Black-footed Ferrets in Captivity (Paul E. Marinari 

and Julie S. Kreeger) .....................................................................................................................23

Use of Reproductive Technology for Black-footed Ferret Recovery (JoGayle Howard, Rachel 

M. Santymire, Paul E. Marinari, Julie S. Kreeger, Linwood Williamson, and David E. 

Wildt) ................................................................................................................................................28

The Genetic Legacy of the Black-footed Ferret: Past, Present, and Future (Samantha M. 

Wisely)..............................................................................................................................................37

Section III. Searching for Wild Black-footed Ferrets..........................................................................45

A History of Searches for Black-footed Ferrets (Louis R. Hanebury and Dean E. Biggins)............47

Section IV. Locating and Evaluating Habitat.........................................................................................67

Areas Where Habitat Characteristics Could Be Evaluated To Identify Potential Black-footed 

Ferret Reintroduction Sites and Develop Conservation Partnerships (Robert J. Luce).....69

A Habitat-based Technique To Allocate Black-footed Ferret Recovery Among Jurisdictional 

Entities (Andrea E. Ernst, Aaron L. Clark, and Donald R. Gober) ............................................89

Habitat Restoration and Management (Joe C. Truett, Kristy Bly-Honness, Dustin H. Long, 

and Michael K. Phillips).................................................................................................................97

Black-tailed Prairie Dog Interactions with Other Herbivores: Mediation via Alterations of 

Vegetation (James K. Detling)....................................................................................................111

Shooting Prairie Dogs (Archie F. Reeve and Timothy C. Vosburgh)..................................................119

Habitat Preferences and Intraspecific Competition in Black-footed Ferrets (Dean E. Biggins, 

Jerry L. Godbey, Marc R. Matchett, and Travis M. Livieri)....................................................129

Evaluating Habitat for Black-footed Ferrets: Revision of an Existing Model (Dean E. Biggins, 

J. Michael Lockhart, and Jerry L. Godbey)..............................................................................143

Section V. Reestablishing Populations ................................................................................................153

Monitoring Black-footed Ferrets During Reestablishment of Free-ranging Populations: 

Discussion of Alternative Methods and Recommended Minimum Standards (Dean E. 

Biggins, Jerry L. Godbey, Marc R. Matchett, Louis R. Hanebury, Travis M. Livieri, and 

Paul E. Marinari)...........................................................................................................................155

Radio Telemetry for Black-footed Ferret Research and Monitoring (Dean E. Biggins, Jerry L. 

Godbey, Brian J. Miller, and Louis R. Hanebury).....................................................................175

Postrelease Movements and Survival of Adult and Young Black-footed Ferrets (Dean E. 

Biggins, Jerry L. Godbey, Travis M. Livieri, Marc R. Matchett, and Brent D. Bibles) .......191

Does Predator Management Enhance Survival of Reintroduced Black-footed Ferrets? 

(Stewart W. Breck, Dean E. Biggins, Travis M. Livieri, Marc R. Matchett, and 

Valerie Kopcso).............................................................................................................................203

Section VI. Managing Diseases ............................................................................................................211

Recent Trends in Plague Ecology (Kenneth L. Gage and Michael Y. Kosoy) ...................................213
iv

Exposure of Captive Black-footed Ferrets to Plague and Implications for Species 

Recovery (Jerry L. Godbey, Dean E. Biggins, and Della Garelle).........................................233

Use of Pesticides To Mitigate the Effects of Plague (David B. Seery)..............................................238

Vaccination as a Potential Means To Prevent Plague in Black-footed Ferrets (Tonie E. Rocke, 

Pauline Nol, Paul E. Marinari, Julie S. Kreeger, Susan R. Smith, Gerard P. Andrews, 

and Arthur W. Friedlander) .........................................................................................................243

The Quest for a Safe and Effective Canine Distemper Virus Vaccine for Black-footed 

Ferrets (Jeffrey Wimsatt, Dean E. Biggins, Elizabeth S. Williams, and Victor 

M. Becerra) ...................................................................................................................................248

Section VII. Extended Abstracts............................................................................................................269

Identifying Focal Areas for Conservation of Black-footed Ferrets and Prairie Dog 

Associates (Jonathan Proctor,Bill Haskins, and Steve Forrest)..........................................271

White-tailed Prairie Dog Population Survey and Habitat Evaluation in Western Wyoming 

(John A. Baroch and David A. Plume) ......................................................................................275

Can the Systemic Insecticide Nitenpyram Be Used for Flea Control on Black-tailed 

Prairie Dogs? (Jeff N. Borchert and Jeff J. Mach).................................................................278

Fleas and Small Mammal Hosts Within and Adjacent to the Coyote Basin White-tailed Prairie 

Dog Colony in Northeastern Utah (Lianna K. Etchberger, William E. Stroh, 

Brent D. Bibles, Matthew R. Dzialak, and Richard C. Etchberger).......................................281

Effects of Moonlight on Cover Usage and Spatial Learning of Black-footed Ferrets (Samantha 

N. Marcum,Dean E. Biggins, and Jennifer A. Clarke)............................................................284

Modeling Black-footed Ferret Energetics: Are Southern Release Sites Better? (Lauren A. 

Harrington, Dean E. Biggins, and A. William Alldredge)........................................................286
v

Acknowledgments

The Symposium on the Status of the Black-footed Ferret and Its Habitat, held January 28–29, 

2004, in Fort Collins, Colo., and the resulting proceedings contained herein would not have been 

possible without the dedicated efforts of numerous individuals and agencies. Financial support 

for the meeting itself and for publication of this volume was provided by Aaron and Gretchen 

Clark of the Black-footed Ferret Recovery Foundation, by the U.S. Fish and Wildlife Service, 

and by the U.S. Geological Survey. Sandi Godbey, ably assisted by Jenny Benson, Matt Dunfee, 

Darren Long, Noe Marymor, Adam Phillips, Adrianna Siniawski, and Ashley Skeen, attended to 

the infinite variety of details that make a meeting run smoothly. We gratefully acknowledge Dr. 

Andrew Dobson, Princeton University, Department of Ecology and Evolutionary Biology, for the 

keynote address, as well as the assistance of Della Garelle, Mike Lockhart, Bill Van Pelt, and 

Chris Brand, who served as session moderators. We thank Mike Lockhart, Paul Marinari, and 

Julie Kreeger for hosting a field trip to the National Black-footed Ferret Conservation Center, 

which at the time of the symposium was under construction but is now a fully functioning 

facility. We are indebted to Dale Crawford, Delia Story, Jennifer Shoemaker, and especially 

Dora Medellin for their able assistance in preparing the text and graphics for publication and 

to Heather Branvold, Patricia Mendoza, and Dale Crawford for graciously allowing us to use 

their artwork. Finally, we wish to thank the many organizations and countless individuals who 

have contributed to the Black-footed Ferret Recovery Program. While space does not allow us to 

mention them individually, it is obvious that neither the symposium nor this volume would have 

been possible without their unflagging efforts on behalf of one of North America’s most engaging creatures.

vii

Dedication

In the period between January 2004, when this symposium took place, and publication of 

the proceedings presented here, partners in the Black-footed Ferret Recovery Program suffered 

enormous personal and professional losses through the deaths of Tom Thorne, Beth Williams, 

Bill Russell, and Stan Anderson.

Tom and Beth, a husband and wife team of professional veterinarians and wildlife biologists, were killed in a traffic accident on their way home to Laramie, Wyo., on December 29, 

2004. Both Tom and Beth were responsible for much of the original recovery program leadership and were deeply involved in all elements of black-footed ferret management and research. 

Their expertise and many notable scientific contributions significantly advanced our knowledge 

and capabilities related to black-footed ferret captive breeding, disease risks and management, 

vaccine efficacy and safety, and reintroduction.

Bill Russell died at Ivinson Hospital in Laramie on August 16, 2005. Bill was a Professor of Animal Science at the University of Wyoming for 25 years and was the official Blackfooted Ferret Species Survival Plan® studbook keeper and genetic advisor for many years. Bill 

provided invaluable advice and recommendations on captive black-footed ferret management 

and breeding.

Stan Anderson died at his home in Laramie on September 1, 2005, following an extended 

illness. Stan started the University of Wyoming Cooperative Fish and Wildlife Research Unit 

in 1980 and served as a graduate advisor for many biologists who obtained advanced degrees 

studying various aspects of prairie dog conservation and black-footed ferret recovery.

These proceedings are dedicated to the memory and accomplishments of these long-time 

ferret recovery advocates, scientists, advisors, administrators, and mentors. The loss of these 

fine individuals leaves a substantial void in the institutional knowledge and foundation of the 

Black-footed Ferret Recovery Program and in the wildlife conservation community as a whole. 

They will be deeply missed, but their legacy will endure.

Section I. Background

The first paper of this section illustrates relationships between this symposium and the 

Black-footed Ferret Recovery Program, placing it in the context of other such symposia and 

workshops, the objectives of the recovery plan, and noteworthy events in the recent history of 

black-footed ferret conservation. The second paper describes the Black-footed Ferret Recovery Program. Its authors present their treatise as a personal commentary based on their own 

experiences. Their perspectives were formed during work with the rediscovered population of 

ferrets at Meeteetse, Wyo., and leadership of the captive breeding program that followed (Tom 

Thorne), and as a result of guiding the overall recovery program as the U.S. Fish and Wildlife Service’s Black-footed Ferret Recovery Program Coordinator (presently Mike Lockhart; 

formerly Pete Gober).

The black-footed ferret (Mustela nigripes) is a member 

of the weasel family (Mustelidae) and is closely related to the 

Siberian polecat (M. eversmannii) of Asian steppes and the 

European polecat (M. putorius). Compared to its relatives, 

the black-footed ferret is an extreme specialist, depending on 

the prairie dogs (Cynomys spp.) of North American grasslands for food and using prairie dog burrows for shelter. The 

black-footed ferret’s close association with prairie dogs was an 

important factor in its decline. Prairie dogs were regarded as 

an agricultural pest as human settlement progressed westward, 

and they became important hosts for plague as that disease 

colonized eastward from its sources of introduction on the 

west coast. Prairie dog numbers were dramatically reduced 

by poisoning, cropland conversions, and plague during the 

first half of the 20th century, and black-footed ferret populations declined precipitously. The black-footed ferret was 

included on the first lists of endangered species, and its status 

was precarious by the time the Endangered Species Act of 

1973 was passed. Its rebound from a low point of 10 known 

individuals in spring of 1985 (Biggins and others, 2006) is 

impressive, but the species is not yet “recovered” in either the 

biological or legal sense (for further details, see Lockhart and 

others, this volume).

Conservation activities to assist black-footed ferrets have 

extended through the past five decades. Included in those 

activities were three previous workshops and a symposium 

organized to facilitate interchange of ideas and information. 

The contents of their published proceedings illustrate changes 

in emphasis regarding issues important to black-footed ferret 

recovery. Placing these meetings in a chronological context 

of major events in ferret conservation (fig. 1) helps to explain 

motives for convening them and content of the papers, and 

provides context for the current volume.

The first workshop on black-footed ferrets and prairie 

dogs (Linder and Hillman, 1973) focused primarily on the 

rangewide status of the ferret and its prairie dog habitat, with 

The Symposium in Context

By Dean E. Biggins1

1

U.S. Geological Survey, Fort Collins Science Center, 2150 Centre Ave., 

Bldg. C, Fort Collins, CO 80526.

Figure 1. Timeline relating recent symposia and workshops to noteworthy events and periods in 

black-footed ferret (Mustela nigripes) research and recovery.
a single paper summarizing new information on the biology 

of the ferret. Despite 10 years of research on the black-footed 

ferret in South Dakota (fig. 1), Ray Erickson concluded that it 

remained “one of the least well known of all of the endangered 

mammals of the United States” (Erickson, 1973, p. 156). 

Rumors began circulating in the late 1970s that the blackfooted ferret was extinct, but these contentions were shortlived because of the discovery of a population of ferrets near 

Meeteetse, Wyo., in 1981 (Biggins and others, 2006). 

The second workshop on black-footed ferrets (Anderson

and Inkley, 1985) occurred near the end of a rather brief 

period of intensive research on the Meeteetse population 

of ferrets and after the first attempt to captive breed South 

Dakota ferrets at the U.S. Fish and Wildlife Service’s (now 

U.S. Geological Survey’s) Patuxent Wildlife Research Center 

in Maryland (fig. 1). A single paper in that workshop was 

devoted to captive breeding, summarizing the failed Patuxent 

attempt but prophetically predicting success if the venture 

were to be repeated with ferrets from the Meeteetse population 

(Carpenter, 1985). The proceedings had a wide variety of other 

papers on the status of prairie dogs and ferrets, institutional 

and procedural issues, searches for more ferrets, and research 

needs. Six papers summarized original field research, much 

of which was conducted on the Meeteetse population. By 

summer of 1985, 10 months after the workshop, plague caused 

extensive declines in the Meeteetse prairie dog population, 

canine distemper was discovered in the ferrets, and the ferret 

population plummeted. These events caused an abrupt end to 

field research on ferrets and forced the beginning of the second 

captive breeding program following emergency rescue of the 

remaining animals. Carpenter’s (1985, p. 12.11) admonition 

(presumably motivated in part by his Patuxent experience) to 

avoid the “tendency to initiate propagation programs as a last 

resort, when few animals are available for captive breeding” 

could not be heeded. Research at Meeteetse also served as the 

primary motivation for publication of a second collection of 

black-footed ferret papers the following year as number 8 of 

the Great Basin Naturalist Memoirs (Wood, 1986), although 

that volume was not the result of a symposium or workshop.

The third workshop, held in 1986, was conceived primarily in response to information needs for the newly developing 

second effort to maintain and produce black-footed ferrets in 

captivity (fig. 1). It blended developing theories in conservation biology with existing biological information on ferrets 

and polecats. This workshop resulted in a book (Seal and 

others, 1989) with chapters covering systematics, population 

biology, reproduction, captive propagation, and conservation.

The fourth meeting was a symposium convened in the 

summer of 1989. Although the captive breeding program had 

a tenuous beginning (Biggins and others, 2006; Lockhart and 

others, this volume), evidence predicting ultimate success had 

emerged by 1988, and thoughts were turning toward planning 

for reintroduction. Reports in the proceedings (Oldemeyer 

and others, 1993) focused primarily on habitat for reintroduction of ferrets. Plague received increased recognition as an 

ominous threat to ferret habitat, with several papers dedicated 

to discussion of that disease.

The symposium culminating in the papers presented 

herein was held on January 28–29, 2004, in Fort Collins, Colo. 

It had been more than 10 years since the previous symposium, 

and much new information on the biology of the black-footed 

ferret had been accumulated. Many of the papers published 

here resulted from information collected as captive breeding became more efficient and as black-footed ferrets were 

released back into native habitats (fig. 1). The symposium 

was organized into sections based on the principal topics in 

the stepdown outline of the Black-footed Ferret Recovery 

Plan (U.S. Fish and Wildlife Service, 1988). The relationship 

between the recovery plan and the papers of this symposium 

illustrates that work is following a somewhat orderly progression guided by principal topics in the recovery plan. A brief 

description of that relationship introduces each section of this 

volume.

This volume and the five that preceded it, including 

proceedings from the three workshops and the symposium, 

plus the Great Basin Naturalist volume (Wood, 1986), cover 

a broad spectrum of work on prairie dogs and black-footed 

ferrets. Although these volumes certainly are not exhaustive in 

their coverage of ferret and prairie dog research and conservation activities, they collectively provide a solid foundation for 

future conservationists working with ferrets and chronicle a 

long-term recovery program for one of North America’s most 

endangered vertebrates.

 

References Cited

Anderson, S.H., and Inkley, D.B., eds., 1985, Black-footed 

ferret workshop proceedings: Cheyenne, Wyoming Game 

and Fish Department, p. 1.1–33.12.

Biggins, D.E., Miller, B.J., Clark, T.W., and Reading, R.P., 

2006, Restoration of an endangered species—the blackfooted ferret, in Groom, M.A., Meffe, G.K., and Carroll, 

C.R., eds., Principles of conservation biology (3d ed.): 

Sunderland, Mass., Sinauer Associates, p. 581–585.

Carpenter, J.W., 1985, Captive breeding and management of 

black-footed ferrets, in Anderson, S.H., and Inkley, D.B., 

eds., Black-footed ferret workshop proceedings: Cheyenne, 

Wyoming Game and Fish Department, p. 12.1–12.13.

Erickson, R.C., 1973, Some black-footed ferret research needs, 

in Linder, R.L., and Hillman, C.N., preparers, Proceedings 

of the black-footed ferret and prairie dog workshop: Brookings, South Dakota State University, p. 153–164.

Linder, R.L., and Hillman, C.N., preparers, 1973, Proceedings 

of the black-footed ferret and prairie dog workshop: Brookings, South Dakota State University, 208 p.

 Recovery of the Black-footed Ferret
Oldemeyer, J.L., Biggins, D.E., Miller, B.J., and Crete, R., 

eds., 1993, Management of prairie dog complexes for the 

reintroduction of the black-footed ferret: Washington, D.C., 

U.S. Fish and Wildlife Service, Biological Report 13, 96 p.

Seal, U.S., Thorne, E.T., Bogan, M.A., and Anderson, S.H., 

eds., 1989, Conservation biology and the black-footed 

ferret: New Haven, Conn., Yale University Press, 302 p.

U.S. Fish and Wildlife Service, 1988, Black-footed ferret 

recovery plan: Denver, Colo., U.S. Fish and Wildlife 

Service, 154 p.

Wood, S.L., ed., 1986, Great Basin Naturalist Memoirs No. 8: 

Provo, Utah, Brigham Young University, 208 p.

The Symposium in Context 
Abstract

This paper is a personal commentary by the authors on 

the background and historical development of the Blackfooted Ferret Recovery Program. The black-footed ferret 

(Mustela nigripes) was recognized as imminently endangered 

in the original Endangered Species Act and has a recovery 

history accentuated with near catastrophes and remarkable 

successes. In this paper, we examine the species’ near demise, 

wild black-footed ferret populations, captive breeding efforts, 

and attempts to restore ferret populations into native habitats. 

We provide our personal perspectives on many lessons learned 

during these program stages, the social and political factors 

affecting species recovery, past and present biological obstacles, and insights relevant to the future of the species.

Keywords: black-footed ferret, captive breeding, endangered species, Mustela nigripes, recovery, reintroduction

Introduction

Rather than a technical presentation of data or a literature 

review, this paper is a personal commentary on historical 

aspects of the Black-footed Ferret Recovery Program. We 

offer our recollections and observations only as reflections 

of our own experiences. We acknowledge that many people 

contributed to black-footed ferret (Mustela nigripes) conservation over this period and that others may view program events 

and our conclusions differently; however, we submit that our 

unique positions of program responsibility over the years 

provide broad perspectives that others not directly involved in 

day-to-day ferret recovery, or involved in limited areas, may 

not have gained.

Collectively, as representatives of lead agencies responsible for ferret recovery, we have been directly involved in ferret 

conservation matters on a daily basis from 1981 to the present. 

The Wyoming Game and Fish Department (WGFD) was a 

primary participant in ferret recovery both in the field and 

in captive breeding from 1981 to 1996 and remains active in 

reintroduction and program planning activities today. The U.S. 

Fish and Wildlife Service (FWS) has been involved with ferret 

conservation since before the species was originally listed 

as endangered in 1967. Moreover, FWS has been directly 

involved with captive breeding since 1996 and has coordinated 

all other recovery activities since that time. In addition, FWS 

has investigated the likely effects of habitat loss on ferrets 

as a result of the decline of its principal prey—prairie dogs 

(Cynomys spp.).

In this paper, we discuss ferret conservation activities 

related to wild population management, captive breeding, 

and reintroduction into the wild. In particular, we address the 

significant biological, political, and social issues that affected 

species recovery. We focus only on the major highs and 

lows of ferret recovery as we view them; we defer the many 

important details to other participants in this symposium. 

Additionally, we characterize our observations of various 

efforts as successes, failures, or lessons learned. Finally, we 

provide recommendations linked to these conclusions that 

may contribute to future recovery of ferrets and perhaps other 

endangered species.

The opinions and recommendations presented in this 

paper are those of the authors and do not represent official 

positions of either the FWS or the WGFD. We also wish to 

acknowledge the extraordinary dedication, hard work, and 

contributions accomplished by the many State, Federal, tribal, 

zoo, and conservation organization partners on behalf of 

black-footed ferret recovery. Overall program success is the 

result of enormous efforts by these many program cooperators.

Management of Free-ranging Populations

Ferrets received little attention until the species was listed 

as endangered in 1967, one of several species to be designated 

under the first version of the Endangered Species Act (ESA). 

Early accounts, from Audubon’s description of the type 

A Historical Perspective on Recovery of the Blackfooted Ferret and the Biological and Political Challenges 

Affecting Its Future

By J. Michael Lockhart,1 E. Tom Thorne,2 and Donald R. (Pete) Gober3

1

U.S. Fish and Wildlife Service, National Black-footed Ferret Conservation 

Center, P.O. Box 190, Wellington, CO 80549.

2

Deceased; Wyoming Game and Fish Department, 709 University Ave., 

Laramie, WY 82072.

3

U.S. Fish and Wildlife Service, 420 S. Garfield Ave., Suite 400, Pierre, SD 

57501.
A Historical Perspective on Recovery 

specimen in the 19th century to anecdotes from rodent control 

personnel in the mid-20th century, largely treated the species 

as a novelty. Its secretive and nocturnal lifestyle in remote 

parts of western North America assured relative anonymity. 

Conversely, the ferret’s principal prey received much more 

attention: prairie dogs were regarded as vermin, competitors 

with the agriculture industry, and impediments to western 

settlement. Accordingly, prairie dog populations were targeted 

for destruction, and decades of eradication efforts significantly 

impacted populations of prairie dogs and ferrets, the prairie 

dog’s most highly specialized obligate predator.

Five species of prairie dogs occur in North America. 

Three of these species cover most of the collective prairie dog 

(and ferret) historical range: the black-tailed prairie dog (C. 

ludovicianus), the white-tailed prairie dog (C. leucurus), and 

the Gunnison’s prairie dog (C. gunnisoni). No documented 

occurrence of ferrets has been linked to either the Utah prairie 

dog (C. parvidens) or the Mexican prairie dog (C. mexicanus). 

These two species have ranges that are relatively small and 

disjunct from those of the other three prairie dog species. 

Black-tailed prairie dogs occurred over the largest expanse of 

land (approximately 160 million ha), from southern Canada 

to northern Mexico between the 98th meridian and the Rocky 

Mountains. White-tailed prairie dogs and Gunnison’s prairie 

dogs occurred over approximately 80 million ha to the west 

of the range of the black-tailed prairie dog. Perhaps 10–20 

percent of the range of all of these prairie dog species was 

physically occupied before western settlement. The actual 

location of prairie dogs varied with topography, soils, rainfall, 

fire, bison (Bison bison) activity, and other factors. Regardless 

of the dynamics of these important and variable biological and 

ecological phenomena, it is obvious that ferrets had an enormous habitat base, and many thousands undoubtedly existed 

across the landscape.

Ferret populations declined over millions of hectares of 

occupied prairie dog habitat for three principal reasons. First, 

a major conversion of native prairie to cropland began late in 

the 19th century, continued steadily through the Dust Bowl 

years of the 1930s, and to a lesser extent continues today. 

Approximately one-third of black-tailed prairie dog potential 

habitat was rendered useless for prairie dogs by cropland 

conversion. In comparison, far less of the available habitat of 

white-tailed and Gunnison’s prairie dogs was physically lost 

to land conversion because, outside of riparian corridors and 

proximate irrigated lands, much of the habitat occupied by 

these species is not suitable for crops.

Second, poisoning of prairie dogs as a means of reducing 

competition with domestic livestock for forage accelerated 

with agricultural policies in the United States around the time 

of World War I. Significant Federal funds became available for 

poisoning programs across the West from approximately 1918 

to 1971, after which the use of many of these chemical rodenticides was banned. Tens of millions of hectares of occupied 

prairie dog habitat were eliminated during this period. Many 

poisoned prairie dog complexes have never recovered to levels 

that could support ferret populations. 

Third, the exotic disease sylvatic plague, foreign to 

the evolutionary history of prairie dogs, was inadvertently 

introduced into North America around 1900. The impact of 

this disease on prairie dogs and ferrets has been significant. 

Plague has been documented over all of the ranges of the three 

principal prairie dog species, except for approximately the 

eastern third of the black-tailed prairie dog range.

Biologically, the prairie dog ecosystem was devastated 

by factors described above, and the consequences to ferret 

populations were even greater. Politically, failure to consider 

the ramifications of these impacts also resulted in diminished 

populations of many other species and in several cases led to 

later consideration of further regulatory protection, such as 

listings under the ESA. Socially, the domination of farming 

and ranching activities on most lands in the American West 

has more severely impacted some species than others. The 

nadir of occupied prairie dog habitat probably occurred around 

1971, when certain toxicants were banned for prairie dog 

poisoning. Many poisoned prairie dog populations apparently 

have increased severalfold since that time but remain low relative to historical numbers. Nevertheless, the limited recovery 

of some prairie dog populations is important in the context of 

potential ferret recovery and long-range management. 

Ferret occurrence undoubtedly mirrored fluctuations in 

prairie dog populations. Steadily declining numbers of credible ferret sightings occurred during the 1950s and 1960s. The 

last population of a few dozen animals was thought to have 

been located in Mellette County, S. Dak., in 1964 (fig. 1). This 

population was studied through 1974, and a few animals were 

captured for a captive breeding trial (see below).

Biologically, the initial field studies of wild ferrets in 

South Dakota provided a starting point for later reintroduction 

efforts—a small success; however, this population continued 

to decline in the face of reduced and progressively fragmented habitat. The political will to conserve this individual 

population through regulatory action did not exist in the era 

preceding passage of the ESA. This lack of action represented 

a notable conservation failure inasmuch as, however difficult the challenges of recovering wild populations in native 

habitat may be, those challenges pale in comparison to the 

trauma, demands, and resources required for last-ditch captive 

breeding and reintroduction efforts. Such invasive, intensive 

recovery programs add many other management dimensions 

and require more adaptive and risky decisions.

Socially, at the national level, the American public was 

just becoming aware of the demise of a number of species 

but had not reacted sufficiently to spur government action to 

conserve even this last ferret population. At the local level, 

“business as usual” ranching practices continued to pursue 

complete eradication of prairie dogs because of their real 

and perceived competition with domestic livestock. There 

was little recognition of the ecological importance of prairie 

dogs and there were no incentive-based initiatives available to 

conserve this important resource. The lack of understanding 

and will to maintain viable prairie dog habitats for associated 

species was a marked failure.
 Recovery of the Black-footed Ferret

Are we any more prepared today to deal with such a 

crisis in a nonregulatory manner? Are means to settle such 

diametrically opposed concerns readily available if similar 

circumstances occur again? Unfortunately, we believe the 

answer to these questions is “no” and reflects another substantial failure in the ability of divergent interest groups, State and 

Federal agencies, and tribes to find reasonable compromises 

needed to preserve sensitive species and biological diversity. 

The inability to find a single ferret in Mellette County, S. Dak., 

after 1974 was widely regarded at that time as the final demise 

of the species and must be viewed as a catastrophic conservation failure.

Despite unconfirmed reports, many biologists thought 

that ferrets were extinct until a ranch dog killed a wild ferret 

near Meeteetse, Wyo., in 1981. This event ushered in a long 

recovery process that was widely heralded as a remarkable 

second chance to conserve a species thought to be lost forever. 

Intensive attention was focused on this population of over 100 

animals (20−40 adults). Considerable field data were acquired 

from 1981 to 1986 until epidemics of sylvatic plague and 

canine distemper took a heavy toll on both prairie dogs and 

ferrets. In hopes of salvaging the species, all remaining wild 

ferrets were removed from the Meeteetse population between 

1985 and 1987 to initiate a captive breeding program.

It must also be acknowledged that underlying social 

support for endangered species conservation was just beginning to emerge in the 1960s and 1970s. Accordingly, no 

clear decisionmaking responsibilities were established for 

ferret recovery during this period, leading to later disputes 

and second guessing among involved entities. As a means of 

Meeteetse

Historical ranges

Prairie dog/black-footed ferret

Red wolf

California condor

Reintroduction sites

Black-footed ferret

Red wolf

California condor

Mellette

County

2

5

3

6

4

8

7

9

1

Figure 1. Location of black-footed ferret (Mustela nigripes) reintroduction sites overlaid on the collective ranges of three major prairie 

dog (Cynomys) species that are considered their obligate prey. Numbers represent the chronological sequence of ferret reintroductions. Also shown are locations of California condor (Gymnogyps californianus) and red wolf (Canis rufus) reintroduction sites in relation 

to their historical ranges.
A Historical Perspective on Recovery 

soliciting advice from experts and interested parties and to 

help prescribe management direction, WGFD established a 

Black-footed Ferret Advisory Team (BFAT) in 1982. Considerable acrimony existed within the recovery program during 

the 1980s and early 1990s, and, although it is never clear when 

criticism ceases to be constructive, we believe that more assertive and effective leadership by FWS during this period might 

have helped reduce conflict. We address this issue further in 

our Summary and Recommendations section.

Despite nearly two decades of extensive and intensive 

searches, and apart from occasional, unsubstantiated reports, 

no wild ferrets outside of reintroduction areas have been 

detected following capture of the last Meeteetse ferret in 1987. 

Further, we do not believe, given the passage of time and the 

expansion of plague in western environs, that any undiscovered ferret populations of wild origin exist anywhere in North 

America today.

With reduced size and quality of prairie dog complexes 

across most of North America and the presence of plague in 

many western States, the sustainability of reintroduced ferret 

populations has also been suspect until very recently. We 

believe that a wild, self-sustaining ferret population now exists 

at Conata Basin, S. Dak., and perhaps on Cheyenne River 

Sioux tribal lands in South Dakota and at Shirley Basin, Wyo. 

The ability to crop a harvestable surplus of ferret kits from 

Conata Basin for translocation to other reintroduction areas is 

a recovery program benchmark of exceptional importance. The 

Conata Basin ferret population likely represents the largest 

and most sustainable population that has existed since species 

listing in 1967, and perhaps for decades before.

Management of wild populations of ferrets (table 1) can 

be divided into three phases. In comparing these three phases, 

phase 1, from the mid-1960s to the mid-1970s in Mellette 

County, S. Dak., ended in disappointment and was greeted 

largely with resignation. Management efforts during that 

period were low in intensity and had few available resources. 

Phase 2, from 1981 to 1987 at Meeteetse, Wyo., was a 

catastrophe and was largely viewed as a second chance that 

nearly slipped away. Biological studies at Meeteetse enjoyed 

considerable resources and were much more intensive, but the 

existing recovery plan provided little realistic management 

guidance. Agencies responsible for ferret recovery at that 

time often disagreed on research and management needs, and 

required action was sometimes slow to develop. Phase 3, from 

about the year 2000 and into the future, has a much broader 

base of support, including a sustainable captive population and 

multiple reintroduction sites, yet presents new challenges such 

as habitat improvement needs, genetic management concerns, 

demographic supplementation in response to adverse stochastic events, diminishing financial resources, and so on. In short, 

the challenges of continued success now require anticipation 

of, and responses to, yet unidentified limitations in ferret 

recovery. Only time will tell whether program collaborators 

and resources will be adequate to address future uncertainties, but the organizational structure and depth of the recovery 

program partnership is a successful achievement in its own 

right and will perhaps ensure more effective responses to any 

forthcoming environmental or social obstacles.

Captive Breeding

A notable advance in black-footed ferret conservation 

was the capture in 1971 of six animals from the Mellette 

County, S. Dak., population for a landmark captive breeding 

trial. This action was deemed essential because, during the 

preceding 6 years of field studies in South Dakota, no other 

black-footed ferret population was discovered nor was any 

other population known to exist within the ferret’s historical 

range. The 1972 captive breeding attempt was conducted at 

FWS’s (now U.S. Geological Survey’s) Patuxent Wildlife 

Research Center (Patuxent), where management preparations 

for captive breeding had begun in 1968 with breeding tests of 

surrogate species (domestic ferrets, Mustela putorius furo, and 

later Siberian polecats, Mustela eversmannii ). Three additional South Dakota animals were captured and transported to 

Patuxent for captive breeding in 1972−73.

Efforts to breed black-footed ferrets at Patuxent were 

crippled from the outset by vaccine-induced canine distemper. Scientists at Patuxent were aware of the susceptibility of 

domestic ferrets to canine distemper and tested an attenuated 

distemper vaccine on domestic ferrets to determine safety and 

efficacy. Although the vaccine was proven safe in domestic 

ferrets, it induced fatal distemper in four of six vaccinated 

black-footed ferrets, demonstrating extreme susceptibility to 

this common viral disease.

Although only nine black-footed ferrets were available 

for breeding at Patuxent, four of which were lost to vaccineinduced canine distemper, the effort was modestly successful. 

Two litters of five kits each were born in successive years 

to a single female. Unfortunately, no kit survived more than 

a couple of days. The remaining captive ferrets eventually 

died, and, unfortunately, the breeding research program using 

surrogate animals was abandoned.

The Patuxent experience demonstrated that black-footed 

ferrets could be bred in captivity and that captive breeding 

might be successful if sufficient animals and resources were 

available. This experience also provided valuable lessons 

regarding diseases and endangered species recovery. Testing 

an attenuated vaccine on a surrogate species proved not to be 

the fail-safe procedure for ensuring vaccine safety for a highly 

susceptible, highly endangered species. Infectious diseases and 

their impacts on small populations became obligatory considerations for future recovery of ferrets and other endangered 

species.

Early ferret recovery efforts at Meeteetse were marred 

by poor planning, inadequate resources, conflict, controversy, 

and crisis. Events subsequent to discovery of the Meeteetse 

population have been, and will continue to be, used as both 

good and bad examples of endangered species management. 
10 Recovery of the Black-footed Ferret

Year Wild populations Captive populations Reintroduced populations

1964 Small wild population found in South 

Dakota

1972 First captive breeding trials at 

Patuxent Wildlife Research Center, 

Laurel, Md.

1974 South Dakota population extirpated

1979 Last captive from South Dakota 

dies; species presumed extinct

1981 Last known free-ranging population 

discovered near Meeteetse, Wyo.

1986 First captive breeding effort using 

Wyoming stock

1987 Last live ferret removed from Meeteetse;

population extirpated

First successful reproduction and 

weaning in captivity

1989 First of several additional captive 

breeding facilities established

1991 First reintroduction: Shirley Basin, 

Wyo.

1994 Second and third reintroductions: 

Conata Basin/Badlands, S. Dak., 

and southern Phillips County, 

Mont.

1996 Fourth reintroduction: Aubrey 

Valley, Ariz.

1997 Fifth reintroduction: Fort Belknap, 

Mont.

1999 Captive population objective 

established in Species Survival 

Plan® reached; captive population 

considered stable

Sixth reintroduction: Colorado/

Utah border

2000 First reintroduced population with 

harvestable surplus of kits for 

translocation to other sites

Seventh reintroduction: Cheyenne 

River Sioux tribal lands, South 

Dakota

2001 Eighth reintroduction: Janos, 

Chihuahua, Mexico

2002 Wild ferrets exceed captive ferrets in 

number

2003 Ninth reintroduction: Rosebud 

Sioux tribal lands, South Dakota

Table 1. Key events in recovery of the black-footed ferret (Mustela nigripes).
A Historical Perspective on Recovery 11

Black-footed ferret recovery efforts in the 1980s and early 

1990s were highly scrutinized and frequently criticized, 

often by individuals uninformed about recovery events 

and/or Federal and State laws regarding endangered species 

management. Although some reviewers and participants were 

well-intentioned and constructive, others ignored difficult 

problems and overlooked lessons learned in earlier ferret 

recovery efforts and other endangered species programs.

The Black-footed Ferret Recovery Plan of 1978 was 

written at a time when ferrets were thought to be extinct, and it 

provided little or no effective guidance for management of the 

Meeteetse population. In March 1982, WGFD quickly formed 

the aforementioned BFAT. This multiagency and private 

sector committee provided advice and guidance to WGFD in 

determining management direction for the Meeteetse population. Black-footed ferret numbers at Meeteetse increased 

from discovery in 1981 through 1984, when the population 

comprised approximately 40 adults and 90 juveniles. Based 

on this somewhat robust population and because no other wild 

population had been found (despite increased interest and 

improved search techniques), WGFD and FWS jointly decided 

in May 1985 to capture a small number of black-footed ferrets 

the following fall to begin a modest captive breeding trial. The 

WGFD’s Sybille Wildlife Research and Conservation Education Center (Sybille) near Wheatland, Wyo., was selected as 

temporary quarters for captured ferrets. Ultimately, knowledge 

gained from Patuxent was used to develop technology and 

facilities at Sybille to support a long-term captive breeding 

program that would eventually provide animals for reintroduction and protect the species from extinction in the event of an 

unexpected catastrophe at Meeteetse.

The decision to initiate a captive breeding program did 

not come easily or without conflict. It took optimism generated by the high number of black-footed ferrets documented in 

the fall of 1984 and assurances that funding would be supplied 

by FWS in 1986 or 1987 for WGFD to commit to captive 

breeding. Although captive breeding was started less than 4 

years after discovery of the Meeteetse population, in hindsight 

it should have been started earlier. The delay is understandable, however, and can be largely attributed to uncertainties in 

the availability of funding and suitable facilities and to the fear 

of animal loss and breeding failure. Lessons learned through 

this process were twofold: captive breeding of critically endangered species should be initiated early, and adequate advance 

planning and committed funding for personnel and facilities 

are essential. These lessons are not unique to the black-footed 

ferret program and were applicable to other endangered 

species, including the red wolf (Canis rufus) and California 

condor (Gymnogyps californianus).

The importance of these lessons became evident in the 

summer of 1985, even before the first attempts to capture 

ferrets for breeding. In June 1985, sylvatic plague was identified in Meeteetse prairie dog colonies. Sylvatic plague usually 

causes extensive die-offs in affected prairie dogs. In an attempt 

to halt the disease episode (by killing flea vectors of plague), 

FWS and WGFD conducted a massive plague-control program 

and dusted approximately 80,000 prairie dog burrows with 

the insecticide carbaryl. Nevertheless, the epizootic reduced 

Meeteetse prairie dog colonies by about 20 percent in the first 

year. Although the susceptibility of prairie dogs to sylvatic 

plague was well known, at that time black-footed ferrets were 

thought not to be susceptible. Siberian polecats, domestic 

ferrets, and other mustelids and carnivores are largely immune 

to sylvatic plague. Several years later, it was discovered that 

black-footed ferrets are actually exceptionally sensitive to 

plague. Black-footed ferret numbers at Meeteetse declined 

sharply over the summer of 1985, and only 58 animals were 

documented by August.

In September and October 1985, six ferrets were captured 

for captive breeding trials and moved into temporary quarters 

at Sybille. These captures led to discovery of another cause 

of the decline of ferret numbers at Meeteetse. One of the last 

two animals trapped died from canine distemper, an illness 

undoubtedly contracted before capture. Based on the experience at Patuxent with vaccine-induced canine distemper and 

the highly contagious nature of this disease, it was predicted 

that the remaining black-footed ferrets originally transferred 

to Sybille would succumb to the disease, which proved true. 

It was also predicted that most, if not all, animals in the 

free-ranging population at Meeteetse would be lost. Although 

extensive precautions had been taken to prevent introduction 

of diseases from outside sources, no precautions were taken 

to guard against known ferret diseases within the free-ranging 

colony. Another lesson learned.

At that point, management of the free-ranging blackfooted ferret population and the fledgling captive breeding 

program were thrown into crisis. Given the reduced numbers 

of ferrets, it was unlikely that simultaneous efforts to maintain 

a wild population and start a captive breeding program would 

succeed; given the presence of disease, it was questionable 

whether a wild population could realistically be preserved. 

The WGFD, an agency accustomed to managing free-ranging 

wildlife, quickly, but with difficulty, decided to begin a second 

capture effort to obtain founders for captive breeding. This 

action was taken with recognition that it would likely lead to 

extirpation of the wild population. Interestingly, opposition 

to this second capture effort came primarily from individuals 

who disagreed with human interference and felt that blackfooted ferrets should be allowed to go extinct.

In October and November 1985, six more ferrets were 

captured and placed in strict isolation quarters at the University of Wyoming until it was certain that all were free of 

canine distemper. Capture efforts were halted in late fall in the 

hope that any free-ranging black-footed ferrets remaining at 

Meeteetse would breed in the wild. Also during this period, 

WGFD invited the Conservation Breeding Specialist Group 

(CBSG) of the Species Survival Commission of the International Union for the Conservation of Nature and Natural 

Resources to provide expert advice on developing a captive 

breeding program.

Captive ferrets did not breed during the 1986 season, 

probably because of male immaturity and stresses associated 
12 Recovery of the Black-footed Ferret

with capture and a captive environment. Recovery partners 

experienced especially difficult times in 1985 and 1986. The 

responsible agencies received pointed and outspoken criticism; 

accusations ranged from allegations of mismanagement to 

intentional exposure of ferrets to canine distemper. Conflicting 

recommendations were received, including capturing all freeranging black-footed ferrets immediately, removing captive 

animals from WGFD’s care and terminating its role, curbing 

all further management intervention in the fate of the wild 

population, and so on. In spite of stresses, alliances between 

WGFD, FWS, and other recovery partners began to solidify. 

Five free-ranging black-footed ferrets survived the 

canine distemper epizootic at Meeteetse, and two females 

subsequently produced litters in the spring of 1986. According 

to a contingency plan prepared by WGFD with assistance of 

CBSG and approved by FWS, the WGFD decided in August 

1986 to capture all remaining animals. The last of 12 known 

wild black-footed ferrets was trapped in February 1987, and 

all captured ferrets were taken to a newly completed captive 

breeding facility at Sybille. The free-ranging black-footed 

ferret population at Meeteetse was thus extirpated, and the 

remaining captive individuals became one of the most endangered mammals in the world, as well as the last hope for the 

species. Survival and future recovery of the black-footed ferret 

now depended on development of an effective captive breeding 

program, which at that time had no proven track record.

Events from October 1981 through early 1987 occurred 

rapidly and developed in the absence of a current or applicable 

recovery plan, but critical decisions were still necessary. 

Perhaps issues faced by WGFD, FWS, BFAT, and CBSG 

could have been better anticipated and addressed, including 

analysis and consideration of probabilities of extinction of the 

small Meeteetse black-footed ferret population; when and how 

to best initiate captive breeding efforts; how to fund captive 

breeding; potential responses to epizootics of canine distemper 

and sylvatic plague; when and how to remove the last freeranging animals to prevent extinction; appropriate responses 

to discovery of another wild ferret population; and advance 

identification and preparation of suitable reintroduction sites.

By spring of 1987, with funding assistance from FWS, 

the captive black-footed ferret colony was moved into permanent facilities at Sybille. With the assistance of CBSG and the 

University of Wyoming, WGFD improved captive husbandry 

protocols, began a genetic management plan, and began intensively monitoring black-footed ferret reproductive cycles and 

pairing events. Captive breeding techniques were developed 

with the intent of minimizing stress and injury and maximizing longevity, productivity, and genetic contribution of founder 

animals without promoting domestication. Two litters were 

born, and seven kits were weaned in 1987. Production of these 

kits was exceptionally significant, not because of the number 

of kits weaned or their genetic makeup but because it was 

finally demonstrated that black-footed ferrets could be bred 

and reared successfully in captivity.

The CBSG, WGFD, and FWS held a workshop on blackfooted ferret conservation biology in 1986. A captive breeding 

program plan (A Strategic Plan for the Management of Blackfooted Ferrets in Wyoming), with time-specific objectives, was 

written in 1987. This plan recognized that few animals were 

available for captive breeding and that many were related. It 

called for maintaining approximately 90 percent of the original 

genetic heterozygosity of founder ferrets over a relatively short 

period of 50 years. This would be accomplished by establishing a captive population of 500 animals with a stable age and 

sex distribution, which would provide an effective population 

of about 250 ferrets and approximately 200 breeding-age 

animals. During the early stages of the program, increasing the 

population had priority over managing genetics. Subsequently, 

ferret pairings were planned to maximize retention of founder 

alleles.

Another objective of the strategic plan was to raise 

enough animals to establish a second geographically removed 

population to protect the species from catastrophic loss. The 

two-facility objective was reached in 1988 when a few ferrets 

were moved from Sybille to the Henry Doorly Zoo, Omaha, 

Nebr., and the National Zoological Park’s Conservation & 

Research Center, Front Royal, Va. Demographic and genetic 

data were maintained in a detailed studbook. Separate captive 

populations were genetically managed and bred as a single 

population. The strategic plan also called for initial ferret 

reintroductions to begin in 1991, provided that the captive 

population reached 500 animals with 200 breeding-age adults.

A revised Black-footed Ferret Recovery Plan was 

completed by FWS in 1988. It included most of the goals of 

the Wyoming strategic plan and called for ensuring immediate survival of black-footed ferrets by increasing the captive 

population to 200 breeding adults by 1991. Additionally, it 

included a downlisting goal of establishing a prebreeding 

population of 1,500 free-ranging, breeding-age ferrets in 10 or 

more populations, as widely distributed as possible over the 

historical range, by the year 2010.

Over the next few years, additional captive populations 

were established at the Cheyenne Mountain Zoo, Colorado 

Springs, Colo.; the Louisville Zoo, Louisville, Ky.; the 

Metropolitan Toronto Zoo, Toronto, Ontario, Canada; and the 

Phoenix Zoo, Phoenix, Ariz. The contribution of these zoos 

to ferret recovery was enormous. They received no monetary 

support from WGFD or FWS and initially were not allowed to 

use ferrets for exhibit. Another requirement was that participating zoos be accredited by the American Zoo and Aquarium 

Association (AZA), which had established a program to 

develop Species Survival Plans® to facilitate captive breeding 

of endangered and declining species. With growing involvement of AZA zoos and maturation of the captive breeding 

program in 1989, the technical advisory role previously filled 

by CBSG was vested in AZA through an established Blackfooted Ferret Species Survival Plan (SSP) and associated 

Management Group of facility specialists.

The partnership between the AZA’s SSP, WGFD, and 

FWS was, and remains, successful, and black-footed ferret 

recovery has benefited greatly from this relationship. Participants in the Black-footed Ferret SSP Management Group 
A Historical Perspective on Recovery 13

included representatives from involved zoos, WGFD, and 

FWS, as well as numerous outside advisors who provided 

extensive expertise in husbandry, veterinary care, disease, 

reproductive management, population management, and genetics. The commitment of the SSP Management Group, CBSG, 

and their many advisors was rewarded in 1991 when captivebred black-footed ferrets were first reintroduced into the wild 

at Shirley Basin in central Wyoming.

With expansion of the reintroduction phase of blackfooted ferret recovery to sites outside Wyoming, and with 

dwindling financial resources within WGFD, management 

responsibility for the Sybille captive breeding facility shifted 

to FWS in 1996. The breeding facility is now part of the 

National Black-footed Ferret Conservation Center and has 

moved to a new site in northern Colorado. The new facility 

became operational in the fall of 2005.

A review of the Black-footed Ferret Recovery Program, 

with particular attention to captive breeding elements, was 

undertaken by AZA in 1995, and a similar analysis and update 

was accomplished by CBSG in 2003. These critical reviews 

of the history, progress, and operations of captive breeding 

projects have been instrumental in evaluating problems and 

providing important input into the direction and management 

of captive breeding. 

Beginning in 1997, an effort was undertaken to increase 

overall captive production and kit quality for reintroduction 

purposes. Ferrets excess to the SSP were used to develop 

limited field breeding projects in association with reintroduction efforts in Arizona, Colorado, and Montana. A separate 

field breeding facility (no accompanying reintroduction 

effort) was constructed near Raton, N. Mex., by the Turner 

Endangered Species Fund. Although many of the basic SSP 

husbandry and breeding procedures were employed, these 

projects were initially designed to test more hands-off breeding strategies in outdoor pens with well-developed prairie dog 

burrow systems. Breeding success at these facilities has been 

mixed and has varied substantially between sites. In addition, 

the expense of operating pen breeding facilities has often been 

greater than anticipated and, as a consequence, has affected 

other elements of reintroduction projects. The objective of 

increasing production and potentially improving the survival 

of released ferrets, however, remains a guiding principle of the 

recovery program and has been a recurring recommendation in 

all formal program reviews. Additional evaluation and refinements of pen breeding capabilities warrant further attention.

Although the black-footed ferret captive breeding 

program will continue to evolve, it has been a great success: 

the first animals were captured for captive breeding 4 years 

after discovery of the Meeteetse population; the program 

survived the sylvatic plague and canine distemper crisis of 

1985 and extirpation of the species in the wild by 1987; and 

the first experimental reintroduction occurred just 6 years 

after the first animals were captured to initiate captive breeding. The captive breeding program has produced and weaned 

over 4,800 ferrets (through 2003) and has supported several 

reintroduction efforts across the former range of the species. 

In addition, the program has provided animals for essential 

disease and vaccine research, for survival enhancement, and 

for educational exhibit. For the foreseeable future, blackfooted ferret recovery hinges on the continued success and 

management of the captive population by involved agency and 

zoo partners. 

Reintroduction

With increasing success in black-footed ferret captive 

breeding efforts in the late 1980s, recovery program partners 

focused greater attention on restoring wild ferret populations. 

Initial interest was directed at reestablishing a ferret population at the site of their last wild origin near Meeteetse, Wyo., 

but sylvatic plague substantially reduced overall habitat quality on the Meeteetse prairie dog complex, thus rendering the 

site unsuitable for ferrets. Today, prairie dog populations in the 

Meeteetse area are still depressed, which highlights a serious 

obstacle to ferret recovery. In the absence of effective plague 

intervention and management capabilities, many affected prairie dog colonies may never regain historical population levels 

and may never support viable ferret populations. This issue is 

discussed at greater length below.

The primary goal of the black-footed ferret recovery 

program is to reestablish a sufficient number of viable, wild 

ferret populations in order to downlist and recover the species, 

remove it from ESA protections, and terminate the expensive 

captive breeding program now necessary to support species 

survival and recovery efforts. Between 1991 and 2003, 12 

discrete reintroduction projects were initiated at nine reintroduction areas in six western States (Arizona, Colorado, 

Montana, South Dakota, Utah, and Wyoming) and at one 

site in northern Chihuahua, Mexico (fig. 1). The history and 

results of these specific reintroduction efforts are addressed 

elsewhere and will not be detailed here. Instead, we highlight 

overall direction, research, monitoring efforts, ferret survival, 

and success in general terms and from our personal views and 

recommended direction.

As of 2003, over 1,800 ferrets had been reintroduced 

into the wild. Success of these efforts, in terms of establishing self-sustaining populations, has been mixed and affected 

most significantly by habitat suitability (which, in turn, is most 

affected in recent years by the presence of sylvatic plague 

throughout most of the historical ranges of prairie dog species 

and ferrets). Only in South Dakota do large, relatively contiguous, and plague-free prairie dog complexes remain, and the 

greatest reintroduction success to date has been at the Conata 

Basin site (Buffalo Gap National Grasslands) in south-central 

South Dakota. Conata Basin experienced exponential growth 

in the wild ferret population following only 3 years of reintroduction and a total release of 165 captive animals. Ferrets at 

Conata Basin may have spread through most of the available 

habitat and are possibly approaching population saturation 

levels, having produced a relatively consistent 60 to 70 litters 
14 Recovery of the Black-footed Ferret

annually over the past 3 years. Conata Basin is the only site to 

have reached a success level that allows translocation of wildborn kits to other reintroduction sites, and kits from Conata 

Basin have been used to support reintroductions in Colorado 

and at two other sites in South Dakota. Similar successes are 

expected at two more recent South Dakota reintroduction 

sites that also support relatively large, plague-free prairie dog 

complexes (Cheyenne River Sioux and Rosebud Sioux tribal 

lands). In contrast, plague has been documented either in or 

around five of the other reintroduction areas outside of South 

Dakota. 

To date, there have been six reintroduction projects in 

black-tailed prairie dog habitat (Montana, Mexico, and South 

Dakota), two in white-tailed prairie dog complexes (central 

Wyoming and an area straddling the Colorado/Utah border), 

and one in Gunnison’s prairie dog habitat (northwestern 

Arizona). Differences in prairie dog colony size, density, and 

life history (e.g., hibernation) exist both between and within 

species (over the extent of each species’ range) and undoubtedly influence site quality and success of ferret reestablishment. For the most part, black-tailed prairie dogs currently 

occupy definable “towns” of varying size (but historically 

included enormous colonies), exist at greater densities, and 

typically do not undergo extended torpor. On the other hand, 

Gunnison’s and white-tailed prairie dogs are more scattered 

and less dense over areas they occupy and hibernate for 

extended periods.

Other regional factors such as geography, vegetation, 

annual precipitation, and drought also affect site suitability 

and reintroduction potential. Land ownership patterns, land 

use, access, and vehicle use constraints further affect implementation and monitoring efficacy of reintroduction projects. 

As a consequence of such dynamics, the recovery program has 

experienced both outstanding and rapid reintroduction success, 

as well as projects that have struggled to maintain even minimal populations. Still, no reintroduction project is regarded as 

a failure, and all have provided vital experience and information to help foster ongoing and future recovery efforts across 

the ferret’s historical range. Moreover, given the severe limitations in available reintroduction sites in North America today, 

it would be imprudent to give up on any reintroduction area 

that has the necessary land base and potential for improved 

habitat conditions and ferret recovery.

Only ferrets considered excess to the captive population 

have been used to support reintroduction projects. The captive, 

SSP population is regarded as the essential foundation of the 

species today. And with the exception of ferret releases in 

Mexico, all reintroductions have been accomplished under 

a special provision of the ESA (section 10(j)) that provides 

for designation of reintroduced populations as “nonessential, 

experimental.” Ferrets released into nonessential, experimental population areas (as established via Federal rulemaking 

and ESA consultation procedures) are no longer classified or 

managed as endangered but are given protection and management flexibility similar to that provided for “candidate” 

species (candidates for listing under ESA but not yet listed). 

This relaxed management flexibility was necessary to gain 

the support of State governments and private landowners for 

releases of an endangered species into areas with fragmented 

ownership patterns of public and private lands.

The section 10(j) nonessential, experimental provisions 

facilitated ferret reintroduction trials; it is unlikely that most 

projects would have been successfully implemented without 

10(j) or a similar mechanism to reduce the perceived consequences of potential expansion of endangered ferrets onto 

private lands. The nonessential, experimental designation has 

other limitations that impede ferret recovery, however, and 

a review of the utility of 10(j) and reexamination of other 

options to tailor reintroductions to site-specific situations are 

warranted. More discussion is provided below.

With the recovery program becoming more focused on 

reestablishing wild ferret populations in the early to mid1990s, other forms of partner acrimony began to surface. 

Disputes over “soft” versus “hard” release techniques, ferret 

preconditioning and predator avoidance training, predator 

control, use of radio telemetry to document survival, how 

and where to prioritize excess ferrets for release, State versus 

Federal authorities, and other research issues, some of which 

first began to surface during the Meeteetse era, seemed to 

deepen divisions among some participants.

Despite individual and agency conflicts and occasional 

setbacks, a committed partnership of biologists and administrators set aside differences (or, perhaps more accurately, 

worked around them) to focus attention on biological and 

social impediments to ferret recovery. As a consequence, 

reintroduction efforts continued to gather momentum between 

1991 and 2003, and a wealth of information was gathered over 

that period about how to effectively reestablish ferret populations and respond to obstacles.

Also, the recovery program experienced a somewhat 

unexpected and positive turnaround in the status of two 

reintroductions over the past several years. Reintroductions at 

Shirley Basin, Wyo., were suspended in 1994 because of an 

extensive plague outbreak. Subsequent monitoring suggested 

that the small remaining ferret population was likely to be 

lost by the end of the 1990s; however, starting in 2002, the 

Shirley Basin population exhibited its own exponential growth 

and appears to be rapidly developing into a self-sustaining 

population. Prairie dog populations in Shirley Basin are also 

rebounding. Likewise, wild ferret production at Aubrey Valley 

in northwestern Arizona has recently increased following 

years of reintroduction attempts, probably as a result of spring 

ferret releases (releases designed to place ferrets in the wild 

when they can best exploit prairie dogs emerging from hibernation and young pups).

Perhaps one of the most significant findings from the 

early reintroduction development stages was recognition of the 

importance of preconditioning captive ferrets prior to release 

in the wild and an associated partner commitment to expand 

preconditioning capacity by constructing a number of outdoor 
A Historical Perspective on Recovery 15

facilities at or near reintroduction sites in Arizona, Colorado, 

Montana, New Mexico, and South Dakota. In simplest terms, 

preconditioning is the exposure of captive-produced ferrets to 

a more natural outdoor environment with relatively large pens, 

prairie dog burrow systems, and live prairie dog prey. In these 

pens, cage-reared ferrets become accustomed to the security 

and life needs of prairie dog colonies; essentially, they learn 

to behave like wild ferrets. As addressed above, outdoor pen 

facilities in Arizona, Colorado, Montana, and New Mexico 

were also used to conduct experimental breeding trials to 

increase the number of animals available for reintroduction.

Certainly, much has been learned about reintroducing 

ferrets. Still, many pressing uncertainties remain, particularly 

those dealing with management of ferrets in plague-affected 

environs, and additional research and reintroduction trials 

are warranted. And perhaps one of the best ways to speed 

recovery is to “cast a wide net” by placing ferrets in as many 

potential sites as possible and letting ferrets ultimately reveal 

what constitutes favorable habitat and management conditions.

As noted above, in 1995 the AZA was contracted by 

FWS to conduct a comprehensive review of the ferret recovery 

program. In this review, the AZA examined the status and 

success of captive breeding and reintroductions, as well as 

the administration and general decisionmaking procedures of 

the program. Much of the information was gathered through 

a series of partner workshops. A final report to FWS in 1996 

included many valuable and specific recommendations on the 

biological and administrative needs of the recovery program. 

With waning funding in WGFD and expanding recovery 

efforts beyond the State of Wyoming, in 1996 FWS assumed 

primary responsibility for operation of the Sybille breeding 

facility and management of captive and field recovery activities. Following guidance provided in the AZA report and as 

set forth in ESA, FWS also established the Black-footed Ferret 

Recovery Implementation Team (BFFRIT) in 1996 and invited 

participation of agencies, organizations, and tribes directly 

involved in ferret recovery activities.

The BFFRIT replaced the original BFAT and subsequent 

Interstate Coordinating Committee as the vehicle for maintaining partner coordination and input into recovery program 

direction. The BFFRIT is composed of 26 cooperating State 

and Federal wildlife and land management agencies, tribes, 

zoos, conservation organizations, and the National University 

of Mexico. The BFFRIT charter established the Executive 

Committee (administrators of agencies/organizations who 

address recovery program policy and funding issues), the 

Conservation Subcommittee (composed of technical experts to 

deal with field reintroduction and research elements), and the 

Education Outreach Subcommittee (to expand public awareness of the recovery program and help pursue outside funding 

opportunities). The SSP Management Group, established 

originally by the AZA in 1991 and made up principally of zoo 

representatives, also effectively serves as a technical subcommittee to the BFFRIT and provides input and expertise on 

management of captive breeding programs.

Although FWS retains ultimate authority, the BFFRIT 

provides essential input and recommendations on all matters 

related to ferret recovery and has effectively guided program 

direction since its inception. To establish more balanced and 

objective procedures for allocating ferrets for reintroduction and research purposes, FWS (through the BFFRIT) also 

developed an annual ferret allocation and project evaluation process in 1996. This allocation process sets priorities 

for ferret distribution based on the biological suitability of 

proposed release sites, overall project and/or research merit, 

and potential recovery program benefits. Allocation proposals, 

which provide details on habitat attributes of project areas, 

disease and predator presence and management capabilities, 

project/research design and implementation capabilities, and 

so on, are submitted to FWS by mid-March each year. The 

proposals are then distributed to BFFRIT members for review.

The FWS awards a preliminary ferret allocation in late 

May via a report that fully discloses (albeit anonymously) 

comments and recommendations provided by BFFRIT 

members and a justification of FWS findings. The number of 

ferrets identified in the preliminary allocation is determined 

from an expected production level based on a 5-year average recruitment rate from the number and age distribution of 

female ferrets presently in the SSP-managed population. For 

now, and into the foreseeable future, first priority for ferrets 

goes to SSP facilities to maintain the genetic representation 

and viability of the essential captive population. Final allocation decisions are made in late summer and depend on the 

actual production achieved by SSP facilities and field breeding 

projects, as well as on resolution of any permitting or other 

project implementation deficiencies (e.g., funding, partnership 

commitments).

The current organizational structure of the Black-footed 

Ferret Recovery Program has had the benefit of time, conflict, 

and critical analysis to evolve. Fortunately, ferrets have 

persisted during periods of unforeseen crises and human 

mistakes. The BFFRIT is a product of important lessons 

learned over the history of the ferret program, and we 

believe it offers a good example of an effective strategy for 

management of large, complex, endangered species recovery 

programs. Instead of a typical, more academically based 

and smaller recovery team, a recovery implementation team 

provides a transparent decision process and equal voice to a 

large number of involved agencies, tribes, and organizations. 

Although FWS retains ultimate authority and responsibilities as specified in the ESA, the BFFRIT is relied on to help 

make informed program decisions, help resolve partner 

disagreements and other program conflicts, and generally keep 

recovery efforts moving in a positive direction. We regard 

such continued partner participation as essential to successful 

recovery of the black-footed ferret. 

Finally, the most pressing limitation to ferret recovery 

is availability of suitable habitat to restore and support wild 

populations. Although program partners have always understood that habitat availability is key to recovery, other program 
16 Recovery of the Black-footed Ferret

imperatives between 1985 and 1997 (i.e., development of 

captive breeding and reintroduction capabilities) somewhat diluted focus on habitat issues. In 1998, the BFFRIT 

Conservation Subcommittee was tasked to identify and prioritize the top 10 sites that could support ferret reintroduction 

across North America, as well as secondary areas that could 

possibly be developed into additional reintroduction sites. 

In 1988, only 10 years earlier, an internal FWS document 

suggested that as many as 38 suitable ferret reintroduction 

areas existed in the United States. That document was based 

solely on suggestions from program partners and did not 

attempt to accurately assess current prairie dog populations, complex size and quality, the status of plague, or other 

practical biological and political factors (e.g., land ownership 

patterns) necessary to determine reintroduction potential.

With more detailed surveys of BFFRIT partners, the 

1998 assessment of site potential indicated that only nine sites 

could immediately support ferret reintroductions across North 

America. Five of these sites were already engaged in reintroduction projects, and three more were activated between 1998 

and 2003. Since 1998, a few other potential release areas have 

been identified, but it is clear that prairie dog habitat throughout the historical range of the ferret has been so severely 

degraded that ferret recovery is not feasible without restoration 

of large, healthy prairie dog complexes.

What constitutes a suitable prairie dog complex for 

ferrets is a question still under scrutiny by program partners 

and is a pertinent issue to be addressed in an upcoming and 

long overdue revision of the Black-footed Ferret Recovery 

Plan. In examining the results of reintroduction efforts to date, 

the only relatively large, self-sustaining, wild population of 

ferrets (ca. 250–400 animals) is at Conata Basin, a site that 

contains some 6,070 ha of closely distributed and relatively 

dense black-tailed prairie dog colonies—prairie dog colonies 

that are also free of sylvatic plague and are managed to 

preserve high prairie dog habitat values. Although prairie dogs 

of all three species are well dispersed throughout their former 

ranges, prairie dog complexes are very small and highly 

fragmented compared to historical conditions. There are very 

few places within North America that approximate the quality 

of habitat for ferret recovery exhibited at Conata Basin.

The revised Black-footed Ferret Recovery Plan 

(published by FWS in 1988) sets forth a downlisting objective 

by the year 2010 of 1,500 adult (breeding) ferrets, established 

in no less than 10 separate populations across the historical 

range of the ferret, with at least 30 individuals in each population. Although a prebreeding census of 1,500 adult ferrets 

may be attainable with continued recovery success in the few 

large prairie dog complexes that exist, there appear to be only 

four or five sites today that have the potential to support viable 

ferret populations, the majority of which will likely be in 

plague-free complexes in South Dakota.

Future recovery of the black-footed ferret hinges on our 

ability to successfully reintroduce and reestablish relatively 

large, healthy populations in the wild. Unlike two other 

endangered, high-profile carnivores of the West, the gray 

wolf (Canis lupus) and grizzly bear (Ursus arctos), the entire 

breadth of habitat within the ferret’s historical range has been 

materially impacted: there are few large expanses of Federal 

public land with sufficient prairie dog habitat to support blackfooted ferret populations at the present time. Furthermore, 

although the amount of land potentially available for habitat 

restoration across the historical range of the ferret far exceeds 

potential capabilities for two other critically endangered 

species, the California condor and red wolf (fig. 1), there 

are significant social and political impediments to restoring 

and managing large blocks of grasslands for prairie dogs and 

ferrets. Prairie dogs are still largely regarded as vermin by 

private landowners and agricultural interests, and only small, 

relatively fragmented complexes are typically tolerated, if at 

all.

And so, ironically, we appear to be at yet another 

important crossroad for this species. Having brought the ferret 

back from the brink of extinction, having invested enormous 

national resources to right an ecological wrong, and having 

developed the necessary capabilities and expertise to actually recover this species, the future of the ferret hangs on our 

social and political will to set aside and develop sufficient 

habitats that could be managed for prairie dogs, ferrets, and 

other sensitive prairie wildlife species. Ferret recovery efforts 

have come full circle, and it will be an enormous challenge to 

overcome a prevailing attitude of “not in my backyard” when 

so few suitable, welcoming backyards are presently available. 

Summary and Recommendations

Given the status of the ferret in 1987, when only 18 live 

animals remained, we submit that ferret recovery has been one 

of the most successful endangered species programs to occur 

anywhere, at any time. Table 2 compares several recovery 

parameters for three well-known North American species 

that were listed as endangered in 1967. The black-footed 

ferret, California condor, and red wolf are notable “grandfathered species” that have always been included in the various 

versions of ESA. Each became further imperiled after listing, 

and each was subsequently removed from the wild for captive 

breeding and eventual reintroduction. Figure 1 indicates the 

historical range of these species and the sites where reintroduction has occurred. Clearly, the “recovery glass” is potentially “half full” for the ferret compared to species that mature 

less quickly, have fewer young, and whose range and essential 

habitats have been even more drastically altered.

The lessons taught by ferrets, condors, and red wolves 

should be carefully heeded. Reducing any species to such 

critically low population levels that captive breeding becomes 

the only possible recovery strategy is a poor way to ensure 

persistence, much less recovery. Extinction risks are elevated, 

recovery becomes more expensive, and bringing species into 

captivity may remove assurances that adequate habitats will be 

available for later reintroduction. 
A Historical Perspective on Recovery 17

The ferret recovery program has experienced a number 

of successes, failures, and hard-learned lessons over the years. 

It is important to continually characterize progress so the 

direction and speed of recovery can be adjusted accordingly. 

Overall, we believe that the ferret recovery program will probably continue to be successful, despite many near disasters and 

remaining obstacles. Ferret recovery efforts may be compared 

internally by contrasting results of different activities over 

time. Comparisons of the progress and success of other endangered species recovery efforts may also provide perspective 

that will aid ferret recovery.

To date, we believe that (1) black-footed ferret captive 

breeding has been highly successful, (2) ferret reintroductions 

have achieved a low to moderate degree of success, and (3) 

wild population management has experienced a low degree of 

success. Marked improvements and efficiencies have occurred 

in all of these areas over the last decade, however.

The most notable success of captive breeding has been 

the creation of a mutually supportive network of staff and 

facilities that successfully raise animals in a protected environment to ensure species survival and provide animals for 

reestablishment of wild populations. Problems that have 

occurred in this recovery component have been straightforwardly addressed. One nagging limitation that may or may not 

be possible to fully overcome in a captive environment is the 

relatively low whelping success in females (as compared with 

wild whelping rates). The most important lesson learned over 

the course of the ferret program, however, is that biological 

breakthroughs in complex recovery efforts can only be realized via the successful involvement of many diverse partners.

Ferret captive breeding may become more efficient as 

limitations are identified and addressed, but this recovery 

component is mature, with over 15 years of accumulated experience. Most attainable internal refinements may already have 

been achieved. If increased numbers of animals are required 

for reintroduction, it may be most practical simply to increase 

the number or size of facilities. Captive breeding of ferrets is a 

tightly controlled process with few extraneous factors affecting 

its continued success or failure. Extended captivity may reveal 

future biological constraints (e.g., inbreeding suppression), but 

most political and social obstacles to captive breeding appear 

to have been adequately addressed.

At present, the knowledge and resources needed for 

continued success of the captive breeding program appear to 

be in place. Nevertheless, we recommend that continued and 

rededicated attention to partner coordination and involvement 

be nurtured through the SSP Management Group and BFFRIT. 

Although this recommendation may appear gratuitous given 

present successes, renewed emphasis is essential to address a 

crucial, laborious, and continuing program element whose failure would undercut all other recovery activities. A recurring 

error of many recovery efforts is to gradually pay less attention to successful foundation components when new limiting 

factors are identified. 

Likewise, progress in reintroducing ferrets and establishing viable, wild populations requires continued nurturing of 

program cooperators and development of new partnerships 

with other States across the ferret’s historical range. The most 

notable success of the reintroduction component has been the 

relatively rapid involvement of the few suitable sites for active 

releases, but few large, high-quality prairie dog complexes 

remain, and future recovery depends on both short-term 

and long-term habitat restoration. In concert with renewed 

efforts to identify and enhance potential recovery habitat, new 

Black-footed ferret

(Mustela nigripes)

California condor

(Gymnogyps californianus)

Red wolf

(Canis lupus)

Year species listed 1967 1967 1967

Year extinct in wild 1987 1987 1980

Number removed from wild 18 27 17

Initial number used in breeding 7 14 14

Number propagated in captivity to date 4,800+ 283 727

Number held in captivity, breeding ~400 130 300

Year reintroduction began 1991 1992 1987

Number reintroduced to date ~1,800 ~167 ~120

Number of attempted reintroduction sites 9 7 2

Number weaned or fledged in wild ~1,200 1 289

Number in wild ~600 ~80 ~100

Reintroduction sites, likely viable 3 0 1

Reintroduction sites, not viable 2 0 1

Reintroduction sites, unknown viability 4 7 0

Table 2. Comparisons of some recovery parameters for three North American endangered species removed from the wild, propagated 

in captivity, and reintroduced into their historical ranges, 1967–2003.
18  Recovery of the Black-footed Ferret

reintroduction trials and continuing research to refine vaccines 

and develop other possible disease management capabilities 

are also essential.

The most notable accomplishment of wild population 

management has been the apparent sustainability of at least 

one reintroduced population. The ferret population at Conata 

Basin, S. Dak., is believed resilient enough to withstand 

harvest of surplus animals for translocation to other reintroduction sites. This is a remarkable accomplishment and 

indicates a population likely more stable than any other extant 

population during the last half century.

The review of the Black-footed Ferret Recovery Program 

by CBSG in June 2003 and the subsequent report issued by 

CBSG in January 2004 provided many practical recommendations to further ferret recovery and addressed issues and 

program needs related to captive breeding, reintroduction, 

disease, and habitat. Many of our views and recommendations 

echo information contained in the CBSG report; however, 

given the current status of the recovery program, after almost 

20 years of captive “life support” for the ferret, and the prospect of a difficult recovery future (given habitat limitations), 

perhaps other, more fundamental questions need to be asked 

and other key recovery priorities more fully pursued.

To realistically fulfill our recovery mission, how do 

we secure greater commitments for financial resources, 

private land incentives, and public land-use reforms necessary to set aside, develop, and sustain sufficient habitats 

across the historical range of the ferret?

Given the presence of plague in the environment and 

the timetable likely necessary to restore suitable prairie dog 

complexes, a foundation of available sites needs to be identified and attendant implementation strategies and schedules 

prescribed in a timely manner. It may take 10–20 years of 

intensive management to enhance complexes of prairie dog 

habitat to the point that they can support healthy ferret populations; planning and commitments must start soon. Federal 

public lands (national grasslands, Bureau of Land Management property, national wildlife refuges, national parks and 

monuments, and military lands) should bear a disproportionate 

amount of habitat development. Responsibilities under section 

7(a)(1) of the ESA (a provision requiring all Federal agencies 

to fully promote and support endangered species recovery) 

should be reinforced at the national level, both with funding 

and refocused priority, to establish and manage large prairie 

dog complexes wherever possible. Partnerships with tribes and 

private land owners must be pursued to secure recovery areas 

over those portions of the ferret’s historical range where public 

lands are largely absent. Cooperative efforts with Canada and 

Mexico should continue in order to develop recovery sites 

at the northern and southern extents of the ferret’s historical 

range.

How can administrative procedures be improved to 

more effectively and rapidly develop suitable black-footed 

ferret reintroduction areas and secure long-range 

management assurances necessary to perpetuate viable, 

wild populations? 

All ferret reintroductions in the United States have been 

accomplished under ESA section 10(j) provisions, which relax 

many of the strict prohibitions of the ESA. Section 10(j) has 

been an important management tool and was necessary for 

initial ferret reintroduction efforts. Nevertheless, 10(j) also has 

limitations and liabilities. Despite successful development of 

ferret reintroduction projects over most of the best remaining 

habitats in the United States since 1991, the administrative 

processes required to establish 10(j) experimental population 

sites typically require 2 years to complete and considerable 

investments of staff and funding. It is not a provision that 

allows rapid response to new opportunities. More importantly, 

10(j) is somewhat one-sided in effect and does not provide 

long-range assurances of support by affected parties. It can 

hinder implementation of program changes in response to 

identified needs and has been used by involved agencies to 

justify positions of social and political expediency rather than 

to fulfill conservation obligations. Other than reducing political opposition to initial reintroduction efforts, 10(j) has done 

little to assure reestablishment of ferrets.

Still, we do not advocate stronger regulations or 

constraints to guide ferret recovery; indeed, we suggest the 

opposite. The time to be most careful and restrictive with 

species like the ferret, condor, and red wolf is when they are 

declining, so as to keep them from slipping into such a precarious abyss in the first place. A process is needed through which 

responsible agencies are given sufficient resources and broader 

latitude to quickly develop site-specific strategies that define 

the boundaries of proposed recovery areas, prescribe the scope 

of agreements, and, like 10(j), hold no private parties accountable for uses or development of their property that might result 

in inadvertent losses of endangered ferrets. The ferret program 

needs to become even more proactive and not shy away from 

potential risks of individual project failure and animal losses. 

Again, we need to cast a wide net while reducing the fear of 

repercussions and impacts to private property that accompany 

efforts to recover endangered species. 

How can resources allocated to endangered species 

recovery at the national level be better prioritized and 

distributed to address biological imperatives?

The manner in which endangered species program priorities are established and funded warrants review. Although we 

certainly support other programs and efforts to recover endangered species, overall national priorities should be biologically based and focused on those species in greatest peril and 

for which habitat protection would have the greatest overall 

ecological benefit. Black-footed ferret recovery has achieved 

some remarkable successes despite a history of inadequate 

funding. Increased funding could have substantially accelerated species recovery, focused greater attention on critical 
A Historical Perspective on Recovery 19

program elements such as habitat conservation and restoration 

and expanded partnerships, and perhaps would have helped 

avoid some of the pitfalls encountered. 

Enormous resources have been poured into recovery 

efforts for the gray wolf and grizzly bear in the Rocky 

Mountain region with great success and public benefits, but 

these species enjoy stable habitats over significant portions of 

their historical ranges—habitats that support large, nonthreatened populations to the north. The black-footed ferret has no 

comparable safety net of extant population reservoirs. Moreover, the grassland/steppe ecosystem upon which the ferret 

depends is imminently more threatened, and its loss would 

have far-reaching consequences to a host of other native plant 

and wildlife species.

We recognize that these questions and suggestions 

address larger issues of regulation, policy, and current management direction and practices for numerous State and Federal 

agencies. We also understand that there are no easy fixes and 

that change may be slow in coming. Still, what better example 

of a species recovery program could be used as a springboard 

to critically review functional elements of ESA, interagency 

coordination and management needs, partnership capabilities, and administrative processes needed to secure greater 

habitat stability and foster species recovery? These questions 

and issues are in need of more focused scrutiny and attention 

by FWS, partner agencies, organizations, and tribes of the 

BFFRIT and other endangered species programs.

Acknowledgments

General data on red wolf and California condor used in 

this paper were obtained from FWS recovery program staff. 

The interpretation and analyses of those data presented here 

are our own.

Recovery Plans and Recovery Program 

Analysis Reports

Conservation Breeding Specialist Group, 2004, Black-footed 

ferret population management planning workshop, final 

report: Apple Valley, Minn., IUCN/SSC Conservation 

Breeding Specialist Group, 129 p.

Hutchins, M., Wiese, R.J., and Bowdoin, J., 1996, Blackfooted ferret recovery program analysis and action plan: 

Bethesda, Md., American Zoo and Aquarium Association, 

137 p.

U.S. Fish and Wildlife Service, 1978, Black-footed ferret 

recovery plan: Denver, Colo., U.S. Fish and Wildlife Service, 145 p.

U.S. Fish and Wildlife Service, 1988, Black-footed ferret 

recovery plan: Denver, Colo., U.S. Fish and Wildlife Service, 154 p.

Section II. Managing Captive Populations

When the Black-footed Ferret Recovery Plan of 1988 was written, captive breeding was 

still considered to be a risky venture (similar to opinions voiced during the creation of the original 1978 recovery plan). An effort to produce black-footed ferrets in captivity failed after the 

1978 plan was written. Much of the more recent plan was devoted to captive breeding, which is 

now largely a success story, although research and operational improvements continue. 

Abstract

Management of black-footed ferrets (Mustela nigripes) 

in captivity has historically utilized a flexible, adaptive 

approach as additional information about the species is 

obtained. Increased survivorship at reintroduction sites within 

the ferret’s historical range has further allowed innovative 

approaches to animal husbandry. Ferret recovery has benefited 

from changes in breeding schemes, nutrition, and vaccinations. 

Production of kits increased when animals were paired during 

daytime hours and allowed to remain together for 3 consecutive days. The Toronto Zoo Small Carnivore Diet was found 

to be a well-balanced, cost-effective, nutritious food for ferret 

maintenance and reproduction. PureVax® Ferret Distemper 

Vaccine provided protective titers under a variety of management scenarios. Changing program needs and flexibility in 

animal management are assessed yearly in order to balance 

maintenance of genetic diversity with maximum productivity.

Keywords: adaptive management, black-footed ferret, 

breeding, captive, Mustela nigripes, nutrition, reproduction, 

vaccination

Introduction

Captive management of endangered species requires an 

adaptive approach, incorporating new information relevant to 

changing program goals while ensuring quality animal care. 

Additionally, a system of checks and balances is needed to 

ensure that changes in management do not adversely affect 

either animal health or the primary goals of captive breeding—productivity and maintenance of genetic diversity. The 

fate of the black-footed ferret (Mustela nigripes) was placed in 

the hands of captive breeding efforts in the late 1980s when a 

decline of the last known wild population was identified. The 

decline of this population, located near Meeteetse, Wyo., led 

to the capture of 18 individuals of the species and was the start 

of a complex, multipartner recovery effort, which is conducted 

under the auspices of the American Zoo and Aquarium 

Association’s Species Survival Plan® (SSP) program and is 

guided by the Black-footed Ferret Recovery Plan (U.S. Fish 

and Wildlife Service, 1988) and a manual of husbandry techniques (Lyster and others, 2002).

One of the primary goals identified in the Black-footed 

Ferret Recovery Plan (U.S. Fish and Wildlife Service, 1988) 

involves development and maintenance of a large, stable, 

and genetically managed captive breeding population, which 

has been further defined as a core breeding population of 

240 adults (90 males, 150 females) located in six geographically separated facilities. Facilities currently housing captive 

breeding populations include the U.S. Fish and Wildlife 

Service’s (FWS) National Black-footed Ferret Conservation 

Center (Center) in Wellington, Colo., (originally managed by 

the Wyoming Game and Fish Department and later the FWS 

at Wheatland, Wyo.), the Smithsonian National Zoological 

Park’s Conservation & Research Center (Virginia), the Louisville Zoo (Kentucky), the Cheyenne Mountain Zoo (Colorado), the Toronto Zoo (Ontario, Canada), and the Phoenix 

Zoo (Arizona).

At the outset of the captive breeding program, all facilities tried to follow identical animal husbandry and management protocols based on the initial success of the Wyoming 

Game and Fish Department’s (WGFD) program. Since 1987, 

this has led to the production of over 5,100 black-footed 

ferret kits through natural breeding and assisted reproductive 

technology. During this time, much has been learned about 

ferret behavior, reproductive technology, nutrition, and other 

factors associated with animal husbandry and captive colony 

maintenance. The ability to experiment with different management schemes in the ferret program has been key to these 

achievements. In the mid-1990s, the FWS assumed operational 

oversight of the largest ferret colony, which is located at the 

Center. The Center houses approximately 55 percent of the 

world’s captive black-footed ferrets and serves as the hub 

for all ferret-related activities. Given the large population of 

ferrets housed there, the Center is an ideal place to implement 

a flexible and adaptive approach in all areas of black-footed 

ferret captive management. Additionally, the Center staff 

works both independently and in collaboration with researchAn Adaptive Management Approach for Black-footed 

Ferrets in Captivity

By Paul E. Marinari1 and Julie S. Kreeger1

1

U.S. Fish and Wildlife Service, National Black-footed Ferret Conservation 

Center, P.O. Box 190, Wellington, CO 80549.
ers to investigate questions concerning reproduction, nutrition, 

disease, and genetics as they relate to captive breeding.

Many changes have been incorporated into the captive 

breeding program based on studies addressing factors that 

affect the number of kits produced and weaned (defined as kits 

surviving to 90 days of age). This paper reviews three areas of 

management that have been addressed through studies at the 

Center: breeding strategies, nutrition, and vaccination against 

disease.

Breeding Strategies

A primary goal of the SSP is to optimize genetic management of the captive population (Ballou, 1984; Russell and 

others, 1994; Miller and others, 1996) by maintaining 80 

percent of the genetic diversity present in the founder population for the next 25 years. Since only 7 of the original 18 

animals successfully reproduced, genetic diversity has been 

limited from the outset. Males and females are paired utilizing 

the mean kinship and inbreeding coefficient strategy (Ballou 

and Oakleaf, 1989). This strategy identifies several potential 

males for each female in the SSP breeding population. Efforts 

to equalize founder representation are succeeding (Wisely, 

2001), indicating that the mean kinship and inbreeding coefficient strategy is an appropriate tool for managing the captive 

population.

Prior to 1996, all ferrets at the Center, as well as those 

at other captive breeding facilities, were paired by using 

procedures developed by the WGFD. This involved pairing 

of animals at night with a high degree of human intervention. 

Staff at the Louisville Zoo tried a different, less intensive 

approach to male and female pairings that resulted in very 

successful kit production. In 1996, Center staff decided to 

conduct a more extensive study comparing the Wyoming and 

Louisville methods.

In 1996, the entire ferret colony at the Center was kept 

under strict quarantine procedures during the breeding season. 

Access to the animals was limited to immediate staff. Additionally, before coming in contact with any ferrets, all employees showered and changed into clothes that remained onsite. 

Vehicle traffic near the main breeding building was restricted 

to emergency use in order to minimize possible disturbance to 

the ferrets.

Black-footed ferret females were divided between the two 

breeding schemes being investigated (n = 36, Wyoming; n = 

29, Louisville). Only females aged 1–3 years were included 

in the study, as these age classes form the core of the captive 

breeding population. All females were housed in the main 

breeding building and were treated similarly up to the moment 

of pairing with their chosen male. Each animal was maintained 

on a strict diet, known as 60/40, which consisted of a mixture 

of ground rabbit, commercial mink chow, and various additives, formulated by staff of the WGFD prior to 1996. During 

the breeding season, rendered lard was added to the 60/40 diet 

of expectant females only. Two months prior to the breeding 

season, each ferret was weighed and assigned a target weight 

that was determined by technicians using visual cues of overall 

body condition. Amount of food provided to the animals 

was altered based on weight over a 2-month period. As the 

breeding season progressed, weekly vulvar measurements 

were performed and recorded. Vaginal cytology and vulva size 

were used to monitor reproductive readiness in both groups 

(Williams and others, 1992). In the Louisville method, males 

were brought to the females’ cages and were not given access 

to the upper nest box at night. Additionally, once a positive 

sperm check was obtained, the pair was left together for 3 

consecutive days and nights. In the Wyoming method, females 

were brought to the males’ cages, confined to the cage surface 

(1.2 × 1.2 m), which contained a breeding box, and separated 

from the males during daylight hours.

Peterson (1996) summarized findings of this study for 

the FWS’s 1996 annual progress report. Fecundity, defined 

as the proportion of bred females that whelped, was higher 

with the Louisville method (55 percent versus 36 percent). 

The average litter size per female bred with the Louisville 

method was 2.65 ± 2.31 (mean ± SE), which was significantly 

different from the average litter size for the Wyoming breeding method (1.46 ± 1.82; one-way ANOVA, P = 0.05). The 

average number of kits weaned per whelping female with the 

Louisville method was 3.13 ± 1.93, which was significantly 

different from results of the Wyoming method (1.54 ± 1.56; 

one-way ANOVA, P = 0.02).

The higher whelping rate and greater number of kits 

produced with the Louisville method indicated that this 

management scheme would be beneficial to overall program 

goals—maintenance of genetic diversity and production of as 

many kits as possible. Greater kit production with the Louisville method could be a result of several factors, including 

less stress because of minimal human interactions while males 

and females were paired and more time for the animals to 

copulate, as black-footed ferrets are induced ovulators. Pairs 

were observed copulating multiple times throughout the day, 

perhaps providing greater stimuli for ovulation to occur. There 

is also a cost savings associated with the Louisville method, 

as additional staff are no longer necessary to monitor pairs 

at night. Daytime ferret keepers balance activities related to 

ferret breeding with husbandry chores. The Louisville method 

has now become standard operating procedure at all captive 

breeding institutions. At the Center, further refinements have 

occurred during the past several years to make pairings more 

efficient. Testicular and vulvar measurements have been 

discontinued in favor of electroejaculation and vaginal cytology to determine male and female reproductive readiness. 

Video monitoring has also been discontinued. Sperm checks 

are now performed opportunistically, and staining (Dip Quick; 

Jorgensen Laboratories, Inc., Loveland, Colo.) is used to 

determine the success of pairings.

24 Recovery of the Black-footed Ferret
Nutrition

Black-footed ferrets rely predominantly on prairie 

dogs (Cynomys spp.) for food in the wild (Campbell, 1987). 

Logistically, it would be extremely difficult to feed all ferrets 

maintained in the captive breeding program a diet of 100 

percent prairie dogs, so alternative diets have been investigated. Initially, captive ferrets were fed a mink chow and 

rabbit-based diet (the aforementioned 60/40 diet) that included 

a variety of additives thought to be important for maintaining a healthy breeding population based on information from 

domestic ferret (Mustela putorius furo) and mink (M. vison) 

captive breeding programs. As additional captive breeding 

facilities were incorporated into the program, feeding strategy 

guidelines and protocols were relaxed. Several nutritionists 

were concerned that the 60/40 diet had excessive polyunsaturated fatty acids. Oyarzun and others (1994) analyzed and 

evaluated common diets used throughout the captive breeding 

program in the mid-1990s and determined that the 60/40 diet 

used at the Metro Toronto Zoo (now the Toronto Zoo) greatly 

exceeded dietary recommendations established for mink as 

well as nutrient levels reported in the natural diet of blackfooted ferrets (Dierenfeld and McGuire, 1989). Oyarzun and 

others (1994) stated that, even though mineral levels were not 

high enough to cause acute toxicosis, feeding of higher than 

recommended levels over an extended period of time might 

have adverse effects (Lyster and others, 2002). Nutritionists at 

the Toronto Zoo continued to alter the diet composition so that 

it more closely fit accepted dietary requirements. This research 

led to formulation of the horse-based Toronto Zoo Small 

Carnivore Diet produced by Milliken Meat Products, Ltd., 

Scarborough, Ontario, Canada.

In 2000, a study was conducted at the Center to compare 

the two diets (60/40 and Toronto Zoo Small Carnivore) used 

in the captive breeding program. The primary objective of the 

study was to examine the effect of the diets on reproductive 

output. As sample sizes were small (four females and four 

males in each group), we were looking only for obvious and 

detrimental deviations from results achieved with the standard 

60/40 diet. Larger sample sizes would have provided greater 

statistical power; however, using additional females in the 

study might also have reduced our ability to maintain genetic 

diversity and produce kits for reintroduction. Supplemental 

vitamin K, believed to decrease the frequency of intraabdominal hemorrhage (blue-belly) in kits, was provided to 

bred and nursing females at the same dose and time period for 

both groups. A less labor-intensive strategy to raise kits was 

used on all litters at the Center. This strategy not only reduced 

handling time but also decreased the amount of supplemental 

diet offered to individual kits. Weights of adults were only 

monitored during preliminary stages of the study to determine 

adequate serving sizes and guard against large fluctuations 

in weight. For females, the number of kits born and those 

surviving to 50 days of age were examined. Fifty days of age 

was chosen as a cutoff point for the study because all kits are 

fed identical diets beginning at that age. The response variable 

for males was number of sperm per milliliter in ejaculate. 

A key advantage to the Toronto Zoo Small Carnivore Diet 

is that it is made under strict quality control (Canadian Food 

Inspection Agency). The prepackaged diet was found to be 

very convenient to use, less labor intensive, and more sanitary 

than the 60/40 diet produced in-house. The majority of ferrets 

readily accepted the diet and seemed to prefer it at first feeding. Overall, there was no difference between the two diets in 

the number of kits born per female (3.5) or the number of kits 

per female surviving to 50 days of age (3.25). Sperm production appeared to be lower in males fed the Toronto Zoo Small 

Carnivore Diet, but concentrations were above acceptable 

limits (250 × 106 sperm/mL). Staff at the Center also noticed 

that kits readily ate the Toronto Zoo Small Carnivore Diet at 

early stages of development.

Based on these feeding trials in 2000, the Toronto Zoo 

Small Carnivore Diet was determined to be effective and 

convenient for use at the Center and replaced the 60/40 diet; 

however, in 2003 the United States banned import of all meat 

products from Canada as a result of mad cow disease (bovine 

spongiform encephalopathy). Accordingly, we investigated 

alternative diets, including the Dallas Crown Carnivore Diet 

(Dallas Crown, Inc., Kaufman, Tex.). Earlier feeding trials 

at the Phoenix Zoo found this diet to be suitable for ferret 

maintenance and production. The SSP Nutrition Advisory 

Group also endorsed the diet as an acceptable alternative if 

the Toronto Zoo Small Carnivore Diet was unavailable. In 

keeping with our efforts to improve management in the captive 

breeding program, in 2004 we also evaluated the effect of 

the Toronto and Dallas Crown diets on sperm production and 

compared the results to those from black-footed ferrets fed a 

more natural diet of prairie dogs. There were no significant 

differences in the response variable among any of the three 

diets. The Toronto Zoo Small Carnivore Diet is generally 

preferred by ferret caretakers because of its more even consistency, which makes it easier to feed to ferrets.

Vaccination

Transport of black-footed ferrets across State and international borders may require rabies vaccination, depending 

on individual State or country regulations. If required, blackfooted ferrets over 3 months of age are vaccinated with Imrab®

3 (Merial, Inc., Athens, Ga.). This vaccine is approved for use 

in domestic ferrets and recommended for yearly revaccination. It is also recommended that ferrets in outdoor pens be 

vaccinated in areas where rabies is endemic. Rabies vaccination must be by or under the direct supervision of a licensed 

veterinarian. 

Management of Captive Black-footed Ferrets 25
Prevention of canine distemper in captive black-footed 

ferrets has been an important management consideration since 

the inception of the captive breeding program. Captive ferrets 

have succumbed to both natural (Williams and others, 1988) 

and vaccine-induced canine distemper virus (CDV) infections 

(Carpenter and others, 1976). The search for a safe and 

effective canine distemper vaccine for use in captive and freeranging black-footed ferret populations has been a priority for 

ferret recovery (Wimsatt and others, this volume).

Historically, ferrets in the captive breeding program were 

vaccinated against CDV with an inactivated virus plus adjuvant. The vaccine was prepared yearly by Dr. M.J.G. Appel 

of Cornell University. Adjuvant was prepared separately. This 

vaccine was used until 2002, but no data on duration of immunity and protection against CDV were collected. The recent 

availability of a commercial, monovalent, canary pox-vectored 

vaccine for use in domestic ferrets, PureVax® Ferret Distemper 

Vaccine (Merial, Inc., Athens, Ga.), provided a new possibility 

for vaccination of captive black-footed ferrets. The vaccine 

had been tested in the Siberian polecat (Mustela eversmannii), a species closely related to the black-footed ferret, with 

promising results (Wimsatt and others, 2003). 

Since management of young black-footed ferrets varies 

according to their ultimate fate (i.e., release to the wild or 

captive breeding), we examined several different paradigms 

of vaccination and the resultant serum neutralization titers. 

Vaccine used in these studies was a generous donation from 

Merial. The Wyoming State Veterinary Laboratory in Laramie 

performed serum neutralization titers.

Three groups of young of the year black-footed ferrets 

were used. Group 1 consisted of 13 ferrets from multiple 

litters. These animals were vaccinated intramuscularly with 

PureVax Ferret Distemper Vaccine. Vaccinations were administered in the right thigh at approximately 60, 74, and 88 days 

of age. Blood for titers was drawn from the external jugular 

vein of anesthetized ferrets every 2 weeks, coinciding with 

vaccinations to minimize handling of young animals. Final 

blood samples were drawn 2 weeks after the last vaccination. 

Group 2 included 12 ferrets subjected to vaccination and blood 

sampling protocols identical to those for group 1, except the 

PureVax vaccine was given subcutaneously. Group 3 consisted 

of nine animals subjected to a protocol chosen to approximate 

the preconditioning of young ferrets for release to the wild. 

Young preconditioned ferrets are placed in outdoor pens at an 

early age and often not recaptured for months. Ferrets in this 

group were vaccinated subcutaneously at 60 days of age and 

again at 120 days of age. Blood samples were taken at initial 

vaccination and at 74, 120, and 134 days.

Serum neutralization titers of >1:128 are considered 

protective (E. Williams, oral commun., 1999). Prior to 

vaccination, all groups had median titers of <1:8. All ferrets 

developed protective titers of >1:128 following the second 

vaccination. Subsequent vaccinations resulted in increased 

titers. Ten of the ferrets that received intramuscular injections 

of PureVax were tested 1 year postvaccination, and eight (80 

percent) had protective titers.

Results of these studies indicate that black-footed ferrets 

vaccinated with a minimum of two doses of PureVax Ferret 

Distemper Vaccine developed protective titers. Three doses of 

vaccine resulted in increased titers and provided 80 percent 

of vaccinated ferrets with protective titers up to 1 year later. 

Based on these results, captive breeding facilities are now 

encouraged to administer a minimum of two canine distemper 

vaccinations in young ferrets, beginning at the age of 60 days.

Conclusions

It is apparent that black-footed ferret captive breeding 

facilities benefit from a flexible management approach. The 

ability of the program to use adaptive techniques in all areas 

of ferret management is inextricably linked to the success of 

reintroduction activities. A stable captive breeding population 

and an increased number of reintroduction sites have allowed 

facilities to be more creative in their management decisions. 

Changes in the program are acceptable only if they do not 

have a detrimental effect in terms of overall production of 

animals. Production of kits has been consistent over time, and 

weaning success continues to improve. Many other changes 

have been made throughout the history of captive management, including the use of ALPHA-driTM bedding material 

(Shepherd Specialty Papers, Watertown, Tenn.), elimination 

of additional vitamin K supplement, and less labor-intensive 

dietary management of kits. We will continue to assess our 

methods on an annual basis and adapt our management in 

order to provide a stable source of animals to meet the everchanging needs of the black-footed ferret recovery program.

References Cited

Ballou, J.D., 1984, Strategies for maintaining genetic diversity 

in captive populations through reproductive technology: 

Zoo Biology, v. 3, p. 311–323.

Ballou, J.D., and Oakleaf, R., 1989, Demographic and genetic 

captive-breeding recommendations for black-footed ferrets, 

in Seal, U.S., Thorne, E.T., Bogan, M.A., and Anderson, 

S.H., eds., Conservation biology and the black-footed ferret: 

New Haven, Conn., Yale University Press, p. 247–267.

Campbell, T.M., 1987, Food habits of Wyoming black-footed 

ferrets: American Midland Naturalist, v. 117, p. 208–210.

Carpenter, J.W., Appel, M.J.G., Erickson, R.C., and Novilla, 

M.N., 1976, Fatal vaccine-induced canine distemper virus 

infection in black-footed ferrets: Journal of the American 

Veterinary Medical Association, v. 169, p. 961–964.

Dierenfeld, E.S., and McGuire, J.T., 1989, Seasonal variation 

in prairie dog carcass composition: New York Zoological 

Society, 33 p.

26 Recovery of the Black-footed Ferret
Lyster, J., Lockhart, M., Marinari, P., and Garelle, D., eds., 

2002, Black-footed ferret husbandry manual: Unpublished 

report available from the National Black-footed Ferret Conservation Center, 118 p.

Miller, B., Reading, R.P., and Forrest, S., 1996, Prairie night— 

black-footed ferrets and the recovery of endangered species: 

Washington, D.C., Smithsonian Institution Press, 254 p.

Oyarzun, S.E., Self, K., Valdes, E.V., Carnio, J.S., Shamir, 

A.B., Wrobel, M., and Musson, A., 1994, Black-footed 

ferret SSP 1994 nutrition report for the Metro Toronto Zoo: 

Toronto, Metro Toronto Zoo, 20 p.

Peterson, D., 1996, A comparison of black-footed ferret 

(Mustela nigripes) productivity under different captive management regimes: Laramie, Wyo., U.S. Fish and Wildlife 

Service, 26 p.

Russell, W.C., Thorne, E.T., Oakleaf, R., and Ballou, J.D., 

1994, The genetic basis of black-footed ferret reintroduction: Conservation Biology, v. 8, p. 263–266.

U.S. Fish and Wildlife Service, 1988, Black-footed ferret 

recovery plan: Denver, Colo., U.S. Fish and Wildlife Service, 154 p.

Williams, E.S., Thorne, E.T., Appel, M.J.G., and Belitsky, 

D.W., 1988, Canine distemper in black-footed ferrets (Mustela nigripes) from Wyoming: Journal of Wildlife Diseases, 

v. 24, p. 385–398.

Williams, E.S., Thorne, E.T., Kwiatkowski, D.R., Lutz, K., 

and Anderson, S.L., 1992, Comparative vaginal cytology of 

the estrous cycle of black-footed ferrets (Mustela nigripes), 

Siberian polecats (M. eversmanni), and domestic ferrets (M. 

putorius furo): Journal of Veterinary Diagnostic Investigation, v. 4, p. 38–44.

Wimsatt, J., Biggins, D., Innes, K., Taylor, B., and Garell, D., 

2003, Evaluation of oral and subcutaneous delivery of an 

experimental canarypox recombinant canine distemper vaccine in the Siberian polecat (Mustela eversmanni): Journal 

of Zoo and Wildlife Medicine, v. 34, p. 25–35.

Wisely, S.M., 2001, Conservation biology of the black-footed 

ferret (Mustela nigripes)—changes in genetic diversity and 

morphology before, during and after a population bottleneck: Laramie, University of Wyoming, Ph.D. dissertation, 

94 p.

Management of Captive Black-footed Ferrets 27
Abstract

Assisted reproductive techniques such as artificial insemination (AI), in vitro fertilization, and sperm cryopreservation 

have been postulated to be “high-tech” strategies for saving 

endangered species from extinction; however, there has been 

limited application of assisted breeding in wildlife management. This report illustrates how reproductive technology has 

been utilized in an integrated conservation program to save the 

endangered black-footed ferret (Mustela nigripes). Considered 

extinct until a remnant population was discovered in Wyoming 

in 1981, the last remaining 18 black-footed ferrets were 

captured to establish a captive breeding program. In 1988, 

the U.S. Fish and Wildlife Service developed a Black-footed 

Ferret Recovery Plan, which emphasized species preservation 

through natural breeding, development of assisted reproductive 

technology, and establishment of multiple reintroduction sites, 

among others. A multi-institutional propagation program has 

been highly successful. Approximately 250 animals currently 

reside in six breeding facilities, and >500 black-footed ferrets 

survive in the wild from reintroduced animals. Methods for 

semen collection, AI, and sperm cryopreservation were developed first in domesticated ferrets (Mustela putorius furo) and 

the closely related Siberian polecat (Mustela eversmannii) and 

now are used routinely in black-footed ferret management to 

(1) assess sperm status prior to natural breeding; (2) circumvent cases of sexual incompatibility; (3) enhance reproduction 

in nonbreeding individuals to retain existing genetic diversity; 

(4) increase founder representation; (5) establish a genome 

resource bank to preserve valuable germ plasm; and (6) 

produce additional offspring for reintroduction. To date, 128 

black-footed ferret kits have been produced by AI using fresh 

or frozen semen. The black-footed ferret represents a model 

for reproductive biotechniques contributing to a multidisciplinary species recovery and reintroduction program.

Keywords: artificial insemination, assisted reproduction, 

genetic management, genome resource banking, semen

Introduction

The value of reproductive technologies to ex situ and 

in situ wildlife management in helping conserve genetic and 

biological diversity has been considered for years. Assisted 

reproductive technologies such as artificial insemination (AI), 

in vitro fertilization (IVF), embryo transfer, and gamete/

embryo cryopreservation offer many advantages for managing 

small populations, largely by ensuring that all genetically valuable animals reproduce (Ballou, 1984; Howard, 1993, 1999; 

Wildt and Roth, 1997; Wildt and others, 1997). The potential 

of assisted reproduction could be enhanced further by developing genome resource banks (repositories of cryopreserved 

sperm, eggs, and embryos), thus preserving valuable genetic 

material for future generations. The combined use of assisted 

breeding and germ plasm banks also has potential for infusing 

genetic material from wild-born individuals into genetically 

stagnant ex situ populations or even for exchanging genetic 

material between isolated wild populations (Holt and others, 

1996; Wildt and others, 1997).

Despite these advantages, assisted reproduction has not 

been used consistently in practical wildlife management and 

conservation, largely for one reason. Until recently, no wildlife 

species had been sufficiently studied that its reproductive 

biology was so comprehensively understood that assisted 

breeding could become routine. It commonly is assumed 

that reproductive knowledge and techniques established for 

laboratory rodents, domestic farm species, and even humans 

are readily adaptable to propagating or overcoming infertility 

in wild animals (Wildt and others, 2001a,b). This is a misperception because all species have naturally evolved, unique, 

species-specific reproductive mechanisms, most of which have 

not yet been elucidated. Without such specific information, no 

assisted breeding technique can ever become routine.

There still is a need to demonstrate how such reproductive strategies can be used pragmatically. In this paper, 

we demonstrate how management and conservation of an 

endangered carnivore, the black-footed ferret (Mustela 

nigripes), have benefited from the application of the reproducUse of Reproductive Technology for Black-footed Ferret 

Recovery

By JoGayle Howard,1 Rachel M. Santymire,1 Paul E. Marinari,2 Julie S. Kreeger,2 Linwood Williamson,1 and 

David E. Wildt1

1

Smithsonian’s National Zoological Park, Conservation & Research Center, 

1500 Remount Rd., Front Royal, VA 22630.

2

U.S. Fish and Wildlife Service, National Black-footed Ferret Conservation 

Center, P.O. Box 190, Wellington, CO 80549.
Use of Reproductive Technology 29

tive sciences, including assisted breeding. We assert that the 

lessons learned from (1) working in partnership with wildlife managers and ex situ breeding institutions, (2) taking a 

systematic basic and multidisciplinary research approach, and 

(3) integrating knowledge have helped to recover and reintroduce this endangered species into nature.

Black-footed Ferret Recovery

Between the fall of 1985 and spring of 1987, the 

Wyoming Game and Fish Department, in cooperation with the 

U.S. Fish and Wildlife Service, captured the last 18 free-ranging black-footed ferrets from Meeteetse, Wyo., a location 

known to have sylvatic plague and canine distemper (Forrest 

and others, 1988; Williams and others, 1988; Thorne and 

Oakleaf, 1991). In the spring of 1987, captive breeding was 

successful, and two litters of black-footed ferret kits were 

born. Given the species’ critical status, a recovery plan for 

ex situ propagation and reintroduction was a high priority. A 

workshop was held in 1986, facilitated by the Conservation 

Breeding Specialist Group (CBSG) of the Species Survival 

Commission of the World Conservation Union (International 

Union for the Conservation of Nature and Natural Resources). 

Widespread stakeholder participation was emphasized, and 

the workshop was attended by representatives from State and 

Federal wildlife and land management agencies as well as 

experts in mustelids, small population biology, reproduction, 

nutrition, veterinary medicine, and genetics. Using workshop 

information (Seal and others, 1989), the U.S. Fish and Wildlife 

Service developed an official Black-footed Ferret Recovery 

Plan in 1988 (U.S. Fish and Wildlife Service, 1988) that 

emphasized species preservation through research, a multiinstitutional ex situ propagation program, and establishment of 

multiple reintroduction sites. The goal of the ex situ breeding 

program was to maintain ~240 ferrets (90 males, 150 females) 

in captivity but in multiple institutions to avoid a catastrophe 

that might affect any single facility. The aim of the eventual 

reintroduction program was to establish a total of 1,500 ferrets 

in at least 10 self-sustaining, free-ranging populations by the 

year 2010. The wild populations, scattered geographically 

within the ferret’s former range, each were to be composed of 

at least 30 breeding adults.

Throughout discussions of the ex situ and (eventually) 

in situ metapopulation structure, the role of sound scientific 

research, including the potential of reproductive technologies, was always recognized. Managers were especially keen 

to determine whether such techniques could be useful for 

evaluating fertility and for developing AI with fresh or cryopreserved spermatozoa, all for the purpose of supporting ex 

situ breeding and especially avoiding further losses in genetic 

diversity. One early concept was to establish a black-footed 

ferret genome resource bank, a frozen repository of spermatozoa from the most genetically valuable males, especially those 

that failed to reproduce by natural breeding.

Ex Situ Natural Breeding

Intensive management by the Wyoming Game and Fish 

Department resulted in production of offspring in 1987 and 

all subsequent years, which allowed dividing the ex situ 

population into six subcolonies at zoological institutions in 

North America. The U.S. Fish and Wildlife Service assumed 

the responsibility of managing the Wyoming breeding facility in 1996 and renamed it the National Black-footed Ferret 

Conservation Center. Since 1987, the multi-institutional ex situ

breeding program has produced >5,100 ferrets (Marinari and 

Kreeger, this volume) while generating extensive knowledge 

on ferret biology. 

For the past decade, the cooperative effort among the 

breeding facilities has been guided by the Black-footed Ferret 

Species Survival Plan® (SSP), a population management strategy of the American Zoo and Aquarium Association designed 

to maintain a self-sustaining ex situ population while providing animals for reintroduction. Breeding recommendations 

also are provided in an attempt to equalize genetic representation of the few original wild-born founders. SSP managers 

determine specific pairs for breeding on the basis of a mean 

kinship value, a measure of how related an individual is to the 

remaining population (Ballou and Lacy, 1995; Wisely, this 

volume). Demographic data, including reproductive lifespan, 

fecundity, age distribution, and sex ratio, are considered in 

predicting population stability and growth rate over time. 

Currently, there are ~250 black-footed ferrets maintained in 

the SSP program at six locations.

Reintroduction

The ability to produce ferrets in captivity allowed 

reintroduction to begin in 1991, initially into the Shirley Basin 

of southeastern Wyoming (Miller and others, 1993; Biggins 

and others, 1997). A few animals survived over winter, and 

additional ferrets were released at the same site in subsequent 

years. In 1995, reintroduction efforts were suspended in 

Wyoming because of a sylvatic plague outbreak and the loss of 

vital prairie dog (Cynomys spp.) colonies on which the ferrets 

depend for food and shelter. Subsequent ferret reintroduction sites were established in Conata Basin in South Dakota 

(Badlands National Park and Buffalo Gap National Grassland 

in 1994) and Montana (Charles M. Russell National Wildlife 

Refuge in 1994; Fort Belknap Indian Reservation in 1996). 

Arizona became the fourth State in the reintroduction program 

in 1996 with ferret releases in Aubrey Valley. Utah was added 

as the fifth State in 1999 when ferrets were released in Coyote 

Basin near the Colorado border. In the fall of 2000, the Cheyenne River tribal lands in South Dakota received ferrets. After 

extensive planning, a site near Janos, Chihuahua, Mexico, 

received releases in 2001. Successful reproduction and 

offspring produced in the wild from released ferrets have been 

documented at all release sites. Survival of released ferrets has 
30 Recovery of the Black-footed Ferret

improved because of preconditioning, the exposure of ferrets 

to large outdoor pens with prairie dog burrow systems and 

live prey prior to reintroduction. Although success varies, the 

highest survival has occurred at Conata Basin (South Dakota), 

with >70 percent of captive-born kits and >90 percent of 

identified wild-born kits surviving over winter and through 

spring (T. Livieri, oral commun., 2005). As of fall 2005, the 

wild population was ~500 black-footed ferrets (T. Livieri, oral 

commun., 2005).

Development of Reproductive 

Technologies in Animal Models

Potential benefits of reproductive technologies were 

recognized from the onset of the recovery program. The 

recovery plan of 1988 (U.S. Fish and Wildlife Service, 1988), 

the original and official guide for species rescue, encouraged 

development of methods for reproductive assessment and 

assisted breeding. It was realized that AI with fresh or frozen 

spermatozoa could help retain genetic diversity by ensuring 

reproduction in every valuable individual that failed to breed 

naturally. Additionally, a genome resource bank containing 

cryopreserved spermatozoa could preserve extant genes for 

the future, as well as assist in the genetic management of this 

small population. 

The National Zoological Park’s Conservation & Research 

Center was invited to take a lead role in studying ferret 

reproductive biology as well as to participate in the ex situ 

breeding program. We began our reproductive investigations 

by using the domestic ferret (Mustela putorius furo) and the 

closely related Siberian polecat (Mustela eversmannii) as 

animal models, first to understand general ferret biology and 

then to use that knowledge for developing assisted breeding 

(Wildt and others, 1986). Molecular analyses revealed that 

the domestic ferret, the Siberian polecat, and the black-footed 

ferret are taxonomically related (O’Brien and others, 1989). 

All of these species are seasonal breeders with reproductive 

activity stimulated by long-day photoperiod (Hillman and 

Carpenter, 1983; Miller and others, 1988; Mead and others, 

1990; Miller and Anderson, 1990; Carvalho and others, 

1991). Testis size gradually increases beginning in January 

or February, peaks from March through June and then gradually declines (Neal and others, 1977). The female’s breeding 

season is monoestrus and restricted to the months of March to 

June, and is characterized by changes in vaginal cytology and 

an increase in vulvar size. Finally, these species are classified as induced ovulators, with ovulation occurring ~30 hours 

after a single copulation or an injection of human chorionic 

gonadotrophin (hCG) or luteinizing hormone (LH) (Mead and 

others, 1988). 

Extensive studies were conducted on domestic ferrets 

to develop a reliable approach for collecting, processing, and 

analyzing fresh or cryopreserved spermatozoa (Curry and 

others, 1989; Wildt and others, 1989; Howard and others, 

1991; Van der Horst and others, 1991). More than 300 electroejaculates from nine males were collected to address the (1) 

effect of temporal spermatogenesis patterns on sperm viability; 

(2) comparative effectiveness of vaginal versus uterine insemination via an atraumatic laparoscopic approach; (3) influence 

of sperm number, dilution medium, and time of hCG administration on pregnancy success, gestation interval, and number of 

offspring produced; and (4) influence of cryodiluent, freezing 

method, and thawing temperature on the biological competence of frozen-thawed ferret spermatozoa (Wildt and others, 

1989; Howard and others, 1991; Howard, 1999). Such basic 

studies were crucial to developing reliable assisted breeding 

techniques. An effective electroejaculation protocol was developed in the domestic ferret to consistently collect high-quality spermatozoa from anesthetized males (table 1). Vaginal 

insemination was determined to be ineffective for producing 

offspring; none of 10 females became pregnant after spermatozoa were deposited intravaginally (Wildt and others, 1989). 

In contrast, transabdominal-intrauterine sperm deposition via 

laparoscopy resulted in high pregnancy success. Seventeen of 

24 ferrets (70.8 percent) inseminated in this fashion became 

pregnant and delivered live young (Wildt and others, 1989). In 

addition, embryo transfer was developed in the domestic ferret 

to nonsurgically transfer preimplantation embryos (Wildt 

and Goodrowe, 1989; Kidder and others, 1999). Comparative 

assessments of 12 cryopreservation methods determined that 

a combination of an egg-yolk/lactose cryodiluent, the pellet 

freezing method, and a 37°C thawing temperature was effective for freeze-thawing ferret sperm and recovering maximal 

motility and acrosomal integrity.When this cryomethod was 

Table 1. Mean (± SE) ejaculate traits, sperm morphology, and 

acrosomal integrity in the domestic ferret (Mustela putorius 

furo), Siberian polecat (M. eversmannii), and black-footed 

ferret (M. nigripes). 

Domestic 

ferret

(n = 4 males)a

Siberian

polecat

(n = 8 males)b

Black-footed 

ferret

(n = 97 males)b

Sperm 

motility (%) 80.7 ± 1.0c 80.6 ± 2.9c 51.2 ± 1.8d

Sperm 

progression 

(0–5; 

5 = best)

3.3 ± 0.1 3.0 ± 0.2 2.5 ± 0.1

Normal 

sperm (%) 67.3 ± 1.3c 74.5 ± 2.6c 21.0 ± 1.5d

Normal 

intact acrosome (%)92.4 ± 0.5c 96.8 ± 1.0c 67.2 ± 2.8d

a

Data based on 52 ejaculates from four males (Howard and others, 1991).

b

Data based on one ejaculate per male (Howard and others, 1996; J. 

Howard, unpub. data, 1996).

c,dWithin rows, values with different superscripts are different (P < 0.05).
Use of Reproductive Technology 31

used, 7 of 10 females (70.0 percent) inseminated in utero with 

frozen-thawed ferret spermatozoa became pregnant (Howard 

and others, 1991). Overall, reproductive efficiency was high 

(70.6 percent) after laparoscopic intrauterine AI with fresh or 

frozen semen (table 2). 

The strategy developed for the domestic ferret was 

subsequently applied to the Siberian polecat and finally to 

the black-footed ferret. Although sperm motility traits were 

similar among the three species, there were significantly fewer 

structurally normal spermatozoa in the black-footed ferret 

compared to the domestic ferret and polecat (table 1; Howard 

and others, 1991, 1996). After cryopreservation and thawing, 

sperm motility and membrane integrity also were less in the 

black-footed ferret compared to the other species (Howard 

and others, 1991, 1996). These differences in sperm viability 

were assumed to be related to the restricted founder base and 

reduced genetic variation in the black-footed ferret. Nevertheless, the laparoscopic intrauterine AI technique, developed 

in the domestic ferret, proved effective in its close relatives. 

Eight of 10 (80.0 percent) Siberian polecats inseminated with 

fresh or cryopreserved semen became pregnant (table 2), and 

this high rate provided the confidence to apply the procedure 

to the rarer black-footed ferret. Four of six (66.7 percent) 

black-footed ferrets inseminated with fresh or frozen-thawed 

semen became pregnant and delivered live young (table 2) 

(Howard and others, 1996; Howard, 1999). 

Assisted Breeding to Enhance Reproduction in Black-footed Ferrets

It soon was realized that the reintroduction goal (1,500 

breeding ferrets in 10 free-ranging populations by the year 

2010) was not achievable at the current rate of propagation 

in the ex situ natural breeding program. Early experiences 

revealed that some animals consistently failed to reproduce. 

Analysis of breeding records indicated that most females 

(>90 percent) demonstrated a spring estrus on the basis of 

vaginal cytology changes (markedly increased numbers 

of superficial, cornified squamous epithelial cells; fig. 1) 

(Williams and others, 1992; Brown, 1997); however, there was 

a high incidence (~40 percent) of pseudopregnancy wherein 

matings were observed (via video camera) and ovulation 

was confirmed (by an abrupt decrease in superficial cornified cells), but no pregnancy occurred (Williams and others, 

1991). Fecal oestradiol and progestogen metabolite profiles in 

pregnant versus pseudopregnant females were similar (fig. 1; 

Brown, 1997), suggesting that endocrine dysfunction was not 

contributing to the problem. 

Table 2. Comparison of laparoscopic intrauterine artificial 

insemination with fresh or frozen-thawed spermatozoa in 

closely related ferret species. Data from Wildt and others 

(1989), Howard and others (1991, 1996), and Howard (1999).

Domestic 

ferret 

(Mustela 

putorius furo)

Siberian 

polecat 

(Mustela 

eversmannii)

Black-footed 

ferret

(Mustela 

nigripes)

Number 

of females 

inseminated

34 10 6

Number of 

pregnant 

females

24 8 4

Pregnancy rate 

(%) 70.6 80.0 66.7

Number of kits 

born 116 42 9

Mean (± SE) 

number of 

kits/litter

4.8 ± 0.8 5.2 ± 1.0 2.3 ± 0.6

Figure 1. Mean (± SE) percent superficial cells in vaginal lavages 

(A) and fecal oestradiol (B) and progestogen (C) metabolite concentrations in pregnant (n = 7) and pseudopregnant (n = 9) blackfooted ferrets (Mustela nigripes). Day 0 is the time of first mating. 

(From Brown, 1997. Reprinted with permission of the Journal of 

Wildlife Management.)
32 Recovery of the Black-footed Ferret

Interestingly, records analysis indicated that a remarkably high proportion (>50 percent) of prime breeding-age 

males (1–3 years old) inexplicably failed to sire offspring in 

captive breeding situations. In 1995, there were 40 such adult 

males (54.8 percent of the breeding-age male population) 

that were exposed to prime age, estrual females and yet did 

not produce young. Simultaneous evaluations also revealed 

a genetic problem, largely that one of the original wild-born 

ferret founders was poorly represented in the modern population. This underrepresented lineage had only 43 descendants 

compared to more than 300 descendants from each of the 

remaining founder lineages. To help preserve original gene 

diversity, it was imperative to balance founder representation. This situation was confounded by another challenge in 

the underrepresented lineage—these males were consistently 

sexually incompatible with designated mates, largely because 

of aggression. Together, these issues prompted an examination 

of the value of reproductive technology and assisted breeding.

At the request of black-footed ferret managers, we agreed 

to (1) assess reproductive traits and breeding behavior in males 

with proven versus unproven fertility; (2) establish a genome 

resource bank containing cryopreserved spermatozoa from the 

most genetically valuable males; and (3) use AI for improving 

reproductive efficiency in nonbreeders, all for the ultimate 

purpose of increasing the number of kits for reintroduction. 

A survey was conducted in 1996 and 1997 to determine 

the precise number of prime breeding-age males not siring 

offspring and the reasons for failed reproduction. As in 1995, a 

high percentage of 1- to 3-year-old males did not sire young in 

1996 (38 of 69 males, 55.1 percent) or 1997 (35 of 60 males, 

58.3 percent) (Wolf and others, 2000b). Semen evaluations 

determined that there were no differences in sperm concentration, motility, or morphology between proven and unproven 

breeders (Wolf and others, 2000b). A detailed review of breeding data revealed that males failed to reproduce because of 

improper breeding position, behavioral incompatibility (e.g., 

aggression), and poor testes development (Wolf and others, 

2000b). As much of the problem was behaviorally based, we 

speculated that assisted reproduction could be beneficial for 

improving reproductive efficiency. 

A systematic strategy was used to establish the genome 

resource bank. Using the computer software program 

(SPARKS; International Species Information System, Eagan, 

Minn.) developed for SSP programs, sperm donors could 

be selected on the basis of founder representation and mean 

kinship (Ballou and Lacy, 1995; Johnston and Lacy, 1995; 

Wisely, this volume). High-priority black-footed ferret males 

were selected for inclusion in the bank, with semen samples 

collected and cryopreserved for AI (as part of routine management) or for long-term storage (as a hedge repository of valuable genes). 

At the National Zoological Park’s Conservation & 

Research Center, the natural breeding program for blackfooted ferrets was modified to include AI. The goals were 

to (1) produce offspring from behaviorally incompatible 

animals, especially nonbreeding males, to meet reintroduction 

demands and (2) increase founder representation in the 

underrepresented lineage. In achieving these goals, other 

opportunities arose, including examining the impact of male 

age on reproductive success. From 1996 through 2003, 

nonbreeding males of high genetic value were chosen as 

candidates for assisted reproduction. Overall, 66 females were 

monitored for natural estrus and were administered hCG or 

LH (to induce ovulation) 5 to 7 days after maximal vulvar 

swelling and >90 percent superficial cornified vaginal cells. 

Twelve to 20 hours later, each female was anesthetized and, 

under laparoscopic observation, inseminated in utero with 

fresh or frozen-thawed spermatozoa. Five of six (83.3 percent) 

females inseminated with semen from founder descendants 

became pregnant and produced 16 kits (table 3). Males were 

determined to produce excellent quality semen through 5 years 

of age, 2 years longer than the normal female reproductive 

life span (Wolf and others, 2000a). Eight of 11 (72.7 percent) 

females inseminated with semen from 5-year-old males 

produced 17 kits (table 3). A total of 28 of 49 (57.1 percent) 

females gave birth to 95 kits following AI with semen from 

genetically valuable, nonbreeding males (table 3). Overall, AI 

resulted in 128 additional black-footed ferret kits, offspring 

that never would have been born from natural mating. 

There were other by-products as well. For example, we 

observed that a high proportion of 1-year-old males produced 

aspermic (no sperm) ejaculates during the breeding season 

(Howard and others, 1998). These males experienced increases 

in seasonal testicular tumescence (albeit somewhat slower than 

elders; fig. 2) and copulated with females; however, systematic 

seminal evaluations revealed that these yearlings produced 

spermic ejaculates at least 4 weeks later in the breeding season 

than older counterparts (fig. 2). This asynchrony in sperm 

Table 3. Use of laparoscopic artificial insemination to enhance 

propagation in nonbreeding founder descendants, 5-year-old 

males, and genetically valuable male black-footed ferrets (Mustela nigripes) from 1996 through 2003.

Founder 

descendantsa

(n = 3 males)

5-year-old 

malesb

(n = 5 males)

Genetically

valuable 

malesc

(n = 27 males)

Number of females

inseminated 6 11 49

Number of 

pregnant 

females (%)

5 (83.3) 8 (72.7) 28 (57.1)

Number of kits 

born 16 17 95

a

Males were descendants of a wild-caught founder whose genetic lineage 

was underrepresented in the ex situ population.

b

Reproductive competence was assessed in 5-year-old males.

c

Despite numerous breeding opportunities, reproductive failure in these 

genetically valuable males was due primarily to behavioral incompatibility 

(aggression or shyness) or inappropriate breeding position.
Use of Reproductive Technology 33

production probably influenced the incidence of pseudopregnancy because aspermic males can induce ovulation following 

copulation. This was an important observation with significant 

implications for routine ferret management. Until this finding, males used for breeding were selected on the basis of 

enlarged testis size. Now, only males with spermic electroejaculates are allowed access to females for natural breeding, 

and introducing this simple semen assessment technique to the 

management protocol in 1998 resulted in a striking 20 percent 

increase in pregnancy success and 59 additional kits available 

for reintroduction (table 4).

Priorities for the Future

The black-footed ferret is a provocative example of how 

reproductive technologies integrated with both ex situ and 

in situ management plans can benefit species conservation. 

The positive results are evidence that reproductive techniques 

are valuable for (1) generating new knowledge of relevance 

to natural and assisted breeding and (2) producing living, 

genetically valuable offspring useful for breeding stock and/or 

reintroduction. Priorities for this species extend far beyond 

reproductive biology; adequate survival after reintroduction 

continues to be essential to the black-footed ferret’s future. 

Urban sprawl, sylvatic plague, and poisoning of prairie 

dogs appear to be never-ending, severe threats. Today, only 

2 percent remains of the ~100 million acres of the original 

prairie dog ecosystem of the Great Plains (Miller and others, 

1996). Monitoring for presence of sylvatic plague as well 

as canine distemper is essential for long-term protection of 

both prairie dogs and ferrets. Research into the development 

and use of a sylvatic plague vaccine is ongoing (Rocke, this 

volume). Even when a vaccine becomes available, there will 

be enormous distribution challenges. Also, persistent poisoning campaigns and recreational shooting continue to contribute 

to the collapse of the prairie dog ecosystem. 

Finally, a high priority will continue to be education 

programs, which play a crucial role in public awareness of 

black-footed ferret conservation issues. Currently, over 30 

zoos and wildlife agencies sponsor educational exhibits, often 

using “ambassador” black-footed ferrets. Gaining public 

support, especially from landowners who consider prairie dogs 

pests, is critical to reintroduction success. Many landowners 

continue to have serious concerns about endangered species, 

especially perceived governmental interference and restrictions 

on land use (Reading and Kellert, 1993; Miller and others, 

1996). A key factor facilitating landowner support for ferrets 

has been the designation of the reintroduced population as 

experimental and nonessential under the Endangered Species 

Act (see Lockhart and others, this volume, for additional 

information). Although resulting in a lower level of protection 

for released ferrets, this strategy has gained local rancher and 

farmer cooperation while providing some assurance that reintroduced ferrets and traditional land uses can be compatible. 

Table 4. Propagation in the black-footed ferret (Mustela 

nigripes) using enlarged testes versus spermic ejaculate as the 

criterion for selecting males for natural breeding.

Enlarged

testes

Spermic

ejaculate

Number of females bred 84 86

Number of litters 50 69

Pregnancy success (%) 59.5 80.2

Number of kits born 190 249

Figure 2. Influence of age on testes development (A) and total 

sperm/ejaculate (B) in 1-year-old versus 2- and 3-year-old male 

black-footed ferrets (Mustela nigripes). Asterisks indicate differences (P < 0.05) between age groups within a month.
34 Recovery of the Black-footed Ferret

Conclusion

It is apparent that reproductive sciences can play a vital 

role in a holistic, integrated conservation program to save an 

endangered species. The contemporary story of the blackfooted ferret illustrates the potential for species recovery and 

reintroduction based on partnerships and multidisciplinary, 

sound science. Perhaps most important has been the cooperative feature, collaboration among over 30 organizations, 

including State and Federal agencies, conservation groups, 

and zoos, that worked together to return ferrets to their former 

grassland habitats of the Great Plains. Reproductive technologies, including AI and a genome resource bank, have been 

integrated successfully into the black-footed ferret recovery 

program to maintain genetic diversity, enhance reproductive 

efficiency, and produce additional animals for reintroduction.

Acknowledgments

The authors acknowledge the U.S. Fish and Wildlife 

Service and the staff of the National Black-footed Ferret 

Conservation Center for continued support. We thank graduate students Karen Wolf and Rachel Santymire and Dr. Janine 

Brown for generating some of the data presented. We are 

indebted to Drs. Julie Kreeger and Mitchell Bush for animal 

health and veterinary assistance and to Linwood Williamson 

and the National Zoological Park’s Conservation & Research 

Center animal keeper staff for animal management and care.

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Abstract

The black-footed ferret (Mustela nigripes) evolved in 

Beringia sometime in the early to middle Pleistocene. By 

35,000 years before present the species was distinct from 

its sister taxon, the Siberian polecat (Mustela eversmannii). 

Genetic analysis revealed that historical populations had 

restricted gene flow prior to human disturbance, which had 

consequences for the conservation of genetic diversity in 

the species. Most genetic diversity in the species was lost 

when Great Plains populations were extirpated, leaving the 

last surviving population genetically distinct and depauperate. Further genetic losses occurred when almost half of the 

animals captured from the last population failed to breed in 

captivity. Once established in captivity, however, maintenance 

of remaining genetic diversity was within the goals of genetic 

management mandated by the recovery plan. Reintroduced 

populations of black-footed ferrets maintained genetic diversity, but were slightly differentiated from one another because 

of differences in population founders. Wild-born animals were 

less inbred than captive-released animals, suggesting that 

inbreeding avoidance mechanisms may operate in the wild. 

Although much diversity has been lost, inbreeding depression 

has not been confirmed. Future management efforts should 

maintain vigilance to conserve remaining genetic diversity 

both in captivity and in reintroduced populations. 

Keywords: captive breeding, genetic diversity, genetic 

drift, Pleistocene refugia, population bottleneck, reintroduction

Introduction

Eighteen years have passed since the first genetic study 

of the black-footed ferret (Mustela nigripes) was completed 

(Kilpatrick and others, 1986). Although techniques have 

advanced and our understanding of genetic processes has 

expanded, the story revealed by this species’ genes remains 

unwaveringly clear: the genetic uniformity measured in this 

species is unprecedented and rivaled by perhaps only one other 

carnivore, the cheetah (Acinonyx jubatus; O’Brien and others, 

1983). Unlike the cheetah, however, whose Holocene population bottleneck remains shrouded in mystery, the black-footed 

ferret’s demise was witnessed and documented by museum 

collectors, commercial trappers, animal control agents, and 

biologists (Anderson and others, 1986), providing a clear 

understanding of the demography of the bottleneck. Although 

the dramatic nature of the bottleneck was unfortunate for 

conservation of the species, it provides conservation-based 

science with the opportunity to study the genetic consequences 

of rapid and dramatic population loss outside of the laboratory 

setting.

Population bottlenecks occur when population size is 

rapidly reduced; the severity of a bottleneck depends on the 

minimum population size attained and the duration of time it 

remained small (Frankham and others, 2002). Although the 

immediate consequence of small population size is increased 

risk of extinction as a result of demographic stochasticity, 

long-term consequences result from reduced genetic diversity and increased inbreeding (Frankel and Soulé, 1981). By 

increasing the expression of deleterious alleles, inbreeding 

reduces individual fitness, further increasing the likelihood 

of extinction. Inbreeding depression caused by a population bottleneck has been documented in a variety of species, 

including the Florida panther (Puma concolor coryi; Roelke 

and others, 1993), fritillary butterflies (Melitaea cinxia; 

Saccheri and others, 1998), and koalas (Phascolarctos 

cinereus; Seymour and others, 2001). Reduced genetic diversity also has the insidious and difficult to measure effect of 

reducing a species’ ability to adapt to a changing environment. 

Examples in the literature are few but include endemic rainforest fruitflies (Drosophila spp.; Hoffman and others, 2003). 

The black-footed ferret experienced serious population 

decline beginning in the mid- to late 1800s as people migrated 

west and converted grasslands to agriculture. By 1981 only 

one population remained, and the species reached its nadir in 

1987 when the last of 18 individuals were taken into captivity 

from Meeteetse, Wyo. (Clark, 1994). This was the first time 

the U.S. Fish and Wildlife Service (FWS) had managed an 

endangered species by removing all living individuals from the 

wild. Ironically, FWS made that decision twice in 1987, when 

the last remaining California condors (Gymnogyps californianus; Seal, 1989; Snyder and Snyder, 2000) were also removed 

from the wild and placed in a captive breeding program. 

Unfortunately, animal husbandry practices were not fully 

The Genetic Legacy of the Black-footed Ferret: Past, 

Present, and Future

By Samantha M. Wisely1

1

Division of Biology, Kansas State University, Manhattan, KS 66506.
established in the initial years of captive breeding, and only 

seven founding ferrets are represented in the current captive 

population (Garell and others, 1998). Nonetheless, more than 

4,000 individuals have been produced, and today approximately 240 animals exist in captivity and 500 in the wild. With 

a founder genome equivalent (the number of unique genomes 

represented in the current population) of 4.1 (Russell and 

others, 1994), the species currently contains a fraction of the 

genetic diversity once present. 

Several demographic events transpired to reduce the 

population size of this species. Habitat conversion, poisoning campaigns aimed at prairie dogs (Cynomys spp.), and 

exotic diseases decreased the population by 99 percent over 

approximately 100 years. In the last historical population, 

simultaneous epizootics of canine distemper virus and sylvatic 

plague (caused by the bacterium Yersinia pestis) caused 

dramatic population decline in less than a year. In order to 

understand the consequences of these demographic events, 

one must first understand the context in which they occurred. 

How much genetic diversity did the species have prior to 

anthropogenic disturbance? How much genetic diversity 

did black-footed ferrets have prior to the bottleneck of the 

Meeteetse population? How did population structure affect 

the rate of loss of genetic diversity? Once ferrets were taken 

into a captive breeding program, how well was the remaining 

genetic diversity conserved? Finally, as reintroduced populations continue to be established, it is crucial to understand 

how the process of reintroduction affects genetic diversity and 

structure. In the following chapter, I synthesize what is known 

about the genetic legacy of this species as it passed through the 

processes of population bottleneck, captive management, and 

reintroduction.

Pleistocene Colonization of North 

America

The black-footed ferret is a relatively recent immigrant to 

North America via the Bering land bridge (Youngman, 1994). 

The earliest fossil record of a black-footed ferret in North 

America is from Cathedral Cave in eastern central Nevada 

(Owen and others, 2000). This specimen dates approximately 

800,000 years before present based on paleomagnetic data 

and biochronology of arvicoline rodents collected at the site 

(Owen and others, 2000). In support of a middle Pleistocene 

invasion, molecular clock estimates based on 41 isozymes 

suggest that the black-footed ferret split from its sister species 

the Siberian polecat (Mustela eversmannii) between 0.5 and 2 

million years ago (O’Brien and others, 1989). Thus it appears 

that the ancestral black-footed ferret crossed the land bridge 

approximately 1 to 2 million years ago. The species gained 

access to the grasslands and shrublands of North America via 

an interglacial, ice-free corridor (Anderson, 1989; Youngman, 

1994) and was established at least as far south as Nevada 

by 800,000 years ago. Once established, the species spread 

rapidly throughout the Great Plains. By 35,000 years before 

present the black-footed ferret was morphologically distinguishable from the Siberian polecat. In fact, the two species 

appear to have existed sympatrically; fossils of both species 

found in eastern Beringia as recently as 35,000 years ago 

suggest a period of secondary contact after differentiation 

(Youngman, 1994). 

Molecular data suggest that black-footed ferret populations had restricted gene flow and high population differentiation that was influenced by both natural history and the ebb 

and flow of Pleistocene glaciers (Wisely and others, 2002). 

Genetic variation becomes partitioned among subpopulations 

when isolating mechanisms, such as Pleistocene glaciers and 

unsuitable habitat, prevent gene flow and increase genetic drift 

(Frankham and others, 2002). Using microsatellite markers, 

Wisely and others (2002) reported an average FST of 0.53. FST

is a pairwise measure of genetic variation that is partitioned 

among populations. This parameter measures genetic structure 

and gene flow between subpopulations and ranges in value 

from 0 (no allelic differentiation or structure) to 1 (maximum 

allelic divergence). The value found for black-footed ferrets 

is one of the highest reported for a mammalian carnivore and 

likely reflects aspects of their unusual ecology. Their diminutive body size, coupled with their semifossorial lifestyle, likely 

limited their dispersal capabilities (when compared to other 

carnivorans) and therefore induced genetic drift within populations and created genetic differentiation among populations 

(Wisely and others, 2002). Subpopulation isolation was likely 

exasperated by habitat barriers that formed during glacial 

maxima.

Historical Genetic Diversity and Structure

The historical population structure of the black-footed 

ferret in North America greatly influenced the amount of 

genetic diversity that was lost. The magnitude of loss of 

genetic diversity was exasperated by the especially isolated 

nature of the last population. Located on the periphery of the 

historical distribution near the town of Meeteetse, Wyo. (Hillman and Clark, 1980), this population was likely a refugium 

during the last glacial maximum and remained isolated from 

other populations throughout the Holocene (Wisely and others, 

2002). Measures of genetic distance used to estimate genetic 

differences among black-footed ferrets from three historical 

populations confirm that the Meeteetse population was the 

most highly differentiated and therefore isolated of the three 

populations (Wisely and others, 2002). Thus, when ferret 

populations from the Great Plains were extirpated, the majority of genetic diversity found in the species was lost (table 1, 

fig. 1). Indeed, the Meeteetse population was different enough 

from other ferret populations that the addition of even four 

individuals from Mellette County, S. Dak. (the second to last 

population of black-footed ferrets), would have increased allelic 

diversity in extant ferrets by 50 percent (Wisely and others, 2002).

38 Recovery of the Black-footed Ferret
Diversity and Structure in Captivity

Although Meeteetse animals were first captured in 1985, 

successful breeding was not achieved until 1987 (Clark, 

1994). Of the 18 animals that survived in captivity, only 8–11 

founders (unknown paternity of some wild-caught litters 

created some uncertainty as to the exact number) were initially 

represented in the pedigree; 20 years later, only 7 founders 

are represented in the extant population (Garell and others, 

1998). The consequences of this bottleneck were measurable. 

Had 5 more of the original 18 ferrets bred successfully, genetic 

diversity of the extant population would have increased by 

30 percent (fig. 1; Wisely and others, 2002). Once animal 

husbandry was understood and disease concerns were alleviated, the population quickly expanded, and the remaining 

genetic diversity was conserved (Wisely and others, 2003). 

The dramatic loss of approximately 90 percent of the 

species’ genetic diversity necessitated conservation of that 

which remained. Because all captive populations are susceptible to problems associated with small population size, 

including inbreeding, inbreeding depression, and genetic 

drift (de Boer, 1994), management of the remaining genetic 

variation was a high priority (Ballou and Oakleaf, 1989). 

Various approaches have been used to maximize retention 

of genetic variability; for the black-footed ferret, the mean 

kinship strategy augmented with line breeding of underrepresented founders was recommended (Ballou and Oakleaf, 

1989). Briefly, mean kinship strategy finds suitable breeding 

pairs that maximize the representation of the most underrepresented founders of the captive population. Over time, this 

strategy is predicted to maximally conserve genetic diversity. 

Empirically, it appears that this strategy has succeeded in 

adequately preserving genetic diversity. Founders’ genes were 

more evenly represented in the captive population in 1999 than 

in the first generation of captive black-footed ferrets (fig. 2; 

Wisely and others, 2003), and even representation of founders 

maximally conserves genetic diversity in a pedigreed population. Likewise, a pedigree-based estimate of loss of heterozygosity was 12 percent, which meets the goal established by the 

American Zoo and Aquarium Association’s Species Survival 

Plan® of retaining 80 percent of the genetic diversity of the 

founding population for 25 years (Garell and others, 1998). 

Molecular-based estimates revealed no loss of allelic diversity; 

all alleles present in the founders were present in the extant 

population (Wisely and others, 2003). 

Diversity and Structure of Reintroduced 

Populations

Once captive-born animals were released into the wild, 

further challenges faced the recovery program. A successful 

captive breeding reintroduction program involves substantive 

Table 1. Observed (Ho) and expected (He) heterozygosities for 14 microsatellite loci in black-footed ferrets (Mustela nigripes) from 

Wyoming, South Dakota, and Kansas combined (representing the species prior to disturbance; n = 20) and from Wyoming only (n = 12), 

and HWE P, the probability associated with an exact test of Hardy-Weinberg equilibrium. Seven of 14 loci were monomorphic in the 

Wyoming population. HWE was tested only in polymorphic loci from Wyoming. (From Wisely and others, 2002. Reprinted with permission of Oxford University Press, Oxford, U.K.)

 Microsatellite

 loci names Ho He Ho He HWE P

Mvis002 0.65 0.48 0.50 0.39 0.53

Mvis9700 0.33 0.36 0.33 0.39 1

Mvis072 0.47 0.65 0.67 0.58 0.77

Mer095 0.20 0.27 0.17 0.16 1

Mer049 0.35 0.69 0.50 0.51 1

Mvi57 0.10 0.54 0.17 0.16 1

Mvis022 0.11 0.63 0.08 0.23 0.13

Gg4 0.05 0.14 - - -

Mvis075 0.17 0.53 - - -

Mvi87 0.00 0.43 - - -

G1A 0.10 0.19 - - -

Mvi232 0.20 0.19 - - -

Mer022 0.11 0.45 - - -

Mer009 0.06 0.06 - - -

 Wyoming, South Dakota, Kansas Wyoming only

Genetic Legacy of the Black-footed Ferret 39
but reconcilable tradeoffs. Demographic and genetic attributes 

affect the success of reintroduced populations (Jiménez and 

others, 1994; FitzSimmons and others, 1997), yet selection of 

animals for release removes them and their potentially unique 

genes from the captive breeding pool. This sets up a potential 

conflict between the goals of captive breeding and reintroduction (Earnhardt, 1999). Because of the tenuous nature 

of the early captive breeding program and the recognition 

that captive breeding would need to be maintained for many 

years, inbred animals and animals with a mean kinship >0.125 

were designated for release (Ballou and Oakleaf, 1989). This 

strategy would maximize retention of genetic diversity in the 

captive population. 

Inbreeding in 1991, estimated from pedigree analysis, 

was higher in released captive-bred animals (F = 0.092) than 

in animals retained in the captive population (F = 0.052; 

Russell and others, 1994), as would be expected by the 

designation criteria for animals retained versus released from 

the captive population. By 1999, overall inbreeding was higher 

(as would be expected in a small, closed population), but 

the difference between captive breeders and captive releases 

was negligible (F = 0.12 and 0.11, respectively; Wisely and 

others, 2003). The gap between breeders and releases was 

closed because founder genes were more evenly represented in 

the 1999 than in the 1991 captive population (fig. 2), resulting in a panmictic population. Equal founder representation 

reduced the variance in mean kinship of individuals, which in 

turn decreased the difference in genetic composition between 

breeders and releases (Wisely and others, 2003).

As of 1999, no difference in genetic diversity had been 

detected between captive-reared releases and their wild 

descendants from Charles M. Russell National Wildlife 

Refuge, Mont., and Conata Basin, S. Dak. (table 2; Wisely and 

others, 2003). This finding suggests that initial releases and 

subsequent augmentation were sufficient to halt drift-induced 

losses in diversity. Small but statistically significant population 

differentiation (FST = 0.09, 95 percent CI = 0.04–0.13) between 

wild descendants of two reintroduced populations suggested 

that even with augmentation of captive animals to the reintroduced population, these two populations had measurable 

genetic differences attributable to random differences in the 

founders of each reintroduction site (Wisely and others, 2003). 

Whether this statistically significant difference is also biologically significant is difficult to say (Hedrick, 1999). It is likely, 

however, that, as populations grow and augmentation ceases, 

continuing genetic drift will decrease genetic diversity within 

populations and increase genetic distance among populations.

Because wild-born animals were descended from animals 

with higher than average mean kinship and inbreeding (as 

calculated from the pedigree and estimated from microsatellites), it was predicted that the offspring would be inbred. In 

fact, this was not the case (table 2; Wisely and others, 2003). 

This result was surprising inasmuch as free ranging populations were smaller than the captive population, released 

animals were the descendants of overrepresented individuals, 

Figure 1. A timeline of genetic diversity in the black-footed ferret 

(Mustela nigripes). A, the average number of alleles per locus 

(triangle); He, expected heterozygosity (diamond); and P, the proportion of polymorphic loci (square) were at their highest values in 

1871. In 1972 only two populations remained, in Mellette County, S. 

Dak., and Meeteetse, Wyo. Note that although many populations 

throughout the Great Plains were extirpated, genetic diversity did 

not appreciably decrease. With the loss of the Mellette County 

population, substantial amounts of genetic diversity were lost. 

Only the Meeteetse population remained in 1982. Further loss of 

genetic diversity occurred because of a bottleneck in the last population from 1982 to 1987. (From Wisely and others, 2002. Reprinted 

with permission of Oxford University Press, Oxford, U.K.)

Figure 2. Founder representation in the first generation of captive breeding (i.e., all the direct descendants) (dotted bars) and 

in 1999, after 14 years of captive breeding (cross-hatched bars) 

of black-footed ferrets (Mustela nigripes). Only founders with 

genes represented in the extant population are considered. (From 

Wisely and others, 2003. Reprinted with permission of Wiley-Liss, 

Hoboken, N.J.)

40 Recovery of the Black-footed Ferret
and litter mates often were released together. A possible explanation for the lack of inbreeding in the wild populations is a 

behavioral response of inbreeding avoidance. Several researchers have proposed that closely related individuals avoid 

mating in such taxa as black-tailed prairie dogs (Cynomys 

ludovicianus; Hoogland, 1982), olive baboons (Papio anubis; 

Packer, 1979), and the marsupial genus Antechinus (Cockburn and others, 1985). Given that the reintroduced founding 

populations were small and that black-footed ferrets avoided 

close kin matings, survival and reproduction of founding 

populations may have been less than optimal as reintroduced 

individuals avoided breeding with one another and dispersed 

to find unrelated mates (Wisely and others, 2003).

The Future

The consequence of dramatic loss of genetic diversity 

in a species is unclear. Some taxa, such as felids, are highly 

susceptible to inbreeding depression, while other taxa appear 

unaffected (Ralls and Ballou, 1983; Lacy, 1997). Small, 

inbreeding populations have a higher rate of expression of 

deleterious alleles, which can lead to extinction when the 

forces of genetic drift are greater than natural selection. When 

selection exceeds drift, however, small, inbreeding populations 

can purge deleterious alleles, ultimately making them less 

susceptible to inbreeding depression. To date, no physiological 

abnormalities have been linked to inbreeding depression in 

black-footed ferrets, although abnormalities exist (Howard and 

others, this volume). Indeed, fecundity of females (measured 

as kits surviving per litter) was virtually the same (3.1–3.3 

kits per female) for animals observed in the historical populations of Mellette County, S. Dak. (Hillman and Carpenter, 

1980), and Meeteetse, Wyo. (Forrest and others, 1988), and 

the reintroduced population in Conata Basin, S. Dak. (U.S. 

Department of Agriculture, 2000).

Without question the two biggest hurdles to recovery 

of this species are lack of suitable habitat for reintroduction 

and sylvatic plague (Conservation Breeding Specialist Group, 

2004). Where ample, plague-free habitat exists, populations 

appear to flourish despite reduced genetic diversity. The 

dramatic loss of genetic diversity in this species should not 

discourage biologists from planning for recovery. With careful 

management of remaining genetic resources, this species will 

likely persist. Continued, vigilant conservation of genetic 

diversity in the captive population will be critical to the 

long-term success of this recovery effort. Likewise, genetic 

management of the reintroduced populations will be critical 

once populations become self-sustaining. Currently, only two 

populations are self-sustaining: those of Shirley Basin, Wyo., 

and Conata Basin, S. Dak. All other reintroduction sites rely 

on yearly augmentation to maintain their populations. Translocations for genetic augmentation may be necessary if reintroduced populations lose genetic diversity because of drift. 

The dramatic loss of genetic diversity in this endangered 

species should serve as a reminder to conservation practitioners that proactive management of population structure 

(conserving as many individuals from as many geographic 

locations as possible) can have a profound effect on the 

conservation of genetic resources for a species. Furthermore, 

rapid breeding of as many founders as possible within the 

first few generations of captive breeding will maximize the 

retention of the remaining genetic diversity and increase the 

likelihood of persistence into the future.

Acknowledgments

Partial financial support for preparation of this manuscript was provided by the David Challinor Postdoctoral 

Fellowship at the National Zoological Park.

Table 2. Measures of genetic diversity, Ho, the observed heterozygosity, and He, the expected heterozygosity, for five groups of blackfooted ferrets (Mustela nigripes), and HWE P, the probability of heterozygote deficiencies (an indicator of inbreeding) within groups. He

and Ho values were averaged over the seven microsatellite loci that were polymorphic in the historical Wyoming population. Each of the 

seven loci had two alleles per locus in each group of ferrets. Both captive breeders and captive releases showed evidence of inbreeding as predicted by pedigree-based methods. (From Wisely and others, 2003. Reprinted with permission of Wiley-Liss, Hoboken, N.J.)

Group n Ho He + 2 SE HWE P

Founders 7 0.40 0.33 + 0.14 0.76

Captive breeders 29 0.32 0.41 + 0.12 0.03

Captive releases 36 0.31 0.38 + 0.14 0.01

Wild, Montana 81 0.33 0.33 + 0.14 0.47

Wild, South Dakota 41 0.43 0.38 + 0.12 0.87

Genetic Legacy of the Black-footed Ferret 41
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Genetic Legacy of the Black-footed Ferret 43

Section III. Searching for Wild Black-footed Ferrets

Because of the potential for a highly inbred and genetically unrepresentative blackfooted ferret population founded solely on individuals rescued from the failed population 

near Meeteetse, Wyo., there was much emphasis in the 1988 recovery plan on finding more 

ferrets. Although extensive effort was undertaken to locate other populations of free-ranging 

ferrets, additional ferrets were not found, as described in the single paper in this section. Future 

organized efforts to find additional populations of free-ranging ferrets are not presently planned 

(and probably are not warranted), even though the discovery of such a population would be of 

great value to the recovery program. 

Abstract

Studies of wild populations of black-footed ferrets 

(Mustela nigripes) in South Dakota in the 1960s, in Wyoming 

in the 1980s, and of captive-bred ferrets reintroduced to unoccupied habitat in Wyoming, South Dakota, and Montana in the 

1990s contributed to our understanding of ferret behavior and 

improved techniques to find ferret populations. We chronicle 

the efforts of private, State, and Federal institutions that used 

these techniques to locate remaining populations of ferrets. 

During the 1980s, a renewed survey effort and solicitation 

of new sightings, coupled with a monetary reward program, 

failed to locate ferrets. We believe that the probability of finding ferrets from noncaptive stock is already small and diminishes with each passing year. 

Keywords: black-footed ferret, Mustela nigripes, reward, 

sighting, spotlighting, survey technique

Introduction

The original recovery plan for the endangered blackfooted ferret (Mustela nigripes) was approved in June 1978, 

and a revised recovery plan was approved in August 1988 

(U.S. Fish and Wildlife Service, 1978, 1988). These plans 

established objectives and outlined steps for recovery that 

would provide for viable black-footed ferret populations in 

captivity and in the wild throughout the historical range. A 

common element in both plans was locating additional wild 

populations both for preservation and as a source of genetic 

diversity for the captive population. Brussard and Gilpin 

(1989) believed that any ferret still extant in the wild should 

be captured to augment the gene pool available to the captive 

breeding program. In addition, a multitude of individuals and 

organizations began work on delineating the historical range 

of the black-footed ferret, defining and identifying suitable 

habitat, and developing methodologies and techniques to find 

remaining populations. This paper presents an overview and 

update on efforts to locate an undiscovered population of 

ferrets.

Techniques for Finding a Wild Population 

of Ferrets

Methodologies to locate black-footed ferrets were first 

developed during the 11 years (1964–74) that a South Dakota 

population was studied (Hillman, 1968a,b; Sheets, 1970; 

Fortenbery, 1972; Hillman and Linder, 1973). Henderson and 

others (1969) presented important life history and behavioral 

characteristics, in addition to techniques for studying and 

locating black-footed ferrets. Nevertheless, Hillman and 

Linder (1973) emphasized the need to develop more efficient 

and conclusive techniques for detecting the presence of ferrets 

on prairie dog (Cynomys spp.) towns.

After the South Dakota population disappeared (the last 

wild ferrets in South Dakota were trapped in 1973; Carpenter 

and Hillman, 1978), the U.S. Fish and Wildlife Service’s 

(FWS) Denver Wildlife Research Center (DWRC), Section 

of Wildlife Ecology on Public Lands, conducted black-footed 

ferret surveys on Federal lands (Martin and Schroeder, 1979, 

1980; Smith and others, 1982; Martin, 1983). The objectives 

of those activities were to search for black-footed ferrets and 

test new or alternative methods for their detection. These 

methods included searches for sign (i.e., diggings, tracks, 

bones, scat, plugged burrows) by foot, horseback, snowmobile, 

and aircraft during daylight hours, as well as searches for 

ferrets at night using spotlights (on foot and from vehicles). 

Other techniques involved observing prairie dog behavior, 

using night vision equipment, and using scent dogs. A partial 

listing of such searches conducted by DWRC in Wyoming 

revealed a total of 1,166 person-hours of night searches 

with spotlights and much more time spent in day searches. 

Spotlight searches resulted in sightings of 54 coyotes (Canis 

latrans), 168 badgers (Taxidea taxus), and 15 long-tailed 

weasels (Mustela frenata), but there were no sightings of 

ferrets.

 In 1981, a new population of black-footed ferrets was 

discovered near Meeteetse, Wyo. (Schroeder and Martin, 

A History of Searches for Black-footed Ferrets

By Louis R. Hanebury1 and Dean E. Biggins2

1

U.S. Fish and Wildlife Service, 2900 4th Ave. N., Suite 301, Billings, MT 

59101.

2

U.S. Geological Survey, Fort Collins Science Center, 2150 Centre Ave., 

Bldg. C, Fort Collins, CO 80526.
48 Recovery of the Black-footed Ferret

1982). Four years of studies by FWS, Biota Research and 

Consulting, Inc., and the Wyoming Game and Fish Department increased our knowledge about locating and monitoring 

black-footed ferrets (Biggins, 1983; Biggins and Fagerstone, 

1983; Clark and others, 1984b, 1986, 1988; Anderson and 

Inkley, 1985; Campbell and others, 1985; Clark, 1985; 

Richardson and others, 1985, 1987; Biggins and others, 1986; 

Fagerstone and Biggins, 1986; Johnson and others, 1986; 

Morkill, 1987). Although Clark and Campbell (1981a) had 

already devised ferret search guidelines, information from 

the Meeteetse studies assisted in formulating updated search 

techniques (Clark and others, 1984a, 1988). It also enabled 

FWS to develop black-footed ferret survey guidelines for 

compliance with the Endangered Species Act (ESA) of 1973, 

as amended (16 U.S.C. 1531–1543) (Schroeder, 1985; U.S. 

Fish and Wildlife Service, 1986). The Wyoming Cooperative 

Fish and Wildlife Research Unit, in cooperation with FWS, 

initiated workshops on black-footed ferret survey techniques 

in 1987. These workshops demonstrated the most current 

methodology for finding ferret populations. Training and certification were necessary to promote uniformity in techniques 

used by consultants and agency personnel. Workshops were 

held periodically until 1994, when Badlands National Park in 

South Dakota hosted the final training course. Today, skills to 

survey for black-footed ferrets are developed by individuals 

participating in field work at one of the active reintroduction 

sites in six States and in Mexico.

Guidelines were prepared by FWS to locate at least 

one animal of a population within three consecutive nights 

of spotlight surveys in a portion of the habitat. Using these 

survey guidelines (U.S. Fish and Wildlife Service, 1986), the 

probability of detecting ferrets was found to be high under 

simulated field conditions (Lindzey and Marinari, 1992; 

Marinari, 1992). Unpublished data from spotlight surveys for 

the reintroduced population of black-footed ferrets studied 

in the mid-1990s on the UL Bend National Wildlife Refuge 

in Montana confirmed the effectiveness of spotlight surveys 

using the FWS protocol (R. Matchett, oral commun., 2003). 

Spotlight surveys conducted while telemetry crews were 

monitoring radio-tagged ferrets suggested that over 90 percent 

of the ferrets above ground (as indicated by telemetry) were 

observed by search crews. Similarly, analysis of ferret observations over a 10-year period in the same area suggested high 

detectability of ferrets, given adequate search effort (Biggins, 

Godbey, Matchett, and others, this volume).

Henderson and others (1969) first proposed the use of a 

helicopter or airplane to locate ferrets during winter. Martin 

and Schroeder (1980) tested both fixed-wing aircraft and a 

helicopter for winter surveys to locate ferret sign. They found 

helicopters more practical because badger diggings were more 

easily found from helicopters. Biggins and Engeman (1986) 

found fixed-wing aircraft acceptable for locating ferret sign in 

winter. Aerial ferret surveys from helicopters were used effectively to survey the large white-tailed prairie dog (C. leucurus) 

complex in northwest Colorado and northeast Utah (U.S. Fish 

and Wildlife Service, 1989). Aerial surveys with helicopters 

or fixed-wing aircraft are currently used at some reintroduction sites and have been used successfully to locate ferrets that 

have dispersed into outlying prairie dog colonies (R. Matchett, 

oral commun., 2003).

In 1978, the DWRC began testing the use of scent dogs 

to locate black-footed ferrets or their sign (Conway and Dean, 

1979; Southwest Research Institute, 1979; Martin and Schroeder, 1980). Two dogs were trained with scat obtained from 

captive black-footed ferrets held at FWS’s (now U.S. Geological Survey’s) Patuxent Wildlife Research Center in Laurel, 

Md. These dogs were later tested at Meeteetse, Wyo., in 1981 

and demonstrated the ability to identify burrows known to be 

occupied by ferrets. Matchett and Smith (2001) successfully 

located reintroduced ferrets in Montana with trained scent 

dogs. Reindl (2004) proposed further testing and use of scent 

dogs in locating black-footed ferrets dispersing from reintroduction sites. Although variability of individual scent dog 

performance remains a problem, these studies indicate that 

dogs may be a useful supplement to spotlight surveys, particularly at sites where conventional search efficiency is poor. 

For example, scent dogs will be used during 2005 in remote, 

hard-to-search areas of the Colorado/Utah black-footed ferret 

reintroduction site (S. Reindl, oral commun., 2004).

Before the discovery of the Meeteetse population, Clark 

and Campbell (1983) tested a track station survey method 

using a variety of lures to detect nocturnal mammalian 

carnivores. Hammer and Anderson (1985) further studied the 

usefulness of track stations and numerous attractants to determine whether black-footed ferrets were present. Tracking and 

camera stations did not record any visitation in areas occupied 

by ferrets in the Meeteetse habitat. Scent stations were tested 

in ferret-occupied habitat in South Dakota with similar results 

(T. Livieri, oral commun., 2005). Scent attractants, track 

stations, and remote cameras have not been proven effective 

for locating ferrets in the wild.

Weasel-like scats have been collected during nocturnal 

and daylight surveys for ferrets and ferret sign (Henderson 

and others, 1969; Fortenbery, 1972; Martin and Schroeder, 

1979; Clark and others, 1984a, 1988; Richardson and others, 

1987). Typical black-footed ferret scat has been described, 

but identification of mustelid scat to species is often problematic. Johnson and others (1986) compared the fecal bile acid 

characteristics of known black-footed ferret scat and other 

known small carnivores and concluded that these acids did not 

enable positive identification of individual ferret scats. They 

did, however, suggest that ferret scats might be identifiable 

with reasonable confidence using gas-liquid chromatography, 

a technique yet to be thoroughly tested. Recent advances in 

DNA testing may provide a reliable and practical method 

to identify black-footed ferret scats where other sign is not 

discernible.

Reintroduced black-footed ferrets afforded renewed 

opportunities to gain knowledge on ferret behaviors, thereby 

increasing our ability to detect free-ranging ferrets. The first 
A History of Searches for Black-footed Ferrets 49

reintroduction of captive-raised black-footed ferrets at Shirley 

Basin, Wyo., yielded much information regarding behavior, 

dispersal, and postrelease survival (U.S. Fish and Wildlife 

Service, 1992; Wyoming Game and Fish Department, 1992, 

1993, 1994, 1995; Oldemeyer and others, 1993). Subsequent 

reintroductions in South Dakota, Montana, Arizona, Colorado, 

and Utah have each added new data, which collectively have 

contributed to validating and refining effective search techniques.

Historical Sighting Reports and 

Surveys

Anderson and others (1986) provided an exhaustive 

summary of black-footed ferret specimens from North 

America, beginning with the first specimen collected by Audubon and Bachman in 1851. One of the earliest efforts to solicit 

black-footed ferret sightings occurred in 1952 (Cahalane, 

1954). During the period 1946–53, 42 black-footed ferrets 

were reported from 42 localities, mostly in South Dakota, 

Montana, Nebraska, and Colorado (Cahalane, 1954). About 

one-third of the animals observed were found dead or were 

trapped (killed), shot, hit by vehicles, or died in captivity. 

Cahalane’s (1954) call for a life history study of the 

black-footed ferret was answered in 1964 with the discovery 

of the population in Mellette County, S. Dak. Before the South 

Dakota population disappeared, a black-footed ferret and prairie dog workshop in 1973 brought together State, Federal, and 

academic interests to present historical and current knowledge 

on prairie dogs and black-footed ferrets (Linder and Hillman, 

1973). Eleven States, Federal agencies, and academic institutions provided information on available habitat, historical 

and recent black-footed ferret sightings, and efforts to locate 

additional populations (Clark, 1973; Grondahl, 1973; Henderson and Little, 1973; Lewis, 1973; Lewis and Hassien, 1973; 

Locke, 1973).

Kansas was one of the first States to have an active 

“Wanted: Black-footed Ferret” program. Historical ferret 

sightings, ferret specimen records, and new sighting reports 

formed the foundation for actively looking for additional 

populations (Henderson and Little, 1973). Letters and 

pamphlets were widely disseminated, supported by articles in 

local newspapers and magazines and by public radio and television announcements (Henderson, 1969). A color “Wanted 

Alive” poster was later produced and sent to all States within 

the original range of the black-footed ferret (appendix, fig. 

A1). Clark (1973, 1978, 1980) and Clark and Campbell 

(1981b) took a similar approach in Wyoming, identifying 

habitat and gathering historical and new specimen records 

and sighting reports. Campbell (1989) described searches to 

locate black-footed ferret populations conducted in Montana 

between 1984 and 1989. Crete (1985) discussed FWS’s efforts 

to work with State agencies and private entities to find other 

wild populations of ferrets. A second major black-footed ferret 

workshop was held in 1984 in Laramie, Wyo. Federal and 

State agencies summarized new efforts to handle black-footed 

ferret sighting reports (Cada, 1985; Grode, 1985; Hammer, 

1985; Hasenyager, 1985; Lengkeek, 1985).

Because of funding constraints and the lack of any legal 

mandate, searches for remaining populations of black-footed 

ferrets were at times limited. For example, despite the blackfooted ferret’s inclusion in the first list of rare and endangered 

wildlife by the U.S. Department of the Interior in 1964 (Clark, 

1989), in the 1966 Endangered Species Preservation Act, and 

in the Endangered Species Conservation Act of 1969, there 

were no prohibitions of harm to a listed species (“taking” was 

prohibited only within national wildlife refuges) and therefore 

no requirements to determine whether black-footed ferrets 

were present prior to authorizing projects that might harm 

ferrets or modify their habitat. By 1965, the Department of 

the Interior had established a policy on precontrol surveys 

for prairie dog control programs throughout the range of 

the black-footed ferret on all classes of land (Berryman and 

Johnson, 1973). This policy was initially for, and first applied 

to, prairie dog control programs on Pine Ridge Indian Reservation in South Dakota (Hanson, 1988, 1993). New Mexico 

also initiated precontrol surveys for black-footed ferrets 

during that time (Hubbard and Schmitt, 1983). The policy on 

precontrol surveys was further refined by Executive Order 

11643 (Berryman and Johnson, 1973). Jobman and Anderson 

(1985) reviewed other Federal authorities that might affect 

or be used in locating funds or facilitating ferret recovery 

activities. Schroeder (1988), however, noted the requirement 

for ferret surveys that was triggered by the ESA of 1973. 

Under section 7(a)(2), Federal agencies are required to consult 

with FWS to ensure that any action they authorize, fund, or 

carry out is not likely to jeopardize the continued existence of 

any endangered or threatened species or result in the destruction or adverse modification of a species’ designated critical 

habitat. If suitable habitat (i.e., capable of supporting at least 

one black-footed ferret) is present within the action area, FWS 

has the authority to recommend that a ferret survey precede 

the project. In 1986, to provide some consistency in survey 

recommendations, FWS developed standard survey guidelines 

(Schroeder, 1985; U.S. Fish and Wildlife Service, 1986). 

When properly implemented, the prescribed strategy has 

good potential to detect a population of black-footed ferrets. 

Validating whether suitable habitat is occupied by ferrets is 

necessary to determine if an action may adversely affect the 

species. Because of the policy to include precontrol surveys 

for black-footed ferrets after 1965, the mandatory consultation 

requirements for Federal agencies in the ESA of 1973, and 

pesticide registration label statements (U.S. Environmental 

Protection Agency, 1987), surveys for black-footed ferrets 

by Federal agencies and their consultants have been occurring for 40 years. Black-footed ferret populations could have 

escaped detection because some surveys were inadequate or 

because some suitable habitats were never surveyed. Nonethe-
50 Recovery of the Black-footed Ferret

less, these surveys have covered large areas without finding a 

population of living ferrets (but they have resulted in discovery 

of old black-footed ferret remains). 

In the initial black-footed ferret recovery plan (U.S. Fish 

and Wildlife Service, 1978), one recovery task was to map 

the amount of prairie dog habitat occupied by black-footed 

ferrets, to be accomplished by compiling sighting reports for 

each State within the historical range (Jobman and Anderson, 

1981a). The FWS’s Pierre, S. Dak., office was designated as 

the receiving station for all black-footed ferret sightings; this 

responsibility was moved to Grand Island, Nebr., in 1985. A 

questionnaire and letter requesting ferret sightings between 

January 1, 1970, and January 1, 1981, were sent to Provincial (Canadian), Federal, State, and private (tribal, industry, 

conservation, and recreation groups) institutions (Jobman and 

Anderson, 1981b), resulting in reports of 228 sightings. Sightings were classified as confirmed, probable, or unconfirmed. 

Periodic updates (W. Jobman, written commun., 1987–92) to 

the original report added the following additional sightings: 

232 (1987), 51 (1988), 25 (1989), 26 (1990), 31 (1991), and 

25 (1992). Partly because of the paucity of additional sightings, updates were discontinued in 1992, and sighting records 

are no longer formally maintained by FWS. Individual State or 

FWS offices may maintain records, however.

There are six reintroduced populations of black-footed 

ferrets that are designated nonessential, experimental in accordance with section 10(j) of the ESA. The black-footed ferret 

reintroduction sites represented the best habitat available and 

would seem to have been likely places to find any extant ferret 

populations. To comply with Section 10(j), all of these release 

sites were surveyed for resident black-footed ferrets. For 

example, there were 350 black-footed ferret surveys conducted 

on lands occupied by prairie dogs at the first site designated 

(Shirley Basin/Medicine Bow, Wyo.) to receive black-footed 

ferrets in 1991 (U.S. Fish and Wildlife Service, 1991). Other 

areas designated as nonessential, experimental populations 

received similar search efforts (U.S. Fish and Wildlife Service, 

1993, 1994, 1996, 1998). Resident populations of ferrets were 

not found in any of the six areas.

Early Reward Programs

Throughout the 1970s, many States within the historical 

range of the black-footed ferret solicited sightings of ferrets. In 

1974, Dr. Tim Clark, through an effort funded by the National 

Geographic Society and the National Academy of Sciences, 

solicited sightings from Wyoming and all States within the 

historical range of the black-footed ferret (Campbell, 1989). 

Clark went a step further by offering a $50 reward for a photograph or other information leading to the discovery of ferrets 

in the wild. “Wanted” posters (appendix, fig. A2) were widely 

distributed, and the reward was increased to $250 in 1980. 

Other States, such as Oklahoma (Hassien, 1976) and New 

Mexico (Hubbard and Schmitt, 1983), had active publicity 

programs to solicit sighting reports of black-footed ferrets. 

The New Mexico Department of Game and Fish during 1978–

81 and the New Mexico State Office of the Bureau of Land 

Management in 1982 conducted well-organized campaigns 

to solicit black-footed ferret sightings (Hubbard and Schmitt, 

1983). The program was well publicized through posters, 

postcards, newspapers, magazines, and television. None of the 

78 records produced was considered reliable evidence for the 

continued existence of black-footed ferrets in New Mexico 

(Hubbard and Schmitt, 1983).

Clark’s $250 reward was paid to the finders of the 

Meeteetse, Wyo., ferret that led to the discovery of the last 

known extant population. Following that seminal event, 

biologists developed a program to locate ferrets in Montana 

by offering a monetary reward (Campbell, 1989). In 1983, 

Montana Fish, Wildlife and Parks and Biota Research and 

Consulting, Inc., developed a reporting system designed to 

standardize and assess ferret sightings, presenting criteria 

to evaluate each sighting and a protocol to follow if ferrets 

were discovered. The publicity resulted in 69 ferret reports 

by August 1986, but none resulted in locating and capturing 

a live black-footed ferret. In August 1986, Montana Fish, 

Wildlife and Parks initiated a new program soliciting ferret 

sightings but now offering a monetary reward (Flath, 1987). 

It included a mailer (appendix, fig. A3) describing the reward 

program and a standardized reporting form (appendix, fig. 

A3). A $5,000 reward for information leading to the discovery 

of a wild population of black-footed ferrets in Montana was 

offered by Wildlife Conservation International (a subsidiary 

of the New York Zoological Society). This program not only 

offered a significant reward but also made it the responsibility of the person submitting the sighting to include adequate 

information, limiting the need for follow-up on reports that 

were questionable. The reward program was aggressively 

advertised and included the distribution of “Wanted” posters 

(appendix, fig. A4) in post offices, public buildings, and businesses. The monetary reward program generated 66 additional 

responses (Campbell, 1989), but none led to the discovery of 

additional ferrets.

Recovery Activities at the National 

Ecology Research Center

In 1988, FWS approved a revised black-footed ferret 

recovery plan (U.S. Fish and Wildlife Service, 1988). The 

FWS’s National Ecology Research Center (NERC; now the 

U.S. Geological Survey’s Fort Collins Science Center) was 

responsible for completion of many of the revised recovery 

tasks outlined in the plan. One of these tasks was finding 

additional ferrets to bolster the depauperate genetic representation of ferrets in the captive breeding program. Biggins 

and Crete (1989), Hanebury and Biggins (1989), and Godbey 

and Biggins (1994) discussed FWS activities associated with 

finding ferrets under the new Black-footed Ferret Recovery 
A History of Searches for Black-footed Ferrets 51

Plan. To locate additional ferrets, NERC expanded Montana’s 

black-footed ferret reward program to other States and 

renewed black-footed ferret surveys on the best remaining 

ferret habitat.

National Reward Program

The Black-footed Ferret Interstate Coordinating Committee (ICC) was established in 1987 to improve communication and promote ferret recovery in 12 States, two Canadian 

Provinces, and the State of Chihuahua, Mexico. In 1987, the 

ICC identified the need for a national reward program and 

recommended a program similar to that used by Montana. In 

the fall of 1987, Wildlife Conservation International agreed to 

apply its $5,000 reward to any State within the former range of 

the black-footed ferret that wished to participate. States in the 

program had to be prepared to follow up on all reported sightings within a reasonable amount of time. Montana, Wyoming, 

Colorado, South Dakota, Utah, Nebraska, Oklahoma, Texas, 

and Arizona participated in the program. The Navajo Nation, 

encompassing a large area in both New Mexico and Arizona, 

also joined the effort. A new poster applicable to all States 

offering the $5,000 reward was prepared, and by February 

1988, 10,000 copies were distributed. The revised posters 

(appendix, fig. A5) included a photo of a distinctive ferretdigging on snow and sketches of ferret tracks; to qualify for 

the reward, the observer needed to submit a photograph or 

information that resulted in verification of one or more live 

black-footed ferrets. To increase the quality and quantity 

of responses, Wildlife Conservation International agreed to 

increase the reward to $10,000 (appendix, fig. A6), effective 

March 3, 1989. Thousands of updated $10,000 reward posters 

were mailed to participating States.

New black-footed ferret sighting report forms were 

developed, incorporating the knowledge gathered from studying the Meeteetse population and experience gained from 

earlier ferret surveys in response to sighting reports. Ranking 

criteria and instructions were also developed. A sighting report 

was scored as highly probable, likely, fair, or unlikely. Such 

a classification was intended to limit follow-up investigations 

to the most probable sightings. This conservative approach 

addressed concerns about “probable” and “confirmed” sighting classifications used by Jobman and Anderson (1981a,b). 

They defined a probable sighting as one made by a qualified 

observer or a competent observer who was not positive about 

a sighting. A confirmed sighting was defined as one made by 

a competent and dependable observer who had no doubts that 

the animal seen was a black-footed ferret observed in or near 

suitable habitat (Jobman and Anderson, 1981a,b). 

As part of the national reward program, an extensive 

effort was made to advertise the reward through all forms 

of local, statewide, and national media. The communications division of the Wyoming Game and Fish Department 

assisted NERC in producing audio and video public service 

announcements on the reward offer, which were sent to radio 

and television stations. Press releases describing summer and 

winter ferret behavior and sign, along with the posters, were 

distributed to the print media. Articles on the black-footed 

ferret reward program appeared in local and major newspapers 

and magazines. Major networks ran segments on programs 

such as Missing: Reward and NBC’s Today Show, and special 

presentations such as the British Broadcasting Corporation’s 

“Wildlife On One: Wanted Alive,” which aired on National 

Geographic Explorer.

Responses to the reward program are difficult to quantify 

because each State handled incoming reports independently. 

Reports came directly to NERC or were forwarded from the 

receiving States. Although respondents provided descriptive 

photographs, drawings, and detailed characteristics, the photographs typically were of domestic ferrets (Mustela putorius 

furo) from both within and outside the black-footed ferret’s 

historical range (e.g., California, Colorado, Florida, Kentucky, 

Nevada, Texas, and Utah), as well as photographs of longtailed and bridled weasels (Mustela frenata), badgers, and 

prairie dogs. Photographs of black-footed ferrets were received 

from individuals in South Dakota and Alberta, Canada. Both 

of the photographs matched older photos taken by others, and 

both reports were judged to be hoaxes. The $10,000 reward 

offer was terminated on December 31, 1990.

Renewed Search Efforts

During the time of the national reward program, NERC 

formed black-footed ferret survey teams that were prepared 

to respond to valid sighting reports and to make one final 

effort to look for ferrets in suitable habitat. Between 1984 

and 1988, six black-footed ferret sightings were reported in 

or around Waterton Lakes National Park in Alberta, Canada 

(Laing, 1988). Those reports were investigated in 1989 by 

Waterton Lakes National Park personnel, a local naturalist, 

NERC, and FWS personnel (Hanebury, 1989; Harvie, 1989; 

McGill, 1989). To lend credence to the reports, there were 

previous specimens or sighting records from Alberta and 

Saskatchewan (Russell, 1985; Anderson and others, 1986; 

Laing, 1987; Laing and Holroyd, 1989). In Canada, blackfooted ferret surveys before this new effort were limited to 

those by Millson (1976), Laing (1987), and Laing and Holroyd 

(1989). Laing and Holroyd (1989) listed 15 recent sightings 

from 1967 to 1986 and surveyed all reported sites. No blackfooted ferrets were confirmed by either the past efforts or the 

more recent efforts by NERC in and around Waterton Lakes 

National Park. Highly ranked reports on Navajo Nation lands 

in New Mexico were also investigated with negative results 

(Hanebury, 1988a). A brief search (26 person-hours) in the 

largest black-tailed prairie dog complex in North America, in 

Chihuahua, Mexico (Ceballos and others, 1993), did not detect 

any black-footed ferrets (Hanebury, 1988b), but there were 

33 sightings of coyotes and 4 sightings of badgers. A partial 

tally of accessible data revealed 978 person-hours of spotlight 

searches conducted by NERC personnel in response to rangewide ferret sighting reports. The searches resulted in sightings 

of many other predators, including 187 coyotes, 193 badgers, 
52 Recovery of the Black-footed Ferret

and 17 long-tailed weasels, but the widely advertised $10,000 

reward and subsequent investigations produced no proof of 

free-ranging ferrets.

In addition to responding to the sighting reports, NERC 

survey crews directed efforts to the locations that seemed most 

likely to harbor ferrets as determined by specimen records, 

clustered sighting reports, and information suggesting the 

presence of high-quality habitat (Jobman and Anderson, 

1981b; Anderson and others, 1986; W. Jobman, written 

commun., 1984, 1992). These areas were located in South 

Dakota, Montana, and Wyoming. In the mid-1970s, when no 

remaining South Dakota ferret populations could be found, 

searches throughout the historical range of the black-footed 

ferret were undertaken by State resource agencies, private 

consultants, industry, university students, private citizens, and 

Federal agency biologists. Because no centralized repository 

for black-footed ferret survey data existed, it is impossible 

to quantify the hours devoted to spotlighting for ferrets, the 

area covered, or how many times the same area was searched 

through time. 

Other evidence of a declining black-footed ferret population includes the number of ferret specimens obtained from 

poisoning, trapping, vehicle collisions, or other sources. Peak 

numbers occurred during the 1920s and 1930s (fig. 1), perhaps 

the period when the largest areas of prairie dog colonies were 

poisoned. Biggins and Schroeder (1988) speculated that this 

increase probably reflected increased attention given the 

species rather than a change in the population. No specimens 

were retrieved during the late 1970s and early 1980s. Although 

trapping probably decreased during that time, vehicular traffic 

and miles of roads increased. Cahalane (1954) reported that 

out of 42 sighting records from the period 1946–53, 17 ferrets 

were killed before or after the sighting. Four of those deaths 

were road kills between 1948 and 1953. During studies of the 

ferret population in south-central South Dakota in Mellette 

County, eight road-killed ferrets were documented in about 

8 years (Hillman and Linder, 1973). There were no reported 

ferret road kills during the decade of the 1980s. It was not 

until 1994, after captive-bred black-footed ferrets were reintroduced to unoccupied habitat, that ferret specimens again began 

to be collected as road kills (fig. 1).

Summary

Since the decline of the last known ferret population in 

South Dakota, substantial effort has been devoted to identifying viable ferret habitat and locating any remaining isolated 

ferret populations. Survey techniques were developed and 

used as a reliable standard to find black-footed ferrets. Search 

efforts increased after the establishment of a policy for prairie 

dog precontrol surveys in 1965 and following implementation of the ESA in FWS field offices throughout the historical range of the ferret. None of the searches performed to 

implement recovery plan tasks, to comply with ESA section 

7 consultation requirements (including pesticide registration), 

and to ensure compliance with the “take” prohibitions of 

section 9 of the ESA, nor heroic efforts by private individuals 

and conservation groups, have found any black-footed ferrets 

in the wild. At some locations, the lack of success in finding 

wild ferrets, combined with the desire by some agencies and 

organizations to expedite projects (e.g., prairie dog control, 

oil and gas development) in ferret habitat (i.e., prairie dog 

colonies), has resulted in requests for FWS to declare areas 

entirely “ferret free” (i.e., to “block-clear” the area from the 

need for preproject ferret searches) (Campbell and others, 

1990). Today, requirements for preproject ferret surveys have 

been either officially eliminated or deemphasized in all of the 

12 States composing the historical range of the black-footed 

ferret. The majority of the ferret range in South Dakota has 

been either block-cleared or exempted from the need for ferret 

surveys because of designation of experimental areas for ferret 

reintroduction through deliberative processes (South Dakota 

Department of Agriculture and South Dakota Department of 

Game, Fish and Parks, 2003). Other States with significant 

remaining areas of viable ferret habitat (active prairie dog 

colonies) have officially block-cleared habitat not considered valuable for ferret recovery (Colorado, R. Krueger, oral 

commun., 2005; Wyoming, M. Jennings, written commun., 

2004). For the most part, North Dakota (B. Bicknell, oral 

commun., 2005), Nebraska (B. Harms, oral commun., 2005), 

Kansas (D. Mulhern, oral commun., 2005), Oklahoma (S. 

Harmon, oral commun., 2005), Utah (R. Chi, oral commun., 

2005), and Texas (J. Hughs, oral commun., 2005) do not 

require preproject ferret surveys for section 7 consultation. 

New Mexico considers the black-footed ferret to be extirpated 

and therefore does not require preproject surveys (M. Murphy, 

oral commun., 2005).

Some organizations have promoted block-clearing as 

a strategy to improve public sentiment toward black-footed 

ferret recovery and prairie dog conservation (Patton and 

Figure 1. Number of black-footed ferret (Mustela nigripes) specimens collected by decade from Anderson and others (1986) and 

Clark (1989), including the decade after ferret reintroductions.
A History of Searches for Black-footed Ferrets 53

Leachman, 1991). Further, the now widely held view that the 

probability of ferrets persisting in the wild is low, combined 

with the expense of conducting guideline-standard ferret 

searches, has caused FWS to relax section 7 consultation 

requirements (M. Lockhart, written commun., 2003) and 

propose that tasks relating to additional ferret searches be 

deemphasized in a second revision of the black-footed ferret 

recovery plan (U.S. Fish and Wildlife Service, 2004). A 

review of ferret survey needs is still in progress, however, and 

will be reflected in the final revised recovery plan.

Over 15 years ago, Lacy and Clark (1989) examined 

genetic variability in black-footed ferret populations and 

stated that it was unlikely that a long-term viable population 

of ferrets existed in the wild. We believe that the probability 

of finding ferrets that stem from noncaptive stock is already 

small and diminishes with each passing year. There are, 

however, several remaining considerations. With the reintroduction of over 1,900 captive-raised black-footed ferrets 

and with much recruitment of wild-born kits since 1991, the 

possibility of newly established populations in the wild will 

increase. The example of the remarkable persistence of ferrets 

in the disease-prone, vast, but fragmented habitat of Shirley 

Basin, Wyo. (Grenier and others, 2004), gives us hope that 

free-ranging ferrets will persist in other States as reintroductions continue. In addition to the need to monitor reestablished 

ferret populations, there will be a continued need for improved 

monitoring methodologies and searches to locate future populations established by dispersing young.

References Cited

Anderson, E., Forrest, S.C., Clark, T.W., and Richardson, 

L., 1986, Paleobiology, biogeography, and systematics of 

the black-footed ferret, Mustela nigripes (Audubon and 

Bachman), 1851: Great Basin Naturalist Memoirs No. 8, p. 

11–62.

Anderson, S.H., and Inkley, D.B., eds., 1985, Black-footed 

ferret workshop proceedings: Cheyenne, Wyoming Game 

and Fish Department, p. 1.1–33.12.

Berryman, J.H., and Johnson, N.C., 1973, Ferret and prairie dog programs on public lands—a perspective and 

some facts, in Linder, R.L., and Hillman, C.N., preparers, Proceedings of the black-footed ferret and prairie dog 

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56 Recovery of the Black-footed Ferret

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58 Recovery of the Black-footed Ferret

Appendix. Posters Used To Solicit Reports of Black-footed Ferret Sightings
A History of Searches for Black-footed Ferrets 59

Figure A1. The first poster used to solicit information about locations of black-footed ferrets (Mustela nigripes) (original poster was 

in color).
60 Recovery of the Black-footed Ferret

Figure A2. A 1974 poster distributed by Tim Clark, offering a $50 reward for information leading to discovery of black-footed ferrets 

(Mustela nigripes).
A History of Searches for Black-footed Ferrets 61

Figure A3. A pamphlet and report form, distributed in Montana starting in 1983, advertising a $5,000 reward for a verified black-footed 

ferret (Mustela nigripes) sighting.
62 Recovery of the Black-footed Ferret

Figure A3. A pamphlet and report form, distributed in Montana starting in 1983, advertising a $5,000 reward for a verified black-footed 

ferret (Mustela nigripes) sighting.—Concluded.
A History of Searches for Black-footed Ferrets 63

Figure A4. A poster used to further advertise the Montana $5,000 reward supported by the New York Zoological Society, distributed 

in 1986–87.
64 Recovery of the Black-footed Ferret

Figure A5. The poster used to advertise the New York Zoological Society’s $5,000 reward after the reward was offered nationally during 

1987–89.
A History of Searches for Black-footed Ferrets 65

Figure A6. The poster used to advertise the New York Zoological Society’s national reward of $10,000 offered in 1989.

Section IV. Locating and Evaluating Habitat

Unlike the relative success of captive breeding, major challenges remain for securing 

adequate habitat for black-footed ferrets. Procedures for evaluating prairie dog colonies and 

complexes are being refined as more is learned about both ferret biology and prairie dog 

ecology. We have gained a greater appreciation of the interactions of prairie dogs with their 

environment and management options for prairie dogs. There is presently too little habitat to 

effectively implement recovery, however, and much of the remaining habitat is threatened by 

human activities and plague (also see papers in Section V).

Abstract

This paper is an attempt to develop a new, broad list of 

potential black-footed ferret (Mustela nigripes) reintroduction sites across its historical range. I reviewed reports and 

publications that identified active, inactive, and potential 

reintroduction sites, including unpublished reports generated 

by State wildlife agencies and universities. I contacted local 

experts and reviewed the published and unpublished literature 

describing colony locations of three species of prairie dogs 

(Cynomys spp.). I list active reintroduction sites and others 

already planned and identify 70 other sites in the historical 

range of the black-footed ferret that might meet the biological 

and habitat suitability requirements for reintroduction of the 

species within 3–10 years, contingent upon directed management emphasis, State and Federal agency management priority, and, if on private land, landowner concurrence through 

agreements or incentives. I present this conceptual effort in the 

hope that identification of sites at this level will prompt discussion, revisions, additions, and deletions and will result in the 

formation of conservation partnerships that will contribute to 

black-footed ferret recovery.

Keywords: black-footed ferret, conservation, Cynomys, 

endangered species, Mustela nigripes, prairie dog, reintroduction

Introduction

Although many known, large prairie dog (Cynomys spp.) 

complexes have previously been identified, I believe that this 

paper is the first serious attempt to develop a new, broader list 

of potential reintroduction sites across the historical range of 

the black-footed ferret (Mustela nigripes). Some of these sites 

have been considered before, but many have not, or at least 

not in the same context as in the current effort. I present this 

conceptual effort in the hope that identification of the sites 

at this level will prompt discussion, revisions, additions, and 

deletions, and result in the formation of conservation partnerships that will contribute to black-footed ferret recovery.

Past efforts to identify sites have been constrained by the 

need to immediately take into account land ownership, plague 

history, and other factors that do not constrain the current 

conceptual effort. I hope that this paper prompts many who 

have not considered contributing to black-footed ferret recovery to get involved with a site in their locality. Several States 

that have not been involved in black-footed ferret recovery in 

the past have not previously participated in site identification. 

I recognize that there are issues other than ecological 

ones that must be addressed when identifying potential reintroduction sites; however, I believe that recovery of the blackfooted ferret depends first and foremost upon identifying and 

conserving areas that meet or have the potential to meet the 

biological parameters for establishment and long-term survival 

of viable populations. I believe that social and economic 

issues, including private land rights, economic concerns 

related to forage competition between livestock and prairie 

dogs, and others, are vitally important. I also believe, however, 

that a start must be made. Changes in Federal land management priorities, cooperative management planning on Federal 

lands, and financial incentives or regulatory assurances for 

private landowners or tribal governments must logically follow 

after habitat suitability has been established. 

Recovery efforts for the endangered black-footed ferret 

have faced numerous and significant challenges, including 

extirpation of the species in the wild, development of captive 

breeding techniques and reintroduction methods, lack of 

adequate financial resources, and organizational inefficiencies 

(Forrest and others, 1985; Clark, 1986; U.S. Fish and Wildlife 

Service, 1988; Miller and others, 1996). Much work has been 

accomplished, and much remains to be done in these areas 

and others, but at present I believe that the most fundamental 

obstacle to meaningful recovery of the black-footed ferret in 

the wild is the availability of suitable habitat, both in quantity 

and quality; that is, prairie dog colonies of sufficient size and 

proximity to other colonies (Chaplin and others, 1996; Lomolino and others, 2002; Luce, 2003). There is a critical need to 

identify suitable sites and begin management of those sites for 

reintroduction and recovery. In fact, this may be the ultimate 

challenge to black-footed ferret recovery because it involves 

the greatest potential conflict with other land-use interests. 

Political and social barriers often surpass in difficulty those in 

the biological arena.

Areas Where Habitat Characteristics Could Be Evaluated 

To Identify Potential Black-footed Ferret Reintroduction 

Sites and Develop Conservation Partnerships 

By Robert J. Luce1

1

P.O. Box 7, Sierra Vista, AZ 85636.
70 Recovery of the Black-footed Ferret

In the late 1980s, spurred by the need to utilize animals 

produced by captive breeding, biologists identified several 

potential reintroduction sites. In 1988–89, R. Luce (written 

commun., 1995) developed a list of 18 potential reintroduction sites in Wyoming by using data from a variety of sources. 

Conway (1989) evaluated six of those sites and concluded 

that only two had prairie dog numbers suitable for blackfooted ferret reintroduction. Closer examination of other 

sites in Wyoming, as well as sites in Arizona, Colorado, 

South Dakota, and Utah, revealed that many were more or 

less unsuitable at the time of evaluation for various reasons, 

principally because prairie dogs did not occupy the sites to the 

extent that earlier evaluations had recorded or assumed (M. 

Lockhart, written commun., 1999–2003). Ranking of sites 

suitable for black-footed ferret reintroduction and recovery 

has emphasized the importance of large complexes of prairie 

dog colonies and identification of multiple sites. Additionally, 

it has been assumed that more densely occupied black-tailed 

prairie dog (C. ludovicianus) colonies are preferable to less 

dense white-tailed (C. leucurus) or Gunnison’s (C. gunnisoni) prairie dog colonies and that a plague-free environment 

is preferable. New data documenting maintenance and/or 

growth of both prairie dog and black-footed ferret populations at reintroduction sites on Gunnison’s and white-tailed 

prairie dog complexes where plague is present in Arizona (B. 

Van Pelt, oral commun., 2004) and Wyoming (M. Grenier, 

oral commun., 2004) indicate that these assumptions warrant 

further investigation.

A revision of the current Black-footed Ferret Recovery 

Plan (U.S. Fish and Wildlife Service, 1988) is underway, so it 

is important to note that I do not intend to supersede the site 

selection process that will be a part of the revised plan. The 

revised plan may include new downlisting and delisting goals 

for number of black-footed ferrets and number or location 

of reintroduction sites, but in either case a large number of 

potential reintroduction sites must be identified. I offer a new 

baseline list that includes contributions from all portions of 

the species’ historical range, both previously overlooked sites 

and recently identified sites. I do not attempt to identify longterm black-footed ferret recovery needs for various areas of 

the species range because a rangewide delisting goal has not 

been identified and because a related method for apportioning 

recovery responsibilities among political jurisdictions has not 

been formalized to date (see Ernst and others, this volume).

The most promising recovery sites already have active 

reintroduction programs in place. I believe that several new 

sites with potential for adequate occupied habitat to be present 

within 3–10 years should be identified for each of the political 

jurisdictions within the historical range of the black-footed 

ferret. It is not appropriate to wait for a definitive answer as to 

the number of black-footed ferrets necessary for delisting or 

the amount of actual habitat that will be needed. Many more 

sites must be evaluated than are currently being considered 

because environmental unknowns, especially plague and 

drought, affect the viability of individual sites; therefore, 

longevity cannot be predicted or guaranteed. In addition, 

political and social attitudes may change, resulting in loss of 

support for maintaining adequate occupied prairie dog habitat 

at a given site. I identify a large number of sites so that no one 

site will be under pressure for rapid development, but yet the 

presence of the sites on the list will allow agencies to begin 

planning toward management of those sites, potentially allowing a significant number of them to be available for blackfooted ferret reintroduction in 3–10 years. 

Methods

I reviewed previous efforts that identified active, inactive, 

and potential black-footed ferret reintroduction sites, including 

the U.S. Fish and Wildlife Service (FWS) reintroduction site 

list (Conway, 1989; M. Lockhart, written commun., 1999–

2003; fig. 1). I also reviewed published literature, including 

Lair and Mecham (1991), Vanderhoof and Robel (1994), Ernst 

(2001), and Johnson and others (2003). In addition, I reviewed 

available information regarding other potential sites, including unpublished reports generated by State wildlife agencies 

Figure 1. Location of eight active black-footed ferret (Mustela 

nigripes) reintroduction sites (1990–2004); three Immediate Potential Sites (1–3 years); and 70 Intermediate Potential Sites, at which, 

pending further evaluation, opportunities for reintroducing ferrets 

may exist in 3–10 years.
Identifying Potential Reintroduction Sites 71

and universities, and contacted local experts. I had personal 

communication with Steve Whiteman, Southern Ute Tribe; 

Craig Knowles, FaunaWest Wildlife Consultants; Derrick 

Holdstock and Heather Whitlaw, Texas Parks and Wildlife 

Department; Julianne Hoagland, Oklahoma Department of 

Wildlife Conservation; Pamela Schnurr, Colorado Division 

of Wildlife; Dave Wagner, Northern Arizona University; Bill 

Woodson, U.S. Army; Mike Albee, U.S. Bureau of Land 

Management; Tim Byer and Dave Augustine, U.S. Forest 

Service; Joe Truett, Turner Endangered Species Fund; Allison 

Puchniak, Montana Department of Fish, Wildlife and Parks; 

Terry Enk, New Mexico Department of Game and Fish; Pete 

Gober, Randy Matchett, Scott Larson, John Nysted, and Lou 

Hanebury, U.S. Fish and Wildlife Service; Mark Lomolino, 

State University of New York, College of Environmental 

Science and Forestry; Amy Seglund and Craig McLaughlin, 

Utah Division of Wildlife Resources; Pat Fargey, Grasslands 

National Park, Canada; Martin Grenier, Wyoming Game and 

Fish Department; Tim Vosburgh, Intertribal Black-tailed Prairie Dog Coordinator; Bill Van Pelt, Arizona Game and Fish 

Department; Rurik List, Instituto de Ecologia, Ciudad Universitaria Coyoacan, Mexico; Travis Livieri, Prairie Wildlife 

Research; Mike Fritz, Nebraska Game and Parks Commission; 

and Sandy Hagen, North Dakota Game and Fish Department. 

Information was acquired for 12 States within the historical range of the black-footed ferret, five Native American 

reservations, two States in Mexico, and one Canadian Province. The foundation for this effort was provided by intensive 

and extensive inventories and preparation of management 

plans for black-tailed prairie dogs, as summarized in Luce 

(2003); white-tailed prairie dog survey data, as summarized 

in Seglund and others (2005a); and Gunnison’s prairie dog 

survey data, as summarized in Seglund and others (2005b).

I use the following terminology. Active Sites are those 

at which black-footed ferrets have been previously released 

and are being actively managed. Immediate Potential Sites are 

those already identified by the Black-footed Ferret Recovery 

Implementation Team and upon which reintroduction work 

has begun. Intermediate Potential Sites are those at which 

opportunities may exist in the 3- to 10-year time frame.

Planning efforts conducted by recovery partners require 

a queue of potential sites. I provide a locally specific list of 

all potential black-footed ferret reintroduction sites across the 

species’ historical range but focus on Intermediate Potential 

Sites since these provide the next step in black-footed ferret 

reintroduction beyond management of Active Sites. Reintroduction efforts could begin at an Intermediate Potential Site 

before the minimum occupied habitat identified was available 

if expansion could be reasonably anticipated within a decade. 

Therefore, sites that are now below the minimum threshold for 

occupied habitat are also listed in this paper, anticipating that 

they have potential to meet or exceed the minimum within 10 

years. Although I surmise that long-term potential sites may 

exist, I do not list those here.

At existing black-footed ferret reintroduction sites, as 

well as in State black-tailed prairie dog management plans, 

contiguous habitat is defined as a complex of colonies in which 

no colony is farther than 7 km from another colony (Biggins 

and others, 1993). A colony is defined as a concentration of 

black-tailed prairie dogs with an average density of at least 4.05 

individuals/ha (Luce, 2003) or as a concentration of white-tailed 

prairie dogs with a minimum of 20 burrow openings/ha on 5-ha 

parcels (Biggins and others, 1993; Seglund and others, 2005a). 

Colony has not yet been defined for Gunnison’s prairie dogs, 

but the species is biologically similar to the white-tailed prairie 

dog. Although this rigorous definition was not used to identify 

the Intermediate Potential Sites in this paper, it must be assumed 

that sites will be required to meet a similar standard eventually 

before their full potential for maintenance of a long-term, viable 

black-footed ferret population can be achieved. 

Based on bioenergetic (Biggins and others, 1993) and 

behavioral considerations (R. Matchett and T. Livieri, oral 

commun., 2003) and known densities of the respective species, 

I began with the premise that the minimum adult population of 

30 individuals identified in the 1988 recovery plan (U.S. Fish 

and Wildlife Service, 1988) might require 1,215 ha of contiguous, occupied black-tailed prairie dog habitat; 1,823 ha of 

contiguous, occupied Gunnison’s prairie dog habitat; or 2,430 

ha of contiguous, occupied white-tailed prairie dog habitat. I 

recognize that prairie dog densities vary between sites and at 

individual sites on an annual basis, but I found it necessary to 

use averages in this evaluation process. 

I also worked from the premise that the amount of extant, 

occupied habitat noted above may not be necessary to identify 

potential reintroduction sites and perhaps begin black-footed 

ferret releases. I suggest that 607.5 ha of contiguous, occupied black-tailed prairie dog habitat; 911.3 ha of contiguous, 

occupied Gunnison’s prairie dog habitat, or 1,215.0 ha of 

contiguous, occupied white-tailed prairie dog habitat may 

be sufficient to begin management planning or possible 

experimental release of black-footed ferrets. The choice of 50 

percent was arbitrary and assumes that prairie dog colonies 

will grow. Of course, many other factors may affect suitability 

of a reintroduction site, but I believe that these rough measures 

may allow preliminary identification of a queue of sites that 

can be further evaluated.

I characterized sites in regard to the species of prairie 

dog present, the amount of occupied prairie dog habitat, and 

disease status in a manner similar to that used by M. Lockhart 

(written commun., 1999–2003). Many of these sites have been 

recently identified as a result of ongoing inventories of prairie 

dog habitat.

Results

Current and potential black-footed ferret reintroduction sites are listed below for U.S. States and some Native 

American tribal lands, Canadian Provinces, and Mexican 

States having historical prairie dog habitat. Each is preceded 

by background information related to prairie dog popula-
72 Recovery of the Black-footed Ferret

tions. Many sites are in the early stages of identification and 

mapping; some may not yet be fully mapped, and some have 

no data on the amount of occupied prairie dog habitat or 

density of prairie dogs. Sites are summarized in table 1 (Active 

and Immediate Potential Sites) and table 2 (Intermediate 

Potential Sites), and locations are illustrated in figure 1. 

Arizona 

Black-tailed and Gunnison’s prairie dogs occurred in 

Arizona historically. The black-tailed prairie dog was extirpated from Arizona in the 1930s; therefore, reintroduction 

of black-tailed prairie dogs would be necessary before their 

colonies could serve as reintroduction sites for black-footed 

ferrets. In 2002, Wagner and Drickamer (2002) collected data 

from all potential sources and identified 400 locations with 

Gunnison’s prairie dog colonies. They revisited 293 colonies 

in 2000 and 2001 and found that 270 were active. Gunnison’s 

prairie dogs are located in northern Arizona from the Colorado 

River to Flagstaff and eastward along the Little Colorado 

River. No survey data are available for the Navajo Indian 

Reservation, which may comprise as much as one-third of the 

potential range. 

Active Sites

Aubrey Valley

Arizona has one active black-footed ferret reintroduction 

site on a Gunnison’s prairie dog complex in Aubrey Valley 

(Coconino, Yavapai, and Mojave Counties) in the northwestern part of the State (fig. 1). Reintroduction efforts began in 

1996. The site is designated a black-footed ferret nonessential 

experimental population, and releases of captive black-footed 

ferrets are ongoing. Approximately 25 black-footed ferrets 

occur in the wild there at present. Total occupied prairie dog 

habitat is approximately 12,039 ha on a mixture of private, 

State, and Hualapai Indian Reservation lands. Monitoring at 

this site has not documented plague during the last 20 years, 

although it has been noted in the region. Prairie dog populations can be severely affected by drought at this site (M. 

Lockhart, written commun., 1999–2003).

Intermediate Potential Sites

East of Seligman

Approximately 2,502 ha of active Gunnison’s prairie dog 

colonies were present on-site in 1992. The site is a large open 

grassland bisected by I-40. Occupied habitat was reduced considerably in 1996 because of a plague epizootic, but recovery began 

in 2001. This area is <10 km from Aubrey Valley (Wagner and 

Drickamer, 2002; D. Wagner, oral commun., 2003).

West of Dilkon, Navajo Indian Reservation

The Navajo Natural Heritage Program surveyed Gunnison’s prairie dogs in this area to investigate its potential as a 

black-footed ferret reintroduction site. The survey documented 

approximately 3,200 ha of occupied habitat. This area was 

affected by plague in 1996, and there has been little recovery to date (Wagner and Drickamer, 2002; D. Wagner, oral 

commun., 2003).

State Site name Nearest town Plague status

Active Sites

Arizona Aubrey Valley Seligman Not present

Colorado Colorado/Utah Dinosaur Present

Montana North-central Phillips County Malta Present

South Dakota Cheyenne River Indian Reservation

Conata Basin/Badlands National Park Wall Not present

Rosebud Indian Reservation Winner Not present

Utah Colorado/Utah Dinosaur, Colo. Present

Wyoming Shirley Basin Medicine Bow Present

Chihuahua, Mexico Janos Janos Not present

Immediate Potential Sites

Montana Custer Creek Miles City Unknown

Utah Cisco Desert Green River Present

Wyoming Thunder Basin National Grassland Bill Present

Table 1. Sites at which black-footed ferrets (Mustela nigripes) have been reintroduced and are being managed (Active Sites), and sites 

identified by the Black-footed Ferret Recovery Implementation Team where some work preparatory to reintroduction has been done (Immediate Potential Sites).
Identifying Potential Reintroduction Sites 73

State or Province Site name Nearest town Plague status

Arizona East of Seligman Seligman Present

West of Dilkon, Navajo Indian Reservation Dilkon Present

West of Wupatki National Monument Flagstaff Present

Colorado Pueblo County Pueblo Present

Weld County Greeley Present

Bent County Lamar Present

Baca County Springfield Present

Crowley County Rocky Ford Present

Pueblo Army Depot Pueblo Present

Fort Carson Colorado Springs Present

Comanche National Grassland, Carrizo Unit Pritchett Present

Comanche National Grassland, Timpas Unit, and Pinon 

Canyon Maneuver Site

La Junta Present

Cimarron National Grassland Springfield Present

BLM Twin Lakes Allotment Alamosa Present

Parlin Gunnison Present

Kansas Z-Bar Ranch Medicine Lodge Plague free

Logan County Colby Plague free

Northern Kearny County Garden City Plague free

Greeley County Horace Plague free

Rawlins County Atwood Plague free

Hamilton County Syracuse Plague free

Southern Kearny County Garden City Plague free

Sherman County Colby Plague free

Montana Leachman complex Billings Present

Northern Cheyenne Indian Reservation Colstrip Present

Miles City BLM District Miles City Present

Fort Benton-Roundup-Harlowton area Roundup Present

Nebraska Blue Creek Ranch Oshkosh Plague free

Oglala National Grassland Chadron Plague free

New Mexico Vermejo Park Ranch Raton Unknown

Quay/Curry County interface Tucumcari Unknown

Roosevelt County/Grulla National Wildlife Refuge Portales Unknown

Lea County Lovington Unknown

Union County Clayton Unknown

North Dakota Horse Creek area, Little Missouri National Grassland Williston Unknown

Standing Rock Indian Reservation North Lemmon Unknown

South Unit, Theodore Roosevelt National Park Dickinson Plague free

Little Missouri River Bowman Plague free

Table 2. Sites at which, pending further evaluation, opportunities for reintroducing black-footed ferrets (Mustela nigripes) may exist in 

3–10 years (Intermediate Potential Sites; n = 70).
74 Recovery of the Black-footed Ferret

State or province Site name Nearest town Plague status

Oklahoma Southwest Cimarron County Boise City Plague free

Texas County No. 1 Guymon Plague free

Texas County No. 2 Guymon Plague free

Beaver County No. 1 Beaver Plague free

Beaver County No. 2 Beaver Plague free

South Dakota Pine Ridge Indian Reservation Pine Ridge Plague free

Standing Rock Indian Reservation Lemmon Plague free

Lower Brule Indian Reservation Pierre Plague free

Wind Cave National Park Hot Springs Plague free

Grand River National Grassland Lodgepole Plague free

Bad River Ranches Pierre Plague free

Smithwick area, Buffalo Gap National Grassland Hot Springs Plague free

Texas Rita Blanca National Grassland Dalhart Unknown

Muleshoe National Wildlife Refuge Lubbock Present

Sherman County Dumas Unknown

Deaf Smith County Amarillo Unknown

Utah Buckhorn and Crescent Junction Price Present

Twelvemile Flat Green River Present

Eightmile Flat (Myton Bench) Green River Present

Sunshine Bench/Brush Creek Green River Present

Buckhorn Flat Price Present

Wyoming Meeteetse Meeteetse Present

Bolton Ranch Saratoga Present

Carter Kemmerer Present

Cumberland Kemmerer Present

Fifteenmile Worland Present

Flaming Gorge Green River Present

Shamrock Hills Rawlins Present

Kaycee Kaycee Unknown

Sheridan Local Training Center Sheridan Unknown

Saskatchewan, Canada Grasslands National Park Swift Current Unknown

Table 2. Sites at which, pending further evaluation, opportunities for reintroducing black-footed ferrets (Mustela nigripes) may exist in 

3–10 years (Intermediate Potential Sites; n = 70)—Concluded. 
Identifying Potential Reintroduction Sites 75

West of Wupatki National Monument

Gunnison’s prairie dogs are present at this site north of 

Flagstaff. A complex of 950 ha was mapped in 2001. Plague 

has occurred, but the extent has not been quantified (Wagner 

and Drickamer, 2002; D. Wagner, oral commun., 2003).

Colorado

Black-tailed prairie dogs, white-tailed prairie dogs, and 

Gunnison’s prairie dogs occur in Colorado. Complete location data are not available for Gunnison’s prairie dogs since 

some potential habitat in southwestern Colorado has not been 

surveyed. White-tailed prairie dogs are also currently being 

surveyed in northwestern Colorado. Black-tailed prairie dogs 

occur in all counties in the historical range in the eastern onethird of the State, and recent surveys indicate 255,596 ha of 

occupied habitat (Colorado Division of Wildlife, 2003). Location data from that survey are not available to the author at this 

time, however. EDAW, Inc. (2000) identified the 10 counties 

with the largest amount of active, occupied habitat in the State: 

Pueblo (8,989 ha), Weld (8,146 ha), Bent (6,914 ha), Baca 

(5,816 ha), Crowley (5,475 ha), Adams (5,372 ha), Prowers 

(5,161 ha), Boulder (4,668 ha), Cheyenne (3,717 ha), and 

Kiowa (3,629 ha). EDAW, Inc. (2000) identified 17 colonies 

>405 ha and 45 colonies from 203 to 405 ha in the black-tailed 

prairie dog range in Colorado. 

Active Sites

Colorado/Utah

White-tailed prairie dogs occupy the only active blackfooted ferret reintroduction site in Colorado. The site is located in 

northwestern Colorado in Moffat and Rio Blanco Counties and 

extends into Utah (Uintah County) and Wyoming (Sweetwater 

County). The Wyoming portion of the site, called Kinney Rim, 

has virtually no active colonies at the current time. Reintroduction 

efforts began in 1998. The site is designated a black-footed ferret 

nonessential, experimental population, and releases of captive 

black-footed ferrets are ongoing. A small population of blackfooted ferrets occurs in the wild there at present. Total occupied 

prairie dog habitat is approximately 20,250 ha, primarily on U.S. 

Bureau of Land Management (BLM) land, and plague is present. 

Potential habitat present in the Colorado portion of this site is estimated at 45,553 ha (M. Lockhart, written commun., 1999–2003).

Intermediate Potential Sites

Pueblo County

Black-tailed prairie dogs occupy this site. The northern 

half of the county, north of the City of Pueblo, has the largest 

concentration of colonies and the majority of the 8,989 ha of 

colonies identified in the county (EDAW, Inc., 2000). This 

county is primarily private land; therefore, development of 

black-footed ferret reintroduction sites would require participation by private landowners.

Weld County

Black-tailed prairie dogs occupy this site. The northeastern half of the county, northeast of the City of Greeley, has the 

largest concentration of colonies and the majority of the 8,146 

ha of colonies identified in the county (EDAW, Inc., 2000). 

This county is primarily private land; therefore, development of black-footed ferret reintroduction sites would require 

participation by private landowners.

Bent County

Large black-tailed prairie dog colonies occur in the 

northern and western parts of the county, encompassing the 

majority of the 6,914 ha identified (EDAW, Inc., 2000). This 

county is primarily private land; therefore, development of 

black-footed ferret reintroduction sites would require participation by private landowners.

Baca County

The western one-half of the county, centered on the town 

of Pritchett, has the largest concentration of black-tailed prairie dog colonies and has the majority of the 5,816 ha identified 

in the county (EDAW, Inc., 2000). This county is primarily 

private land; therefore, development of black-footed ferret 

reintroduction sites would require participation by private 

landowners.

Crowley County

Large black-tailed prairie dog colonies occur in several 

places in the county, encompassing 5,475 ha (EDAW, Inc., 

2000). This county is primarily private land; therefore, 

development of black-footed ferret reintroduction sites would 

require participation by private landowners.

Pueblo Army Depot

Black-tailed prairie dogs occupy this site, which is on a 

U.S. Army installation. Approximately 1,066 ha of occupied 

habitat were present before a plague outbreak in 2003. The site 

is managed by the military and is protected from shooting and 

poisoning except where black-tailed prairie dogs may constitute a 

human health hazard (B. Woodson, oral commun., 2003). A large 

area of occupied habitat also occurs on private lands adjacent to 

Pueblo Army Depot in El Paso County (EDAW, Inc., 2000).

Fort Carson

Black-tailed prairie dogs occupy this site, which is on a 

U.S. Army installation. Approximately 1,418 ha of occupied 

habitat were present before a plague outbreak occurred in 2002 

or 2003. The site is managed by the military and is protected 

from shooting and poisoning except where black-tailed prairie 
76 Recovery of the Black-footed Ferret

dogs may constitute a human health hazard (B. Woodson, oral 

commun., 2003). A large area of occupied habitat also occurs 

on private lands adjacent to Fort Carson, particularly along the 

southern boundary in Pueblo County (EDAW, Inc., 2000).

Comanche National Grassland, Carrizo Unit

Recent GIS analyses identified 46,395 ha of potential 

black-tailed prairie dog habitat on this site in Baca County. 

Potential habitat was defined as land with clay or loamy soil 

and <5 percent slope. Of this potential habitat, 1,622 ha are 

currently occupied, with an additional 450 ha occupied outside 

of potential habitat (primarily on lands mapped as sandy soils, 

most likely because of inaccurate generalities in the soil map). 

The Carrizo Unit has extremely fragmented land ownership. 

Intermingled private lands have even higher densities of 

colonies (due to higher grazing intensity), but landowners have 

strongly negative attitudes toward black-tailed prairie dogs. 

Approximately 2,076 ha of occupied black-tailed prairie dog 

habitat occurs on National Forest lands, and the amount of 

occupied habitat on intermingled private lands is unknown (D. 

Augustine, written commun., 2003).

Comanche National Grassland, Timpas Unit, and Pinon 

Canyon Maneuver Site

Black-tailed prairie dogs occupy this site, which is on 

the Timpas Unit and the adjoining U.S. Army Pinon Canyon 

Maneuver Site. Together these areas provide a large block of 

land in public ownership with little fragmentation. The Timpas 

Unit includes a number of private inholdings but is far less 

fragmented than the Carrizo Unit (above). The amount of 

occupied habitat in the Timpas Unit is lower than in the past 

because of plague. A total of 35,917 ha of potential habitat 

exists, of which 192 ha are currently occupied. An additional 

41 ha are outside the area mapped as suitable habitat, for a 

total of 233 ha on the Timpas Unit. Occupied habitat on the 

Pinon Canyon Maneuver Site totaled 143 ha when last mapped 

(D. Augustine, written commun., 2003). 

Cimarron National Grassland

Black-tailed prairie dogs occupy this site, which has 

approximately 16,200 ha of potential habitat, 1,296 ha of 

which were occupied in 2003. The area is bounded on the 

north by cropland and on the south by riparian/sand sagebrush 

(Artemisia filifolia) habitat. The Cimarron is separated from 

the Comanche by sand sagebrush habitat unsuitable for blacktailed prairie dog expansion (D. Augustine, written commun., 

2003).

Bureau of Land Management Twin Lakes Allotment

Gunnison’s prairie dogs occur at this site on public land 

in Conejos County, approximately 32 km south of Alamosa. 

The area supports a large complex of colonies dating back 

to the 1970s, many of which are old or inactive. Existing 

occupied habitat is approximately 512 ha (M. Albee, oral 

commun., 2003).

Parlin

Gunnison’s prairie dogs occur at this site, which is on 

public land 19 km southeast of Gunnison in Gunnison County. 

The amount of occupied habitat in 1980 was 497 ha (M. 

Albee, oral commun., 2003).

Kansas

Only black-tailed prairie dogs occur in Kansas. Recent 

surveys estimate 52,861 ha of occupied habitat in western 

Kansas (Kansas Black-tailed Prairie Dog Working Group, 

2002). The estimate of suitable habitat in Kansas based on the 

Bailey Ecoregion habitat model (Luce, 2003) is a minimum of 

60,181 ha.

Intermediate Sites

Z-Bar Ranch

Black-tailed prairie dogs occupy this site, which is on 

property owned by Turner Enterprises, Inc., approximately 40 

km southwest of Medicine Lodge in Barber County. The site 

currently supports 101 ha of occupied habitat and is growing 

steadily. Grassland conservation and black-tailed prairie dog 

expansion are high priority management objectives (J. Truett, 

oral commun., 2003).

Logan County

This county contained the largest complex (3,522 ha) of 

black-tailed prairie dogs in Kansas in 2001 (Kansas Blacktailed Prairie Dog Working Group, 2002).

Northern Kearny County

The northern part of this county contained the second 

largest complex (1,104 ha) of black-tailed prairie dogs in 

Kansas in 2001 (Kansas Black-tailed Prairie Dog Working 

Group, 2002).

Greeley County

This county contained the third largest complex (826 ha) 

of black-tailed prairie dogs in Kansas in 2001 (Kansas Blacktailed Prairie Dog Working Group, 2002).

Rawlins County

This county contained the fourth largest complex (448 

ha) of black-tailed prairie dogs in Kansas in 2001 (Kansas 

Black-tailed Prairie Dog Working Group, 2002).
Identifying Potential Reintroduction Sites 77

Hamilton County

This county contained the fifth largest complex (423 ha) 

of black-tailed prairie dogs in Kansas in 2001 (Kansas Blacktailed Prairie Dog Working Group, 2002).

Southern Kearny County

The southern part of this county contained the sixth largest complex (400 ha) of black-tailed prairie dogs in Kansas in 

2001 (Kansas Black-tailed Prairie Dog Working Group, 2002).

Sherman County

This county had the highest number of colonies and 

highest occupied area in the 1990–92 survey: 60 colonies and 

1,420 ha (Vanderhoof and Robel, 1992, 1994). It also had 

significant occupied black-tailed prairie dog habitat in 2001 

(Kansas Black-tailed Prairie Dog Working Group, 2002).

Montana

Both black-tailed and white-tailed prairie dogs occur 

in Montana. White-tailed prairie dogs are confined to a very 

small area near the border with Wyoming and occupy roughly 

40 ha of habitat at the present time; therefore, no black-footed 

ferret reintroduction potential exists for the foreseeable future. 

Black-tailed prairie dogs occur in the eastern part of the State, 

and the best estimate of occupied area is 36,450 ha (Montana 

Prairie Dog Working Group, 2002). The estimate of suitable 

habitat based on the Bailey Ecoregion habitat model (Luce, 

2003) is a minimum of 97,349 ha.

Active Sites

North-central Phillips County

Black-tailed prairie dogs occupy this site. Black-footed 

ferret releases have occurred since 1994. Occupied prairie dog 

habitat was 12,014 ha in the mid-1990s, with 5,457 ha occurring on Fort Belknap Indian Reservation, 4,472 ha on BLM 

lands, and 2,085 ha on Charles M. Russell National Wildlife 

Refuge. The area was heavily affected by plague in the late 

1990s. The black-footed ferret population is very low at the 

current time. Land ownership is mixed private, Federal, and 

tribal (M. Lockhart, written commun., 1999–2003). 

Immediate Potential Sites

Custer Creek

Black-tailed prairie dogs occupy this site in Prairie and 

Custer Counties, which contains >100 colonies and 1,705 ha 

of occupied habitat on a mixture of State, private, and BLM 

lands. Plague has not been documented since 1996. Since this 

site is in an area of checkerboard land status, private interests 

control the site potential (M. Lockhart, written commun., 

1999–2003).

Intermediate Potential Sites

The following locations were identified in the Conservation Plan for Black-tailed and White-tailed Prairie Dogs in 

Montana (Montana Prairie Dog Working Group, 2002) as 4 of 

the 10 largest known prairie dog complexes in Montana in 2000.

Leachman Complex

This site is entirely on tribal land in the northwest portion of 

the Crow Indian Reservation in Yellowstone and Big Horn Counties, and once supported an estimated 4,050–4,860 ha of occupied 

prairie dog habitat (L. Hanebury, oral commun., 2003). The site 

included >2,835 ha of occupied prairie dog habitat in recent times 

but suffered a plague outbreak prior to 2003. Approximately 

2,430 ha remained in two colonies in the southwest and central 

portions of the area in 2003. With translocations, this complex 

could be viable within a few years (L. Hanebury, oral commun., 

2003). Since surveys of suitable habitat on the Crow Indian 

Reservation have not been completed, sites other than the Leachman site may also exist (L. Hanebury, oral commun., 2003).

Northern Cheyenne Indian Reservation

Suitable habitat exists on the Reservation along the upper 

Tongue River in Big Horn and Rosebud Counties as well as 

on adjacent U.S. Forest Service and private lands. Occupied 

habitat exceeded 5,265 ha prior to a recent plague outbreak. 

With the help of translocations, this site grew to approximately 

2,025 ha in 2003 (L. Hanebury, oral commun., 2003).

Miles City Bureau of Land Management District

Potential habitat exists in Custer and Prairie Counties. This 

site is mixed private and BLM lands and supported approximately 2,430 ha of prairie dogs in 2000; however, recent plague 

outbreaks have reduced the size of this complex to approximately 

1,337 ha. A change in land ownership resulted in reduced access 

for mapping, which may have exaggerated the apparent decline in 

occupied habitat (L. Hanebury, oral commun., 2003).

Fort Benton-Roundup-Harlowton Area

Suitable habitat exists along the upper Musselshell River 

in Yellowstone, Stillwater, Musselshell, Golden Valley, Wheatland, and Petroleum Counties. The area is mixed private, 

BLM, and FWS lands and supported >2,430 ha of prairie dogs 

in 2000 (L. Hanebury, oral commun., 2003).

Nebraska

Only black-tailed prairie dogs occur in Nebraska. Recent 

surveys estimate 32,400 ha of occupied habitat (M. Fritz, oral 
78 Recovery of the Black-footed Ferret

commun., 2003) in western Nebraska. The estimate of suitable 

habitat based on the Bailey Ecoregion habitat model (Luce, 

2003) is a minimum of 55,588 ha.

Intermediate Potential Sites

Blue Creek Ranch

This site, which is owned by Turner Enterprises, Inc., 

is 16 km northeast of Oshkosh and currently has 8 ha of 

occupied habitat, which is expanding. Grassland conservation 

and black-tailed prairie dog expansion are high management 

priorities (J. Truett, oral commun., 2003).

Oglala National Grassland

This site is located in Sioux and Dawes Counties and 

currently has 284 ha of occupied black-tailed prairie dog 

habitat. The Oglala National Grassland will require time to 

expand existing prairie dog habitat and to consolidate the land 

base to improve the management potential (S. Larson, written 

commun., 2003).

New Mexico

Black-tailed and Gunnison’s prairie dogs occur in New 

Mexico. Recent black-tailed prairie dog surveys estimate 

24,300 ha of occupied habitat (Johnson and others, 2003) in 

eastern New Mexico. The estimate of suitable habitat based on 

the Bailey Ecoregion habitat model (Luce, 2003) is a minimum of 35,288 ha. Surveys are ongoing for Gunnison’s prairie 

dog, but there is no estimate of current occupied habitat.

Intermediate Potential Sites

Vermejo Park Ranch

This site, which is owned by Turner Enterprises, Inc., is 

located 40 km southwest of Raton and currently has 689 ha 

of occupied habitat, which is expanding rapidly. Grassland 

conservation and black-tailed prairie dog expansion are high 

priorities (J. Truett, oral commun., 2003).

Quay/Curry County Interface

This site is south of Tucumcari and contains >3,848 ha of 

occupied habitat, with >2,025 ha contiguous. The mean size of 

colonies is 19 ha, and the maximum area of a single colony is 

152 ha (Johnson and others, 2003).

Roosevelt County/Grulla National Wildlife Refuge

This site is south of Portales and contains >5,265 ha of 

occupied habitat, with >2,025 ha contiguous. The mean size of 

colonies is 35 ha, and the maximum size of a single colony is 

339 ha (Johnson and others, 2003).

Lea County

This site is northeast of Lovington and contains approximately 9,720 ha of occupied habitat, with >2,025 ha contiguous. The mean size of colonies is 60 ha, and the maximum 

area of a single colony is 956 ha (Johnson and others, 2003). 

Plague has recently been active in this area, but impacts have 

not been quantified (P. Gober, oral commun., 2003).

Union County

This site is southwest of Clayton and contains approximately 3,240 ha of occupied habitat. The mean size of 

colonies is 41 ha, and the maximum area of a single colony is 

292 ha (Johnson and others, 2003).

North Dakota

Only black-tailed prairie dogs occur in North Dakota. 

Recent surveys estimate 8,303 ha of occupied habitat 

(Knowles, 2003) in western North Dakota. The estimate of 

suitable habitat based on the Bailey Ecoregion habitat model 

(Luce, 2003) is a minimum of 40,723 ha.

Intermediate Potential Sites

Horse Creek Area, Little Missouri National Grassland

Black-tailed prairie dogs occupy 162 ha at this site in 

McKenzie County in western North Dakota. The site has 

strong potential to reach biological readiness for black-footed 

ferret reintroduction within 10 years, but local support cannot 

be predicted at this time. The site is included in the most 

recent land management plans for Little Missouri National 

Grassland and is plague free (S. Larson, written commun., 

2003).

Standing Rock Indian Reservation

Black-tailed prairie dogs occupy 1,215 ha at this site in 

Sioux County. Colonies are scattered over a large area, and 

the land base is a checkerboard of private and tribal lands. The 

area is plague free (S. Larson, written commun., 2003).
Identifying Potential Reintroduction Sites 79

South Unit, Theodore Roosevelt National Park

Black-tailed prairie dogs occupy 729 ha at this site in 

Billings County. In 2002, 61 active colonies were mapped 

(Knowles, 2003). Knowles (2003) predicted that the site 

potential on the national park is >2,633 occupied ha based 

on the amount of suitable habitat present. Additional suitable 

habitat occurs on adjacent private land, and the area is plague 

free (Knowles, 2003).

Little Missouri River

Black-tailed prairie dogs occupy this site in Slope 

County. The site had 345 ha of occupied habitat in 2002. 

Significant biological potential exists if private land issues can 

be addressed. The area is plague free (Knowles, 2003).

Oklahoma

Only black-tailed prairie dogs occur in Oklahoma. Recent 

surveys estimate 26,007 ha of occupied habitat (J. Hoagland, 

oral commun., 2003) in western Oklahoma. The estimate of 

suitable habitat based on the Bailey Ecoregion habitat model 

(Luce, 2003) is a minimum of 27,806 ha.

Intermediate Potential Sites

Sites in Oklahoma have previously been described as 

clusters of colonies (M. Lomolino, written commun., 2003).

Cimarron County

This site is in the southwestern corner of the county. 

Cluster A had 12 colonies totaling 345 ha, and Cluster B had 

6 colonies with a total of 652 ha when mapped in 1996–98 

(Lomolino and Smith, 2001; M. Lomolino, written commun., 

2003).

Texas County No. 1

This site is in the north-central part of the county. Cluster 

C had 12 colonies with a total of 332 ha when mapped in 

1996–98 (Lomolino and Smith, 2001; M. Lomolino, written 

commun., 2003).

Texas County No. 2

This site is in the east-central part of the county. Cluster 

D had 18 colonies with a total of 302 ha when mapped in 

1996–98 (Lomolino and Smith, 2001; M. Lomolino, written 

commun., 2003).

Beaver County No. 1

This site is in the east-central part of the county. Cluster E 

had 10 colonies with a total of 93 ha when mapped in 1996–98 

(Lomolino and Smith, 2001; M. Lomolino, written commun., 

2003).

Beaver County No. 2

This site is in the south-central part of the county. Cluster 

F had 34 colonies with a total of 319 ha when mapped in 

1996–98 (Lomolino and Smith, 2001; M. Lomolino, written 

commun., 2003).

South Dakota

Only black-tailed prairie dogs occur in South Dakota. A 

2001 survey estimated 64,800 ha of occupied habitat (South 

Dakota Prairie Dog Work Group, 2001) in western South 

Dakota. The estimate of suitable habitat based on the Bailey 

Ecoregion habitat model (Luce, 2003) is a minimum of 

80,786 ha.

Active Sites

Cheyenne River Indian Reservation

Black-tailed prairie dogs occupy this site in Dewey and 

Ziebach Counties. Total occupied habitat is 17,861 ha in three 

separate complexes, one of which is 8,424 ha. An operational 

prairie management program is currently pursuing blackfooted ferret reintroduction. There is no history of plague in 

the area (M. Lockhart, written commun., 1999–2003).

Conata Basin/Badlands National Park

Black-tailed prairie dogs occupy this site in Pennington, 

Shannon, and Jackson Counties. Total occupied habitat is 

6,116 ha, with 4,779 ha on U.S. Forest Service lands and 1,337 

ha on National Park Service lands. The estimated potential 

for the area based on suitable habitat is 7,128 ha. There is no 

history of plague in the area (M. Lockhart, written commun., 

1999–2003).

Rosebud Indian Reservation

Black-tailed prairie dogs occupy 28,350 ha at this site in 

Todd and Mellette Counties, 18,225 ha of which is on tribal 

trust lands. There is no history of plague in the area (M. Lockhart, written commun., 1999–2003).
80 Recovery of the Black-footed Ferret

Intermediate Potential Sites

Pine Ridge Indian Reservation

Black-tailed prairie dogs occupy 20,250–40,500 ha on 

tribal lands at this site in Shannon County. The site has the 

biological capacity to support a large black-footed ferret population but may be constrained by social, cultural, and political 

factors (S. Larson, written commun., 2003). 

Standing Rock Indian Reservation

Black-tailed prairie dogs occupy 2,835 ha at this site in 

Corson County. Black-tailed prairie dogs are scattered over a 

large area, and the land base is a mixture of private and tribal. 

There is no history of plague in the area (S. Larson, written 

commun., 2003).

Lower Brule Indian Reservation

Black-tailed prairie dogs occupy 11,745 ha at this site in 

Stanley and Lyman Counties. There is no history of plague in 

the area (S. Larson, written commun., 2003).

Wind Cave National Park

Black-tailed prairie dogs occupy 689 ha at this site 

in Custer County. Biologically, this site could be ready for 

black-footed ferret reintroduction within a few years, and the 

National Park Service is supportive. There is no history of 

plague in the area (S. Larson, written commun., 2003).

Grand River National Grassland

Black-tailed prairie dogs occupy 648 ha at this site in 

Perkins and Corson Counties. Biologically, this site is not 

ready for black-footed ferret reintroduction, as it needs time 

for black-tailed prairie dogs to expand occupied habitat. 

The U.S. Forest Service needs to consolidate its land base; 

however, it has identified the site for prairie dog expansion in 

the most recent land management plan. There is no history of 

plague in the area (S. Larson, written commun., 2003).

Bad River Ranches

Black-tailed prairie dogs occupy this site on lands owned 

by Turner Enterprises, Inc., in Stanley and Jones Counties, 

16 km southwest of Pierre. The site currently has 506 ha of 

occupied habitat and is growing steadily. Grassland conservation and black-tailed prairie dog expansion are high priorities. There is no history of plague in the area (J. Truett, oral 

commun., 2003).

Smithwick Area, Buffalo Gap National Grassland, Fall 

River Ranger District

Black-tailed prairie dogs occupy 405 ha at this site in 

Custer County. From a biological standpoint, the site could 

be ready for black-footed ferret reintroduction within 5 years. 

The site was included in the most recent land management 

plan for Buffalo Gap National Grassland. There is no history 

of plague in the area (S. Larson, written commun., 2003).

Texas

Only black-tailed prairie dogs occur in Texas. Ongoing 

surveys currently estimate 79,785 ha of occupied habitat in 

western Texas (D. Holdstock, oral commun., 2003). The estimate of suitable habitat based on the Bailey Ecoregion habitat 

model (Luce, 2003) is a minimum of 118,717 ha.

Intermediate Potential Sites

Rita Blanca National Grassland

Black-tailed prairie dogs occupy this site north of 

Dalhart in Dallam County. The site was identified by Lair and 

Mecham (1991) as having >4,050 ha of occupied habitat, with 

49 colonies >41 ha in size and >1.0 percent of the land area in 

prairie dog colonies (Lair and Mecham, 1991; Ernst, 2001). 

Muleshoe National Wildlife Refuge

Black-tailed prairie dogs occupy this site northwest 

of Lubbock in Bailey County. It was identified by Lair and 

Mecham (1991) as having >2,835 ha of occupied habitat, with 

25 colonies >41 ha in size and >1.0 percent of the land area in 

prairie dog colonies (Lair and Mecham, 1991; Ernst, 2001). 

Sherman County

Black-tailed prairie dogs occupy this site north of Dumas. 

It was identified by Lair and Mecham (1991) as having >3,240 

ha of occupied habitat, with 32 colonies >41 ha in size and 

1.5 percent of the land area in prairie dog colonies (Lair and 

Mecham, 1991; Ernst, 2001). 

Deaf Smith County

Black-tailed prairie dogs occupy this site southwest of 

Amarillo. It was identified in Lair and Mecham (1991) as 

having >5,670 ha of occupied habitat, with 55 colonies >41 ha 

in size and 1.5 percent of the land area in prairie dog colonies 

(Lair and Mecham, 1991; Ernst, 2001).
Identifying Potential Reintroduction Sites 81

Utah

Gunnison’s prairie dogs and white-tailed prairie dogs 

occur in Utah. Data on locations and occupied area are still 

being developed for both species.

Active Sites

There is one active black-footed ferret reintroduction site 

in Utah (see discussion under Colorado).

Immediate Potential Sites

Cisco Desert

White-tailed prairie dogs occur at this potential site 

identified by the Black-footed Ferret Recovery Program. 

The site was mapped in 1986 (Boschen, 1986) and again in 

2002 (Seglund and others, 2005a). The site is on public land 

in Grand County in east-central Utah along I-70 from east of 

Green River to the Colorado border. Land ownership is mixed 

private, State, and Federal (M. Lockhart, written commun., 

1999–2003).

Intermediate Potential Sites

Buckhorn and Crescent Junction

White-tailed prairie dogs occupy this site in Emery 

and Grand Counties in south-central Utah. According to C. 

McLaughlin (oral commun., 2003), Cedar Creek Associates 

mapped 7,644 ha, including both active and inactive colonies, 

in this complex on public lands in 1985. The area mapped 

extended south of Huntington to I-70 along State Highway 

10, east to State Highway 6, and along I-70 to Thompson 

Springs. In 2002, mapping within the same area recorded 

7,881 ha, including active and inactive colonies, approximately a 3 percent increase from 1985 (C. McLaughlin, 

written commun., 2003). 

Twelvemile Flat

White-tailed prairie dogs occur at this site on public 

lands in the BLM Diamond Mountain Resource Area west and 

north of Green River in northeastern Utah. Twelvemile Flat 

contained 363 ha of occupied habitat in 1985. The site was 

resurveyed in 1992–93 (Cranney and Day, 1994) and found 

to have 771 ha of occupied habitat, slightly over double the 

amount present in 1985. In 2002, mapping located 365 ha of 

occupied habitat (C. McLaughlin, written commun., 2003).

Eightmile Flat (Myton Bench)

White-tailed prairie dogs occur at this site on public lands 

in the BLM Diamond Mountain Resource Area west and north 

of Green River in northeastern Utah. Eightmile Flat contained 

2,673 ha of occupied habitat in 1985. The site was resurveyed 

in 1999 and found to have increased by 9 percent, to 2,936 ha 

of occupied habitat (C. McLaughlin, written commun., 2003).

Sunshine Bench/Brush Creek

White-tailed prairie dogs occur at these sites on public 

lands in the BLM Diamond Mountain Resource Area west 

and north of Green River in northeastern Utah. The sites were 

mapped to evaluate their suitability for black-footed ferret 

reintroduction in 1992–93 (Cranney and Day, 1994). The 

Sunshine Bench complex contained 2,085 ha of occupied 

habitat in 1992–93, while the adjacent Brush Creek area 

contained 145 ha of occupied habitat. The combined occupied 

area of Sunshine Bench and Brush Creek was 7,837 ha in 2002 

(C. McLaughlin, written commun., 2003).

Buckhorn Flat

White-tailed prairie dogs occur at this site on public lands 

56 km south of Price. The estimated occupied habitat at the 

site is 2,412 ha (A. Seglund, written commun., 2003).

Wyoming

Black-tailed and white-tailed prairie dogs occur in 

Wyoming. Black-tailed prairie dogs occur in the eastern 

one-third of the State. Recent occupied habitat estimates range 

widely, but the current estimate is 50,625 ha (M. Grenier, 

written commun., 2003). The estimate of suitable habitat 

based on the Bailey Ecoregion habitat model (Luce, 2003) is 

a minimum of 64,059 ha. White-tailed prairie dogs occur in 

the west-central part of the State, and surveys are underway to 

estimate occupied habitat.

Active Sites

Shirley Basin

Shirley Basin/Medicine Bow is the only active blackfooted ferret reintroduction site in Wyoming and occurs in the 

white-tailed prairie dog range. The site was fully mapped in 

1989 (Conway, 1989) and again in 1990 by using a combination of aerial transects and ground verification (Hnilicka and 

Luce, 1992). In 1990, intensive mapping showed the complex 

to contain 59,726 ha (Parrish and Luce, 1990). Captive-bred 
82 Recovery of the Black-footed Ferret

black-footed ferrets were released from 1991 to 1994, and the 

highest number of black-footed ferrets found on subsequent 

surveys was in 2004, when 85 individuals were located during 

spotlight surveys (Grenier and others, 2004) of less than 20 

percent of the occupied habitat (based on 1990 mapping data). 

Therefore, considerable potential exists for a large, contiguous 

population of black-footed ferrets or several subpopulations. 

It is important to note that both prairie dogs and black-footed 

ferrets have persisted with plague present since at least 1987 

(Orabona-Cerovski, 1991).

Immediate Potential Sites

Thunder Basin National Grassland

Black-tailed prairie dogs occur at this site in Campbell, 

Converse, and Weston Counties. The site is identified as a 

black-footed ferret reintroduction site in the current Forest 

Plan for the Medicine Bow National Forest/Thunder Basin 

National Grassland. There was no history of plague before 

2001 when an extensive die-off occurred, reducing occupied 

habitat by over 4,050 ha. Recovery is occurring. Prior to the 

plague outbreak, occupied black-tailed prairie dog habitat was 

8,079 ha, including 7,290 ha on U.S. Forest Service land and 

789 ha on State land. The U.S. Forest Service estimates that 

there are 193,590 ha of potential habitat on its lands in this 

area of Wyoming (T. Byer, written commun., 2003).

Intermediate Potential Sites

Meeteetse

White-tailed prairie dogs occupy this site west of Meeteetse in Park County. This site, from which all of the blackfooted ferret captive breeding stock was taken, had 4,930 ha of 

occupied habitat in 1982, just after black-footed ferrets were 

first discovered, and a high population of 129 black-footed 

ferrets (43 adults, 25 litters) in 1984. Because of plague in 

white-tailed prairie dogs, occupied habitat was reduced to 

roughly 2,029 ha by 1989, 2 years after all extant black-footed 

ferrets were captured for captive breeding (Black-footed Ferret 

Advisory Team, 1990). The site has not shown significant 

recovery of prairie dogs since 1989 (Biggins, 2003). The 

habitat capability of the site remains, including old burrow 

systems, so the potential exists for recovery to sufficient 

occupied habitat for black-footed ferret reintroduction within 

10 years.

Bolton Ranch

White-tailed prairie dogs occupy this site west of Saratoga in Carbon County. Land ownership is a checkerboard of 

public and private lands. The site had 4,500 ha of occupied 

habitat in 1989 when it was first surveyed (Conway, 1989). No 

surveys have been conducted since then (Grenier and others, 

2003; R. Luce, written commun., 1995).

Carter

White-tailed prairie dogs occupy this site 32 km southeast 

of Kemmerer, on BLM lands in Lincoln County. The site has 

not been fully mapped or surveyed to determine prairie dog 

density. It contained more than 4,050 ha of occupied habitat 

when partially mapped in the 1980s (Grenier and others, 2003; 

R. Luce, written commun., 1995). The Carter site is potentially connected to another site (Moxa) which is 32 km north 

of Kemmerer, indicating that an extremely large complex 

may exist in this area. Moxa was identified in the mid-1990s 

when 17,415 ha of occupied habitat were mapped, and the site 

has not been resurveyed (Grenier and others, 2003; B. Luce, 

unpub. data, 1995). 

Cumberland

White-tailed prairie dogs occupy this site southwest of 

Kemmerer in Lincoln County. Land ownership is a checkerboard of public and private lands. The site was fully mapped 

and preliminary density data were collected in the 1980s 

(Clark and Campbell, 1981). Occupied habitat was 4,293 ha. 

The site has not been remapped.

Fifteenmile

White-tailed prairie dogs occupy this site on BLM land 

40 km west of Worland in Hot Springs County. The site 

contained 3,078 ha of occupied habitat when mapped in the 

1980s and has not been remapped (Grenier and others, 2003; 

R. Luce, written commun., 1995). 

Flaming Gorge

White-tailed prairie dogs occupy this site on BLM land 

64 km south of Green River in Sweetwater County. The site 

was intensively mapped in 1989 and contained 3,049 ha of 

occupied habitat (Martin and Luce, 1990). It has not been 

remapped.

Shamrock Hills

White-tailed prairie dogs occupy this site on BLM land 

16 km north of Rawlins in Carbon County. The site was 

mapped in the 1980s and had >4,050 ha of occupied habitat. 

The site has not been remapped (Grenier and others, 2003; R. 

Luce, written commun., 1995).

Kaycee

Black-tailed prairie dogs occupy this site west of the town 

of Kaycee in Johnson County, primarily on private land. This 

site was discovered recently and has not been mapped, but 
Identifying Potential Reintroduction Sites 83

it is estimated that >1,215 ha of occupied habitat are present 

(R. Luce, unpub. data, 2003). 

Sheridan Local Training Center

Black-tailed prairie dogs occupy this site on a U.S. Army 

installation adjacent to Sheridan in Sheridan County. The site 

contained 284 ha of occupied habitat in 2001, and adjacent 

private and State lands had a substantial amount of additional 

occupied habitat (R. Luce, unpub. data, 2003). 

Canada

Only black-tailed prairie dogs occur in Canada, which is 

the northern extent of the range of the species. 

Intermediate Potential Sites

Grasslands National Park and Vicinity

Black-tailed prairie dogs occur at this site in Saskatchewan, 160 km south of Swift Current. The site has 25 colonies 

containing a minimum of 1,044 ha. It has been partially 

mapped since 1993 but was fully mapped for comparative 

purposes from 1998 to 2002 and had a stable occupied area for 

that time period (P. Fargey, written commun., 2003).

Mexico

Black-tailed prairie dogs occur in northern Mexico, the 

southern extent of the range, and are the only species of prairie 

dog in Mexico in the historical range of the black-footed 

ferret.

Active Sites

Janos

Black-tailed prairie dogs occur at this site north of Nuevo 

Casas Grandes in Chihuahua. Estimated occupied prairie 

dog habitat is 19,845 ha, and the potential suitable habitat is 

55,080 ha. Land ownership is divided between Federal Ejidos 

and private ownership. This is a large prairie dog complex and 

may have the potential for one contiguous black-footed ferret 

population or several subpopulations. No management plan 

exists for the area (R. List, oral commun., 2003).

Intermediate Potential Sites

There are no Intermediate Potential Sites in Mexico.

Discussion

It is clear from past efforts that a “best and only” methodology for successful black-footed ferret reintroduction has not 

been unequivocally established. The 1988 recovery plan (U.S. 

Fish and Wildlife Service, 1988) encourages experimentation. 

It also emphasizes a management philosophy important for 

both establishing and maintaining reintroduced populations 

whereby the broadest possible distribution of black-footed 

ferrets might be achieved. This risk management approach is 

important to protect the species overall from adverse impacts 

that may occur locally, especially disease.

Preparation of this paper does not constitute a proposed 

State or Federal action at any of the proposed sites; it is merely 

a conceptual approach to aid in black-footed ferret recovery. 

Many steps will be required before any site can eventually 

receive ferrets; however, I do not believe that it is necessary 

or appropriate to wait for final biological, social, and political issues to be addressed at a given site in order for it to be 

considered for the list of potential reintroduction sites. This 

conceptual exercise identifies sites based entirely on either a 

minimum area of occupied prairie dog habitat or a small but 

increasing prairie dog population at a site that has the habitat 

characteristics necessary to support black-footed ferrets. I 

recognize that myriad actions would be necessary before 

black-footed ferrets could actually be released at a given site, 

especially where private lands are involved. 

The general limitation of lack of habitat or habitat 

availability is shared with many other species. But in the 

case of the black-footed ferret, which is a highly specialized 

prey/habitat obligate of prairie dogs, dependence has proven 

to be especially catastrophic because of the dramatic reduction 

of its prey over the past century by adverse land-use practices 

such as prairie conversion to cropland, poisoning to reduce 

forage competition with domestic livestock, and sylvatic 

plague, an exotic disease catastrophic to prairie dogs (Cain and 

others, 1972; Hansen, 1988; Cully, 1993; Van Pelt, 1999; U.S. 

Fish and Wildlife Service, 2000; Cully and Williams, 2001; 

Antolin and others, 2002; Luce, 2003). Despite these potential 

conflicts and future challenges, identification of appropriate 

sites for black-footed ferret reintroduction has been ongoing 

for over two decades.

Although occupied prairie dog habitat has been significantly reduced since western settlement (Hoogland, 1995; 

Miller and Cully, 2001), it has been only in the last decade that 

the degree of both the quantity and quality of this loss relative 

to potential black-footed ferret recovery has been recognized. 

At present there may not be sufficient occupied prairie dog 

habitat in total in the historical ranges of the black-tailed prairie dog, white-tailed prairie dog, and Gunnison’s prairie dog, 

either in quantity or quality, for the black-footed ferret to be 
84 Recovery of the Black-footed Ferret

fully recovered, especially if black-footed ferret populations 

are to be broadly represented geographically as a precaution 

against depressant stochastic influences (M. Lockhart, written 

commun., 1999–2003).

The 1988 Black-footed Ferret Recovery Plan (U.S. Fish 

and Wildlife Service, 1988) set a downlisting goal for the 

species at 1,500 adults in 10 or more populations dispersed 

across its historical range, with no single population being 

less than 30 adults. Downlisting the species would move it 

from endangered to threatened status but would not represent 

complete recovery. Delisting the black-footed ferret through 

recovery sufficient to obviate its endangered status and permit 

its removal from the endangered species list (pursuant to the 

Endangered Species Act of 1973, as amended) would require 

even more recovery sites.

I suggest that it may be necessary to evaluate an order 

of magnitude more sites to achieve complete recovery and 

delisting, or 100 sites across the historical range of the species. 

These sites should be widely dispersed and represent the 

variety of habitats available, including different prairie dog 

species, ecological circumstances, disease prevalence, and the 

like. Since some sites may prove not to be usable for biological, social, or other reasons, or may not be successful, it will 

be necessary to consider many.

Plague is a confounding factor. Annual monitoring to 

document plague activity and the amount of habitat affected 

would assist prairie dog and black-footed ferret management. 

Continuing research on the mechanisms by which plague is 

spread, pretreatment of prairie dogs, and posttreatment of 

burrows to kill fleas and thus reduce the magnitude of an 

epizootic may allow practical management of the disease in 

the next 10 years. Meanwhile, maintaining spatial distribution 

of prairie dog complexes and isolated colonies over the entire 

range to act as reservoirs to replace prairie dogs lost to plague, 

as well as development of black-footed ferret reintroduction 

sites east of the plague line (in the plague-free area), will 

greatly assist in managing the impacts of the disease on prairie 

dogs. 

In my opinion, data presented by Cully and Williams 

(2001) suggest that a fundamental change may be occurring in prairie dog ecology whereby some large colonies, 

especially those of black-tailed prairie dogs, may not persist 

when repeatedly challenged by plague. Persistence of only 

small colonies or complexes may have serious implications 

for black-footed ferret recovery. Extensive habitat will be 

necessary for reintroduction success, especially in the absence 

of management, and few large sites may persist at their full 

habitat capability in the face of repeated plague epizootics. 

On the other hand, recent surveys of white-tailed prairie dogs 

and black-footed ferrets in Shirley Basin, Wyo., indicate 

that these areas may have proportionately higher value than 

previously thought because both prairie dogs and black-footed 

ferrets have maintained significant populations in the presence 

of plague since monitoring was begun in 1991 (Luce, 2002; 

Grenier and others, 2004). In fact, both white-tailed prairie 

dog and black-footed ferret numbers increased despite more 

than 10 years of active plague (Grenier and others, 2004). 

Status of Prairie Dog Conservation

Since black-footed ferret recovery and prairie dog 

management issues are closely tied, the future of the blackfooted ferret essentially depends on developing effective 

management of black-tailed, white-tailed, and Gunnison’s 

prairie dogs. The Black-tailed Prairie Dog Conservation 

Team (later just the Prairie Dog Conservation Team), which 

includes representatives from 12 State wildlife agencies, has 

been working since 1998 to develop effective conservation for 

prairie dogs. The team first developed the Black-tailed Prairie 

Dog Conservation Assessment and Strategy (Van Pelt, 1999), 

which was followed by an addendum called the Black-tailed 

Prairie Dog Multi-State Conservation Plan (Luce, 2003), a 

guideline for development of State black-tailed prairie dog 

management plans. Black-tailed prairie dog management plans 

have been completed in Colorado, Kansas, Montana, New 

Mexico, North Dakota, Oklahoma, and Texas. Draft management plans are moving toward finalization in South Dakota 

and Wyoming. Arizona has a draft management plan and is 

currently evaluating black-tailed prairie dog reintroduction, 

while Nebraska does not expect to continue development of a 

management plan.

The Black-tailed Prairie Dog Multi-State Conservation 

Plan includes several provisions that are important to blackfooted ferret recovery, two areas of which are of the greatest 

significance. First, the objectives for occupied area, shown in 

table 3, indicate a commitment on the part of a majority of the 

States with black-tailed prairie dogs to increase the occupied 

area from 631,127 ha to 685,946 ha by 2011 (Luce, 2003). 

Second, the Multi-State Conservation Plan sets other target 

objectives for the United States as follows:

1. Maintain at least the current occupied area of blacktailed prairie dog habitat in the two complexes greater 

than 2,025 ha that now occur on and adjacent to Conata 

Basin-Buffalo Gap National Grassland, S. Dak., and 

Thunder Basin National Grassland, Wyo. 

2. Develop and maintain a minimum of nine additional 

complexes greater than 2,025 ha (with each State managing or contributing to at least one complex) by 2011. 

A State could contribute to a 2,025 ha complex along 

a State boundary by cooperating with the adjacent 

State to manage part of the complex. A similar agreement could be developed between a State and a Native 

American tribe.

3. Achieve and maintain at least 10 percent of total occupied habitat in colonies or complexes greater than 405 

ha by 2011.
Identifying Potential Reintroduction Sites 85

State

Historical potential 

habitat1 (ha)

Current occupied 

habitat2 (ha) Gross habitat3 (ha)

Suitable habitat4 and 

minimum 10-year 

objective5 (ha)

Arizona 2,854,090 0 2,854 1,861

Colorado 11,077,916 255,596 110,779 103,588

Kansas 14,513,206 52,861 61,039 60,181

Montana 24,479,316 36,450 120,401 97,349

Nebraska 14,594,350 32,400 59,430 55,588

New Mexico 15,803,686 24,300 39,148 35,288

North Dakota 4,473,334 8,303 44,733 40,723

Oklahoma 8,750,479 26,007 28,702 27,806

South Dakota 11,851,333 64,800 88,339 80,786

Texas 31,829,943 79,785 125,933 118,717

Wyoming 8,937,378 50,625 75,524 64,059

Total 149,165,031 631,127 756,882 685,946

Table 3. Estimates of historical, current, gross, and suitable black-tailed prairie dog (Cynomys ludovicianus) habitat, and the 10-year 

minimum habitat objective (Luce, 2003). Native American tribes in Montana, South Dakota, and North Dakota will set an occupied-area 

objective independent of the States.

1

Historical potential habitat = total potential habitat (not occupied habitat) encompassed within the range of the black-tailed prairie dog (as mapped by Hall, 

1981). See Luce (2003) for further explanation.

2

Current occupied habitat = estimates provided by the individual States.

3

Gross habitat = total area of core range × 0.01 + area of secondary range × 0.001. Core range was defined as Bailey Ecosections dominated by shortgrass 

prairie plants and having black-tailed prairie dogs on the list of native fauna. Secondary range was defined as Bailey Ecosections dominated by plants not associated with shortgrass prairie, or having historically suitable habitat but a current sociopolitical climate unfavorable for prairie dog management. See Luce (2003) 

for additional details.

4

Suitable habitat = gross habitat minus habitat with >10% slope and habitats such as large bodies of water, badlands, wetlands, forests, or other features not 

used by prairie dogs. Agricultural lands were included if they met the slope criterion.

5

Minimum 10-year objective = objective for minimum area of occupied prairie dog habitat in each State, and total for the 11 States, by 2011.

4. Maintain distribution across at least 75 percent of the 

counties in the historical range or at least 75 percent 

of the historical geographic distribution. Ten States 

currently meet this objective (Arizona does not since 

the black-tailed prairie dog was extirpated), and all but 

Nebraska and Arizona have black-tailed prairie dogs in 

100 percent of the counties in the historical range. This 

objective addresses the need to maintain all prairie dog 

colonies, whatever the size or location, throughout the 

range. State management plans will deal directly with 

management of complexes and individual, isolated 

colonies.

Management strategies for black-tailed prairie dogs on 

tribal lands were prepared for the Intertribal Prairie Ecosystem Restoration Consortium in January 2002 (T. Vosburgh, 

oral commun., 2003). The goal is to develop and implement 

management programs for the conservation of prairie dog 

habitat. These management strategies were revised on February 4, 2002, following review and comment from participating 

tribes, the U.S. Fish and Wildlife Service, and the Interstate 

Coordinator for the 12-State Prairie Dog Conservation Team. 

The consortium convened twice in 2002 and is working with 

other groups and agencies to move prairie dog management 

and conservation forward. The tribes have drafted plans to 

ensure that prairie dog populations and habitat are maintained. 

The Lower Brule and Fort Belknap Indian Reservations have 

final prairie dog management plans in place, and draft plans 

have been prepared for the Fort Berthold, Northern Cheyenne, 

Crow Creek, and Rosebud Indian Reservations. 

The States of Wyoming, Colorado, Utah, and Montana 

developed a conservation assessment for the white-tailed 

prairie dog in 2005 (Seglund and others, 2005a), as did the 

States of Arizona, New Mexico, Colorado, and Utah for the 

Gunnison’s prairie dog (Seglund and others, 2005b). When a 

conservation strategy is developed for the Gunnison’s prairie 

dog, complexes of colonies will be identified, and other sites 

with black-footed ferret reintroduction potential may thus 

become apparent.
86 Recovery of the Black-footed Ferret

Summary and Recommendations

The black-footed ferret recovery program has faced and 

overcome several obstacles to reach the point where it is today. 

Foremost were capture of the wild population at Meeteetse, 

Wyo., captive breeding, development of release strategies, and 

release site identification based on habitat suitability and other 

factors. Given that those obstacles to success were overcome, I 

believe that, at the present time, continued progress on blackfooted ferret recovery depends upon identification and active 

management of additional reintroduction sites. To that end, 

I identify 70 sites in the historical range of the black-footed 

ferret that might meet the biological and habitat suitability 

requirements for reintroduction of black-footed ferrets within 

3–10 years, contingent upon directed management emphasis, 

State and Federal agency management priorities, and, if on 

private land, landowner concurrence based on agreements or 

incentives.

The Black-footed Ferret Recovery Implementation Team 

and Prairie Dog Conservation Team are encouraged to:

• Cooperate closely with State and Federal agencies and 

eight tribal governments to move toward the targets set 

in the Black-tailed Prairie Dog Multi-State Conservation Plan and State and tribal management plans.

• Assist the White-tailed and Gunnison’s Prairie Dog 

Working Groups to develop management plans for 

both species.

• Cooperate to evaluate the sites presented in this paper 

and develop strategies to begin management of as 

many sites as possible for black-footed ferret reintroduction within 10 years.

• Support and advance the High Plains Partnership 

landowner incentive program and/or other programs 

designed to bring about landowner participation in 

grassland species management.

Acknowledgments

Pete Gober provided insight for, and extensive assistance with, preparation of this paper. Brian Miller, Sterling 

Miller, Larry Nelson, Bob Oakleaf, and Bill Van Pelt provided 

comments and suggestions. Charlene Beskin produced 

figure 1.

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Abstract

We offer a technique to allocate a hypothetical blackfooted ferret (Mustela nigripes) recovery goal in an equitable 

fashion across the historical range of ferrets. A geographic 

information system (GIS) was used to predict the distribution 

of prairie dog (Cynomys spp.) habitat where the black-footed 

ferret historically occurred. Proportions of predicted habitat by 

jurisdictional entity provided a foundation to allocate a hypothetical delisting of the black-footed ferret. Subject to modification, this technique is presented as an example to bring 

long-term ferret recovery into finer focus at a national scale. 

In addition, we offer this technique to encourage a broader 

assessment of future reintroduction sites, to inspire creative 

thinking on how recovery goals could be allocated across the 

historical range, and to motivate collaborative efforts among 

Federal and State agencies, conservation groups, and private 

landowners to increase the likelihood of successful recovery of 

the black-footed ferret. 

Keywords: black-footed ferret, Cynomys, geographic 

information system, GIS, Mustela nigripes, prairie dog, 

predicted habitat model, recovery

Introduction

The ultimate goal of the Endangered Species Act 

is recovery and subsequent preservation of threatened or 

endangered species (U.S. Fish and Wildlife Service, 2002a). 

Achievement of this goal can be defined in terms of downlisting, which is the reclassification of a species from endangered 

to threatened status, or delisting, which is the removal of a 

species from the Federal List of Endangered and Threatened 

Wildlife and Plants (Cole, 1989; U.S. Fish and Wildlife 

Service, 2002a). Downlisting and delisting result from 

successful recovery efforts; delisting occurs when protection 

of a species is no longer deemed necessary. To coordinate 

recovery efforts among Federal, State, and local agencies, the 

U.S. Fish and Wildlife Service prepares recovery plans that 

outline necessary procedures to achieve downlisting and delisting. Recovery plans identify specific tasks aimed at making a 

species a viable, self-sustaining component of its ecosystem 

(Cole, 1989; U.S. Fish and Wildlife Service, 2002b).

The first recovery plan for the critically endangered 

black-footed ferret (Mustela nigripes) was approved in 1978. 

At that time, no ferrets were known to exist in the wild (U.S. 

Fish and Wildlife Service, 1988; Cole, 1989). The subsequent 

discovery of a wild population of ferrets in Wyoming necessitated revision of the recovery plan. The main revision was 

a shift in management emphasis from free-ranging ferret 

populations to captive breeding and reintroduction (Biggins 

and Thorne, 1994). The revised recovery plan (U.S. Fish and 

Wildlife Service, 1988) placed the ferret program in a national 

scope and outlined steps “to ensure immediate survival of the 

black-footed ferret by: (1) increasing the captive population 

of black-footed ferrets to a census size of 200 breeding adults 

by 1991; (2) establishing a pre-breeding census population of 

1,500 free-ranging black-footed ferret breeding adults in 10 

or more populations with no fewer than 30 breeding adults in 

any population by the year 2010; and (3) encourage the widest 

possible distribution of reintroduced black-footed ferret populations” (U.S. Fish and Wildlife Service, 1988, p. 19). 

As stated in the third step in the recovery plan, reintroduction of ferrets should be considered in the context of their 

historical geographic range. Selection of reintroduction sites 

should be based on several biological considerations, including the vulnerability of ferrets to demographic stochasticity 

(survival of population subgroups); environmental stochasticity (diseases, changes in predator densities); and genetic 

stochasticity (effects of inbreeding and loss of genetic variation through drift) (Shaffer, 1981; Groves and Clark, 1986; 

Clark, 1994). To be successful, however, black-footed ferret 

recovery must also involve more than biological considerations (Kleiman and others, 2000), and a variety of issues, 

including availability and ownership of potential habitat, 

should be considered when selecting reintroduction sites. 

To date, selection of reintroduction sites has focused on 

identifying, protecting, and developing the most promising and 

A Habitat-based Technique To Allocate Black-footed 

Ferret Recovery Among Jurisdictional Entities

By Andrea E. Ernst,1 Aaron L. Clark,2 and Donald R. (Pete) Gober3

1

New Mexico Cooperative Fish and Wildlife Research Unit, New Mexico 

State University, P.O. Box 30003, MSC 4901, Las Cruces, NM 88003.

2

Black-footed Ferret Recovery Foundation, 1133 Pennsylvania St., Denver, 

CO 80203.

3

U.S. Fish and Wildlife Service, 420 S. Garfield Ave., Suite 400, Pierre, SD 

57501.
largest reintroduction locations; however, large reintroduction 

sites may not be developed as rapidly as needed, and availability of these sites should not limit overall ferret recovery (Clark, 

1994). New sites need to be identified, and maintenance of a 

few large sites should not necessarily preclude other, smaller 

recovery areas. To contribute to the overall recovery effort and 

to fulfill State recovery objectives, a strategy that incorporates 

recovery areas of various sizes would maximize the potential 

to secure ferret populations in the wild (U.S. Fish and Wildlife 

Service, 1988). 

Additional challenges in ferret recovery include successful ferret reintroduction and effective long-term management 

of the sites (Reading and Miller, 1994). Selection of potential 

reintroduction sites is problematic and controversial and has 

suffered from disagreements among multiple interest groups, 

conflicting objectives, biological uncertainty, sociopolitical 

constraints, and intense public scrutiny (Maguire and others, 

1988). Given these challenges, field biologists, veterinarians, 

and administrators representing Federal, State, and private 

agencies must provide a means by which to allocate ferret 

recovery in an equitable fashion. Equitable allocation will 

encourage participation by all entities and help place longterm ferret recovery in a national scope. To assist in meeting 

these challenges, we offer a habitat-based technique to allocate 

reintroduction efforts among jurisdictional entities. This technique is based on quantifying the relative amount of potential 

habitat across the geographic range. We offer this technique 

only as a test case to help bring long-term ferret recovery into 

finer focus at the national scale. Further, our technique will 

potentially broaden current assessments of future reintroduction sites and encourage cooperation across the extended 

network of people involved in the survival of the black-footed 

ferret. 

Methods

Digital Data Layers

Recent advances in computer-aided mapping, combined 

with accessibility of geographic information system (GIS) 

data sets, enable production of digital maps depicting distributions of predicted habitat at a spatially detailed, landscape 

scale. Historical black-footed ferret specimens were recorded 

in association with three species of prairie dogs (Cynomys 

spp.), including the black-tailed prairie dog (C. ludovicianus), 

white-tailed prairie dog (C. leucurus), and Gunnison’s prairie 

dog (C. gunnisoni) (Anderson and others, 1986). Further, 

black-footed ferret habitat is often defined in terms of prairie 

dog colonies. Thus, we created predictive habitat distribution 

models for the three species of prairie dogs. We defined and 

restricted the geographic area used in the predictive models by 

using the comprehensive prairie dog range maps as described 

by Hall (1981). These maps characterize the extremes of the 

area where prairie dog species were found historically and 

incorporate all known specimen records, including marginal 

habitats and disjunct populations. The range distribution 

maps provided by Hall (1981) were scanned with a desktop 

scanning device at 800 dots per inch. The digital images were 

saved in a tagged image file format to provide baseline GIS 

coverages. These images were registered to geographic coordinates, and distribution boundaries were digitized for each 

prairie dog species. We did not include the Utah prairie dog 

(C. parvidens) because evidence suggests that black-footed 

ferrets were not associated with this species (Anderson and 

others, 1986).

Collection records demonstrate that ferrets, until the first 

decades of the 20th century, were distributed over 40 million ha 

in 12 States and 2 Canadian Provinces (Anderson and others, 

1986; Clark, 1986, 1987). County jurisdictional boundaries 

were acquired from the National Atlas of the United States®

(http://www.nationalatlas.gov), imported into ArcGIS® 8.3 

software (Environmental Systems Research Institute, Inc., 

Redlands, Calif.), and dissolved by State attributes, producing 

boundaries at a scale of 1:100,000 for the 12 States in which the 

black-footed ferret historically occurred.

Digital data sets depicting landscape attributes were 

chosen based on the availability and uniformity of data across 

the geographic range. We used the U.S. Geological Survey 

(USGS) National Land Cover Dataset (NLCD; http://landcover.usgs.gov/natllandcover.asp) to provide an estimate of 

current land cover. This data set depicts generalized land cover 

categories labeled agriculture, urban areas, forests, wetlands, 

grasslands, and shrublands with a 30-m spatial resolution. The 

NLCD was created from Landsat 5 Thematic Mapper satellite 

imagery digitally captured in 1992, produced by the Earth 

Resources Observation System Data Center. The NLCD was 

downloaded in complete State sections, which included a 300-

m (10-pixel) buffer added to each outer State boundary. The 

data were then imported into ERDAS IMAGINE® 8.6 (Leica 

Geosystems Geospatial Imaging, LLC, Norcross, Ga.) and 

projected into a common coordinate system. Each State was 

clipped to the individual jurisdictional boundaries. 

We used the USGS National Elevation Dataset (NED) 

to provide continuous, seamless elevation information at a 

30-m spatial resolution. We downloaded the NED (http://ned.

usgs.gov) in individual 1:250,000 quadrangles. The individual 

quadrangles were then map-joined to create one complete 

data layer for each State. Each data layer, as with all GIS data 

used in the model, was projected to a common coordinate 

system (Albers Equal Area projection). This projection is 

used in the United States and other countries that have a larger 

east-west than north-south extent because it preserves the area 

of the displayed features over the entire map with the same 

proportional relationship as the actual geographic areas they 

90 Recovery of the Black-footed Ferret
represent (Kennedy and Kopp, 2000). The individual data sets 

were then clipped to the State jurisdictional boundaries. 

Predicted Habitat Models

We created digital models of predicted prairie dog habitat 

based on a set of landscape attributes and wildlife-habitat 

relationships. The wildlife-habitat relationships were based 

on attributes important in defining prairie dog habitat, such as 

land cover and topographic gradient (Koford, 1958; Clippinger, 1989). Generalized land cover categories considered 

suitable prairie dog habitat were grassland, shrubland, small 

grains, row crops, and pastures. Land cover types considered 

unsuitable were forests, water, and snow. Residential, wetland, 

and fallow land cover types may provide some prairie dog 

habitat; however, we considered these contributions minimal 

and placed these land cover types in the unsuitable category. 

Topographic gradient was an additional landscape attribute used to predict prairie dog habitat. We used an algorithm 

in ERDAS IMAGINE 8.6 to derive percent slope from the 

NED. Slopes of 0–10 percent were considered suitable habitat. 

Although prairie dogs may occur on slopes greater than 10 

percent, black-tailed prairie dogs usually build on slopes of 

less than 10 percent (Koford, 1958; Dalstead and others, 1981; 

Clippinger, 1989). Therefore, the remaining slope categories 

(11 percent and greater) were considered unsuitable habitat for 

all prairie dog species.

The Spatial Modeler module of ERDAS IMAGINE 8.6 

was used to produce the individual predicted habitat models 

for each State. We used the additive overlay technique, which 

combined each individual data layer as an equally weighted 

component in the model. Although this process is referred to 

as additive, the file produced depicts the specific combination 

of the appropriate land cover and slope attributes selected as 

suitable prairie dog habitat. The predicted models for each 

State were then clipped to the individual range boundaries and 

merged into one complete data set. The result was a predicted 

habitat model for each prairie dog species. 

The final step in modeling predicted habitat was removal 

of small, isolated tracts. Our models were produced at a 

30-m spatial resolution, which we considered to be below the 

minimum habitat area required for black-footed ferrets. Minimum habitat area can be defined as the minimum amount of 

contiguous habitat that is required before an area will be occupied by a species (Clippinger, 1989). Because the different 

prairie dogs species afford different ferret carrying capacities, 

the size of suitable reintroduction areas ultimately depends 

on densities of prey. For example, ferrets have been shown to 

occur at densities of one adult black-footed ferret per 40–60 ha 

in white-tailed prairie dog colonies (Forrest and others, 1985; 

Richardson and others, 1986; U.S. Fish and Wildlife Service, 

1988). Hillman and others (1979) found that 6 of 11 observed 

ferret litters occupied black-tailed prairie dog colonies greater 

than 40 ha. Further, black-tailed prairie dogs tend to be more 

gregarious and thus occur in more dense populations. Therefore, the minimum area of black-tailed prairie dog colonies 

that can support ferrets may be smaller than that for other 

prairie dog species (Clark, 1994).

We removed patches that were below the minimum size 

suitable for black-footed ferret survival in each habitat model 

with the Clump and Eliminate commands in ERDAS IMAGINE. We filtered predicted habitat based on the minimum area 

suitable for black-footed ferret survival. We used a minimum 

patch size of 40 ha in the black-tailed prairie dog range, 60 ha 

in the Gunnison’s prairie dog range, and 80 ha in the whitetailed prairie dog range.

Although the ability of various habitats to support populations of a given size will only be known from the results of 

reintroductions, at present it appears that large complexes are 

necessary for viable ferret populations (U.S. Fish and Wildlife 

Service, 1988). The minimum areas we chose may be reasonable based on available bioenergetic and behavioral information, however, and we offer them as working hypotheses in 

presenting our methodology for allocating ferret recovery.

Ferret Allocation

The 1988 recovery plan deferred specification of a delisting population size pending outcomes of reintroductions and 

accumulation of additional management experience (U.S. Fish 

and Wildlife Service, 1988). We offer a hypothetical delisting 

population size of 15,000 ferrets, an order of magnitude larger 

than the downlisting objective specified in the 1988 recovery 

plan. We chose this value based on several lines of reasoning. 

First, large prairie dog colonies such as those currently used 

for reintroductions may be scarce (Dobson and Lyles, 2000). 

Additional, smaller populations may be necessary to meet 

any delisting objective. Second, a larger number of smaller 

populations may help protect against catastrophic events 

(e.g., disease outbreaks) that can decimate entire populations 

(Forrest and others, 1988). Third, fossil evidence supports 

the hypothesis that black-footed ferrets may have been more 

common throughout the historical range (Linder and others, 

1972; Choate and others, 1982; Hubbard and Schmitt, 1984; 

Anderson and others, 1986). 

Although the majority of habitat occurs in the blacktailed prairie dog range, we suggest larger than proportional 

allocations of black-footed ferrets in the white-tailed and 

Gunnison’s prairie dog ranges. We suggest 8,625 ferrets (57.5 

percent) allocated to the black-tailed prairie dog range; 3,375 

ferrets (22.5 percent) to the Gunnison’s prairie dog range; 

and 3,000 ferrets (20 percent) to the white-tailed prairie dog 

range. To equitably divide ferret recovery across jurisdictional 

entities, we calculated the total amount of predicted habitat 

in the individual prairie dog ranges, calculated the percent of 

predicted habitat in each State, and then used those percentAllocating Recovery Among Jurisdictional Entities 91
ages to apportion black-footed ferrets by State and by prairie 

dog species.

Results and Discussion

Black-tailed Prairie Dog

Using the model described above, we calculated about 

128.9 million ha of predicted habitat in the black-tailed 

prairie dog range (table 1), or about 71 percent of the range 

distribution as described by Hall (1981). The largest amount 

of predicted habitat occurred in Texas and encompassed over 

29.2 million ha. New Mexico provided the second largest 

amount of predicted habitat with ~16.0 million ha. Nebraska, 

Kansas, and Montana had similar amounts of predicted 

habitat, with approximately 14 million ha each. North Dakota 

and Arizona, both considered range extremes, had the smallest 

estimate of predicted habitat with ~3.5 million ha and ~1.5 

million ha, respectively. Texas was allocated 1,957 individual 

black-footed ferrets, and New Mexico was allocated 1,072 

ferrets. South Dakota, where the last known extant populations of ferrets occurred in the black-tailed prairie dog range, 

was allocated 746 black-footed ferrets, and Wyoming was 

 State

Predicted

habitat (ha)

Percent of predicted 

habitat within each 

jurisdictional entity

Number of

ferrets allocated

Minimum habitat 

required (ha)

Minimum habitat

as a percent of total

Black-tailed prairie dog (C. ludovicianus)

Arizona 1,484,257 1.2 99 3,960

Colorado 9,870,127 7.7 660 26,400

Kansas 13,977,156 10.8 935 37,400

Montana 13,719,492 10.6 918 36,720

Nebraska 14,660,668 11.4 981 39,240

New Mexico 16,024,114 12.4 1,072 42,880

North Dakota 3,520,025 2.7 236 9,440

Oklahoma 7,764,139 6.0 520 20,800

South Dakota 11,145,988 8.6 746 29,840

Texas 29,248,634 22.7 1,957 78,280

Wyoming 7,486,045 5.8 501 20,040

Total 128,900,645 100.0 8,625 345,000 0.27

Gunnison’s prairie dog (C. gunnisoni)

Arizona 5,338,155 39.4 1,331 79,860

Colorado 2,206,766 16.3 551 33,060

New Mexico 5,505,857 40.7 1,373 82,380

Utah 482,473 3.6 120 7,200

Total 13,533,251 100.0 3,375 202,500 1.50

White-tailed prairie dog (C. leucurus)

Colorado 934,483 8.9 268 21,440

Montana 53,308 0.5 15 1,200

Utah 1,075,817 10.3 309 24,720

Wyoming 8,394,910 80.3 2,408 192,640

Total 10,458,518 100.0 3,000 240,000 2.29

Table 1. Amount of predicted habitat by prairie dog (Cynomys) species and jurisdictional entity, and resulting black-footed ferret (Mustela nigripes) allocations based on the hypothetical delisting objective of 15,000 individuals.

92 Recovery of the Black-footed Ferret
allocated 501. The State with the lowest ferret allocation was 

Arizona.

Based on our calculations (table 1), the minimum amount 

of habitat needed in the range of the black-tailed prairie dog 

was about 345,000 ha. Texas, with 23 percent of the predicted 

habitat, required a minimum of ~78,000 ha, and New Mexico 

required ~43,000 ha. Arizona could contribute ~4,000 ha. 

Overall, the minimum amount of habitat needed to achieve the 

hypothetical delisting objective was less than 1 percent of the 

total predicted habitat.

The amount of predicted habitat was calculated from 

input variables based on our model. We recognize that different definitions of suitable land cover could result in different 

amounts of predicted habitat and different ferret allocations. 

For example, we included agricultural land in our model based 

on the recognition that large areas of historically suitable 

prairie dog habitat were converted to cropland after settlement 

because prairie dogs prefer deep, relatively level soils—the 

same land preferred for agricultural development (Choate and 

others, 1982; Clark, 1986). Although we do not assume that 

land under current cultivation practices would be converted 

back to rangeland solely to provide black-footed ferret habitat, 

some agricultural practices may be compatible with blackfooted ferret and prairie dog management, provided that 

prairie dogs can be tolerated (U.S. Fish and Wildlife Service, 

1988). Overall, the agriculture class was approximately 44.7 

million ha or 34 percent of the predicted habitat in the blacktailed prairie dog range. Oklahoma and Kansas had the largest 

proportions of agriculture, with more than 52 percent of the 

area under cultivation. In New Mexico, agriculture totaled over 

48 percent of the area.

Gunnison’s Prairie Dog

The amount of predicted habitat in the Gunnison’s prairie 

dog range was over 13.5 million ha or 40 percent of the range 

distribution as described by Hall (1981). New Mexico had 

~5.5 million ha of predicted habitat, followed closely by 

Arizona with ~5.3 million ha. Colorado had ~2.2 million ha of 

predicted habitat and Utah ~482,000 ha (table 1).

Based on our calculations, New Mexico and Arizona 

were allocated a similar number of black-footed ferrets, 

approximately 1,350 individuals. Colorado and Utah 

combined were allocated 671 ferrets. Our results indicate 

that the minimum amount of habitat needed to achieve the 

hypothetical ferret recovery goal was 1.5 percent of the total 

predicted habitat in the Gunnison’s prairie dog range.

Unlike the black-tailed prairie dog range, inclusion of the 

agriculture land cover class did not have much impact in the 

Gunnison’s prairie dog range. Only 5 percent of the area was 

estimated to be in agriculture; however, additional information, as it becomes available at a regional scale, might improve 

the model. For example, soil type, soil depth, and rock ground 

cover are important variables in defining Gunnison’s prairie 

dog habitat (Wagner and Drickamer, 2004). These variables 

should be included in the model when the spatial data become 

available.

White-tailed Prairie Dog

White-tailed prairie dogs afforded the least amount of 

predicted habitat, ~10.5 million ha or 45 percent of the range 

distribution as described by Hall (1981). The majority of 

predicted habitat in the white-tailed prairie dog range occurred 

in Wyoming, which had over 8.3 million ha. Montana was 

estimated to have less than 1 percent of the total predicted 

habitat (table 1).

Based on our estimates (table 1), Wyoming could host 

2,408 black-footed ferrets, Utah 309, Colorado 268, and 

Montana 15. Overall, in the white-tailed prairie dog range, the 

minimum amount of habitat needed to reach the hypothetical black-footed ferret recovery goal was 240,000 ha, with 

Wyoming contributing most of the potential habitat. The minimum amount of habitat estimated to achieve our hypothetical delisting objective was 2.3 percent of the total predicted 

available habitat. 

As with the Gunnison’s prairie dog predicted model, 

inclusion of agriculture did not strongly affect the outcome for 

white-tailed prairie dogs, with only 7 percent of the area classified in the agriculture land cover type; however, the whitetailed prairie dog model could be improved with more detailed 

land cover information. For example, the NLCD shrubland 

cover class may be too general to define white-tailed prairie 

dog habitat. Although white-tailed prairie dogs occur in shrubland habitats, shrub height and density (Collins and Lichvar, 

1986) may be better predictive variables.

Distribution of Resources

Based on our model, Wyoming received the largest 

allocation of black-footed ferrets with approximately 2,909 

individuals. New Mexico was allocated 2,445 individuals and 

Texas 1,957. The total amount of predicted habitat across 

all prairie dog species was 152.9 million ha. We calculated 

a minimum of 787,500 ha of habitat needed to attain the 

hypothetical delisting of the black-footed ferret, or less than 1 

percent of the potential available habitat. Our results support 

the conclusion in the 1988 recovery plan that sufficient habitat 

to meet downlisting is less than 0.1 percent (75,000–100,000 

ha) of western rangelands (U.S. Fish and Wildlife Service, 

1988).

Conclusion

Our technique has several underlying assumptions. The 

principal assumption is that all prairie dog habitat is suitable 

black-footed ferret habitat. We recognize that black-footed 

ferret habitat is more restricted, requiring complex spatial 

Allocating Recovery Among Jurisdictional Entities 93
configurations of prairie dog colonies, specific distances 

between those colonies, and substantial prairie dog densities (Stromberg and others, 1983; Houston and others, 1986; 

Biggins, Lockhart, and Godbey, this volume). Another 

assumption of our technique is that land cover data identified 

from modern remote sensing platforms can reasonably predict 

prairie dog habitat. Nevertheless, we offer this technique as a 

test case and encourage modifications and refinements. Future 

efforts should consider using a larger variety of input variables 

with more locally specific information, different classifications 

of land cover or slope categories, and greater spatial resolution.

Our technique (or refinements of it) could be used to allocate black-footed ferret recovery across jurisdictional entities. 

This technique may help place long-term black-footed ferret 

recovery into a national scope based on equitable contributions 

among those entities. In so doing we hope to inspire creative 

thinking on how specific recovery goals might be allocated 

across the historical range. We hope to motivate the collaborative effort among Federal and State agencies, conservation 

groups, and private landowners that will be needed to turn the 

black-footed ferret back from the brink of extinction (Cole, 

1989; Reading and Miller, 1994).

Acknowledgments

We are indebted to T. Schrader for his computer expertise 

and configuration of hardware and software, which enabled 

manipulation of our large digital data sets. Special thanks to K. 

Young for his constructive review of the manuscript. Financial 

assistance for this project was provided by the South Dakota 

Ecological Services Field Office of the U.S. Fish and Wildlife 

Service. The New Mexico Cooperative Fish and Wildlife 

Research Unit at New Mexico State University provided 

access to GIS software.

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Wildlife Management, v. 68, p. 188–197.

Allocating Recovery Among Jurisdictional Entities 95

Abstract

Black-footed ferrets (Mustela nigripes) historically occupied colonies of three prairie dog (Cynomys) species—Gunnison’s (C. gunnisoni), white-tailed (C. leucurus), and blacktailed (C. ludovicianus)—more or less throughout their ranges. 

Historical declines in the abundance of ferret habitat (prairie 

dog colonies) resulted from poisoning of prairie dogs, sylvatic 

plague, conversion of habitat to agriculture, and changes in 

grazing practices to benefit mid-height and tall grasses. Prairie 

dog restoration often involves translocating prairie dogs into 

vacant habitat and managing vegetation to enhance colony 

growth. Sites for reestablishment should be selected with 

attention to ecological suitability, level of plague risk, return 

on economic investment in restoration and management, and 

social acceptability. Plague, conventional grazing and farming 

practices, and hostility of land managers toward prairie dogs 

can depress rates of restoration, but incentives may help overcome these obstacles. Two case histories illustrate restoration 

and management of black-tailed prairie dogs in two grassland 

types—mixed-grass and shortgrass. Options for expanding ferret habitat restoration and management opportunities 

include using small prairie dog complexes for ferret releases, 

introducing more intensive grazing to benefit black-tailed prairie dogs in taller grasslands, and reclaiming retired farmlands 

with shortgrass species beneficial to prairie dogs.

Keywords: black-footed ferret, Cynomys spp., habitat, 

management, Mustela nigripes, prairie dog, restoration

Introduction

Black-footed ferrets (Mustela nigripes) require populations of prairie dogs (Cynomys spp.) for sustained existence 

in the wild. Historical distribution records of ferrets coincide 

closely (though not exactly) with the presence of prairie dog 

colonies and the known historical ranges of three prairie dog 

species—black-tailed (C. ludovicianus), white-tailed (C. 

leucurus), and Gunnison’s (C. gunnisoni). Ferrets collected 

outside prairie dog colonies or ranges could have come from 

ferret populations within colonies (Hubbard and Schmitt, 

1984; Anderson and others, 1986). Efforts to recover ferrets 

proceed under the assumption that wild populations cannot 

long survive without prairie dogs (U.S. Fish and Wildlife 

Service, 1988).

Ferret habitat restoration thus implies restoration and 

management of prairie dogs, which of course requires suitable 

prairie dog habitat. Many landscapes historically occupied 

by black-tailed, white-tailed, or Gunnison’s prairie dogs have 

been changed by conversion to agriculture, alterations in 

large herbivore abundance, or increases in woody vegetation. 

Singly or in combination, these changes have altered habitat 

suitability for prairie dogs (U.S. Fish and Wildlife Service, 

2000; Knowles, 2002). Thus, habitat restoration for ferrets 

often must begin with habitat restoration and management for 

prairie dogs.

We focus herein on restoration and management of prairie dogs as a means of restoring ferret populations. First we 

discuss historical patterns of ferret and prairie dog abundance 

and, partly on that basis, regional priorities for restoration. 

Then we describe prairie dog restoration and management 

methods, challenges to both, and ways of expanding opportunities. Some issues, such as relative habitat quality among 

the prairie dog species, the influences of plague and predation, and the effects of livestock grazing, also are addressed 

elsewhere in this volume.

Ferret Habitat: A Historical Perspective 

Historical information on ferret habitat is limited because 

of the fossorial and nocturnal habits of the species (Biggins 

and Schroeder, 1988) and its early demise. Even so, making 

the most of available data seems imperative; such data not 

only provide a rough template for restoration but also can 

inform the recovery process. The most reliable data primarily include past distributional abundance of ferrets based on 

verified records (usually collections) and the biogeographical 

patterns that can be inferred from these records. We recognize 

that collection records provide a poor surrogate for ferret 

abundance (numerous factors could influence collection 

density, as discussed later), but few other historical data sets 

are as relevant to restoration.

Habitat Restoration and Management

1

Turner Endangered Species Fund, P.O. Box 211, Glenwood, NM 88039. 

2

Turner Endangered Species Fund, P.O. Box 1118, Fort Pierre, SD 57532.

3

Turner Endangered Species Fund, P.O. Box 131, Cimarron, NM 87714.

4

Turner Endangered Species Fund, 1123 Research Drive, Bozeman, 

MT 59718.

By Joe C. Truett,1 Kristy Bly-Honness,2 Dustin H. Long,3 and Michael K. Phillips4
The general picture that emerges from verified records 

shows a ferret distributional range largely overlapping the 

ranges of the three prairie dog species (fig. 1). Black-tailed 

prairie dog range, being much more extensive than ranges of 

white-tailed and Gunnison’s prairie dogs, encompasses most 

of the ferret range and accounts for most of the ferret records 

(Powell, 1982; Anderson and others, 1986). An important 

question for restoration is whether these records suggest 

any apparent preferences of ferrets for prairie dog species or 

biogeographic regions.

If one assumes that density (number per unit area) of 

ferrets collected or otherwise verified in prairie dog range 

correlates with habitat quality or preference, Anderson and 

others’ (1986) distribution maps in most cases suggest no 

clear preference among species within the same regions. Other 

factors, however, such as proportion of prairie dog range 

occupied by colonies, could confound judgments of habitat 

quality based solely on ferret records. Biggins, Lockhart, and 

Godbey (this volume) and Ernst and others (this volume) note 

the likelihood that higher density populations of prairie dogs 

supported more ferrets per unit area, and, as Knowles (2002) 

indicated, black-tailed prairie dogs usually occur in higher 

densities than do the other two species. New Mexico presents 

a conundrum (see also below) in that about four times as many 

ferret records came from Gunnison’s as from black-tailed prairie dog range in the State (Anderson and others, 1986) despite 

the probable greater density of black-tailed prairie dogs and 

the estimated similarity in area occupied by the two species 

(see Hubbard and Schmitt, 1984).

The distribution of ferret records in black-tailed prairie 

dog range suggests that a greater density of ferrets occurred 

in northern parts than in southern parts. The northern half of 

the range produced about eight times as many ferret records 

as did the southern half (calculated from Anderson and others 

[1986]; fig. 1). Furthermore, numbers of ferret records from 

Montana, Texas, and the portion of New Mexico occupied by 

black-tailed prairie dogs (Anderson and others, 1986), viewed 

in light of estimated prairie dog colony area (table 1), show 

ferret records per habitat unit in Montana to be about 50 times 

those in New Mexico and well over 100 times those in Texas. 

Bailey (1905) described a single colony of black-tailed prairie 

dogs in Texas that occupied about 65,000 km2; Anderson and 

others (1986) showed only two to five ferrets verified from 

the region occupied by that colony. In comparison, South 

Dakota’s entire prairie dog range (including the unoccupied 

parts) covered only about twice that area but yielded 99 ferret 

records. Oklahoma, a southern State with roughly the same 

area of prairie dog range as that of South Dakota, yielded only 

four ferret records (Anderson and others, 1986).

Several factors other than habitat quality could have 

contributed to these north-south differences. Flath and Clark 

(1986) may have substantially underestimated the area of 

prairie dog colonies in Montana, and Bailey (1905) may 

have substantially overestimated it in Texas (D. Gober, oral 

commun., 2003). Trapping for furs, which accounted for some 

of the specimens collected (Anderson and others, 1986), may 

have been more intensive in areas producing better furs—that 

is, northern regions. The intrusion of agriculture into eastern 

portions of black-tailed prairie dog range may have occurred 

earlier in southern than in northern States, perhaps biasing 

Figure 1. Collection locations for black-footed ferrets (Mustela 

nigripes) (Anderson and others, 1986) and historical ranges of prairie dogs (Cynomys spp.) across the Great Plains. Each collection 

location (dark triangle) represents >1 verified historical record(s).

Table 1. Black-footed ferret (Mustela nigripes) collection records 

from black-tailed prairie dog (Cynomys ludovicianus) range in 

three states, and densities of records within ferret habitat based 

on reported habitat acreages (i.e., areas occupied by prairie dog 

colonies).

Estimated Ferret

Number area (km2) records/

of ferret of habitat 100 km2 of

State recordsa available habitat

Montana 44 6,000b 0.733

Texas 13 230,000c 0.006

New Mexico 3 ~21,000d 0.014

a

Anderson and others (1986).

b

Flath and Clark (1986).

c

Bailey (1905).

d

Hubbard and Schmitt (1984).

98 Recovery of the Black-Footed Ferret
later collection efforts toward northern States (Anderson and 

others, 1986). Finally, far southwestern (Chihuahuan Desert) 

portions of black-tailed range, having historically lacked large 

wild grazers (Truett, 1996), may have supported low numbers 

of prairie dogs (and few or no ferrets) prior to the proliferation 

of cattle (Bos taurus) (Hubbard and Schmitt, 1984). 

Definitive answers about latitudinal differences in habitat 

quality of black-tailed prairie dog colonies will come only 

with comparisons between ferret releases that span the historical range. To date, colony complexes near Janos, Chihuahua, 

Mexico, host the only ferret releases in southern parts of 

black-tailed prairie dog range. The youth of this release 

program precludes a reliable assessment of its success.

Regional Priorities for Restoration

The Black-footed Ferret Recovery Plan (U.S. Fish and 

Wildlife Service, 1988) calls for establishing the widest 

possible distribution of 10 or more self-sustaining ferret populations. Sites for release of ferrets are selected on the basis 

of several criteria of habitat suitability (Biggins and others, 

1993), key among which are size and expected longevity of 

prairie dog colony complexes. To complement this strategy, 

those planning prairie dog restorations probably should set 

regional priorities. We believe that important criteria for 

setting such priorities include level of plague risk, species of 

prairie dog, and regional differences in habitat quality within 

prairie dog species. All of these criteria will affect relative 

costs of prairie dog restoration and management.

Plague Risk

The sensitivities of prairie dogs and ferrets to plague 

make it the most important long-term threat to ferret habitat 

restoration in regions susceptible to epizootics. The historical 

spread of sylvatic plague eastward from the west coast and the 

apparent termination of this advance at the so-called plague 

line are addressed elsewhere (Cully and Williams, 2001; Gage 

and Kosoy, this volume). At present, plague apparently occurs 

in the wild more or less throughout the ranges of white-tailed 

and Gunnison’s prairie dogs and in black-tailed range to about 

the western borders of South Dakota, Nebraska, Kansas, and 

Oklahoma—the plague line (Cully and Williams, 2001). The 

chances of plague epizootics affecting prairie dogs and ferrets 

west of the plague line seem to vary considerably among 

localities and to diminish as one nears the line.

Prairie Dog Species

Available evidence suggests to us that, among prairie 

dog species, the Gunnison’s ranks lowest in priority for ferret 

habitat restoration and that the black-tailed ranks highest. We 

rank the Gunnison’s prairie dog lowest primarily because of 

the species’ relatively high and persisting losses rangewide 

to plague (Cully and Williams, 2001; Knowles, 2002) and its 

relatively intact (unaltered) habitat (Knowles, 2002); these 

factors suggest that restoration and habitat management efforts 

may lead to little long-term improvement in population status 

of the species. The average low survival and reproduction of 

ferrets released into a large Gunnison’s prairie dog complex 

in Arizona (Conservation Breeding Specialist Group, 2004) 

suggest that, for unclear reasons, ferret habitat quality may be 

poor (plague appears to be absent at release sites).

We rank the white-tailed prairie dog second in priority. Although also at high risk from plague rangewide, this 

species is believed to suffer lower losses to epizootics than do 

Gunnison’s or black-tailed prairie dogs, perhaps because of its 

commonly low population densities (Menkens and Anderson, 

1991; Cully and Williams, 2001). In support of this belief, 

releases of ferrets during 1991–94 into a white-tailed prairie 

dog complex in Wyoming’s Shirley Basin (Luce and others, 

1997) resulted in unexpectedly high numbers of ferrets present in 2003 (Grenier, 2003), despite plague epizootics in the 

interim (Luce and others, 1997; Cully and Williams, 2001). 

Like Gunnison’s prairie dogs, however, white-tails probably 

offer low per capita returns on investment in restoration and 

habitat management because of their low density and relatively 

intact habitat (Knowles, 2002). 

We rank the black-tailed prairie dog highest in priority. A 

substantial proportion of their relatively large range remains 

plague free, densities within colonies (especially in plaguefree areas) tend to be relatively high, and restoration and 

management efforts can yield high per capita returns. Much of 

the habitat within their historical range has been degraded, but 

substantial proportions could be restored. The most successful 

releases of ferrets have been in plague-free parts of blacktailed prairie dog range (Conservation Breeding Specialist 

Group, 2004).

Regions Within Black-tailed Prairie Dog Range

Priority for restoration varies from place to place within 

black-tailed prairie dog range. Most obviously, priority 

increases with decreased risk of plague. Ferrets released east 

of the plague line in South Dakota have survived and reproduced much better than those released west of the plague line 

in Montana (Conservation Breeding Specialist Group, 2004). 

Also, as noted above, if distributional abundance of ferret 

records correlates with habitat quality, restoration priority 

increases with latitude. 

Restoration Methods and Challenges

We discuss two aspects of prairie dog restoration: reestablishment of populations and habitat improvement. Hostile 

traditions toward prairie dogs among land managers represent 

an important socioeconomic challenge to prairie dog restoration; incentives may help address this challenge.

Habitat Restoration and Management 99
Translocation

Timely restoration will require reestablishing prairie dogs 

where they formerly existed. At least three factors will hinder 

natural recolonization: (1) large spatial vacancies within previously occupied ranges, (2) short dispersal distances of blacktailed prairie dogs (Knowles, 1985) and probably the other 

species as well, and (3) infrequency with which new colonies 

originate on their own (Knowles, 1982). Translocations to 

establish new colonies will greatly accelerate the rate of restoration (D. Long and K. Bly-Honness, unpub. data, 2004).

Unlike natural colonization, translocation can space colonies across landscapes to form complexes ideal for ferrets and 

compatible with other land uses (see Bevers and others, 1997; 

Hof and others, 2002). Because small, new colonies expand 

much faster than large, old ones (Knowles, 1982; D. Long and 

K. Bly-Honness, unpub. data, 2004), translocation accelerates 

the rate of population growth. Also, translocation can retard or 

control unwanted expansion in source colonies by removing 

substantial proportions of the populations.

Only Utah prairie dogs (C. parvidens) and black-tailed 

prairie dogs have been extensively translocated (Truett 

and others, 2001a). Translocations of Utah prairie dogs 

commenced in the early 1970s with concern for the imperiled 

status of that species. Large-scale translocations of blacktailed prairie dogs have taken place primarily since 1990 

(Long and others, in press). Methodologies for both species 

have been published elsewhere; below we review and compare 

these methods and recommend approaches that seem to work 

best for ferret habitat restoration.

Black-tailed Prairie Dogs

Source populations for translocating black-tailed prairie 

dogs should be selected with attention to disease risks, potential legal restrictions, genetic makeup, and effect of removal 

on the source population (Truett and others, 2001a; Long and 

others, in press). To date, plague presents the greatest disease 

problem and may indicate the need to quarantine animals 

(Marinari and Williams, 1998) before release. Monkeypox 

is an emerging disease issue but so far is confined to captive 

prairie dogs and other rodents. State or Federal restrictions 

on trapping and transporting prairie dogs may exist; recent 

restrictions related to monkeypox (U.S. Department of Health 

and Human Services, 2003) are the most prohibitive to date 

in that they restrict trapping and transport of all prairie dogs 

without special exemption. With respect to maintenance of 

unique gene pools, some biologists have voiced concern about 

translocating prairie dogs long distances. In practice this 

concern has influenced few translocation programs, although 

in New Mexico we acquired prairie dogs from a specific 

locality to help preserve the gene pool. Using translocations 

to remove unwanted animals is an attractive idea but in fact 

is an inefficient and often ineffective control method, in part 

because most populations seem able to support sustained 

harvests of at least 25–30 percent annually (T. Livieri, unpub. 

data, 2002).

The best sites for releases often have evidence of previous 

occupancy, but risk of plague or encroachment of tall vegetation may have degraded the suitability of such sites (Long and 

others, in press). Sites without evidence of historical occupancy also can be suitable if soils are deep and relatively fine 

textured and slopes are less than about 6 percent (Reading and 

Matchett, 1997). Grass dominance by grazing-resistant species 

is an important indicator of release site suitability (Long and 

others, in press).

Operators capture prairie dogs for translocation usually 

with livetraps but sometimes by pulling them from burrows 

with a vacuum truck or flushing them out with water (Truett 

and others, 2001a; Long and others, in press). We advise 

immediately treating captured animals with a pesticide to kill 

fleas, which can transmit plague, and then transporting them 

in wire-mesh cages to quarantine facilities or release sites. 

Important protocols for handling captive prairie dogs include 

protection from extreme temperatures, provision of adequate 

food and water, euthanization if seriously injured, and necropsy of 

any dying from unknown causes (Marinari and Williams, 1998). 

We and most other practitioners conduct translocations 

during July–September to reduce losses of the very young that 

would occur with translocations in spring and to give released 

animals time to excavate new burrows before winter (Long 

and others, in press). We (Truett and others, 2001a; Long and 

others, in press) mow tall vegetation at release sites to 10 cm 

or less and hold the prairie dogs there for several days in acclimation cages consisting of belowground nest boxes connected 

by an access tube to aboveground retention baskets. The 

acclimation cages contribute greatly to survival by reducing 

dispersal and providing shelter from predators during the first 

few months postrelease while the prairie dogs are excavating new burrows. Predation by coyotes (Canis latrans) and 

badgers (Taxidea taxus) during this period usually accounts 

for most of the postrelease losses; installation of nest boxes at 

least 1.2 m deep, monitoring for predators at release sites for 

2–3 weeks, and selective control of predators during this time 

commonly result in 50 percent or more surviving onsite at the 

end of 2 months. By that time, loss rates decline substantially. 

We usually see recruitment of young at near normal rates the 

following May and June.

In our experience, most operators translocate prairie dogs 

in groups as trapped without trying to retain them in original 

family units or specific sex and age groups. We found no 

significant difference in postrelease survival or recruitment 

between groups of prairie dogs translocated as family units 

(n = 4) and those translocated as mixed-family groups (n = 6) 

(Bly-Honness and others, 2004), but Shier (2004) found that 

five groups she translocated as family units survived and 

reproduced at higher rates than did five groups trapped without 

attention to family unity. We found (insignificantly) greater 

average survival among mixed-family groups translocated 

after being quarantined together for 2 weeks than among those 

100 Recovery of the Black-Footed Ferret
not quarantined (Bly-Honness and others, 2004). Preliminary 

data indicated lower survival in groups containing more than 

about 60 percent juveniles than in groups containing less than 

about 40 percent juveniles (K. Bly-Honness and D. Long, 

unpub. data, 2004).

After several months, released animals have usually excavated numerous new secure burrows, and control of depredating coyotes and badgers becomes less important. Occasionally, 

large losses of prairie dogs at a release site will necessitate 

supplemental releases during the first several months after 

the initial release. Supplements usually survive at higher rates 

than those originally released because they take advantage of 

the burrows excavated by the first contingent. After several 

months to a year, management of colonies established by 

translocation differs little from management of preexisting 

colonies.

Other Prairie Dog Species

The relatively extensive work on translocation of Utah 

prairie dogs may instruct efforts to translocate white-tailed and 

Gunnison’s prairie dogs. Utah prairie dogs are more closely 

related to these two species than are black-tailed prairie dogs, 

and they occupy similar habitats (i.e., intermountain valleys, 

benches, and plateaus; Knowles, 2002). Utah prairie dogs were 

first translocated in 1972, and approximately 20,000 individuals have been moved to date (Long and others, in press). In 

this section we focus on aspects of these translocations that 

are different from those discussed above for black-tailed 

prairie dogs. These differences are rather minor; they include 

primarily release-site selection and preparation and postrelease 

protection and monitoring.

Coffeen and Pederson (1993), citing Crocker-Bedford 

and Spillett (1981), provided criteria for release-site selection for Utah prairie dogs. Sites should be well drained, with 

soils at least 1.2 m deep and not easily collapsible. Vegetation 

should be sufficiently short or sparse to allow good horizontal 

visibility but sufficiently lush to provide forage even in dry 

periods. Evidence of previous occupancy by prairie dogs 

increases a site’s suitability rating.

Treatment of release sites for Utah prairie dogs has 

primarily involved removal of tall, dense vegetation and 

augering of artificial burrows. Player and Urness (1982) 

demonstrated the benefits of shrub removal to postrelease 

survival; removal of plants that obstruct horizontal visibility 

has become standard practice (McDonald, 1993). Augered 

holes 9–15 cm in diameter and 0.5–1.0 m deep at angles into 

the ground provide relief from temperature extremes and 

some level of protection from predators (Player and Urness, 

1982; Jacquart and others, 1986; McDonald, 1993). Covering 

entrances of augered holes with wire-mesh retention baskets 

to temporarily restrain the prairie dogs and acclimate them to 

the site (Player and Urness, 1982; Jacquart and others, 1986) 

appears to improve postrelease survival (McDonald, 1993).

As with black-tailed prairie dogs, mammalian predators, particularly badgers, apparently have caused the greatest 

losses in translocated Utah prairie dogs (Jacquart and others, 

1986; Coffeen and Pederson, 1993; McDonald, 1993). Badger 

damage has been greatest during the first year or two following release, before the prairie dogs have excavated many 

secure burrow systems (Jacquart and others, 1986). In comparison, black-tailed prairie dogs usually seem secure from 

extensive badger depredation after several months (see above). 

Postrelease monitoring for predators and selective control of 

badgers are commonly used to protect Utah prairie dogs at 

release sites (Jacquart and others, 1986; Coffeen and Pederson, 

1993). Even so, loss of released animals to badger predation 

remains a major problem (McDonald, 1993; D. Biggins, written commun., 2003). 

Vegetation Management

For several reasons we address primarily black-tailed 

prairie dogs in this section. This species has a larger historical 

range that has been proportionately more degraded by agriculture and vegetation change than is the case with white-tailed 

and Gunnison’s prairie dogs (Knowles, 2002). Absence of 

plague in substantial portions of black-tailed range, coupled 

with greater average densities of the species, increases the 

unit-area benefits of habitat restoration. Further, more information exists about habitat restoration and management for 

black-tailed than for white-tailed or Gunnison’s prairie dogs, 

although the scarcity of information on the latter can be partly 

offset by the relatively rich database for the Utah prairie dog.

Prairie dogs respond markedly to habitat structure—soil 

texture, slope, and particularly vegetation height and density 

(Slobodchikoff and others, 1988; Reading and Matchett, 1997; 

Truett and others, 2001a). Short vegetation benefits all three 

species (Longhurst, 1944; Knowles, 1982; Slobodchikoff 

and others, 1988), presumably because it facilitates visual 

detection of approaching predators. Black-tailed prairie dogs 

seem more adversely affected by tall, thick vegetation than 

do Gunnison’s or white-tailed prairie dogs (Scheffer, 1947; 

Hoogland, 1981; Hubbard and Schmitt, 1984). This effect 

may be a consequence in part of interspecific differences in 

predator avoidance behavior (Hoogland, 1981). Detection 

of predators by visual cues and intraspecific warning calls 

seem more highly developed in black-tailed prairie dogs, as 

does clipping of vegetation to improve visibility (Tileston and 

Lechleitner, 1966; Hoogland, 1996). These characteristics of 

this species may be evolutionary adaptations to exploit heavily 

grazed landscapes (Truett, 2003).

Many have noted the positive response of black-tailed 

prairie dogs to intensive grazing by large herbivores. Osborn 

and Allan (1949), Snell and Hlavachick (1980), Knowles 

(1982, 1986), and Cable and Timm (1988) documented expansion of colonies with heavy grazing and their stabilization 

or shrinkage without grazing in areas supporting mid-height 

or tall grasses. Truett and others (2001b) and Truett (2003) 

Habitat Restoration and Management 101
discussed historical fluctuations in abundance of black-tailed 

prairie dogs in Great Plains grasslands as a function of changing abundance of large grazers. Other ways of keeping the 

vegetation short, such as burning or mowing, can substitute for 

grazing (Ford and others, in press). 

Only in shortgrass steppe, which occupies a relatively 

small part of their historical range (compare fig. 1 with fig. 2), 

do black-tailed prairie dogs seem relatively free of the need 

for large grazers (D. Long, unpub. data, 2004). In mixed-grass 

and tallgrass prairie, sustained absence of grazing (Osborn and 

Allan, 1949; Knowles, 1982), or simply grazing deferment 

during the growing season (Snell and Hlavachick, 1980; Snell, 

1985), can within a few years or decades exclude black-tailed 

prairie dogs. This may hold true as well in many historically 

occupied sites in Chihuahuan Desert grasslands (Truett and 

Savage, 1998; J. Truett, unpub. data, 2004). 

White-tailed, Gunnison’s, and Utah prairie dogs tolerate tall, dense vegetation better than do black-tailed prairie 

dogs. Hoogland (1981) noted the relatively large numbers of 

shrubs in white-tailed prairie dog colonies (compared with 

black-tailed colonies) and thought they might serve as protective cover. Taylor and Loftfield (1924) and Longhurst (1944) 

noted the tolerance of Gunnison’s prairie dogs for tall grasses 

and shrubs in their colonies. Collier and Spillett (1975) and 

Coffeen and Pederson (1993) indicated that Utah prairie dogs 

often coexist with, and may benefit from, shrubs.

Still, habitat quality for these species often appears to 

decline with increasing shrub density beyond some point. 

Longhurst (1944) described increasing density of Gunnison’s 

prairie dogs with decreasing shrub density and increasing 

visibility. Collier and Spillett (1975) and the U.S. Fish and 

Wildlife Service (1991) attributed declines of Utah prairie 

dogs partly to historical increases in shrub density. As with 

black-tailed prairie dogs, these species may continue to face 

declining habitat quality unless tall vegetation (shrubs in this 

case) can be controlled. The federally threatened status of the 

Utah prairie dog has prompted attempts at habitat rehabilitation by “chopping” (Coffeen and Pederson, 1993), “rotobeating,” “railing,” and burning (Player and Urness, 1982) 

shrubs. Similar efforts to improve habitat for white-tailed and 

Gunnison’s prairie dogs have not been reported.

Socioeconomic Challenges

Aside from plague, the greatest impediment to prairie 

dog restoration may be hostile traditions among rangeland 

owners and managers. The historical demise of prairie dogs 

resulted in large part from control programs aimed at removing 

a presumed competitor with livestock (Merriam, 1902; Mulhern 

and Knowles, 1997). Perceptions molded by a century of institutionalized control of prairie dogs (Reading and others, 1999) 

will be difficult to reverse. To exacerbate the dilemma, livestock 

production on rangelands has long built on the tradition of 

moderate grazing uniformly distributed (Fuhlendorf and Engle, 

2001), which, especially in mixed-grass and tallgrass prairie, 

militates against rapid restoration (Truett, 2003).

At a recent symposium on black-tailed prairie dogs, a 

Colorado rancher was asked why ranchers dislike prairie dogs. 

In response, he largely dismissed the risk of cattle breaking 

their legs in burrow entrances but pointed to the loss of forage 

that could reduce profits. Then, after some hesitation, he 

offered another important insight—prairie dog colonies simply 

look bad. Who wants to see his land blighted by the disturbed 

soil and rodent activity characteristic of prairie dog colonies? 

In word and gesture he portrayed prairie dogs as symbols 

of neglect, pariahs of the range, their presence a sign of lax 

stewardship comparable to an untidy house at Sunday dinner.

Independent of prairie dog control, grazing at light 

to moderate intensities has come to symbolize good land 

stewardship among range managers. To many, heavy grazing 

equates with “overgrazing” and unwise use. This perception 

took root in the early 1900s with Clements’ (1916, 1936) 

model of “proper” grazing as that which maintained grasslands near climax condition (i.e., dominated by the tallest of 

the species at a given site). Historical evidence indicates that 

black-tailed prairie dogs thrived over the moister parts of their 

original range because of heavy grazing, first by bison (Bison 

bison) and then by cattle (Truett, 2003). Unfortunately for 

Figure 2. Collection locations for black-footed ferrets (Mustela 

nigripes) (Anderson and others, 1986) and distribution of Great 

Plains grassland types (Lauenroth and others, 1999). Each collection location (dark triangle) represents >1 verified historical 

record(s).

102 Recovery of the Black-Footed Ferret
ferret restoration, the relatively moist and plague-free areas 

in the Great Plains that can support the greatest densities of 

prairie dogs need the heaviest grazing. Thus, black-tailed 

prairie dog restoration is squeezed between plague risks from 

the west and “good” range management from the east.

Managers’ preferences for tall grass compromise another 

potentially fruitful avenue for prairie dog habitat restoration—reclamation of abandoned farmland (discussed later). 

The traditional maxim that tall grass is better grass leads 

most managers to recommend and use seed mixes containing 

largely tall or mid-height grass species for reclaiming lands 

such as those under the Conservation Reserve Program (CRP) 

of the 1985 Food Security Act. 

In sum, those in the best position to restore prairie dogs 

on private and public lands usually lack the motivation to do 

so. They often come from rural backgrounds, which predisposes them to dislike prairie dogs (Reading and others, 1999). 

They subscribe to rural traditions that for generations have 

seen prairie dogs, and the range conditions associated with 

them, as economically and socially undesirable.

Given the entrenched nature of tradition, must changes 

in attitude await a new generation of managers with different cultural backgrounds? Perhaps not. For one thing, recent 

paradigm changes among professionals about what constitutes 

good conditions on rangelands (discussed later) may legitimize heavy grazing for conservation purposes (Task Group on 

Unity in Concepts and Terminology, 1995). A more immediate 

hope builds around incentives, particularly economic ones. 

Money has a history of reshaping tradition.

Incentives

Landowners, land managers, and agencies that set land 

management policy potentially can be motivated to restore 

prairie dogs through at least three kinds of incentives. The 

most direct and immediately effective incentive is probably 

economic—money offered to induce change. Regulation or the 

threat thereof can be brought to bear through the Endangered 

Species Act (ESA) or other legal means but may generate 

resentment and thus delay response. Self-motivated cultural 

change through education is slower still but usually longer 

lasting. Long-term success in prairie dog restoration may 

require a combination of all three strategies.

Economic incentives can come from private or public 

sources, and we can attest to the effectiveness of both. Turner 

Enterprises, Inc., and the Turner Endangered Species Fund 

(TESF) have supported prairie dog restoration on private 

ranches since 1995. Funding from TESF enabled restoration 

of prairie dog populations on six ranches and also promoted 

the concept of prairie dog restoration through educational 

efforts: technical publications, presentations at symposia and 

meetings, support of university graduate student programs, 

and field tours to educate people from grade schoolers through 

governors. Recently TESF funding has been supplemented 

by matching funds from nongovernment organizations (e.g., 

National Fish and Wildlife Foundation) and Federal agencies 

(e.g., U.S. Fish and Wildlife Service Private Stewardship 

Grants Program, or PSGP). The PSGP awarded grants for 

prairie dog restoration to other private landowners as well. 

In 2005, TESF received additional support through the new 

federally funded State Wildlife Grants Program as matching 

funds to assist with prairie dog restoration in South Dakota.

The U.S. Fish and Wildlife Service (2000) recently 

determined that the black-tailed prairie dog was warranted 

for listing as threatened under the ESA, listing being temporarily precluded by higher priority actions. This finding 

stimulated the States included in the species’ historical range 

to collaborate on a conservation strategy (Luce and others, 

2001). This strategy has involved a variety of actions including 

periodic meetings, interagency memoranda of understanding, and agreements on implementation schedules. Fear that 

management of the species would be assumed by the Federal 

government motivated this collaboration. The States organized 

working groups dedicated in part to planning and carrying 

out restoration actions, and many have completed population 

estimates and status assessments as a first step toward conservation (Luce and others, 2001). It is too early to assess the 

extent to which restoration on the ground will result from this 

action by the Federal government.

Over the longer term, the success of prairie dog and ferret 

restoration will rely on cultural acceptance of these species as 

valuable and appropriate components of grassland ecosystems. 

Private charities, Federal grants, and even government regulations that promote restoration all arose from cultural beliefs 

that more of nature should be preserved than just the parts 

generating income. All of these sources of support can disappear without consistent reinforcement of such beliefs. Maintenance of culture-based incentives will require a continuing 

effort to educate people about the intangible benefits of prairie 

dogs and other species that have little immediate economic 

worth. The most enduring incentives are likely to come 

through intergenerational transmission of values beyond 

money.

Case Histories

For several years the TESF has been restoring blacktailed prairie dogs on private ranches with the intent of eventually releasing ferrets into the habitat developed. Here we 

summarize restoration and management efforts on two of these 

ranches—Vermejo Park Ranch (Vermejo) in shortgrass prairie 

southwest of Raton, N. Mex., and the Bad River Ranches (Bad 

River) in mixed-grass prairie west of Pierre, S. Dak. Bison 

graze both ranches at generally moderate intensities.

Translocations to establish new colonies and protection 

of prairie dogs from poisoning and shooting have been key to 

restoration on both ranches. Most releases used source stock 

from within the respective ranches. Translocation methods 

followed Long and others (in press). Translocated animals 

were held for several days prior to release in acclimation cages 

Habitat Restoration and Management 103
at the release site; these cages had artificial underground nest 

chambers that prairie dogs continued to use after release while 

they excavated new burrows nearby. Predator control focused 

primarily on coyotes (both ranches) and badgers (Vermejo) 

during and for a few months following the translocation 

period. Major field efforts took place during May–October, 

involving one person on each ranch, with temporary help from 

another person for 2–3 months during June–August.

Vermejo

Annual monitoring of colony numbers and sizes 

commenced in 1997. Translocations began in 1999, and from 

then until 2003 we established 35 new colonies. Two colonies 

or fewer originated naturally during the 6-year period 1997–

2003. Forty-six colonies currently exist, a few formed by the 

merging of two colonies that were originally separated.

Total area occupied by colonies increased from 202 ha in 

1997 to 980 ha in 2003, expanding an average of 31 percent 

annually (mean of yearly values). Growth rate varied appreciably among colonies, mostly as a function of colony size. 

Colonies expanded an average of 12 percent per year during 

1998–99 when a few large colonies predominated, but expansion increased to an average of 41 percent per year during 

2000–03, during which time many small, new colonies were 

established by translocation. 

The short-statured vegetation never seemed to offer much 

of an impediment to colony growth. Colony growth during 

1999, when precipitation and vegetative growth substantially 

exceeded average, did not differ from that in 1998, when less 

rain fell. A major drought in 2001 and 2002 (21.8 cm and 

23.9 cm, respectively, of precipitation compared with approximately 36.8 cm annual average) greatly reduced vegetative 

growth and recruitment of young into the prairie dog population but seemed not to influence areal expansion rate of 

colonies.

Bad River

Annual monitoring of colony numbers and sizes began 

in 1999, at which time 35 colonies existed. Translocations 

began in 2000, and from then until 2003 we established 35 

new colonies. Eleven new colonies originated naturally during 

1999–2003, mostly during a drought year (2002), and six 

disappeared during a wet year (2001). Seventy-eight colonies, 

a few having been formed by the merging of two original 

colonies, existed by late 2003.

Total colony area increased from 271 ha in 1999 to 

584 ha in 2003; the average annual increase (mean of yearly 

values) was 25 percent. Smaller colonies grew faster than 

larger ones, but the greatest influence on colony growth 

resulted not from colony size but from grass height and 

density as a function of precipitation. In 2001, when rainfall 

and vegetative growth peaked, total colony area shrank 12 

percent; in the drought year of 2002 colony area increased 72 

percent.

Grazing by bison during years of average or above-average precipitation strongly influenced colony expansion. Heavily grazed colonies in these circumstances expanded at much 

greater rates than did colonies grazed lightly or not at all. 

Successful establishment of new colonies in wet years in the 

absence of grazing required us to mow release sites in summer, 

sometimes repeatedly, to enhance visibility and postrelease 

survival. Colonies in an area intensively managed—by establishment of new colonies, grazing at moderate intensities, and 

mowing as needed—grew 78 percent during the 2-year period 

that they were managed. Colonies outside this area grew by 29 

percent during the same period.

Comparisons and Implications

Colony area in the shortgrass prairie at Vermejo expanded 

faster on average than that in the mixed-grass prairie at Bad 

River, and growth rate varied less among years at Vermejo. 

Our data suggest, however, that the potential average growth at 

Bad River with intensive grazing or drought may be substantially greater than that at Vermejo. This higher growth rate, 

coupled with the nearly threefold greater density of prairie 

dogs at Bad River (D. Long and K. Bly-Honness, unpub. 

data, 2004), illustrates the great potential that exists for ferret 

habitat restoration in taller grass regions of the Great Plains. 

Even so, it may be difficult to maximize this potential without 

changes in grazing management philosophy, which we discuss 

below.

Changing Paradigms, New Opportunities

Habitat scarcity seems a looming bottleneck in ferret 

restoration. The shortage of large prairie dog complexes 

suitable for ferret release coupled with the increase in ferrets 

annually available for release suggests a need to evaluate the 

use of smaller complexes. At the same time, changing philosophies and economics related to the major land uses in ferret 

range (i.e., grazing and farming) may open new avenues for 

habitat restoration and management. Below we assess some of 

the opportunities presented by these changes.

Minimum Size of Prairie Dog Complexes

Clearly, other factors being equal, larger complexes of 

prairie dog colonies offer better ferret habitat than do smaller 

ones. Although a high-density colony of black-tailed prairie 

dogs as small as 10 ha can in theory (Biggins and others, 

1993) and in fact (Hillman and others, 1979) support a family 

of ferrets in the short term, Biggins and others (1993) recommended a minimum 400-ha colony area to sustain a ferret 

population. The Conservation Breeding Specialist Group 

104 Recovery of the Black-Footed Ferret
(2004) estimated that 2,440 ha of high-quality habitat (i.e., 

black-tailed prairie dog colonies in Conata Basin, S. Dak.) 

would be needed to support 120 breeding adult ferrets with 

more than 90 percent probability of persistence over 100 

years. Moreover, they recommended development of 4,050-ha 

complexes to achieve ferret recovery objectives.

Given the current scarcity of large complexes secure from 

poisoning and plague, however, the Conservation Breeding 

Specialist Group (2004) also recommended investigating ways 

to enhance ferret recovery by using small (less than 2,000 ha) 

complexes. Use of smaller sites could attract collaborators 

(e.g., States and private landowners) excluded by large minimum-area requirements and quickly open up options spanning 

the entire historical ferret range. Literally and metaphorically, 

it could plant the seeds needed to ultimately establish larger 

complexes of prairie dog colonies and the widest possible 

distribution of ferrets. 

Probabilities of extinction rise as ferret population 

size declines; thus, maintenance of ferrets in small colony 

complexes might necessitate periodic reintroductions from 

elsewhere. Still, this inconvenience might be trivial given the 

possible rewards—attracting wider public and private support, 

supplying wild-reared kits for release elsewhere, hosting 

research to better inform a variety of restoration schemes, and 

maintaining numerous wild populations as a hedge against 

regional catastrophe. Furthermore, finding ways to use small 

complexes could ultimately lead to shifts in grazing and farming philosophies to benefit ferret recovery.

New Directions in Grazing: Beyond Clements’ 

Climax

Recently, members of the Task Group on Unity in 

Concepts and Terminology (1995) of the Society for Range 

Management laid to rest the conventional notion that grazing 

according to Clements (1916, 1936) (i.e., maintenance of grass 

communities near climax) is the sole gospel of good range 

management. They envisioned an array of potentially “good” 

grazing management options depending on management goals. 

In so doing, they legitimized such previously objectionable 

ideas as intensive grazing in areas of mixed-grass and tallgrass 

climax to benefit shortgrass species. In our view this change in 

perspective opened the door conceptually for extending prairie 

dog and ferret recovery efforts farther eastward into plaguefree terrain.

Most ferret records for the Great Plains came from 

regions where prairie dog populations depended to some 

extent on grazing; that is, regions dominated by mixed or tall 

grasses (fig. 2) Historical accounts suggest that grazing by 

bison, before their demise in the 19th century, facilitated occupancy of these regions by prairie dogs and ferrets; the need for 

intensive and frequent grazing increased with distance eastward (reviewed by Truett, 2003). Bison had been eliminated 

in most Great Plains areas well before most ferret collections 

were made (cf. Anderson and others, 1986; Isenberg, 2000). 

Prior to bison extirpation, ferrets not only might have been 

more abundant in eastern portions of their range than numbers 

collected indicate, but also might have ranged farther east than 

ecologists have assumed.

Can intensive grazing (by livestock) be reinstated in these 

eastern, plague-free areas to pave the way for prairie dogs 

and ferrets? The historical rebound of prairie dogs in some 

of these areas following entry of cattle in very large numbers 

in the last decade or two of the 19th century (Merriam, 1902; 

Truett, 2003) suggests so. Several key management questions 

surround such a concept.

1. How far east can prairie dogs potentially thrive? 

Collection records (Hall, 1981) suggest that prairie 

dogs historically were common farther east than they 

generally occur now except under anomalous circumstances (e.g., predator-unfriendly sites such as remnant 

corners of pivot-irrigated fields or human settlements; 

Sidle and others, 2001; Truett, 2003). Some colonies 

established by people in high-rainfall areas east of 

historical range—for example, Nantucket Island off 

the coast of Massachusetts (Merriam, 1902) and a site 

east of Fort Worth, Tex. (Schmidly, 1983)—apparently 

have thrived. In the relatively cool and moist climate 

of the late Pleistocene, black-tailed prairie dog range 

extended substantially east of its historical limits 

(Goodwin, 1995), possibly because of heavy grazing 

by the numerous megaherbivores of the time (Truett, 

2003). The key to prairie dog survival eastward to the 

limits of historical range and beyond may simply be 

short grass.

2. What vegetative changes come with the intensive grazing associated with prairie dog occupancy of mixedgrass and tallgrass sites? Mid-height and tallgrass 

species decline in dominance, often dramatically, and 

perennial shortgrasses and annuals increase (Detling, 

1998; Truett and others, 2001b). Given availability of 

propagules, shortgrass species such as buffalograss 

(Buchloe dactyloides), blue grama (Bouteloua gracilis), and tumblegrass (Schedonnardus paniculatus) 

increase and often persist in dominance (Archer and 

others, 1987; Weltzin and others, 1997). Net primary 

productivity (indicative of forage quantity annually 

available) typically declines over time, but forage quality increases. Heavy grazing by livestock outside colonies causes similar but usually less dramatic changes 

(reviewed by Truett and others, 2001b).

3. Would these changes reduce profits from ranching 

operations? The many variables involved preclude a 

detailed response, but the short answer is sometimes 

yes and sometimes no (Detling, 1998; this volume). 

Prime among the important variables is the proportion 

Habitat Restoration and Management 105
of the landscape occupied by prairie dog colonies. 

Livestock profits may decline if prairie dog occupancy 

level is high but may increase if occupancy level is 

low. For example, Vanderhye (1985) projected substantial benefits to bison at a site in South Dakota where 

prairie dog colonies occupied only 12 percent of the 

landscape. Moreover, heavy grazing by cattle to benefit 

prairie dogs may under some conditions yield greater 

sustainable profits than would more conventional 

grazing intensities (Manley and others, 1997; Sims and 

Gillen, 1999). 

Reclaiming Retired Farmland

Large proportions of the plague-free part of the Great 

Plains have been converted to agriculture; these proportions 

generally increase with distance eastward and southeastward 

(Lauenroth and others, 1999). Retirement of farm acreages 

under programs such as the CRP may offer the potential for 

prairie dog restoration. Could prairie dogs reoccupy retired 

farmlands? If so, how should reclamation of such lands 

proceed?

Black-tailed prairie dogs readily colonize abandoned 

farmland, often in preference to undisturbed prairie. In 

Montana, Knowles (1982) found that colonies were disproportionately abundant on previously cultivated lands near 

abandoned homesteads. In Colorado, Koford (1958) observed 

that prairie dogs near Fort Collins readily invaded fields 

under cultivation, and D. Seery (oral commun., 2002) noted 

that many prairie dog colonies on Rocky Mountain Arsenal 

National Wildlife Refuge, Colo., occupied long-abandoned 

fields. In Badlands National Park, S. Dak., Langer (1998) 

found more and larger prairie dog colonies on long-abandoned 

farmland than on undisturbed prairie. We observed that prairie 

dogs near Pierre, S. Dak., quickly invaded land last plowed the 

previous year.

As expected, cultivated land with tall vegetation repels 

prairie dogs; land with short or very sparse vegetation attracts 

them (Koford, 1958). Retired farmland reclaimed with perennial shortgrasses should sustain prairie dogs and, in some 

circumstances, limit erosion better than if tallgrasses were 

used in reclamation (see Truett, 2003), the latter a prime goal 

of the CRP. Mid-height and tall species of grass usually dominate CRP seed mixes (Reynolds and others, 1994; Johnson 

and Igl, 1995; Patterson and Best, 1996), however, rendering 

fields reclaimed with such mixes unsuitable for prairie dogs 

and other shortgrass fauna (e.g., see Kamler and others, 2003). 

Retired farmlands seem lucrative targets for prairie dog restoration, but seed mixes dominated by shortgrass species would 

be needed, particularly under programs such as CRP that limit 

grazing on lands enrolled in the program.

Acknowledgments

We thank the Turner family, particularly R.E. Turner, for 

their continuing interest in and financial support of prairie dog 

restoration on Turner-owned ranches and for their commitment 

to dissemination and publication of scientific information. 

Turner ranch managers T. Waddell, D. Vackar, T. LeFaive, S. 

Dobrott, K. Yearout, and J. Hansen assisted with prairie dog 

restoration efforts. We thank A. Ernst for digitizing maps of 

prairie dog, ferret, and grassland type distributions. 

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Habitat Restoration and Management 109

Abstract

Intensive grazing by black-tailed prairie dogs (Cynomys ludovicianus) typically reduces graminoid biomass and 

enhances production and standing crop of less desirable forage 

species; however, the quality of remaining graminoids is often 

increased because of higher crude protein concentrations and 

higher digestibility. Increased forage quality may partially 

account for why some large grazers such as bison (Bison 

bison), pronghorn (Antilocapra americana), and possibly 

cattle (Bos taurus) are attracted to prairie dog colonies as 

preferred sites to graze. In relatively productive grasslands, 

grazing and disturbance of tall vegetation by large herbivores 

apparently allow prairie dogs to expand into areas they might 

not otherwise occupy. These interactions between prairie dogs 

and large herbivores do not appear to be as strong in the more 

arid, less productive shortgrass steppe as in the mixed-grass 

prairie.

Keywords: bison, cattle, Cynomys ludovicianus, forage 

quality, grazing, plant-animal interactions, pronghorn, trophic 

interactions

Introduction

At the time that European settlers first migrated westward across North America, prairie dogs (Cynomys spp.) 

occupied vast areas of the Great Plains grasslands. For 

example, Merriam (1902, p. 258) described a single colony 

that occupied an area of nearly 65,000 km2 and contained, 

by his estimate, 400 million prairie dogs. Much of the area 

originally inhabited by black-tailed prairie dogs (C. ludovicianus) is within the shortgrass and mixed-grass prairies, but 

they also occupy parts of desert grasslands and shrublands in 

southern New Mexico and northern Mexico (Hoogland, 1995; 

Detling, 2006). A large portion of their historical range is now 

either livestock grazing land or cultivated cropland. Because 

they can consume or destroy large quantities of forage by 

clipping, widespread eradication campaigns were mounted 

during the 20th century to eliminate prairie dogs from much 

of their original habitat. These campaigns, together with 

habitat loss and the introduction of bubonic plague into much 

of their former range, has resulted in an estimated 98 percent 

reduction in their populations from a century earlier (Miller 

and others, 1990, 1994). We now know that prairie dogs are 

important in the maintenance of grassland species diversity 

and are essential for survival of black-footed ferrets (Mustela 

nigripes) in the wild. Therefore, a number of ecologists and 

conservation biologists recently have argued for elimination of 

these eradication campaigns (Miller and others, 1990, 1994; 

Wuerthner, 1997; Kotliar and others, 1999), which has in turn 

raised concerns among land managers about how rapidly prairie dog populations might grow, what their effects on grassland 

vegetation might be, and how this might affect livestock or 

populations of native grazers.

This paper reviews extant literature pertaining to the 

above issues with respect to black-tailed prairie dogs. Specifically, I address three questions: (1) How does grazing by 

prairie dogs affect grassland vegetation? (2) What effects 

might these changes have on other herbivores? (3) How might 

grazing by other herbivores affect expansion of prairie dog 

colonies? Knowing the answers to such questions will enable 

us to better understand the nature of the habitat used by prairie 

dogs and associated species such as black-footed ferrets and 

will assist land managers in assessing some of the consequences of managing for increased area of prairie dog habitat.

Effects of Prairie Dogs on Vegetation

Vegetation Characteristics and Prairie Dog 

Diets

Most native shortgrass and mixed-grass prairies are 

dominated by perennial grasses and other graminoids, which 

may compose as much as 90 percent of the aboveground 

biomass (Coupland, 1992; Lauenroth and Milchunas, 1992). 

Although they typically make up a relatively low proportion of 

the biomass, a diverse group of forbs (i.e., herbaceous dicots) 

and woody sub-shrubs contribute substantially to overall plant 

species diversity in most Great Plains grasslands (Sims and 

others, 1978).

Black-tailed Prairie Dog Interactions with Other 

Herbivores: Mediation via Alterations of Vegetation

By James K. Detling1

1

Colorado State University, Department of Biology and Natural Resource 

Ecology Laboratory, Fort Collins, CO 80523.

Black-tailed Prairie Dog Interactions with Other Herbivores 111
Prairie dog diets consist largely of native graminoids, and 

many of the species they consume also compose most of the 

diets of native and domesticated ungulates (Detling, 2006). 

For example, on the shortgrass steppe of Colorado (Hansen 

and Gold, 1977) and the mixed-grass prairie of South Dakota 

(Uresk, 1984), relatively high-quality forage species such 

as blue grama (Bouteloua gracilis), needleleaf sedge (Carex 

eleocharis), western wheatgrass (Pascopyrum smithii), and 

sand dropseed (Sporobolus cryptandrus), all native perennial 

graminoids, made up about 85 percent of prairie dog diets. 

In contrast, forbs such as scarlet globemallow (Sphaeralcea 

coccinea) and a sub-shrub, fringed sagewort (Artemisia 

frigida), accounted for the other 15 percent. In addition to 

consuming vegetation, prairie dogs frequently clip and fell 

taller vegetation, apparently to enhance predator detection 

(Hoogland, 1995).

Changes in Plant Cover, Biomass, and Species 

Composition

As a result of their grazing and clipping behaviors, one 

of the most striking visual effects that prairie dogs have is a 

reduction in height of vegetation on their colonies. Where 

vegetation in uncolonized areas is relatively tall, the visual 

contrast between colonies and adjacent, uncolonized areas 

may be substantial. For example, at several mixed-grass prairie 

sites in South Dakota, vegetation adjacent to prairie dog 

colonies averaged about 25 cm tall while that on the colonies 

averaged <10 cm (Archer and others, 1987; Whicker and 

Detling, 1988a; Russell and Detling, 2003). Another common 

trend following colonization is a reduction in the amount of 

standing dead plant biomass relative to live biomass (Coppock 

and others, 1983a; Detling, 1998). Consequently, prairie dog 

colonies often appear “greener” than surrounding uncolonized 

grassland, reminiscent of classical “grazing lawns” (sensu 

McNaughton, 1984). In drier grasslands with shorter vegetation, such as the shortgrass steppe of eastern Colorado and 

northern New Mexico, differences in height of vegetation 

on and off colonies are much less dramatic (Guenther and 

Detling, 2003), and these colonies may not look greatly different from uncolonized grasslands.

As prairie dog colonies in the northern mixed-grass 

prairie age following initial colonization of a site, perennial 

graminoids become weakened by repeated leaf removal and 

the consequent reduction in their photosynthetic capacity. 

Not only is shoot biomass of graminoids reduced (Coppock 

and others, 1983a), but root biomass also declines markedly, 

particularly in older prairie dog colonies (Ingham and Detling, 

1984; Whicker and Detling, 1988b; Detling, 1998). Consequently, over time graminoids lose their competitive dominance and are replaced by forbs and sub-shrubs (Coppock and 

others, 1983a; Archer and others, 1987). In Badlands National 

Park, S. Dak., for example, 7 of the 10 most abundant species 

sampled off prairie dog colonies were graminoids, while 8 or 

more of the most abundant species on old colonies were forbs 

(Fahnestock and Detling, 2002). Thus, prairie dog colonies 

may consist of a variety of vegetation types. Younger parts of 

colonies are grass dominated and have species composition 

similar to uncolonized sites but lower biomass and cover. The 

oldest, most altered parts of colonies are forb dominated and 

often have little or no graminoid cover or biomass. In addition, as colonies age, the proportion of bare ground tends to 

increase (Whicker and Detling, 1988b; Russell and Detling, 

2003).

Less has been written about vegetation changes following 

colonization of southern mixed-grass prairie sites. At two >50-

year-old colonies in Texas, biomass was only one-third to onefourth as great on colonies as on uncolonized sites because 

mid-height grasses had been nearly eliminated (Weltzin and 

others, 1997a,b). In contrast to northern mixed-grass prairie 

sites, forb biomass was greater off colonies than on colonies, 

and biomass of short grasses did not differ significantly on and 

off colonies.

Available evidence suggests that vegetation is less altered 

by prairie dogs on the semiarid shortgrass steppe than in 

mixed-grass prairies. Nevertheless, the general trends appear 

to be in the same direction as in mixed-grass prairies. For 

example, in a shortgrass steppe in north-central Colorado, forb 

cover was greater (5.7 percent) on a colony than off it (3.4 

percent), while cover of the dominant grass, blue grama, was 

lower on the colony (12.2 percent) than off it (19.2 percent) 

averaged over the growing season (Bonham and Lerwick, 

1976). Similarly, Winter and others (2002) reported relatively 

small differences in vegetation structure and species composition on and off prairie dog colonies in shortgrass steppe in 

southwestern Kansas and southeastern Colorado. Moreover, 

bare ground was not significantly greater on colonies than off 

colonies at the Central Plains Experimental Range in northern 

Colorado (Guenther and Detling, 2003). These patterns of 

relatively small effects of prairie dogs on shortgrass steppe 

vegetation are consistent with the notion that this ecosystem 

has a long evolutionary history of grazing and is very resistant 

to heavy grazing (Milchunas and others, 1988), perhaps, in 

part, a result of the widespread dominance of grazing-resistant 

blue grama in this grassland type (Lauenroth and Milchunas, 

1992).

Changes in Forage Quality

In addition to vegetation changes discussed above, 

grazing by prairie dogs may alter the phytochemistry and 

forage quality of plants. One such change involves nitrogen 

(or crude protein) concentration. In the northern mixed-grass 

prairie, mean live shoot [N] was 1.3 percent (crude protein = 

8.1 percent) in six graminoid species off a prairie dog colony 

and 1.6 percent (crude protein = 10.0 percent) in the same six 

species on the colony when averaged over a growing season 

(Coppock and others, 1983a). Similar trends were observed 

by Krueger (1986). Moreover, the digestibility of graminoids 

was greater on prairie dog colonies than on uncolonized areas 

112 Recovery of the Black-footed Ferret
Black-tailed Prairie Dog Interactions with Other Herbivores 113

adjacent to them (Coppock and others, 1983a). Vanderhye’s 

(1985) model results, reviewed by Detling (2006), suggested 

that changes in forage quality of the magnitude observed on 

these prairie dog colonies could significantly enhance weight 

gain of bison (Bison bison). Thus, heavy grazing by prairie 

dogs apparently results in a tradeoff: the quantity of forage 

species preferred by large grazers declines, but the forage 

quality of those species is enhanced. Determining the consequences of this quantity-quality tradeoff for cattle (Bos taurus) 

or other large grazers is complicated because the magnitude 

of the tradeoff likely depends on a variety of factors. These 

include the type of grassland, length of time a site was inhabited by prairie dogs, past and current management practices, 

weather conditions, and others. Nevertheless, some simple 

calculations based on data from a mixed-grass prairie site 

(Coppock and others, 1983a) might illustrate the approximate 

magnitude of this tradeoff.

Pringle Valley occupies 120 ha in Wind Cave National 

Park, S. Dak., and at the time of the study, prairie dogs 

occupied 36 ha (30 percent) of this valley (table 1). Coppock 

and others (1983a) recognized three zones within the colony 

based on length of time colonized: old colony (occupied >26 

years), young colony (3–8 years), and edge of colony (<2 

years). Since graminoids compose the majority of forage 

used by livestock, I calculated the effects prairie dogs had on 

mean growing season biomass, crude protein, and digestible 

dry matter of graminoids in the valley. These attributes of 

the forage differed as a function of time colonized (table 1). 

For example, at the colony edge, mean graminoid biomass 

per unit area was only 28 percent lower than on adjacent 

off-colony sites, while on the oldest part of the colony it was 

98 percent lower. Because of the higher leaf [N] in plants on 

colonies (Coppock and others, 1983a), however, the mass of 

crude protein per unit area was only 12 percent lower at the 

colony edge (compared to 28 percent lower biomass) than at 

off-colony sites. Similarly, prairie dog-induced reductions in 

mass of crude protein in other zones of the colony were not 

proportionately as great as reductions in graminoid biomass, 

although they were greater than at the colony edge (table 1). 

Similar trends occurred for mass of digestible dry matter per 

unit area, but the magnitude of the compensatory effect was 

not as great (i.e., reductions in digestible dry matter more 

closely matched reductions in graminoid biomass) as it was 

for crude protein (table 1).

The quantity-quality tradeoff also can be illustrated by 

estimating the total reductions in mean biomass, mass of 

crude protein, and mass of digestible dry matter attributable 

to prairie dogs in the entire valley, rather than on a unit area 

basis (table 1). These estimates were made by multiplying the 

mass per unit area by the area in each zone of the prairie dog 

town (table 1) and comparing the totals with the quantity that 

would have been present if prairie dogs were absent (assuming 

the same values on the colony as were present in uncolonized 

grassland). Although the prairie dog colony occupied 30 

percent of the area of Pringle Valley, seasonal mean graminoid biomass was only 17.5 percent lower in the valley with 

prairie dogs present than it would have been with no prairie 

dogs present, while masses of crude protein and digestible dry 

matter were 14.6 percent and 16.6 percent lower, respectively. 

Therefore, had this valley been a paddock on a ranch, available 

graminoid biomass would have been reduced by prairie dogs 

proportionately slightly more than available mass of crude 

protein or digestible dry matter. The difference in the proportional reductions in crude protein and digestible dry matter 

relative to reductions in biomass represents the approximate 

magnitude of the quantity-quality tradeoff. Thus, the compensatory effect of prairie dog grazing on forage quality was small 

compared to their effect on graminoid biomass. It should be 

stressed, however, that these reductions are greatest on the 

oldest part of the colony, which suggests that small, relatively 

young colonies, such as those in areas periodically killed by 

plague, may have only a small effect on carrying capacity for 

large grazing animals.

Responses of Other Herbivores to Prairie 

Dog-Induced Vegetation Changes

Native Herbivores

Some older literature suggests that large native herbivores such as bison and pronghorn (Antilocapra americana) 

may forage preferentially within prairie dog colonies (King, 

1955; Koford, 1958). Most of this early literature was based 

on anecdotal observations and was not well documented with 

data; however, several subsequent studies have confirmed that 

such a positive association between large native herbivores and 

prairie dogs may occur, at least under some conditions.

In northern mixed-grass prairie, Coppock and others 

(1983b) examined bison use of prairie dog colonies at two 

different scales in Wind Cave National Park: (1) parkwide 

use of colonies and (2) use of a single colony in the 120-ha 

Pringle Valley. The parkwide study involved driving a given 

route through the entire park three to four times per week from 

mid-May through mid-October and comparing the proportion 

of all bison observed that were on colonies to the proportion 

of landscape occupied by colonies (12 percent). Bison use 

of prairie dog colonies was greatest during midsummer and, 

when in the western portion of the park that contained most 

of the colonies, bison strongly preferred colonies. When their 

movement patterns took them to the east side of the park 

(which had fewer colonies), however, bison did not show a 

preference for prairie dog colonies. Thus, prairie dog colonies 

did not solely control bison herd movement in the park.

The Pringle Valley study involved mapping, from a 

nearby fire tower, the location of each bison that entered the 

valley (Coppock and others, 1983b). When in the valley, bison 

used the prairie dog colony preferentially over uncolonized 

portions of the valley. From June through mid-October, they 

used graminoid-dominated parts of the town two to three 
times as much as would be predicted by random utilization, 

and grazing was a predominant activity there. Following a fire 

in adjacent, uncolonized grassland, bison continued to use 

the prairie dog colony preferentially but also used the burned 

area preferentially over the remaining uncolonized, unburned 

portion of the valley (Coppock and Detling, 1986).

A subsequent study by Krueger (1986) at Wind Cave 

National Park confirmed and extended this research in several 

ways. First, in a parkwide study similar to that of Coppock 

and others (1983b), Krueger (1986) confirmed that bison 

used prairie dog colonies preferentially. From April through 

November, about 42 percent of all her bison observations 

were on prairie dog towns, which covered 12 percent of the 

sample area. Second, similar to results of Coppock and others 

(1983b), bison strongly preferred graminoid-dominated parts 

of colonies (96 percent of all observations) to forb-dwarf 

shrub-dominated areas (Krueger, 1986). Third, pronghorn also 

used prairie dog colonies (67 percent of all observations) more 

frequently than expected from random use (12 percent), and 

79 percent of all pronghorn Krueger observed on prairie dog 

colonies were in areas dominated by forbs and dwarf shrubs. 

Thus, while bison and pronghorn both used prairie dog colonies preferentially, they made use of different vegetation zones 

and plant resources within the colonies.

The patterns described above may be modified by 

precipitation or other weather that affects resources available 

to grazers. For example, Green (1998) found that bison at 

Wind Cave National Park used graminoid-dominated parts of 

prairie dog colonies in proportion to their availability during 

a year of below average precipitation and forage production; 

however, in the following year when precipitation and forage 

production were above normal, bison used these areas preferentially. During the intervening winter, bison avoided prairie 

dog colonies.

Livestock

Are cattle and other livestock attracted to prairie dog 

colonies as bison and pronghorn apparently often are? This 

topic has not been thoroughly researched, so the answer is 

not clear. On the shortgrass steppe in northern Colorado, 

cattle used prairie dog colonies approximately in proportion 

to their availability; that is, they neither preferred nor avoided 

them (Guenther and Detling, 2003). While on these colonies, 

however, cattle grazed as intensively as they did in grassland 

communities not colonized by prairie dogs. Because the shortgrass steppe is quite different from the mixed-grass prairie, it 

is not possible to say whether the lack of a preference for colonies by cattle was the result of differences in foraging behavior 

between cattle and bison or differences in large ungulate (e.g., 

bison and cattle) grazing behavior on shortgrass steppe versus 

mixed-grass prairie. The result may be more closely related to 

differences in grassland type, since a year of average precipitation on the shortgrass steppe is similar to a dry year such as 

that observed by Green (1998) on the mixed-grass prairie.

While this latter idea is somewhat speculative and not 

supported by data, it is supported by anecdotal observations. 

Off colony Edge of colony Young colony Old colony Total

% change by 

prairie dogs

Area occupied (ha) 84 12 15 9 120 -30

Mass per unit area (kg/ha)

 Live graminoids

 Crude protein

 Digestible dry matter

990

80

499

710 (-28%)

71 (-12%)

383 (-23%)

410 (-59%)

41 (-49%)

221 (-56%)

20 (-98%)

2 (-90%)

11 (-98%)

---

---

Mass in each zone (kg/zone)

 Live graminoids

 Prairie dogs present

 If prairie dogs were absent

 Crude protein

 Prairie dogs present

 If prairie dogs were absent

 Digestible dry matter

 Prairie dogs present

 If prairie dogs were absent

83,160

83,160

6,757

6,757

41,916

41,916

8,520

11,880

852

965

4,596

5,988

6,150

14,850

615

1,207

3,315

7,484

180

8,910

18

724

99

4,491

98,010

118,800

8,242

9,653

49,926

59,879

-17.5

-14.6

-16.6

Table 1. Effects of colonization by black-tailed prairie dogs (Cynomys leucurus) on seasonal mean mass of graminoids, crude protein in 

graminoids, and digestible graminoid dry matter in Pringle Valley, Wind Cave National Park, S. Dak. Values were calculated from data on 

graminoid biomass, nitrogen concentration, and digestibility measured by Coppock and others (1983a) from June 1 to October 1, 1979.

114 Recovery of the Black-footed Ferret
Black-tailed Prairie Dog Interactions with Other Herbivores 115

One such observation came in a letter addressed to me and 

postmarked March 30, 1998, from Mr. Francis Bardanouve, 

a former long-time member of the House of Representatives in Montana. Mr. Bardanouve was writing in response 

to statements attributed to me by Long (1998), in which I 

suggested that bison and pronghorn may graze preferentially 

on prairie dog colonies because of the higher quality forage 

there compared to uncolonized areas. Mr. Bardanouve, a selfdescribed lifelong rancher from an area of mixed-grass prairie 

in northern Montana, wrote:

I never really [saw] many [prairie dogs] until I began 

leasing lands on the Ft. Belknap reservation. In a 

few places it had towns [i.e., colonies] of several 

hundred acres...[Prairie dogs] cut everything off 

down to a height of almost less than an inch...There 

is no grazing left where they are.

However, I have had one mystery which I could 

never explain. I suddenly realized the answer in your 

statement. From time to time I would occasionally 

move cattle within the lease for some reason. What 

I could never explain was I would be moving them 

along fine without any trouble until I hit a prairie 

dog town. It never failed the movement of the bunch 

[of cattle] would come to a screeching halt. The 

bunch would begin grazing grass so short you could 

hardly see it and I could hardly get them moving. I 

would move one side of the bunch ahead a little and 

the rest would not move. I would then rush over and 

shove them ahead and the part that I had just pushed 

would quit moving. This slow zigzag movement 

would continue until we were off the “town site” and 

then the herd would take off at their normal pace.

Clearly, such anecdotal observations should not be taken 

as scientific evidence that cattle in mixed-grass prairie are 

attracted to prairie dog colonies as bison or pronghorn are; 

however, accounts such as these lend some credence to the 

idea and could perhaps be used as a justification for future 

studies to address this question.

Does Grazing by Other Herbivores Affect 

Expansion of Prairie Dog Colonies?

By the early 1900s, settlers and their livestock had 

moved into much of the Great Plains, and to some it was 

evident that prairie dog populations were increasing. C. Hart 

Merriam (1902, p. 263), the former chief of the U.S. Bureau 

of Biological Survey, noted that “prairie dogs are now more 

abundant than formerly and their colonies have overspread 

extensive areas previously unoccupied.” He attributed this 

increase to human-related factors, particularly (1) increasing 

the food supply for prairie dogs via cultivation of the soil and 

(2) decreasing the abundance of their natural enemies such 

as coyotes (Canis latrans), badgers (Taxidea taxus), hawks, 

owls, and snakes. Merriam (1902) recognized that prairie dogs 

caused substantial losses of forage and crops, but he apparently failed to consider that grazing and trampling of vegetation by settlers’ livestock might have contributed to the rapid 

expansion of prairie dog populations.

By the mid-20th century, a number of researchers were 

beginning to recognize that large grazers might be responsible 

for expansion of prairie dog colonies. At the Wichita Mountains 

Wildlife Refuge in southwestern Oklahoma, Osborn and Allan 

(1949) studied a prairie dog colony that had been designated 

for complete protection from poisoning. Following termination 

of all cattle grazing permits in 1937, only native ungulates and 

a small group of longhorns grazed the 24,000-ha refuge, and 

very few grazed in the vicinity of this colony. Over the next 

decade, the study colony completely disappeared. Prairie dogs 

were known to be a shortgrass plains species and had previously been observed to spread into surrounding vegetation 

following overgrazing. Therefore, Osborn and Allan (1949) 

concluded that their initial presence on this site, whose natural 

climax vegetation was dominated by tall grasses, resulted from 

heavy grazing during its earlier use as cattle range. Following 

removal of cattle, they reasoned, grass cover increased in stature 

and density, and the prairie dogs were restricted to smaller 

and smaller areas until they eventually died out. By contrast, 

other colonies at the refuge persisted in spite of at least limited 

poisoning, but these colonies received regular grazing by bison 

and other big game (Osborn and Allan, 1949).

The idea that prairie dogs could expand more readily into 

short vegetation was supported by King’s (1955) observations 

in the mixed-grass prairie of Wind Cave National Park. There, 

he observed that prairie dogs “invaded” a limestone outcrop 

covered with short vegetation about 100 m away from the 

parent colony rather than areas of taller vegetation adjacent to 

the existing colony. King (1955, p. 105) suggested “that short 

vegetation may encourage prairie dogs to settle a new area” 

and that they “seem to select vegetation that is neither too rank 

nor too tall” as they colonize new areas.

Following his study of prairie dog colonies in northern 

mixed-grass prairie, shortgrass steppe, and southern mixedgrass prairie, Koford (1958) also noted that prairie dog expansion was favored by shorter, less dense vegetation. In particular, 

Koford remarked (p. 63) that stands of tall grass surrounding 

prairie dog colonies could act as effective barriers to expansion, 

and that prairie dogs “seldom enter grass so tall and thick that 

they cannot see through or over it.” Moreover, he noted (p. 65) 

that prairie dogs rarely expanded into rangeland that was in 

good to excellent condition and (p. 67) that “heavy grazing [by 

livestock] tends to reduce the barriers and allow the spread of 

prairie dogs.” While Koford (1958, p. 67) felt that “conservative 

grazing” would allow vegetation to grow taller and inhibit prairie dog expansion, he also pointed out that this might not occur 

in more arid grasslands such as the shortgrass steppe. In support 

of this idea, Snell and Hlavachick (1980) observed that, after 4 

years of rest from livestock grazing during the growing season 

in southern Kansas, native grasses on a prairie dog colony had 
become more abundant and the colony had decreased in area 

from about 45 ha to 5 ha.

Results from more recent studies are consistent with 

these earlier anecdotal observations and interpretations. For 

example, in northern mixed-grass prairie in South Dakota, 

Uresk and Bjugstad (1983) found an average of 106 active 

prairie dog burrows per hectare on sites where no cattle grazed 

and more than twice as many (235/ha) where cattle and prairie 

dogs grazed. They attributed the lower burrow density where 

cattle were excluded to taller vegetation there and concluded 

that high prairie dog densities were more likely to occur when 

rangelands are heavily grazed. In northeastern Montana, 

Knowles (1986) found that over 60 percent of the colonies he 

surveyed were on pastures with heavily grazed livestock developments and that nearly all (>97 percent) occurred adjacent 

to trails and roads. Interviews with landowners and managers 

suggested to Knowles (1986) that colonization of these areas 

by prairie dogs followed, rather than preceded, intensive grazing and soil disturbance. Likewise, Licht and Sanchez (1993) 

suggested that creation of cattle point attractants (e.g., water 

tanks and supplementary feeding sites) encourages colonization by prairie dogs after vegetation height around the attractants is reduced by livestock grazing and trampling. Similarly, 

Truett and Savage (1998) noted that expansion of introduced 

prairie dogs into Chihuahuan Desert grasslands typically 

only occurred where vegetation was less than 20–25 cm tall. 

Following mowing of taller vegetation, prairie dogs quickly 

moved into the mowed areas.

Scholarly reviews of the literature and early accounts of 

prairie dog interactions with large native and introduced herbivores such as bison and cattle led Truett and others (2001) and 

Truett (2003) to many of the same conclusions. Specifically, 

prior to extensive settlement of the Great Plains, the distribution of prairie dogs in more productive grasslands was closely 

linked to areas frequented by bison, which kept the grass 

relatively short. Following removal of bison, the range of prairie dogs shrank until cattle were introduced in large numbers, 

thereby allowing prairie dog populations to expand again. In 

areas where cattle were introduced soon after extermination of 

bison, prairie dogs persisted in large numbers; however, severe 

long-term overgrazing by livestock may reduce densities 

of prairie dogs by reducing availability of forage resources 

(Desmond, 2004).

Conclusions

The literature reviewed here suggests a strong interactive relationship between prairie dogs and other grazers, 

particularly in relatively productive grasslands. We have seen 

that, as a result of selectively grazing graminoids and clipping the vegetation to a short height, prairie dogs may greatly 

reduce aboveground plant biomass and cover, change plant 

species composition towards a greater dominance by forbs, 

and enhance the quality of the remaining forage via increases 

in leaf [N] and digestibility. Native grazers, such as bison and 

pronghorn, as well as cattle, may be attracted to these colonies 

as preferred sites to graze. Where densities of large grazers 

are relatively high, their grazing and trampling activities in 

uncolonized grassland may make some sites more suitable for 

colonization by prairie dogs and thereby facilitate expansion 

of prairie dog populations. While much of this latter concept is 

based on anecdotal reports rather than on experimental results, 

the preponderance of similar reports (Truett and others, 2001; 

Truett, 2003) lends credibility to it. In grasslands such as 

the semiarid shortgrass steppe, where vegetation is naturally 

shorter and dominated by species such as blue grama, the 

strength of many of the interactions between prairie dogs and 

large grazers discussed above is apparently not as strong. 

Acknowledgments

Preparation of this manuscript was supported in part 

by National Science Foundation grant DEB-0217631 to the 

Shortgrass Steppe Long Term Ecological Research Project at 

Colorado State University, Fort Collins.

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118 Recovery of the Black-footed Ferret
Abstract

Recreational shooting of prairie dogs (Cynomys spp.) 

has occurred for many years, but interest and intensity have 

increased dramatically in the past decade. Shooting can cause 

prairie dogs to change their behavior and can affect sex and 

age groups differently. Prairie dog populations are capable 

of recovering from shooting or other reductions, but time to 

full recovery depends on demographic parameters (survivorship and fecundity). Simple population growth models with 

demographic variability demonstrate less risk of population 

extinction when shooting is regulated by effort rather than by 

quotas on numbers shot. Landowners might consider allowing 

shooting as a source of income, but, if not closely managed, 

shooters potentially can eliminate small colonies. Predation 

by black-footed ferrets (Mustela nigripes) probably does not 

significantly depress prairie dog populations. Prairie dog 

mortality caused by unregulated recreational shooting can 

vastly exceed predation by black-footed ferrets, affecting prairie dog survivorship and potentially affecting fecundity and 

recruitment. Until effects of shooting prairie dogs as prey for 

black-footed ferrets are better understood, shooting closures 

on reintroduction sites are appropriate.

Keywords: black-footed ferret, Cynomys spp., Mustela 

nigripes, prairie dog, recreational shooting

Introduction

Many long-time residents of western States recall spending summer vacations “plinking” prairie dogs (Cynomys spp.), 

whether by wrist rocket, air gun, or .22 caliber rifle. For over 

100 years, shooting black-tailed prairie dogs (C. ludovicianus) in rural Kansas, for example, was common on Sunday 

afternoons by self-styled “varmint hunters” and by afterschool target shooters (Smith, 1967). Shooting prairie dogs has 

been and continues to be primarily for sport but now involves 

marksmen who utilize high-technology rifles while practicing 

their shooting skills. To hone their skills, many shooters use 

a variety of rifles, scopes, range finders, shooting benches, 

and reloading equipment. Indeed, shooting prairie dogs at 

distances >450 m entitles one to membership in the 500 Yard 

Club, sponsored by the Varmint Hunters Association, and 

some members have registered successful shots >1,350 m.

Many shooters come from out of State (Vosburgh, 

2000; South Dakota Prairie Dog Work Group, 2001), but 

this appears to be a recent phenomenon. In North Dakota, 

for example, nonresidents must purchase either a nongame 

license or a combination nongame and furbearer license to 

shoot prairie dogs; residents are exempt (North Dakota Game 

and Fish Department, 2001). The number of nonresident 

nongame licenses sold increased from 36 in 1975 to 625 in 

2001, while nonresident nongame and furbearer license sales 

increased from 163 in 1989 to 1,326 in 2001 (S. Hagen, written commun., 2003). The recent increase in license sales to 

nonresidents in North Dakota indicates the rise in recreational 

shooting of prairie dogs by people from out of State. Similarly, 

in South Dakota over 35 percent of the estimated 16,011 prairie dog shooters on nontribal land in 2001 were nonresidents 

(Gigliotti, 2001).

Shooting Intensity

Available information indicates that substantial numbers 

of prairie dogs have been shot. In 2000, recreational shooters killed 1,186,272 prairie dogs on nontribal lands in South 

Dakota (South Dakota Prairie Dog Work Group, 2001). The 

number killed on nontribal lands increased to 1,516,174 in 

2001 (Gigliotti, 2001). Shooters spent a total of 75,059 recreation days to kill that many prairie dogs: 54,849 by residents 

and 20,210 by nonresidents (Gigliotti, 2001).

During 1998 in Nebraska, 7,100 shooters spent 33,400 

recreation days killing 301,000 prairie dogs; in 1999, fewer 

shooters (5,970) spent less time (28,300 recreation days) to 

kill more prairie dogs (356,000) (Nebraska Game and Parks 

Commission, 2001). The Colorado Division of Wildlife 

(2002) estimated (with ±95 percent confidence interval) that 

6,070 shooters (±629) killed 418,412 prairie dogs (±75,234) 

during 64,674 recreation days (±825) in the 1998–99 reporting 

period. Based on these data, shooter success rates (number 

killed per recreation day) appear similar among reporting 

States: 6.5 killed per day in Colorado (between 5.2 and 7.7), 

12.6 killed per day in Nebraska in 1999, and 20.3 killed per 

day in South Dakota in 2001.

Shooting Prairie Dogs

1

Black-footed Ferret Recovery Foundation and PIC Technologies, Inc., 309 

S. 4th St., Suite 201, Laramie, WY 82070.

2

Current address: Edge Environmental, Inc., 309 S. 4th St., Suite 201, 

Laramie, WY 82070.

3

Intertribal Black-tailed Prairie Dog Coordinator, P.O. Box 1101, Boulder, 

MT 59632.

By Archie F. Reeve1,2 and Timothy C. Vosburgh3

Shooting Prairie Dogs 119
Estimates of prairie dogs killed in individual States 

depend on shooters’ responses to survey questionnaires, 

which are possible only when shooters are licensed, such as in 

South Dakota and Colorado (South Dakota Prairie Dog Work 

Group, 2001; Colorado Division of Wildlife, 2002, 2003). 

On the other hand, nontribal recreational shooters on some 

tribal lands are required to be accompanied by a guide and 

must fill out a questionnaire that includes the number of days 

spent shooting, number of rounds fired, and estimated number 

of prairie dogs killed. In 2000, 936 shooters fired 156,307 

rounds to kill 57,848 prairie dogs on the Rosebud Indian 

Reservation (T. Vosburgh, unpub. data, 2002). That rate of one 

prairie dog killed per 2.7 rounds fired is similar to an observed 

rate of one prairie dog killed per 3.0 shots fired on the Fort 

Belknap Indian Reservation in northern Montana during 2001 

(Vosburgh, 2000).

The Lower Brule Indian Reservation in central South 

Dakota has collected 9 years of black-tailed prairie dog harvest 

data (table 1). From 1993 to 2001, an average of 121 licensed 

recreational shooters killed an average of 14,200 prairie dogs 

per year while spending an average of 372 recreation days on 

the reservation (Lower Brule Sioux Tribe, 2002). Each shooter 

averaged 118 prairie dogs shot per year or about 38 shot per 

day. That level of success is comparable to nonresident shooters on nontribal lands in South Dakota who, in 2001, spent 

an average of 3.5 days per shooter to kill 36 prairie dogs per 

day (Gigliotti, 2001). The average success rate was higher 

than reported above by Nebraska or Colorado. Relatively 

high levels of shooter success may be due to tribes’ interest in 

recreational shooting as a source of revenue with concomitant 

monitoring of shooting effects on prairie dog populations. 

Also, out-of-State shooters may be especially diligent in 

pursuit of their quarry. During 2001, residents of South Dakota 

shot an average of 14 prairie dogs per day, considerably fewer 

than the 36 prairie dogs per day reported shot by nonresidents 

(Gigliotti, 2001).

Shooter success rates appear related to prairie dog 

densities. Typical densities of black-tailed prairie dogs exceed 

those of white-tailed prairie dogs (C. leucurus) (Tileston and 

Lechleitner, 1966), whereas densities of Gunnison’s prairie 

dogs (C. gunnisoni) are intermediate or overlap those of the 

other two species (Fitzgerald and others, 1994). All three 

species are shot in Colorado, but harvest estimates are not 

reported by species, only by county (Colorado Division of 

Wildlife, 2003). Based on species’ distributions (Fitzgerald 

and others, 1994), we estimated harvest for the three species 

(table 2). Although ranges of shooting success rates overlap, 

shooters in 2002–03 killed more prairie dogs per recreation 

day in counties with black-tailed prairie dogs than in counties inhabited by Gunnison’s and/or white-tailed prairie dogs 

(table 2).

Effects of Shooting on Individual 

Prairie Dogs

Prairie dogs subjected to shooting change their behavior. 

In Montana, black-tailed prairie dogs in colonies with recreational shooting spent less time above ground than did prairie 

dogs in colonies with no shooting. When above ground, the 

former devoted less time to feeding and more time to scanning 

than the latter (Vosburgh and Irby, 1998). Prairie dogs in colonies with recreational shooting are more likely to escape when 

approached on foot or by vehicle, retreating to burrows sooner 

than prairie dogs not subjected to shooting (Vosburgh and Irby, 

1998; Keffer and others, 2000). Increased alertness and early 

escape by prairie dogs are potential problems for recreational 

shooters, though some shooters may by more interested in 

shooting skill and firearm accuracy than in numbers of prairie 

dogs killed.

The timing of shooting prairie dogs may affect reproduction and mortality of various sex and age groups. Shooting 

from March to May is likely to kill pregnant or lactating 

females so that neither they nor their offspring will reproduce 

the following year (Knowles, 1988). Shooters generally cannot 

distinguish between male and female prairie dogs and, except 

during early summer, between adults and juveniles. Juvenile 

prairie dogs are more susceptible than adults to low levels 

Table 1. Data from 9 years of shooting black-tailed prairie dogs (Cynomys ludovicianus) on the Lower Brule Indian Reservation, S. Dak.a

Statistic

Year

1993 1994 1995 1996 1997 1998 1999 2000 2001

Number of licenses sold 115 146 139 127 157 97 114 130 64

Estimated total killed 17,700 28,000 4,600 10,700 15,300 16,700 12,100 14,800 8,069

Total recreation days 367 503 334 486 372 392 363 319 211

Harvest/day/shooter 48 56 14 22 41 43 33 46 38

Average days/shooter 3.2 3.6 2.4 3.8 2.8 2.9 3.2 2.5 3.3

a Lower Brule Sioux Tribe (2002).

120 Recovery of the Black-footed Ferret
Shooting Prairie Dogs 121

of shooting (Keffer and others, 2000). For example, when 

a colony having 35 percent juveniles was subjected to a 10 

percent harvest during early to mid-summer, 53 percent of 

the animals killed were juveniles. On the same colony, adult 

females and adult males (39 percent and 26 percent of the 

population, respectively) were killed proportionately less than 

their occurrence in the population. Higher shooting pressure 

(>20 percent mortality) on another colony also targeted juveniles disproportionately (Keffer and others, 2000).

Adult females, including yearlings, appear more vulnerable to shooting than do adult and yearling males (Vosburgh 

and Irby, 1998; Keffer and others, 2000). During early summer 

1995 in Montana, for example, the ratio of adult males to 

females was 92:100 on nine colonies (Vosburgh and Irby, 

1998). In late summer, after an average of 8.5 hours of 

shooting per colony, the ratio of adult males to adult females 

was 167:100 on the same nine colonies. Survivorship of adult 

females during shooting was only 57 percent of the survivorship of adult males. On eight control colonies with no shooting, adult female survivorship was 122 percent of adult male 

survivorship between early and late summer in the same year 

(Vosburgh and Irby, 1998), which is similar to differential 

survival of unhunted black-tailed prairie dog populations 

elsewhere (Hoogland, 1995). Greater vulnerability of females 

probably exacerbates the impact of shooting by diminishing 

future reproduction.

Reproduction by adult and yearling female prairie dogs 

may be suppressed on colonies that are subject to continual 

recreational shooting. In North Dakota, only 32 percent 

of yearling female black-tailed prairie dogs on colonies 

disturbed by >20 years of heavy shooting reproduced (based 

on placental scars and evidence of ovulation) compared with 

90 percent of yearling females on colonies relatively undisturbed by shooting (Stockrahm and Seabloom, 1988). Counts 

of placental scars in adult females (>2 years old) examined 

from the disturbed colonies were significantly lower than in 

adult females on relatively undisturbed colonies, indicating 

depressed reproduction on the disturbed colonies. These observations do not demonstrate that continual shooting was solely 

responsible for depressed reproduction because the disturbed 

colonies were spatially confined and not growing, whereas 

the undisturbed colonies were not spatially restricted and had 

doubled in size during the previous 5 years (Stockrahm and 

Seabloom, 1988). Depressed reproduction in the disturbed 

though confined colonies, especially by yearling females, may 

indicate density-dependent processes similar to those observed 

by Garrett and others (1982) in South Dakota.

Effects of Shooting on Prairie Dog 

Populations

Populations increase with birth and immigration of 

individuals but decrease with their death or emigration. 

For species such as prairie dogs that reproduce once a year 

(Hoogland, 1995) but die from various sources throughout the 

year, information about the rate of population increase (sometimes called the “finite rate of increase” and symbolized here 

by R; others often use lambda, λ) is important to understanding potential effects of recreational shooting on colonies. The 

equation Nt = N0 Rt can be used to compute population size 

at time t, Nt, if the initial population size, N0, and R are known.

Finite rates of increase in prairie dog colonies with no 

shooting vary from year to year. For example, at one blacktailed prairie dog colony in Wind Cave National Park, S. Dak., 

colony size increased in some years but declined in others. 

Population finite rates of increase at this colony averaged 1.03 

(1 standard deviation = 0.25), with minimum R = 0.70 and 

maximum R = 1.45 (Hoogland, 1995, table 16.1). Because 

this colony was surrounded by unsuitable habitat, its area 

remained constant, so the observed R = 1.0 might have been 

Table 2. Harvest estimates for three species of prairie dogs (Cynomys)—white-tailed (C. leucurus), Gunnison’s (C. gunnisoni), and 

black-tailed (C. ludovicianus)—in Colorado during 2002−03 with rates and ranges of number killed per recreation day.

Speciesa

Number of

counties in

distributiona

Number of

huntersb

Recreation

days spentb

Total

prairie dogs

killedb

Shooter

kill rate

(number/day)

Range

(number/day)

for counties

White-tailed 5 1,063 13,197 30,943 2.34 0.78–5.51

White-tailed with 

Gunnison’s 2 394 12,153 66,772 5.49 4.76−5.71

Gunnison’s 12 827 9,278 31,533 3.40 0.00−6.44

Gunnison’s with 

black-tailed 3 197 1,083 3,762 3.47 3.25−3.65

Black-tailed 20 1,948 17,845 170,867 9.58 1.42−101

a

Counties within species’ distributions as described by Fitzgerald and others (1994).

b

Colorado Division of Wildlife (2003).
expected. Stationary populations often increase in response to 

factors such as addition of food, increase in habitable area, and 

population reduction (Caughley and Sinclair, 1994). Population reduction decreases competition, usually for food. With 

more food per individual, reproduction increases, mortality 

decreases, and the population grows (Caughley and Sinclair, 

1994).

Limited experimental evidence demonstrates that 

removing prairie dogs, by shooting or other means, enhances 

population growth rates. After 2 consecutive years of shooting at two small black-tailed prairie dog colonies, populations 

were reduced or eliminated; in the year after shooting ceased, 

portions of both colonies were still inactive (Knowles, 1988). 

Five years after the shooting program ended, the larger of 

the two shot colonies had expanded to cover 140 percent of 

its preshooting area, and the smaller had grown to cover 90 

percent of its former area (Knowles, 1988). Spatial growth of 

these treatment colonies resulted from increased numbers of 

prairie dogs, but details of population increase—whether by 

immigration from neighboring colonies or as a demographic 

response of the surviving prairie dogs—are unavailable. Data 

on active versus inactive burrow entrances indicate a similar 

response to cessation of shooting at larger black-tailed prairie 

dog colonies in South Dakota. Less than 45 percent of total 

burrows were active while recreational shooting was allowed, 

but, after 4 years without shooting, 74 percent of burrow 

entrances were active (B. Perry, unpub. data, 2000).

Rates of population increase have been documented 

under other conditions that reduce prairie dog populations. 

Knowles (1986) studied the effects of a toxicant, zinc phosphide, on several black-tailed prairie dog colonies by different 

treatment regimes and then observed the population recoveries 

for up to 5 years following the treatments. Knowles computed 

instantaneous growth rates for each year during population 

recovery. The instantaneous growth rate, r, is related to the 

finite rate of increase, R, by R = er , (Akçakaya and others, 

1999) and is employed to predict population growth in continuous time rather than in discrete time, by the equation Nt

 = N0

er t . One month after treatments that attempted to completely 

eradicate two colonies, Knowles (1986) estimated a reduction of 95 percent caused by the treatments. By continuing 

to monitor population recovery, Knowles computed average 

r = 0.916 (R = 2.499) after 1 year, r = 1.069 (R = 2.912) from 

the first to the second year and r = 0.350 (R = 1.419) from 

the second to the fifth year. For the five colonies that received 

partial toxicant treatment, which reduced target populations 

to an average of 19 percent of pretreatment levels, average r = 

1.339 (R = 3.815) after 1 year and r = 0.148 (R = 1.160) from 

the first to the second year (Knowles, 1986).

Values for R reported by Knowles (1986) were higher 

for both study groups after the first year following treatments 

than values observed on a prairie dog colony in Wind Cave 

National Park, S. Dak. That colony expanded from 0.47 ha 

to 1.86 ha over a 3-year period (Garrett and Franklin, 1988) 

when colony size in June increased from 51 to 134 adults and 

juveniles (average R = 1.38). Similarly, a black-tailed prairie 

dog colony near Nunn, Colo., with no population reduction 

treatment, expanded from 2.1 ha to 3 ha in 1 year as the colony 

size in June grew from 28 to 82 animals (Koford, 1958, p. 10, 

table 1). For that colony in that 1-year period, R = 2.93, but 

in the previous year the colony had declined from 50 to 28 

animals (R = 0.56).

Population responses were also tracked following 

reduction of two colonies in South Dakota by translocating 

live-trapped black-tailed prairie dogs (Radcliffe, 1992). After 

intensive removal during June of the first study year, 6 prairie 

dogs remained in one of the colonies, but 10 were present 

by June of the following year. By June of the second year 

following removal, the population had increased to 51 prairie 

dogs, but the increase was mostly attributed to immigration 

(Radcliffe, 1992). The second colony also grew substantially 

after the population was reduced to 10 individuals in June of 

the treatment year. By the next June that colony had grown 

to 23 and by June of the second year had grown to 80 prairie 

dogs; the extraordinary growth rate during the second year was 

R = 3.48. In this second colony, immigration played a minor 

role (three immigrated annually). Population growth mainly 

resulted from increased litter size and higher juvenile survivorship (Radcliffe, 1992).

These data support our earlier generalization that populations can be stimulated to grow by reducing the number of 

animals that compete for a limited resource. Similar population responses were noted in colonies of Gunnison’s prairie 

dog during and after a sylvatic plague epizootic in Moreno 

Valley, N. Mex. Cully (1997) found that after plague killed 

more than 99 percent of the population, the few surviving 

prairie dogs formed two colonies in areas that were previously 

unoccupied. Using Leslie matrix analysis involving demographic parameters (survivorship and fecundity), Cully found 

that the population would be nearly tripling each year. The key 

to this high rate of population growth was found to be yearling 

females reproducing at a rate similar to that of adults and 

having a relatively high survival rate (Cully, 1997). Similarly, 

the survival rate of juveniles (90 percent) in a young, expanding black-tailed prairie dog colony in South Dakota was 

significantly higher than juvenile survivorship (49 percent) in 

an older, nonexpanding colony (Garrett and others, 1982). In 

addition, yearling females on the younger colony were more 

likely to produce and wean a litter than were those on the older 

colony. These two demographic characteristics of juveniles 

and yearlings appear consistent in rapidly growing prairie dog 

populations.

Simulated Effects of Different Shooting 

Strategies

In many of these studies, prairie dog populations 

appear to exhibit density-dependent growth; crowding with 

122 Recovery of the Black-footed Ferret
Shooting Prairie Dogs 123

concomitant diminishing resources available to each individual 

leads to increased mortality (decreased survival), decreased 

reproduction, and/or increased emigration. An assumption of 

density-dependent population growth is that when a population approaches carrying capacity (K) the growth rate declines 

and eventually reaches R = 1.0 when N = K. The value of R 

at time t, Rt, depends on the population size Nt relative to K 

according to the equation, Rt = Rmax

(1 – Nt ∕ K ). When the population Nt

 is small, the exponent (1 – Nt ∕ K) is close to 1 and the 

population’s growth rate Rt is close to the maximum possible, 

or Rmax. As the population grows and approaches its carrying 

capacity, the growth rate R is much less than Rmax, and when 

the population reaches carrying capacity, R = 1.0 because the 

exponent (1 - Nt ∕ K) = 0. 

If prairie dogs are viewed as an economic resource, the 

best strategy is to manage colonies for a sustained yield. A 

landowner or wildlife manager hoping to capitalize on prairie 

dog harvest might allow shooters to kill as many prairie dogs 

in a year as are produced. With density-dependent growth 

(Rmax = 2.00, K = 1,000), a population growing from 20 to 

1,000 animals produces the maximum number of animals 

(maximum yield) when it is approximately at half of carrying 

capacity (Peek, 1986). In this example, maximum yield = 209 

when the population reaches 438 after 5 years of growth.

Harvesting the population at maximum productivity 

maximizes yield, but managing for maximum yield is difficult (Caughley and Sinclair, 1994). Removing animals from 

a population reduces the base population. The difficult task 

is determining what base population produces the best yield 

for the next year. If the harvest exceeds maximum yield and 

continues over time, the population will eventually decline to 

zero (Caughley and Sinclair, 1994).

Two common approaches to control harvest are 

(a) imposing a quota on numbers harvested and (b) regulating 

harvesting effort (Caughley and Sinclair, 1994). Regulating 

harvest by quotas is conceptually attractive: once the quota is 

attained, shooting stops. Determining when quotas for prairie 

dogs are reached might be problematic, however. If shooters 

consistently record prairie dogs killed and number of rounds 

fired, wildlife managers can estimate the number killed from 

the number of shots. Figure 1 demonstrates the problem with 

fixed annual harvest quotas. An initial population of 1,000 

eventually stabilizes at 585 animals in 24 years if 195 prairie 

dogs are shot each year, but if the annual harvest exceeds 209 

animals (the maximum yield when Rmax = 2.00 and K = 1,000), 

then the population declines to extinction, doing so faster with 

larger harvests.

Theoretically, controlling harvest effort removes some 

proportion of the population over time rather than a fixed 

number of animals each year. One way to control harvest 

effort is to limit the timing and duration of the harvest. 

Another way is to limit the number of shooters (Caughley and 

Sinclair, 1994). Figure 2 shows the outcome of various annual 

harvest levels as percentages of the current population. At an 

annual harvest rate of 25 percent, the population stabilizes at 

585 animals, but in only 13 years, while the long-term average 

harvest (from t = 0 to t = 30) is approximately 199 animals.

To this point, population growth was assumed to be 

deterministic with no uncertainty in birth or death rates. 

Environmental variation from year to year, or day to day, and 

from one locale to another causes fluctuations in prairie dog 

populations’ birth and death rates (Hoogland, 1995). In addition, individual animals in the same population have different 

reproductive capabilities or chances of survival. Recreational 

shooting of prairie dogs introduces additional uncertainty in 

Figure 1. Effects of constant annual harvest quotas on a population with density-dependent growth (Rmax = 2.00, K = 1,000) but no 

random variation in Rt . Annual harvest >209 animals (maximum 

yield) cannot be sustained, and the population eventually declines 

to zero.

Figure 2. Deterministic predictions of a population with densitydependent growth (Rmax = 2.00, K = 1,000) subject to different 

levels of proportional harvest annually.

10%

1,000

800

600

400

200

0

0 5 10 15 20 25 30

Time (years)

Population size

5% harvested annually

25%

35%

45%

55% harvested annually
population demographic parameters since age groups and 

sexes may be affected differently. Levels of variability might 

be measured at some point in time but cannot be predicted 

in the future. Stochastic population models provide for such 

uncertainty.

In the simulation examples provided so far, the finite 

rate of increase at time t, Rt, is equal to Rmax

(1 – Nt ∕ K ). Annual 

variation in rates of birth, immigration, death, and emigration 

all contribute to variability of Rt . Annual variation in carrying 

capacity also causes variation in Rt . We introduce variation by 

increasing or decreasing the computed value of Rt by a random 

amount but within some defined limits, for example within 

±20 percent of the computed value for Rt , which includes 

demographic variation as well as random variation in carrying 

capacity.

This simple approach was applied in 100 simulations 

to project population growth from an initial population of 20 

animals with Rmax = 2.0 and K = 1,000. The simulations show 

that the average population size stabilizes at approximately 

1,000 animals (fig. 3) but, because of random variability of Rt, 

the population at t = 15 could range from 797 to 1,230 animals 

in any one simulation.

Random variation, now limited to only ±15 percent of the 

computed value for Rt , for example, is used to predict how an 

initial population of 1,000 (N0 = K) with Rmax = 2.0 responds 

to an annual quota of 195 animals harvested. The results 

(fig. 4) are different from those generated by the deterministic 

model (fig. 1). After 1,000 simulations, the stochastic model 

predicts a population of 406 (ranging from 0 to 819) at the 

end of 30 years with average annual harvest of 183. The 

model also predicts a 23 percent chance that the population 

will become extinct by t = 30. Risk of extinction increases 

with level of random variation in Rt . For example, with 

random variation ±10 percent of Rt , extinction within 30 years 

occurred in 1 percent of the trials, but a 46 percent chance of 

extinction is expected with random variation ±20 percent of Rt

(after 1,000 simulations with an annual quota of 195).

Alternatively, an annual harvest rate of 25 percent 

produces an expected population of 580 animals (ranging from 

439 to 744 animals) after 30 years of simulation with average 

annual harvest of 197 animals but poses no risk of extinction 

(fig. 5), unlike the risk observed with fixed quota harvest (fig. 

4). With demographic and environmental uncertainty, sustainable populations are more likely if harvested proportionally 

rather than by fixed quota. Implicit in modeling these two 

harvest strategies, however, is intensity of harvest management. Once set, the quota of 195 harvested did not change 

over time even though the population may have been declining 

Figure 3. Results of 100 simulations of density-dependent population growth (Rmax = 2.00, K = 1,000), but with random variation in 

the population growth rate each year (within ± 20% of Rt after 

computation as Rt = Rmax

(1 – Nt ∕ K )

).

Figure 4. Results of 1,000 simulations with an annual harvest 

quota of 195 animals, an initial population of 1,000 animals, 

density-dependent population growth (Rmax = 2.00), and random 

variation in the population growth rate each year (within ±15% of 

Rt after computation as Rt = Rmax

(1 – Nt ∕ K )

).

Figure 5. Results of 1,000 simulations with an annual harvest rate 

of 25%, an initial population of 1,000 animals, density-dependent 

population growth with Rmax = 2.00, and random variation in the 

population growth rate each year (within ±15% of Rt after computation as Rt

 = Rmax

(1 – Nt ∕ K )

).

124 Recovery of the Black-footed Ferret
Shooting Prairie Dogs 125

in a given simulation. Alternatively, numbers harvested were 

continuously adjusted when a harvest rate of 25 percent was 

applied. To ensure a sustainable population while realizing a 

desired annual harvest, the manager must monitor the dynamics of the target population and respond accordingly.

Proper application of either harvest strategy, whether by 

regulating harvest quota or by regulating harvesting effort, 

requires knowledge of the target population’s carrying capacity 

and the species’ Rmax. Seldom are these parameters known with 

any certainty. A population at approximately K/2 is expected 

to yield the maximum number of animals that, in theory, could 

be harvested each year as a maximum sustained yield (MSY). 

Nevertheless, stochastic events in the environment and variability among individuals in a population can lead to substantial 

population fluctuations, and harvest should always be well 

below the estimated MSY (Caughley and Sinclair, 1994).

A third approach involves harvesting only when a population exceeds some threshold level, above which excess animals 

are taken (Lande and others, 1997). Threshold harvesting 

requires specific knowledge about population levels but 

produces high annual variation in harvest because populations below the threshold are not harvested (Lande and others, 

1997). Threshold harvesting might be possible if a manager 

or landowner had never attempted to control prairie dogs and 

had monitored population levels under various environmental 

conditions so that average K could be estimated.

Recreational Shooting on Private Land 

for Prairie Dog Conservation

Of >660 livestock and agricultural operators surveyed in 

eastern Wyoming (Wyoming Agricultural Statistics Service, 

2001), 23 percent expressed interest in a program of financial compensation for allowing prairie dogs on their land. 

The survey posed four types of management programs to 

respondents who expressed interest in financial compensation: (a) a shooting management program, (b) a cooperative 

shooter placement program to direct shooters to landowners 

willing to allow shooting, (c) a program to develop markets 

for prairie dogs as pets or for nature photography, and (d) a 

banking program in which other States would compensate 

Wyoming landowners for conserving prairie dogs (Wyoming 

Agricultural Statistics Service, 2001). Of these, prairie dog 

banking was the most popular (59 percent interested), a 

cooperative shooter placement program (57 percent interested) 

was second, followed by interest in shooting management (51 

percent of respondents).

Respondents who expressed interest in programs with 

financial compensation considered $74−$86/ha annually to be 

reasonable ($30−$35/acre, median value). Generally, interest 

in maintaining or increasing the number of acres of prairie dog 

colonies on their land varied directly with the level of financial 

compensation. To attain $74−$86/ha in potential income from 

shooting, a landowner with 405 ha (1,000 acres) of prairie dog 

colonies, for example, could charge four shooters $79−$92 per 

person per day to shoot during the period between Memorial 

Day and Labor Day (approximately 95 days). Four shooters during that period are equivalent to 380 recreation days. 

Applying data from the Fort Belknap Indian Reservation 

(average of 38 prairie dogs shot per recreation day), the annual 

toll would be 14,440 prairie dogs killed, whereas 2,470 killed 

would be expected in a year at the rate of 6.5 prairie dogs per 

recreation day estimated in Colorado.

Densities of black-tailed prairie dogs in Conata Basin, S. 

Dak., range from 8/ha to 41/ha (Severson and Plumb, 1998). 

If that range of densities is applied to the simple example of a 

405-ha colony on private land, then the population, estimated 

between 3,240 and 16,605 prairie dogs, could eventually be 

eliminated by recreational shooting under either the shooter 

success rate on Fort Belknap Indian Reservation or in Colorado. To ensure a future income, the private landowner would 

have to significantly decrease the number of recreation days 

spent shooting, which should concomitantly decrease the 

number of prairie dogs shot. In addition, to attain the desired 

income, the landowner would have to substantially increase 

fees charged per shooter.

Managing prairie dogs on private land for recreational 

shooting might be appropriate for some landowners and not 

others. Still, when faced with the apparent need or desire to 

control prairie dogs, opening land to shooters can provide 

landowners with an additional source of income and thus an 

incentive to support some level of occupied habitat that they 

otherwise would not tolerate.

Recreational Shooting on Black-footed 

Ferret Reintroduction Sites

Recreational shooting of prairie dogs has been totally or 

partially restricted on black-footed ferret (Mustela nigripes) 

reintroduction sites (Colorado Division of Wildlife and others, 

2002), although there are few instances where effects of shooting closures on prairie dog populations have been monitored. 

In some instances, shooting closures coincided with changes 

in statewide prairie dog management practices following 

States’ adoption of the Black-tailed Prairie Dog Conservation Assessment and Strategy and addendum (Luce, 2001). 

Closures to shooting have also been applied to other species 

of prairie dog, as in Arizona where black-footed ferrets were 

introduced in Gunnison’s prairie dog colonies (B. Van Pelt, 

oral commun., 2003). In other cases, shooting closures were 

initiated to improve habitat for introduced black-footed ferrets 

and to ensure that ferrets, especially kits, would not be inadvertently shot (B. Perry, oral commun., 2003). Other wildlife, 

burrowing owls (Athene cunicularia) in particular, can be 

killed during recreational shooting of prairie dogs. Though not 

documented as a consequence of shooting prairie dogs, there 

are instances of substantial burrowing owl mortality by shooting (Haug and others, 1993; James and Espie, 1997).
In its review of a petition to list the black-tailed prairie 

dog under the Endangered Species Act, the U.S. Fish and 

Wildlife Service (1999) evaluated effects of recreational shooting, concluding in part that shooting may be a compensatory 

source of mortality in large populations with substantial reproduction. Where small prairie dog populations are depressed 

by other factors such as disease, shooting may be an additive 

source of mortality. Compensatory mortality, where one 

source of mortality offsets or replaces another source (Mackie 

and others, 1990), whether caused by harvest or predation, 

is most likely to occur in populations near their ecological 

equilibrium or carrying capacity (Peek, 1986; Bartmann and 

others, 1992; Boyce, 2000). In such density-dependent regulated populations, when density is high so are mortality rates, 

and a population decrease by whatever means results in higher 

survivorship in the remaining population, as long as removal 

of animals does not adversely affect reproduction the following year. When removal by harvest and/or predation is high 

enough to affect reproduction in subsequent years, mortality 

from those sources is likely to be additive and, if extreme, can 

force the target population to extinction.

By most accounts, ferret predation does not significantly 

depress prairie dog populations (Fagerstone, 1987) and would 

seem a source of compensatory mortality. Biggins and others 

(1993) estimated annual consumption of 109 prairie dogs 

by one black-footed ferret family group (1 adult female, 3.3 

young, and 0.5 adult male) while recognizing the potential for 

substantial prairie dog predation by other species. Assuming 

moderate levels of mortality by other sources, Biggins and 

others (1993) estimated that a stable population of 763 prairie 

dogs would sustain a ferret family group for 1 year. Using an 

age-dependent predation model of ferrets and prairie dogs, 

Klebanoff and others (1991) concluded that as many as 2,000 

prairie dogs per ferret may be necessary to sustain a stable 

predator-prey system. A stable system can also be attained 

with fewer prairie dogs—though not as few as 763—but 

only if prairie dog survivorship or fecundity rates increase 

(Klebanoff and others, 1991). We are not aware if either 

estimation approach has been field tested.

Prairie dog mortality by unregulated recreational shooters can vastly exceed predation by black-footed ferrets, thus 

affecting prairie dog survivorship and potentially affecting 

fecundity and recruitment. Recreational shooting can be additive mortality, potentially more so on black-tailed prairie dog 

colonies than on Gunnison’s or white-tailed colonies (table 

2). Management agencies have recognized that, even with 

closures of specific areas, recreational shooting has continued 

and that enforcement of shooting closures is problematic (V. 

Kopcso, oral commun., 2003). Until more is known about 

effects of recreational shooting on prairie dogs that are the 

primary prey resource for black-footed ferrets, managers are 

wise to restrict shooting and enforce closures, particularly on 

ferret reintroduction sites inhabited by black-tailed prairie 

dogs.

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128 Recovery of the Black-footed Ferret
Habitat Preferences and Intraspecific Competition in 

Black-footed Ferrets

By Dean E. Biggins,1 Jerry L. Godbey,1 Marc R. Matchett,2 and Travis M. Livieri3

1

U.S. Geological Survey, Fort Collins Science Center, 2150 Centre Ave., 

Bldg. C, Fort Collins, CO 80526.

2

U.S. Fish and Wildlife Service, Charles M. Russell National Wildlife 

Refuge, P.O. Box 110, Lewistown, MT 59457.

3

Prairie Wildlife Research, P.O. Box 515, Wall, SD 57790.

Abstract

We used radio-telemetry data (28,560 positional fixes) 

collected on 153 black-footed ferrets (Mustela nigripes) to (1) 

reexamine the assumed obligate relationship of these ferrets 

to prairie dogs (Cynomys spp.), (2) investigate habitat preferences of ferrets at a small scale (<1 ha), and (3) gain insight 

into competition among ferrets for habitat patches of varying 

quality. We used densities of prairie dog burrows as an indicator of habitat quality because burrows are presumably valuable 

to ferrets as cover and because density of burrows is correlated 

to density of prairie dogs. Burrow density summaries were 

generated from maps of all burrows on ferret reintroduction 

sites in Montana and South Dakota. Aboveground movements 

by ferrets were mostly (89 percent) within the boundaries 

of prairie dog colonies or associated with circuits involving 

return to a colony (10 percent), with no evidence that ferrets 

sought to occupy alternative habitats. Sampling with 0.07-ha 

plots suggested that dispersion of prairie dog burrows within 

colonies was neither uniform nor random. Burrows were 

clumped, and ferrets preferred (P < 0.001) patches of habitat 

with high densities of burrows compared to samples taken at 

random points on the colonies they occupied. The magnitude 

of preference (the difference between use and availability) was 

greatest for resident young ferrets compared to their recently 

released counterparts, whether the newcomers were compared 

with residents of 2–4 weeks (P = 0.039) or >1 year (P = 

0.048). Also, preference was stronger for wild-born young 

ferrets than for young captive-born ferrets released to augment 

the wild population (P = 0.040). This additional evidence 

for competition among ferrets, and for an advantage of prior 

residency, raises conservation concerns. The energetics-based 

model commonly used to predict ferret densities at reintroduction sites does not consider competition, which likely 

leads to overestimation of the densities of ferrets attainable 

in high-quality habitat. During sequential releases of ferrets, 

prior residency may handicap success of newcomers, even 

though the latter may have higher potential fitness. Although 

the manner of initial colonization of available habitat by blackfooted ferrets, and their subsequent competition for it, was 

suggestive of an ideal despotic distribution, we did not assess 

effects of prey density or burrow density on fitness.

Keywords: black-footed ferret, competition, Cynomys, 

endangered species, habitat, ideal despotic distribution, ideal 

free distribution, Mustela nigripes, prairie dog, prior residency

Introduction

Conservation efforts for the highly endangered blackfooted ferret (Mustela nigripes) include a captive breeding 

program that rescued the species from a remnant population of 10 animals in Wyoming (fig. 1) during the winter of 

1985–86. That captive breeding program currently produces 

annual surpluses of 200–300 kits for reintroduction (Marinari 

and Kreeger, this volume). Ferrets have been reintroduced 

at sites in six U.S. States and Chihuahua, Mexico (Lockhart 

and others, this volume). Releases of ferrets into unoccupied 

and occupied habitat, and monitoring of wild-born ferrets, 

provided unique opportunities to evaluate large-scale habitat 

Figure 1. The site near Meeteetse, Wyo., that provided ancestral stock for the captive breeding program, and study sites in 

Montana and South Dakota where black-footed ferrets (Mustela 

nigripes) were released.
use by ferrets (objective 1), habitat preferences at small scales 

(objective 2), and relationships between ferret territoriality and 

habitat quality (objective 3), all of which are relevant to ferret 

conservation.

Considerable evidence supports a strong relationship 

between prairie dogs (Cynomys spp.) and black-footed ferrets. 

A summary by Anderson and others (1986) indicates that 

almost all recent ferret specimens were collected from areas 

within the composite ranges of black-tailed prairie dogs 

(C. ludovicianus), white-tailed prairie dogs (C. leucurus), 

or Gunnison’s prairie dogs (C. gunnisoni), and most of the 

explicit descriptions of locality, where provided, mentioned 

prairie dog colonies. The last extant ferret populations were 

found on prairie dog colonies, and studies of those ferrets 

revealed intensive use of prairie dog colonies (Hillman and 

others, 1979; Biggins and others, 1985). Prairie dogs are the 

predominant prey taken by black-footed ferrets (Sheets and 

others, 1972; Campbell and others, 1987). Strategies for evaluating black-footed ferret habitat (Linder and others, 1972; 

Forrest and others, 1985; Flath and Clark, 1986; Houston and 

others, 1986; Biggins and others, 1993) universally assumed 

that prairie dog colonies were a primary requirement. Others, 

however, have questioned the characterization of black-footed 

ferrets as extremely specialized (Owen and others, 2000). 

One objective of this study was to further document the use 

of habitats by ferrets on a large scale, using data from radio 

tracking and maps of black-tailed prairie dog colonies in 

Montana and South Dakota, to reexamine the degree of dependence of black-footed ferrets on prairie dogs. 

Evaluations of ferret habitat are mostly large scale, 

conducted on colonies hundreds of hectares in size and on 

complexes occupying thousands of hectares, leaving the 

details of how ferrets use their local environments largely 

unexplored. If black-footed ferrets are obligate predators 

on prairie dogs and variation exists in densities of prairie 

dogs and their burrows within their colonies, we predict that 

intensity of ferret activity will correlate positively with density 

of prairie dogs when habitat is examined at scales smaller 

than colonies. Thus, our second objective was to evaluate 

preferences of ferrets by using sample parcels of land <1 ha 

in size. To address small-scale habitat preferences and the 

following objective, we used burrow densities as an indicator 

of habitat quality. Prairie dog burrow densities should give a 

suitable measure of habitat quality for black-footed ferrets, in 

part because they correlate to density of the prairie dog prey 

(Biggins and others, 1993) and in part because burrows have 

intrinsic value to ferrets as refuges from predators and adverse 

weather and as dens to rear young. 

Black-footed ferrets, like many other mustelids, appear 

to be intrasexually territorial (Powell, 1979; Miller and others, 

1996). In typical carnivore fashion, females attempt to control 

access to food resources, while males attempt to control 

access to females (Ewer, 1973). Although several factors in 

varying combinations appear to contribute to an organism’s 

resource holding power (e.g., relative size of contestants, age, 

experience in former contests), prior residency often confers 

significant advantages. The residency advantage is widespread 

among several taxa, including insects (Davies, 1978), arachnids (Riechert, 1978), decapods (Jennions and Backwell, 

1996), fish (Harwood and others, 2003), amphibians (Mathis 

and others, 2000), and mammals (Neumann, 1999). Because 

many of the ferrets we studied were released into unfamiliar 

terrain that was either unoccupied by ferrets or occupied by 

ferrets for known periods of time, it was possible to examine 

the effect of prior residency. 

Release of ferrets into vacant habitat allowed us to assess 

the sequence of occupancy. If habitat patches are heterogeneous, the order in which they become colonized or abandoned should relate to quality of those patches as perceived by 

occupants (Wiens, 1976; Krohn, 1992). Ideal free distribution 

theory predicts such an interrelationship between population 

density and carrying capacity of patches in heterogeneous 

habitats (Fretwell and Lucas, 1970). If order of occupancy 

reflects quality of habitat patches, then assessment of the 

colonization process also may lead to improved understanding 

of source-sink dynamics after habitats become fully populated 

(Howe and others, 1991; Pulliam and Danielson, 1991). To 

evaluate intraspecific competition for habitat and order of 

occupancy of habitat patches, we again utilized radio-telemetry data, overlaying ferret locations onto digitized maps of 

prairie dog burrows within the colonies studied. 

Methods

We radio tracked 153 black-footed ferrets on prairie dog 

colonies at UL Bend National Wildlife Refuge, Mont., and on 

the Buffalo Gap National Grassland, S. Dak., during September–November 1994–97 (figs. 1 and 2). Some of the resulting 

28,560 telemetric fixes were used for multiple studies; the UL 

Bend data from 1994 and 1995, for example, were also used 

in the comparisons of adults and kits reported herein (Biggins, 

Godbey, Livieri, and others, this volume). We affixed transmitters having 20-cm whip antennas to wool collars of 1-cm 

width, using Teflon® (DuPont, Wilmington, Del.) heat-shrink 

tubing (Biggins and others, 1999; Biggins, Godbey, Miller, 

and Hanebury, this volume). We weighed and radio collared 

ferrets that were wild caught or captive bred (while the 

animals were held under isoflurane anesthesia) and inserted 

passive integrated transponder chips for long-term identification (Biggins, Godbey, Matchett, and others, this volume). 

Ferrets from captive breeding facilities were reared under a 

variety of strategies and released during August–November 

with no more than 1-day acclimation in onsite cages (Biggins 

and others, 1998). 

We radio tracked ferrets from fixed stations fitted with 

dual-beam, 11-element Yagi antennas on 6-m masts and 

used null-peak direction finding and triangulation to fix each 

ferret’s position at intervals of 7–60 minutes while the animals 

130 Recovery of the Black-footed Ferret
were active above ground (Biggins and others, 1999; Biggins, 

Godbey, Livieri, and others, this volume). We developed 

station-specific error estimates from test data by using 

differences between telemetry-derived azimuths and azimuths 

to transmitters of known location (Biggins and others, 

1999; Biggins, Godbey, Miller, and Hanebury, this volume). 

Aboveground activity of black-footed ferrets is mostly 

nocturnal (Biggins and others, 1986; Biggins, 2000), so we 

limited monitoring of ferrets to hours of darkness for 2 weeks 

to 2 months postrelease. We recorded estimated locations of 

ferrets and associated error polygons as Universal Transverse 

Mercator coordinates derived from paired azimuths with 

program TRITEL (Biggins, Godbey, Miller, and Hanebury, 

this volume).

We recorded locations of prairie dog burrow openings 

(henceforth, such openings will be referred to as burrows) 

with Global Positioning System (GPS) receivers, differentially 

corrected to provide point estimates with errors of <1 m. 

ArcInfo® Version 8.2 (Environmental Systems Research Institute, Inc., Redlands, Calif.) was used for all vector processing, 

and the GRID module was used for all raster modeling. The 

vector point data for all prairie dog burrows and ferret locations in the study were consolidated into the Universal Transverse Mercator Zone 13 projection using the North American 

Datum of 1927. The GRID module was used to convert the 

vector points to 1-m2 cells. To create a map of each prairie dog 

colony, cells were expanded by 10 m in every direction. Thus, 

the maps of colonies (fig. 2A,B; table 1) can be envisioned to 

include a buffer of 10 m beyond the outermost burrows and to 

exclude spaces within the outer boundary that are >10 m from 

the nearest burrow.

Use of Habitats Other than Prairie Dog Colonies

To investigate the broad-scale preference of black-footed 

ferrets for prairie dog colonies, we examined ferret use of 

the colonies as defined above and their use of noncolony 

areas. Ferret fixes were classified as being on or off colonies. 

Because there were nearby colonies in the South Dakota 

complex that were not mapped with the system described, 

ferret fixes that were not on mapped colonies could have been 

on other colonies. Thus, we did not use South Dakota data for 

these large-scale assessments. Similarly, a subset of ferrets in 

Montana (14 animals living near the eastern boundary of the 

subcomplex) had access to colonies that were not mapped with 

this system and were likewise eliminated from the analysis. 

The remaining data used for this overview included 24,512 

fixes on 108 radio-tagged animals, including released and resident adults and kits. Because ferrets presumably must make 

exploratory moves to assess the distribution of prairie dogs, 

and because some ferrets traversed noncolony areas during 

routine travels between colonies, fixes that were off colonies 

do not necessarily imply that ferrets were actually living in 

areas not occupied by prairie dogs. We estimated the relative 

use of noncolony areas attributable to these phenomena, defining an off-colony excursion as a movement involving ≥2 fixes 

Colony Area (ha) Burrows/ha

Montana

1. South Locke 90.1 57.9

2. North Locke 166.0 48.9

3. Small 5.0 64.1

4. Sagebrush 79.8 49.8

5. South Hawley 102.4 79.4

6. North Hawley 144.0 54.6

7. Wilderness 42.2 62.1

South Dakota

North Sage Creek 160.1 138.9

Table 1. Black-tailed prairie dog (Cynomys ludovicianus) colonies 

where prairie dog burrows were mapped. Numbers for Montana 

colonies correspond to the numbered colonies of figure 2.

Figure 2. Distribution of black-tailed prairie dog (Cynomys ludovicianus) burrows (A) and black-footed ferret (Mustela nigripes) 

telemetric fixes (B) on colonies at UL Bend National Wildlife Refuge, Mont. Each dot is a burrow opening or telemetric fix; density 

of resulting stippling thus reflects density of burrows or fixes. 

Attributes for numbered colonies are summarized in table 1.

Habitat Preferences and Intraspecific Competition 131
away from a colony, followed by return to a colony. We also 

tallied the number of fixes associated with intercolony moves 

and dispersal moves (movement with no return to a colony).

Preferences Within Prairie Dog Colonies

We examined habitat preferences of ferrets within 

colonies at a small scale by comparing counts of the number 

of mapped burrows in circular plots of 0.07 ha (radius = 

15 m) surrounding ferret fixes with counts in similar plots 

surrounding random points on colonies (fig. 3), sampling with 

replacement (plots were allowed to overlap). To be included 

in the analysis, the boundary of a sample plot was required 

to be entirely within a colony as defined above. Ferrets with 

≤3 fixes were excluded. To characterize densities of prairie 

dog burrows on the Montana colonies, we sampled 20,328 

plots at random points and compared those to plots centered 

on 21,185 fixes for 110 ferrets. In South Dakota, we counted 

burrows within plots surrounding 427 fixes (for 19 ferrets) and 

465 random points. Because many ferret fixes for individual 

animals were serially autocorrelated (e.g., the sequential fixes 

of fig. 3), we summarized density of burrows within plots as 

mean densities for each animal and used those means in all 

subsequent analyses. Thus, sample sizes became numbers 

of animals (not numbers of fixes). We further restricted this 

data set to include only those ferrets radio tracked >3 days; 

estimates for animals radio tracked for shorter periods were 

deemed unreliable.

A patchy distribution of habitat (burrows) within prairie 

dog colonies is a prerequisite for allowing choice by ferrets. 

Frequencies of counts within the plots described above would 

be expected to follow a Poisson distribution if dispersion of 

burrow openings on colonies were random (Ricklefs, 1990). 

For a Poisson distribution, the variance in counts is equal 

to the mean; evenly spaced burrow openings will produce a 

variance less than the mean, and clumped burrow openings 

will result in variance greater than the mean. We examined 

the variance:mean ratios for the counts within our samples of 

circular plots to provide an indication of dispersion of burrow 

openings in each colony.

Intraspecific Competition for Habitat

To assess intraspecific competition for habitat, we 

compared habitats occupied by groups of black-footed ferrets 

that were expected to differ in competitive standing. We 

predicted that (1) resident adult ferrets would have a competitive advantage over their wild-born kits, (2) wild-born kits 

would have an advantage compared to released kits, (3) kits 

released first would be more competitive than kits released 

subsequently into the same area in the same year, (4) larger 

kits would have an advantage over smaller kits, and (5) kits 

released into unoccupied habitat during the first year of 

reintroductions at a site would have an advantage over kits 

released in subsequent years to augment a population. As 

outlined above, we assumed burrow density correlated positively with habitat quality. We thus expected dominant ferrets 

to occupy areas of higher burrow density compared to their 

less competitive counterparts. We assessed burrow densities 

estimated from the sample of 0.07-ha plots described above.

As implied by the groups in comparisons 1–5 above, 

various overlapping subsets of animals were used for analyses. 

Montana data were best suited for this assessment because 

ferrets were released in multiple years on several colonies, 

they were released in several consecutive groups in the same 

colonies during 2 years, and resident ferrets were monitored 

during 1 year. As with the broader analysis above, we included 

only those ferrets radio tracked >3 days. Within the Montana 

data set, the comparison of adult and young resident ferrets 

(1 above) was limited to the 1997 subset of data collected 

on Hawley and associated colonies, as was the comparison 

of wild-born and released young ferrets (2). We compared 

groups of young ferrets released sequentially during the same 

years at the same sites (3) within the 1994 and 1995 data 

sets at all colonies. Measures of mass (4) were available for 

Montana animals released in 1994 and 1995, and that variable 

Figure 3. Example of encircling a series of telemetric fixes 

with plots of 15-m radius, within which black-tailed prairie dog 

(Cynomys ludovicianus) burrow entrances were counted, for 

black-footed ferret (Mustela nigripes) no. 32, North Sage Creek, 

S. Dak., on the night of October 26–27, 1997. Overlapping plots 

were allowed for both ferret fixes and random points (sampling 

with replacement).

132 Recovery of the Black-footed Ferret
was included in the assessment of within-year sequential 

releases. Because sexes are dimorphic, we included sex in 

the model to interact with mass. Finally, we compared young 

ferrets released into vacant habitat at Hawley and associated 

colonies in 1995 with young ferrets released into that habitat 

in 1997, when portions of it were occupied by resident ferrets 

(5 above). That 368.3-ha area of prairie dog colonies (the 

four western colonies of fig. 2A,B) was occupied by at least 8 

adults and 19 kits that we marked (not all were monitored via 

the radio tracking of this study).

To provide additional evidence on the effect of competition, we assessed numbers of released ferrets that moved 

between colonies in 1995, when these ferrets were released 

into habitat without a resident population of ferrets, and in 

1997, when ferrets were released into the same prairie dog 

colonies to augment an existing population.

Statistical Evaluation

For statistical comparisons, we reduced burrow density 

data to animal-specific estimates for habitat they used, paired 

with colony-specific estimates for colonies they occupied. 

If an animal occupied more than one colony, we calculated separate pairs of estimates (use and availability) for 

each colony. We used multivariate general linear modeling 

(repeated measures) to evaluate differences between burrow 

densities for colonies and for habitat used by ferrets, assuming 

that all habitat on the colony occupied by a ferret was potentially available to that ferret. General models were reduced to 

more parsimonious versions by backward elimination using 

partial F-tests, when appropriate. Comparisons were judged as 

significant if the probability of committing a Type I error was 

≤0.05. Exact chi-square analyses (Berry and Mielke, 1985) 

assisted in evaluation of proportions of ferrets engaging in 

intercolony movements.

Results

Use of Habitats Other than Prairie Dog Colonies

Of the 24,512 total fixes used, 2,744 (11.19 percent) 

were off colonies. There were 88 instances of intercolony 

movement. Some of the off-colony locations were solitary 

telemetric fixes that could be the result of radio-tracking 

error. Because clusters of sequential fixes provide information on pattern of movement, we assessed off-colony moves 

using groups of ≥2 consecutive fixes away from a colony. 

The number of clustered fixes off colonies was 2,010 in 

474 bouts of movement made by 87 animals with 1 to 24 

bouts per ferret; 1,767 of these (87.91 percent) were associated with exploratory excursions involving returns to the 

colony of origin (fig. 4) and intercolony moves (fig. 5). If 

these cluster-based estimates are applied to the total of 11.19 

percent of fixes away from prairie dog colonies, it appears 

that only about 1.4 percent (0.1119 * 0.1209 = 0.0135) of the 

total number of off-colony fixes may involve dispersal (fig. 

6) without known return to the colony of origin or travel to 

another colony.

Preferences Within Prairie Dog Colonies

Patchiness in dispersion of burrow openings was highly 

evident, as indicated by variance:mean ratios >>1.0 for all 

colonies (fig. 7) sampled by 0.07-ha circular plots. Overall, 

black-footed ferrets preferred patches of habitat with densities 

of prairie dog burrows higher than the averages for colonies 

they occupied (fig. 8). Our general statistical model evaluated overall differences between ferret plots and random plots 

(hereafter referred to as preference) and the effects of sex and 

colony. Sex accounted for relatively little variation (F1,149 = 

0.130, P = 0.719) and was removed from the model. Preference of sites with elevated densities of burrows was consistent 

(F1,154 = 16.996, P < 0.001) among colonies (fig. 8), but the 

magnitude of the differences between burrow densities in 

Figure 4. An example of an exploratory excursion away from a 

black-tailed prairie dog (Cynomys ludovicianus) colony by young 

male black-footed ferret (Mustela nigripes) no. 24, UL Bend 

National Wildlife Refuge, Mont., October 20, 1994. Numbers associated with points are times of day.

Habitat Preferences and Intraspecific Competition 133
ferret and random plots appeared to vary (preference × colony 

interaction; F7,154 = 2.144, P = 0.042).

Intraspecific Competition for Habitat

Three of the four general models in these analyses had 

only class of animal in the repeated measures comparison 

of random and ferret-centered estimates of burrow density; 

these three models were not further reduced. Failure of sex 

and mass (in the 1994 and 1995 Montana data) to explain 

significant variation (P > 0.160) resulted in reduction of that 

model to a simpler submodel resembling the others used to 

evaluate competition. Each of these subsets of data reflected 

the significant habitat preferences of ferrets (P ≤ 0.010) that 

were documented in the more general treatment above. Our 

primary focus in evaluations of competition was centered on 

the interaction term of each model that tested whether classes 

of ferrets influenced variation in differences between habitat 

used and habitat available (preference). In that regard, only 

the comparison between habitat preferences of resident adult 

ferrets and their resident young failed to explain significant 

variation (preference × age interaction; F1,31 = 0.579, P = 

0.452). As predicted, wild-born resident young ferrets were 

Figure 5. An example of an intercolony move by young female 

black-footed ferret (Mustela nigripes) no. 71, UL Bend National 

Wildlife Refuge, Mont., November 5, 1995. Numbers associated 

with points are times of day.

Figure 6. An example of dispersal away from black-tailed prairie 

dog (Cynomys ludovicianus) colonies by young male black-footed 

ferret (Mustela nigripes) no. 213, October 21–22, 1997. Numbers 

associated with points are times of day.

Figure 7. Densities of burrows on study colonies and variance to 

mean ratios (V:M) estimated from samples of 0.07-ha plots. Diameter of symbol is proportionate to V:M ratio within sample of plots.

134 Recovery of the Black-footed Ferret
able to exercise a higher level of preference than did released 

young (fig. 9) (preference × origin interaction; F1,51 = 4.445, P

= 0.040), first-released young ferrets were more selective than 

were young released later the same year (fig. 10) (preference 

× sequence interaction; F1,67 = 4.430, P = 0.039), and young 

ferrets released into vacant habitat were more selective than 

were young used to augment the population in that habitat 

during a later year (fig. 11) (preference × year interaction; F1,62

= 4.063, P = 0.048).

Most (12/13 = 92.3 percent) young ferrets added to the 

resident population in the western colonies of the UL Bend 

complex in 1997 moved between colonies. That proportion 

was significantly different (X2 = 13.789, df = 1, P < 0.001) 

from the corresponding proportion for 1995 (8/27 = 29.6 

percent), when young ferrets were released into the same 

colonies that were then vacant.

Discussion

Use of Habitats Other than Prairie Dog Colonies

The term “preference” suggests that use is compared 

to availability, but we made no explicit attempt to define or 

measure availability of habitat not occupied by prairie dogs. 

Noncolony areas, however, were much more available to 

ferrets (on a large scale at least) than were prairie dog colonies. Thus, the extremely high use of prairie dog colonies by 

black-footed ferrets does indeed suggest strong preference, 

and there was no need to delve into more rigorous analyses of 

preference at that large scale. 

Figure 8. Burrow densities within plots encircling telemetric 

fixes of black-footed ferrets (Mustela nigripes) and within plots at 

random points on black-tailed prairie dog (Cynomys ludovicianus) 

colonies.

Figure 9. Densities of burrows (mean + SE) in areas used by (and 

available to) black-footed ferret (Mustela nigripes) kits released into 

ferret-occupied habitat at Hawley Flats Mont., in 1997, and densities 

of burrows in habitat used by (and available to) the resident wild-born 

ferret kits at that site.

Figure 10. Densities of burrows (mean + SE) in areas used by (and 

available to) the first black-footed ferret (Mustela nigripes) kits 

released at Locke Ranch and Hawley Flat, Mont., in 1994 and 1995, 

and densities of burrows in areas used by (and available to) ferret 

kits after subsequent releases during those years at those sites.

Most ferrets tracked during this study were young of 

the year, and many were captive-born ferrets released onto 

prairie dog colonies. To learn about their new surroundings, 

these naive animals must explore, and some may adopt home 

ranges that include multiple colonies. Thus, the small proportion of telemetric fixes away from prairie dog colonies is 

mostly explained by behaviors that should be expected even 

for a species fully dependent on prairie dogs. Also, a greater 

proportion of off-colony fixes occurred in the 1997 animals 

Habitat Preferences and Intraspecific Competition 135
(16.7 percent), which were subjected to potentially more 

intense intraspecific competition than were the ferrets released 

into unoccupied habitat in 1994 and 1995. Considering that 

the remaining small proportion of “unexplained” off-colony 

moves also involved (1) ferrets that were killed by predators and carried away from colonies, (2) ferrets with whom 

telemetric contact was lost, rendering their future travels and 

fates unknown, (3) predominantly captive-reared ferrets that 

may behave erratically at times, and (4) dispersal that ultimately may lead ferrets to other prairie dog colonies, there 

was little indication that ferrets will attempt to live on habitat 

other than prairie dog colonies, let alone successfully colonize 

other habitats. We documented a high degree of preference 

for prairie dog colonies by ferrets, which weakens the contention that there should be a “broader range of possibilities for 

conservation of the black-footed ferret” (Owen and others, 

2000, p. 422), an argument implying broader habitat tolerances based on similarities between black-footed ferrets and 

Siberian polecats (Mustela eversmannii) and the hypothetical 

niches of North American Pleistocene and Holocene ferrets 

(or polecats). Our data and those of others (e.g., Biggins, 

2000) suggest that natural selection has resulted in considerable divergence of behaviors and nonskeletal features in these 

two extant species of Mustela and that they are “ecological 

equivalents” (Hoffman and Pattie, 1968, p. 57; Lincoln and 

others, 1998, p. 94) only in the broadest sense. Attempts 

to release each species on varying habitats further test this 

hypothesis. Reproductively sterile Siberian polecats persisted 

for only short periods when released on prairie dog colonies 

in Wyoming (16 percent survival for 15 days) and Colorado 

(16 percent survival for 1 day) (Biggins, 2000), and some of 

the polecats used habitats other than the prairie dog colonies. 

Release of Siberian polecats and black-footed ferrets into 

colonies of larger species of North American ground squirrels 

(Spermophilus) has not been attempted but could be informative.

Preferences Within Prairie Dog Colonies

For analyses of habitat preference within colonies, we 

defined as available to a ferret all of the prairie dog colony 

on which it resided. Definitions of availability are always 

somewhat arbitrary but are important because they affect the 

outcome of preference analyses (Johnson, 1980). Prior studies 

of ferret movements (Biggins and others, 1985, 1999; Biggins, 

2000), coupled with the relatively small sizes of the colonies 

of the present study, helped justify our definition. We believe 

that the subjects of our study would not have been physically 

impeded from accessing any portion of the colonies on which 

they resided and were influenced primarily by the variables 

targeted for study (quality of habitat and competition for it). 

Even within the boundaries of prairie dog colonies, therefore, 

ferrets consistently preferred areas with relatively high densities of prairie dog burrows.

The preference of black-footed ferrets for areas on prairie 

dog colonies with high densities of prairie dog burrows was 

made possible by the clumped dispersion of burrows at our 

study sites. This nonrandom and nonuniform arrangement of 

burrow openings may be due to phenomena at several scales. 

Habitat quality for prairie dogs themselves may vary within 

the boundaries of their colonies, resulting from variation in 

soil type, soil depth, slope, and aspect. Vegetative mosaics 

are apparent on some colonies, resulting from these edaphic 

and physiographic attributes and other influences (e.g., plant 

competition) and from grazing by prairie dogs. Thus, the 

patchiness we observed at the scale of our plots (707 m2) is 

likely a reflection of the patchiness at intermediate scales 

(measured in hectares) resulting from the factors mentioned 

above coupled with variation at finer scales caused (at least 

in part) by the social organization of black-tailed prairie dogs 

into coteries and by interconnected burrow openings within 

coteries (Hoogland, 1995). We believe that attention to these 

considerations of scale will be increasingly important in 

gaining a more complete understanding of ferret ecology. 

Former evaluations of habitat for black-footed ferrets (e.g., 

Hillman and others, 1979; Forrest and others, 1985; Houston 

and others, 1986; Miller and others, 1988; Biggins and others, 

1993) heavily emphasized the larger scales of colonies and 

complexes and may have led us to overlook details important to ferrets. Ferret preferences for areas of relatively high 

densities of prairie dog burrows, and the apparent intraspecific competition for those areas, imply qualities that may be 

related to fitness. We hypothesize that the value of clusters of 

burrow openings lies not only in their correlation to clusters 

of prairie dogs as prey but also in the immediacy of protective cover from predators during aboveground movements by 

Figure 11. Densities of burrows (mean + SE) in areas used by (and 

available to) black-footed ferret (Mustela nigripes) kits released 

into ferret-unoccupied habitat at Hawley Flat, Mont., in 1995, and 

densities of burrows in areas used by (and available to) ferret kits 

released to augment the extant population in 1997.

136 Recovery of the Black-footed Ferret
ferrets. Predation appears to be a substantial hazard for ferrets 

(Forrest and others, 1988; Biggins, 2000), causing by far the 

most losses during the repatriation program (Biggins, Godbey, 

Livieri, and others, this volume). Because of the positive association between safety and resources, ferrets are not forced 

into tradeoffs requiring choices between “a productive, but 

risky habitat and a less productive, safer habitat” (Grand and 

Dill, 1999, p. 389). 

Intraspecific Competition for Habitat

Several lines of previous evidence suggest that territoriality is an important feature in the social lives of black-footed 

ferrets. Although direct agonistic encounters between freeranging individual ferrets are rarely seen (Clark and others, 

1986), two adult males were observed in what was described 

as “mortal combat” at the UL Bend in 1997 (Stoneberg, 1997, 

p. 13). Play behaviors in juveniles that may be precursors 

to such behaviors in adults (Poole, 1966, 1967, 1974) were 

commonly seen in free-ranging (Hillman, 1968; Clark and 

others, 1986) and captive (Miller, 1988; Vargas, 1994) litters. 

Agonistic behaviors between captive adult black-footed ferrets 

resembled agonistic interactions of domestic ferrets (Miller, 

1988). General spacing patterns suggest that ferrets occupy 

somewhat distinct territories (Clark, 1989). Scent marking is 

a common behavior in ferrets and is particularly evident for 

males during the breeding season (Miller, 1988). Our understanding of competition among ferrets (especially females) 

for resources or space is nevertheless incomplete. Although 

free-ranging ferrets tend to occupy space that is not used by 

other ferrets of the same sex, occasional sharing of space by 

females during winter (Richardson and others, 1987) and 

even by females with litters (Paunovich and Forrest, 1987) 

raises doubts about exclusiveness of areas of activity. Captive 

Siberian polecats have been held in large cages for prolonged 

periods as same-sex and mixed-sex groups, but, on other 

occasions, aggression has been immediate and severe when 

multiple polecats were introduced into the same space (D. 

Biggins, unpub. data, 1995). Individual black-footed ferrets 

have severely injured their neighbors in conflicts through the 

wire mesh that separated their adjacent outdoor pens, and 

female ferrets have even killed their prospective mates (A. 

Vargas, oral commun., 1995). Simple rules seem inadequate 

for predicting outcomes of interactions. For females especially, 

activity area sizes and their exclusivity in time and space may 

be influenced by habitat quality and variation among individuals (Biggins, 2000), and perhaps nepotism at times masks the 

central tendency of ferrets to defend territories. 

Nonetheless, the general theme of competition among 

black-footed ferrets for possession of space was supported 

by our study; the group that was predicted to be subordinate 

based on prior residency consistently occupied the habitat 

of lower quality. Large body size may be an advantage in 

contests, but we did not detect a significant effect of mass 

in the competition for high-quality habitat among sequentially released young ferrets. Ferrets seemed to follow the 

“bourgeois strategy” (Ramsay and Ratcliffe, 2003, p. 120) in 

which prior residency overwhelms effects of size and other 

factors. The duration of prior residency also may have an 

effect (Harwood and others, 2003). In an experiment involving 

releases of white-throated sparrows (Zonotrichia albicollis) 

into outdoor aviaries, Dearborn and Wiley (1993) noted a 

gradual increase in effect of prior residency from 2–45 days, 

but the increase was most dramatic during the first 14 days. 

Duration of prior residency for ferrets in our sequential release 

experiment was fairly brief, with 2–4 weeks between the first 

and subsequent releases, but duration of residency was >1 year 

for individuals in the extant population that was augmented in 

1997.

As ferret populations are assembled through progressive 

releases and additions of wild-born animals, intraspecific 

competition appears to result in sequential occupation of habitat patches by descending order of burrow (and prey) density. 

As available habitat becomes filled, the additional occupancy 

of sites with lower densities of burrows and prairie dogs is 

expected to increase the variance in burrow density of occupied sites. At sites with low burrow densities, areas of activity 

of ferrets may be largest. These phenomena outwardly resemble the characteristics associated with an ideal free distribution 

or an ideal dominance (despotic) distribution (Fretwell and 

Lucas, 1970). Explorations by released ferrets may be sufficient to impart “ideal” knowledge regarding availability of 

habitat, but territoriality of resident ferrets may prevent “free” 

choice (sensu Fretwell and Lucas, 1970). Further assessment 

of processes involved in ferret habitat occupancy in relation to 

theoretical distributions (Fretwell and Lucas, 1970; Fretwell, 

1972) must consider relative fitness (Messier and others, 1990; 

Beckman and Berger, 2003), a topic we will address separately 

with other data sets. 

Commonly used habitat evaluation systems for blackfooted ferrets (e.g., that of Biggins and others, 1993) likely 

overestimate ferret densities attainable on the best habitats. 

As acknowledged by Biggins and others (1993, p. 75) in the 

introduction to their suggested model, “Social behavior may 

dictate a maximum ferret density regardless of prey abundance.” Mounting evidence regarding territoriality in ferrets 

does indeed suggest that models used to predict carrying 

capacity of habitat for ferrets should include an increasing 

effect of social exclusion of ferrets at high densities of prairie 

dogs. Because the best quality habitats as rated by the model 

of Biggins and others (1993) are presently sustaining ferrets 

at densities almost double those of low-quality habitats, we 

suggest retention of the fundamental structure of the model, 

with modifications recently suggested (Biggins, Lockhart, and 

Godbey, this volume). Although our comparative data suggest 

that competitiveness varies among individuals and has an 

important influence on population assembly (groups varied in 

their ability to control space and resources), we are unable to 

estimate the strength of territoriality at varying prey densities. 

Habitat Preferences and Intraspecific Competition 137
Additional studies on territoriality in male and female ferrets 

could help refine predictions of the model at high prairie dog 

densities. The model also would benefit from an improved 

understanding of habitat limitations for reproductive female 

ferrets inhabiting colonies with low prairie dog densities, a 

subject beyond the scope of this study.

The prior residency advantage raises other issues of 

conservation concern. Quality of ferrets released may vary 

because of prerelease experience (Biggins and others, 1998, 

1999) and age (Biggins, Godbey, Livieri, and others, this 

volume). Preliminary releases of lower quality animals may 

reduce the amount of good habitat available for higher quality 

animals subsequently released if the first animals become 

established. Even if those first residents succumb rather 

quickly to predation, their initial presence could elevate the 

risk to newcomers during the first critical days postrelease. 

Thus, we recommend careful consideration be given to choice 

of sites and sequence of release when habitat will receive 

groups of ferrets varying in prerelease experience, origin, and 

age.

Acknowledgments

Although space prevents us from individually recognizing 

the more than 40 technicians who radio tracked black-footed 

ferrets during these studies, we are grateful for their dedication to those long hours of nocturnal monotony punctuated 

with brief bouts of pandemonium. Funding was provided by 

the Legacy Program of the U.S. Department of Defense; U.S. 

Fish and Wildlife Service (FWS); U.S. Forest Service; U.S. 

Geological Survey; Montana Department of Fish, Wildlife and 

Parks; National Biological Service; National Fish and Wildlife 

Foundation; National Park Service; South Dakota Department of Game, Fish and Parks; U.S. Air Force; U.S. Army; 

and Wildlife Preservation Trust International (now Wildlife 

Trust). We appreciate the assistance from Project Lighthawk 

in moving ferrets from Pueblo, Colo., to Montana. We are 

deeply indebted to those who raised black-footed ferrets for 

the reintroduction efforts, including individuals at the Blackfooted Ferret Conservation Center (FWS and Wyoming Game 

and Fish Department), National Zoo, Henry Doorly Zoo, 

Louisville Zoo, Phoenix Zoo, Toronto Zoo, Cheyenne Mountain Zoo, and the Pueblo experimental rearing facility (FWS). 

Additional efforts were required by dedicated individuals who 

preconditioned ferrets at F.E. Warren Air Force Base, Wyo., 

and Buffalo Gap National Grassland, S. Dak. We appreciate 

the assistance of B. Waltermire and D. Schneider with GIS 

analysis of the prairie dog burrow data. Our generic list of 

agencies and institutions seems inadequate to acknowledge the 

hundreds of individuals who accomplished the multitude of 

tasks necessary to produce these ferrets and move them back 

to their native habitats. The teamwork and coordination were 

mind boggling. We extend genuine appreciation to all, admitting that this is a paltry tribute for accomplishment of such a 

monumentally important task.

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140 Recovery of the Black-footed Ferret


Abstract

Black-footed ferrets (Mustela nigripes) are highly 

dependent on prairie dogs (Cynomys spp.) as prey, and 

prairie dog colonies are the only known habitats that sustain 

black-footed ferret populations. An existing model used 

extensively for evaluating black-footed ferret reintroduction 

habitat defined complexes by interconnecting colonies with 

7-km line segments. Although the 7-km complex remains a 

useful construct, we propose additional, smaller-scale evaluations that consider 1.5-km subcomplexes. The original model 

estimated the carrying capacity of complexes based on energy 

requirements of ferrets and density estimates of their prairie 

dog prey. Recent data have supported earlier contentions of 

intraspecific competition and intrasexual territorial behavior in 

ferrets. We suggest a revised model that retains the fixed linear 

relationship of the existing model when prairie dog densities 

are <18/ha and uses a curvilinear relationship that reflects 

increasing effects of ferret territoriality when there are 18–42 

prairie dogs per hectare. We discuss possible effects of colony 

size and shape, interacting with territoriality, as justification for the exclusion of territorial influences if a prairie dog 

colony supports only a single female ferret. We also present 

data to support continued use of active prairie dog burrow 

densities as indices suitable for broad-scale estimates of prairie 

dog density. Calculation of percent of complexes that are 

occupied by prairie dog colonies was recommended as part of 

the original habitat evaluation process. That attribute has been 

largely ignored, resulting in rating anomalies.

Keywords: black-footed ferret, burrows, carrying capacity, competition, Cynomys, energy, habitat, Mustela nigripes, 

prairie dog, territory

Introduction

By 1988, captive breeding of black-footed ferrets 

(Mustela nigripes) rescued from a failing population in 

Wyoming was becoming successful (Biggins and others, 

1997), and a subcommittee of the Black-footed Ferret Interstate Coordinating Committee (ICC) addressed the challenge 

of locating, evaluating, and comparing sites for black-footed 

ferret reintroduction. Habitat for terrestrial species, including prairie dogs (Cynomys spp.), is commonly evaluated with 

respect to vegetative and physiographic features. Although we 

recognize the crucial link between prairie dogs and their environments, the extreme specialization of the black-footed ferret 

allows us to equate black-footed ferret habitat with prairie dog 

colonies. A habitat model now in common use was developed 

by the ICC to assess the ability of prairie dog colonies and 

complexes to support populations of black-footed ferrets 

(Biggins and others, 1993). The model arose from earlier 

descriptions and models of ferret habitat (Linder and others, 

1972; Hillman and others, 1979; Forrest and others, 1985; 

Houston and others, 1986; Miller and others, 1988), models 

of ferret energetics (Stromberg and others, 1983; Powell and 

others, 1985), data on ferret nutrition and food habits (Sheets 

and others, 1972; Campbell and others, 1987; Joyce, 1988), 

and information on behaviors of free-ranging ferrets (Hillman, 

1968; Biggins and others, 1985; Paunovich and Forrest, 1987; 

Richardson and others, 1987). Biggins and others (1993) also 

provided a method for estimating approximate densities of 

prairie dogs from strip transect samples of active burrows and 

offered a technique for grouping colonies into complexes. 

Complexes were defined as clusters of colonies that could be 

circumscribed with 7-km line segments; colonies are sequentially added to a complex if they are separated by ≤7 km. 

Spaces within a complex that are devoid of prairie dogs are 

defined similarly. 

We herein suggest changes to procedures described 

by Biggins and others (1993), based in part on information 

collected during 1991–2003 from reintroduced populations 

of black-footed ferrets, and we discuss aspects of the existing system needing renewed emphasis. Changes include 

assessing portions of complexes at a smaller scale (called 

subcomplexes), incorporating the effects of ferret territoriality 

Evaluating Habitat for Black-footed Ferrets: Revision of an 

Existing Model

By Dean E. Biggins,1 J. Michael Lockhart,2 and Jerry L. Godbey1

1

U.S. Geological Survey, Fort Collins Science Center, 2150 Centre Ave., 

Bldg. C, Fort Collins, CO 80526.

2

U.S. Fish and Wildlife Service, National Black-footed Ferret Conservation 

Center, P.O. Box 190, Wellington, CO 80549.
144 Recovery of the Black-footed Ferret

in assessments of habitat carrying capacity, and limiting the 

effect of ferret territoriality on small habitat patches where 

social strife is unlikely to influence ferret use. 

Subcomplexes

The initial impetus for considering smaller, more 

compact clusters of prairie dog colonies as subcomplexes 

stemmed from de facto procedures used to select and prioritize 

ferret release sites. Sites were intuitively regarded as high 

quality if colonies were closely spaced or large and if prairie 

dog densities were high. Release of ferrets took place on such 

“core” sites, with much less attention given to the remainder 

of the complex as defined by the 7-km procedure (Biggins and 

others, 1993). We describe a process, involving subcomplexes, 

that has been in practical use since 1999 by the U.S. Fish and 

Wildlife Service and partners to allocate ferrets. 

On occasion, more than one cluster of colonies has been 

used as a release site, but ferret releases in each year have been 

conducted on relatively small portions of complexes. Following initial release(s), ferrets rather quickly populated some of 

these core release areas through natural reproduction; other 

clusters in a complex defined with the 7-km criterion were 

mostly populated with additional releases (e.g., Conata Basin, 

S. Dak.) or natural dispersal over longer time periods (Shirley 

Basin, Wyo.). Although lines of delineation are arbitrary, ferret 

movement within clusters where colonies were separated by 

≤1.5 km was common. At UL Bend National Wildlife Refuge, 

Mont., for example, there were 88 intercolony moves by radiotagged ferrets during 1994, 1995, and 1997 (Biggins, Godbey, 

Matchett, and Livieri, this volume); 77 (85.5 percent) of these 

moves were between colonies separated by <1.5 km, but all 

moves were between colonies separated by <2.1 km. In the 

Meeteetse, Wyo., complex of white-tailed prairie dogs (Cynomys leucurus) that supported the remnant ancestral population 

of ferrets, most colonies were interconnected with a 1.5-km 

maximum distance between them. Based on these experiences and data, we propose defining a subcomplex as a group 

of colonies that can be linked to one another with a series of 

line segments ≤1.5 km in length. The procedure for outlining 

a subcomplex will be further standardized by following the 

method used to circumscribe a complex (Biggins and others, 

1993), but substituting a 1.5-km line segment for the 7-km line 

segment (fig. 1). 

Territoriality and Carrying Capacity

Carrying capacity has been traditionally discussed in 

terms of populations of an organism and their food supply, 

with implications of sustainability of resources (Leopold, 

1933). In that sense, the habitat model of Biggins and others 

(1993) attempts to estimate carrying capacity of prairie dog 

colonies for black-footed ferrets. Many organisms, however, 

seem to space themselves within habitat. Carnivores are 

often intrasexually territorial, but King’s (1990) chapter on 

“adjustable living spaces” provides evidence that there is an 

interaction between habitat quality (mainly abundance of 

prey) and territoriality for other Mustela species. The utility 

of estimating the upper limits of habitat to sustain organisms, 

whether such limits are imposed by food or other mechanisms, 

was evident in early attempts to model regulated growth with 

the logistic equation (Pearl and Reed, 1920) and in Leopold’s 

(1933) discussions of managing game for sustained yields. 

More recent efforts at modeling ferret population fluctuations 

require similar input (Bevers and others, 1997). 

Although somewhat conflicting evidence precluded 

considering ferret territoriality in their earlier model, Biggins 

and others (1993, p. 75) suggested that “social behavior may 

dictate a maximum ferret density regardless of prey abundance.” There is increasing evidence that black-footed ferret 

territoriality does indeed constrain predictions of the energetics model when prey may not be limiting. First, reintroduced 

ferret populations in South Dakota habitats seldom had 

average densities exceeding about 1 female per 30 ha, even 

though the energetics-based model often predicted 1 female 

per 20 ha or less. Additional evidence from ferrets released 

in Montana and South Dakota suggests that there is competition for good quality habitat (Biggins, Godbey, Matchett, and 

Livieri, this volume). These recent results are consistent with 

observations that female ferrets generally do not use overlapping areas (Richardson and others, 1987) and evidence of 

spacing in other Mustela species (Powell, 1979; King, 1990). 

The mounting evidence is sufficiently compelling that we here 

suggest adding a function to the simple linear relationship 

between densities of black-footed ferrets and prairie dogs that 

Figure 1. Procedure for circumscribing a subcomplex of prairie 

dog (Cynomys spp.) colonies by using a minimum intercolony 

distance of 1.5 km. See Biggins and others (1993) for additional 

details on the methodology.
Evaluating Habitat for Black-footed Ferrets 145

will have increasing impact as ferret density rises. A guiding 

principle is parsimony; we do not suggest adding complexity 

that is unsupported empirically. 

We revised the energetics-based model (Biggins and 

others, 1993) to allow an effect of territoriality that is initiated 

at densities of 18 prairie dogs per hectare, gradually increases 

in intensity, and reaches an asymptote of 0.04 ferret families 

per hectare at a prairie dog density of 42/ha (fig. 2). Because 

a black-footed ferret family includes one female, we are 

discussing female ferret density. A quadratic equation (Y = a 

+ bX + cX2) adequately approximates the proposed curvilinear 

relationship within the range 18–42 prairie dogs per hectare, 

where: Y = predicted density of female ferrets, X = density 

of prairie dogs, a = -0.00456329, b = 0.00193283, and c = 

-0.00002083. If there are <18 prairie dogs per hectare, the 

equation for the straight line segment of the graph remains Y 

= 0.00131062X, a slope determined by the existing energetically based estimates and a linear relationship between ferret 

density and prairie dog density. Although white-tailed prairie 

dogs occasionally have been found at densities >20/ha, the 

graph suggests (correctly, we believe) that density of female 

ferrets seldom will be limited by territoriality on white-tailed 

prairie dog habitat. In contrast, we believe that territorial 

behavior of female ferrets will commonly influence their spacing on most black-tailed prairie dog (Cynomys ludovicianus) 

habitat. Under our proposed relationship, that influence will 

increase until female ferrets reach densities of about 1 ferret 

per 25 ha of habitat (the maximum density of 0.04 ferrets per 

hectare). We predict that prairie dog densities above 42/ha will 

not result in increased densities of territorial female ferrets, but 

these higher densities of prairie dogs may affect other population attributes such as ferret survival and productivity.

Habitat-induced Isolation

“Islands” or “peninsulas” of habitat with high densities 

of prairie dogs may support more ferret families than would 

large blocks of uniform habitat because some configurations 

of habitat can reduce among-female interaction. An “island” 

arrangement with a colony small enough to support just one 

female and her litter (figs. 3 and 4) seems likely to eliminate 

any potential for limiting effects of territoriality. A “peninsula” 

configuration removes that effect on two sides, but territorial spacing comes into play for end-to-end territories along 

linear habitat. The example of 20 ha of prairie dog colony 

needed to support a female and her litter (fig. 3) is somewhat 

conservative. Five of the nine ferret litters reported by Hillman 

and others (1979) in Mellette County, S. Dak., were raised 

on colonies <16 ha in area (one was 10 ha). A female ferret 

raised two kits on a 5-ha colony in Montana (fig. 4); however, 

it seems doubtful that the Montana female could have accomplished that feat without seriously depleting the prairie dog 

population, and her small litter suggests that conditions may 

have been suboptimal. At Meeteetse, Wyo., the smallest 

colonies that supported females with litters were about 50 ha, 

but white-tailed prairie dogs at Meeteetse occurred at much 

lower densities (about 7.7/ha, calculated from the visual count 

density of Clark and others (1985) divided by the sightability 

adjustment of 0.495 of Biggins and others (1993)) than did the 

black-tailed prairie dogs discussed above (Hillman and Linder, 

1973). We accommodate the most extreme of these influences 

of colony sizes and shapes into the evaluation procedure with a 

provision that removes the effect of territoriality if a colony is 

sufficiently small and isolated to support just a single female. 

To facilitate evaluation of prairie dog complexes as habitat for 

black-footed ferrets, a spreadsheet with appropriate formulae 

is available from the authors.

Colonies as small as the minimum mapping unit (5 ha) 

suggested by Biggins and others (1993) may support a female 

and her litter. Usually, however, colonies <10 ha will not have 

sufficient numbers of prairie dogs to sustain both themselves 

and a ferret family. Depletion of prairie dogs can be expected 

on colonies <10 ha if they are occupied by a ferret family, 

and it seems unlikely that such small colonies will support 

ferret reproduction in consecutive years. Nevertheless, we 

propose allowing colonies as small as 5 ha to contribute 

to the family rating of a complex by using the direct linear 

equation (Biggins and others, 1993), without the influence of 

Figure 2. Hypothetical relationship between densities of prairie 

dogs (Cynomys spp.) and densities of female black-footed ferrets 

(Mustela nigripes) (individuals/ha), allowing territoriality to modify 

the linear relationship predicted by the energetics-based equation (Y = X/763) of Biggins and others (1993) at prairie dog densities 

>18/ha but defining an upper limit of 0.04 ferrets/ha. At intermediate prairie dog densities (18–42/ha), the increasing influence of 

territoriality is approximated by the quadratic equation Y = a + bX 

+ cX2, where Y = density of ferrets, X = density of prairie dogs, a = 

-0.00456329, b = 0.00193283, and c = -0.00002083.
146 Recovery of the Black-footed Ferret

Figure 3. Comparison of female black-footed ferret (Mustela nigripes) numbers supported by hypothetical prairie dog (Cynomys spp.) 

complexes occupying 640 ha: (A) a complex with a single square colony (n = 16 ferrets); (B) a complex with 20-ha colonies at sufficient 

spacing to allow separate ferret territories (n = 32 ferrets); (C) a complex with a single linear colony (22 ferrets); and (D) a complex 

with a single rectangular colony (18 ferrets). These predictions are based on the following assumptions: (1) ferret territories are 40-ha 

squares, (2) a patch of prairie dog habitat occupying at least 20 ha is centered in the territory, and (3) a habitat patch of 20 ha has sufficient prairie dogs to sustain a ferret family while maintaining its prairie dog population.

Figure 4. Activity areas (circumscribed by minimum convex polygons) for three female black-footed ferrets (Mustela nigripes) with 

litters. These females and their litters were repeatedly relocated 

during summer 1998 at UL Bend National Wildlife Refuge, Mont. 

Heterogeneity in dispersion of black-tailed prairie dog (Cynomys 

ludovicianus) burrows (small dots) is evident. The female ferret on 

the small colony is relatively insulated from repeated contact with 

other females. In this example, areas of dense prairie dog burrows do not form true “islands” of good habitat, but low densities 

of prairie dog burrows in the central portion of the larger colony 

may have influenced separation of ferret activity areas.
Evaluating Habitat for Black-footed Ferrets 147

territoriality. If the rating using the linear equation is less than 

two female ferrets, then a single colony, regardless of size or 

prairie dog density, may be evaluated with the linear relationship (even if the result is a rating <1.0). 

The concept of islands and peninsulas discussed above 

creates an image of prairie dog colonies within landscapes that 

have areas devoid of prairie dogs. Islands with high densities 

of prairie dogs, however, may also be situated within intervening habitat of low prairie dog density. Thus, the island effect 

may be operative within colonies that have heterogeneous 

densities of prairie dogs. The mosaic of prairie dog densities 

is reflected by nonuniform densities of prairie dog burrows. 

Heterogeneity in distribution of burrows may influence separation of activity areas of at least some female ferrets (fig. 4). 

Another Look at Burrow Densities as 

Indicators of Prairie Dog Density

Biggins and others (1993) suggested that densities of 

active burrows were significantly correlated with densities 

of prairie dogs determined from visual counts. Severson and 

Plumb (1998, p. 864), however, failed to detect a relationship 

between densities of prairie dogs and their burrows, concluding that “burrow counts . . . should not be used to estimate 

or index prairie dog numbers.” This theme has a rather long 

history of debate extending to species other than prairie 

dogs, and a full discussion is outside the intended scope of 

this paper. Nevertheless, an overview of the topic and brief 

discussion of the specific criticism noted above are appropriate because working groups responsible for monitoring ferret 

reintroduction sites have made wide use of burrow sampling 

to calculate indices of habitat quality for ferrets. The need 

remains for a practical technique to monitor prairie dog status 

and trends over large scales of space (thousands of hectares) 

and time (decades). Decisions to use some form of capturerecapture method, visual counts, or burrow indices to estimate 

prairie dog abundance and density depend in part on objectives and available resources (Biggins and others, 2006). In 

addition, choice of method will be affected by precision and 

accuracy required.

Biggins and others (1993) provided only correlation 

coefficients for regression relationships between data from 

burrow transects and visual counts. To enhance comparisons 

with other data sets, more information is needed. Their data 

sets were generated from counts and transects on 30 whitetailed prairie dog plots and 39 black-tailed prairie dog plots. 

Using regression models with constants (Biggins and others 

[1993] reported regression through the origin), the relationships between densities of active burrows and density of 

prairie dogs as determined by visual counts were highly 

significant for both species (white-tailed prairie dogs, F1,28 = 

86.282, P < 0.001, R2 = 0.755; black-tailed prairie dogs, F1,37 

= 29.390, P < 0.001, R2 = 0.443). A comparison of the studies 

done by Severson and Plumb (1998) and Biggins and others 

(1993) reveals differences in several key features that collectively may affect the power to detect correlations (table 1). 

Collectively, the relative ranges of values and various levels of 

sampling intensity (e.g., plot size, number of plots, geographic 

coverage) should have given an advantage to the data sets of 

Biggins and others (1993). Intensity of transect sampling to 

estimate burrow density is as important as other features but 

was not reported by Severson and Plumb (1998).

Evidence of the utility of the burrow transect technique 

is also provided by data generated from its use. The overall 

collapse of the Meeteetse complex of white-tailed prairie dogs 

was documented by using densities of active burrows derived 

from strip transect sampling (fig. 5). It would be difficult to 

imagine that the downward trend during the 10-year study 

was an artifact of the sampling procedure, even without the 

corroborative evidence that exists from visual counts (D. 

Table 1. Attributes of two studies on the relationship between 

densities of black-tailed prairie dogs (Cynomys ludovicianus) and 

densities of their active burrows.

Severson and

Plumb (1989)

Biggins and

others (1993)

Number of States 1 3

Number of plots 24 39

Plot size (ha) 4 9

Area sampled (ha) 96 351

Burrow transects (km) ? 248

Lowest prairie dog density 

(no./ha)

8 0.8

Highest prairie dog density 

(no./ha)

46 54.2

Figure 5. Estimates of white-tailed prairie dogs (Cynomys leucurus) 

on the Meeteetse, Wyo., complex, derived from estimates of active 

burrow density (Biggins and others, 1993). (Adapted from Biggins 

and Kosoy, 2001. Reprinted with permission of the Journal of the 

Idaho Academy of Science, Pocatello, Idaho.)
148 Recovery of the Black-footed Ferret

Biggins, unpub. data, 1988–93) that were repeated annually 

over most of that time period.

The purpose of the foregoing discussion is simply to 

provide evidence that burrow indices are a useful tool for 

indexing prairie dog abundance. This does not imply superiority of the technique compared to other tools; methods must 

be matched to objectives, size of area to be sampled, local 

conditions, and available funding.

Reemphasizing Percent of a Complex 

Occupied by Prairie Dog Colonies

Spatial relationships and shapes of prairie dog colonies 

were discussed above in the context of small islands and 

peninsulas of habitat with high prairie dog densities. Others 

(Forrest and others, 1985; Houston and others, 1986; Miller 

and others, 1988; Biggins and others, 1993) have discussed 

spatial arrangements of prairie dog colonies at larger scales, 

implying that some measure of colony arrangement or density 

within a complex is necessary to adequately evaluate habitat quality for black-footed ferrets. The 7-km limitation to 

intercolony spacing (Biggins and others, 1993) was a partial 

solution, but, without additional criteria, vast complexes that 

are thinly populated with prairie dog colonies may provide the 

same rating as complexes with more compact arrangements of 

colonies (fig. 6). There have been theoretical problems with 

measures of colony dispersion such as intercolony distances 

(Biggins and others, 1993), ultimately leading the ICC to 

adopt the conceptually simple tactic of using percent occupied 

(100 × sum of colony area/total area of complex) proposed 

by Miller and others (1988). During the first decade of ferret 

reintroductions, however, the spatial arrangement of prairie 

dog colonies within complexes largely has been ignored. Few 

participants have bothered to calculate the percent occupied 

attribute suggested as an overview of dispersion of colonies. 

By invoking the new procedure for defining subcomplexes 

of colonies spaced at 1.5 km or less, the consequences of 

this oversight are diminished (but not eliminated). It will 

be possible to examine how much of a complex consists of 

high-quality “core” subcomplexes. Subcomplexes should be 

rated separately from 7-km complexes; they should no longer 

be considered as having equal quality to complexes with the 

same cumulative area occupied by prairie dog colonies (fig. 

6C versus 6A and 6B). Nevertheless, calculation of the proportion of complexes and subcomplexes occupied by prairie dog 

colonies will provide useful additional information (e.g., to 

distinguish between complexes such as A and B of fig. 6), 

and we continue to recommend that management teams at all 

reintroduction sites make these simple measurements. The 

technique will allow improved comparisons of complexes and 

subcomplexes among and within ferret reintroduction sites and 

may help characterize the potential for colony expansion.

Summary of the Procedure for Evaluating 

Ferret Habitat

The following steps for evaluating habitat for blackfooted ferrets summarize the approach suggested by Biggins 

and others (1993) and the modifications presented herein.

1. Map the complex of prairie dog colonies. 

2. Circumscribe the complex by using the 7-km criterion.

3. Circumscribe high-quality subcomplexes by using the 

1.5-km criterion.

4. Estimate areas of complex, subcomplexes, and colonies 

with geographic information system (GIS) software if 

maps are digital. Use polar planimeter or other methods (e.g., dot grid) to estimate areas if only hard copies 

of maps are available.

5. Calculate percent of complex and subcomplexes occupied by prairie dog colonies. 

6. Estimate prairie dog densities on colonies by using 

burrow density transects or visual counts.

7. Enter density and area estimates for each colony into 

separate spreadsheets for the overall complex and all 

subcomplexes.

8. Calculate ferret family ratings by using modified 

formulae (example spreadsheets with formulae are 

available from the authors).

Figure 6. In these three hypothetical complexes, total area 

occupied by prairie dog (Cynomys spp.) colonies (shaded squares) 

is the same (1,000 ha), but the percentages of each complex 

occupied by colonies are 4%, 14%, and 57% for A, B, and C, 

respectively. Are the complexes of equal quality as habitat for 

black-footed ferrets (Mustela nigripes)? Arrangement C qualifies 

as a subcomplex because of intercolony spacing of <1.5 km.
Evaluating Habitat for Black-footed Ferrets 149

Assumptions and Unresolved Questions

We believe that the suggested modifications discussed 

above will improve the existing model but reiterate that any 

model is only an approximation of reality (Biggins and others, 

1993). Reflection on the basic assumptions involved in this 

exercise serves as a reminder of its inexact nature. Assumptions include (1) the average prairie dog weighs 760 g, (2) 

a ferret wastes 20 percent of each prairie dog it kills, (3) the 

several steps involved in estimating caloric demands of ferrets 

are correct, (4) losses of prairie dogs to other causes are 250 

percent of losses caused by ferret predation, (5) the intrinsic 

rate of growth for prairie dog populations (λ) is 1.0, and (6) 

prairie dog populations remain stable. A sobering fact is that 

some of these attributes vary widely (e.g., numbers 4 and 5) 

and are in need of further study. The earlier model implicitly 

assumes that all prairie dogs, regardless of sex or age, are 

equally available as prey. If female ferrets selectively prey 

upon juvenile prairie dogs, their own productivity may be 

more closely correlated with prairie dog productivity than with 

prairie dog density. This possibility leads to questions about 

links between forage production, prairie dog production, and 

ferret production and highlights the potential importance of 

local and annual variation in precipitation. 

A better understanding of prairie dog torpor (Lehmer and 

Biggins, 2005), burrow-plugging behavior, and energetics of 

ferret digging behavior could also improve the quality of these 

models. Is the digging involved in excavating hibernating 

prey more energetically costly than hunting of nonhibernating prey (Harrington and others, 2003)? What is the balance 

in tradeoffs between energetic costs of accessing prey and 

risk of injury in killing prey when comparing hibernating and 

nonhibernating prairie dogs? Does the presumably lower risk 

involved in killing hibernating prey allow use of larger prairie 

dogs that might not otherwise be available?

Territoriality in ferrets also remains poorly understood. 

Key questions include the following: (1) At what densities 

of prey does control of minimum space take precedence over 

control of prey resources? Can our proposed curve be further 

refined? (2) Does nepotism affect territory size and overlap 

(i.e., are females more tolerant of their female offspring 

than of less closely related females)? (3) How do shapes 

and arrangements of high-quality patches within and among 

colonies affect territorial behavior?

Some related topics would be appropriate for additional 

investigation. The earlier attempt to define minimum habitat 

attributes necessary to sustain female ferret reproduction 

(Biggins and others, 1993) may be questioned. Further 

study of female ferret behavior on white-tailed prairie dog 

or Gunnison’s prairie dog (Cynomys gunnisoni) colonies, 

where prey densities are low, would help establish the lower 

limits. Preliminary data suggest a positive correlation between 

productivity of female ferrets and density of burrows in the 

habitat they occupy (D. Biggins, M. Matchett, and T. Livieri, 

unpub. data, 1997–2000), a relationship that also suggests 

further research on habitats with low prey densities. Territorial behavior of male ferrets has been ignored but may be 

an important factor in extinction risk for small populations 

(Conservation Breeding Specialist Group, 2004). Increasing 

numbers of black-footed ferrets in reintroduced populations 

are providing more opportunity to investigate these and other 

important aspects of ferret ecology.

References Cited

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Spatial optimization of prairie dog colonies for black-footed 

ferret recovery: Operations Research, v. 45, p. 495–507.

Biggins, D.E., and Kosoy, M.Y., 2001, Disruptions of ecosystems in western North America due to invasion by plague: 

Journal of the Idaho Academy of Science, v. 37, p. 62–65.

Biggins, D.E., Miller, B.J., Clark, T.W., and Reading, R.P., 

1997, Management of an endangered species—the blackfooted ferret, in Meffe, G., and Carroll, R., eds., An 

introduction to conservation biology (2d ed.): Sunderland, 

Mass., Sinauer Associates, Inc., p. 420–426.

Biggins, D.E., Miller, B.J., Hanebury, L.R., Oakleaf, B., 

Farmer, A.H., Crete, R., and Dood, A., 1993, A technique 

for evaluating black-footed ferret habitat, in Oldemeyer, 

J.L., Biggins, D.E., Miller, B.J., and Crete, R., eds., Management of prairie dog complexes for the reintroduction of 

the black-footed ferret: Washington, D.C., U.S. Fish and 

Wildlife Service, Biological Report 13, p. 73–88. 

Biggins, D.E., Schroeder, M., Forrest, S., and Richardson, L., 

1985, Movements and habitat relationships of radio-tagged 

black-footed ferrets, in Anderson, S.H., and Inkley, D.B., 

eds., Black-footed ferret workshop proceedings: Cheyenne, 

Wyoming Game and Fish Department, p. 11.1–11.17.

Biggins, D.E., Sidle, J.G., Seery, D.B., and Ernst, A.E., 2006, 

Estimating the abundance of prairie dogs, in Hoogland, J.L.,

ed., Conservation of the black-tailed prairie dog: Washington, D.C., Island Press, p. 94–107.

Campbell, T.M., III, Clark, T.W., Richardson, L., Forrest, 

S.C., and Houston, B.R., 1987, Food habits of Wyoming 

black-footed ferrets: American Midland Naturalist, v. 117, 

p. 208–210.

Clark, T.W., Richardson, L., Forrest, S.C., Campbell, T.M., 

III, Casey, D., and Fagerstone, K.A., 1985, Black-footed 

ferret prey base, in Anderson, S.H., and Inkley, D.B., eds., 
150 Recovery of the Black-footed Ferret

Black-footed ferret workshop proceedings: Cheyenne, Wyoming Game and Fish Department, p. 7.1–7.7. 

Conservation Breeding Specialist Group, 2004, Black-footed 

ferret population management planning workshop, final 

report: Apple Valley, Minn., IUCN/SSC Conservation 

Breeding Specialist Group, 129 p.

Forrest, S.C., Clark, T.W., Richardson, L., and Campbell, 

T.M., III, 1985, Black-footed ferret habitat—some management and reintroduction considerations: Cheyenne, Wyoming BLM Wildlife Technical Bulletin No. 2, 49 p.

Harrington, L.A., Biggins, D.E., and Alldredge, W., 2003, 

Basal metabolism of the black-footed ferret (Mustela 

nigripes) and the Siberian polecat (M. eversmannii): Journal 

of Mammalogy, v. 84, p. 497–504.

Hillman, C.N., 1968, Field observations of black-footed ferrets 

in South Dakota: Transactions of the North American Wildlife and Natural Resources Conference, v. 33, p. 433–443.

Hillman, C.N., and Linder, R.L., 1973, The black-footed ferret, in Linder, R.L., and Hillman, C.N., preparers, Proceedings of the black-footed ferret and prairie dog workshop: 

Brookings, South Dakota State University, p. 10–20.

Hillman, C.N., Linder, R.L., and Dahlgren, R.B., 1979, Prairie 

dog distributions in areas inhabited by black-footed ferrets: 

American Midland Naturalist, v. 102, p. 185–187.

Houston, B.R., Clark, T.W., and Minta, S., 1986, Habitat suitability index model for the black-footed ferret—a method to 

locate transplant sites: Great Basin Naturalist Memoirs No. 

8, p. 99–114.

Joyce, S.L., 1988, Feeding behavior and water requirements of 

black-footed ferrets (Mustela nigripes): Laramie, University 

of Wyoming, M.S. thesis, 82 p.

King, C., 1990, The natural history of weasels and stoats: 

Ithaca, N.Y., Cornell University Press, 253 p.

Lehmer, E.M., and Biggins, D.E., 2005, Variations in torpor 

patterns of free-ranging black-tailed and Utah prairie dogs 

across gradients of elevation: Journal of Mammalogy, v. 86, 

p. 15–21.

Leopold, A., 1933, Game management: New York, Charles 

Scribner’s Sons, 481 p.

Linder, R.L., Dahlgren, R.B., and Hillman, C.N., 1972, Blackfooted ferret–prairie dog interrelationships, in Symposium 

on rare and endangered wildlife of the Southwestern United 

States: Santa Fe, New Mexico Department of Game and 

Fish, p. 22–37.

Miller, B.J., Menkens, G.E., and Anderson, S.H., 1988, A field 

habitat model of black-footed ferrets, in Eighth Great Plains 

Wildlife Damage Control Workshop Proceedings: U.S. 

Department of Agriculture, Forest Service, General Technical Report RM-154, p. 98–102. 

Paunovich, R., and Forrest, S.C., 1987, Activity of a wild 

black-footed ferret litter: Prairie Naturalist, v. 19, p. 159–

162.

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population of the United States since 1790 and its mathematical representation: Proceedings of the National Academy of Sciences, v. 6, p. 275–288.

Powell, R.A., 1979, Mustelid spacing patterns—variations on 

a theme by Mustela: Zeitschrift für Tierpsychology, v. 50, 

p. 153–165.

Powell, R.A., Clark, T.W., Richardson, L., and Forrest, S.C., 

1985, Black-footed ferret (Mustela nigripes) energy expenditure and prey requirements: Biological Conservation, 

v. 34, p. 1–15.

Richardson, L., Clark, T.W., Forrest, S.C., and Campbell, 

T.M., III, 1987, Winter ecology of black-footed ferrets 

(Mustela nigripes) at Meeteetse, Wyoming (USA): American Midland Naturalist, v. 117, p. 225–239.

Severson, K.E., and Plumb, G.E., 1998, Comparison of methods to estimate population densities of black-tailed prairie 

dogs: Wildlife Society Bulletin, v. 26, p. 859–866.

Sheets, R.G., Linder, R.L., and Dahlgren, R.B., 1972, Food 

habits of two litters of black-footed ferrets in South Dakota: 

American Midland Naturalist, v. 87, p. 249–251.

Stromberg, M.R., Rayburn, R.L., and Clark, T.W., 1983, 

Black-footed ferret energy requirements—an energy balance estimate: Journal of Wildlife Management, v. 47, p. 

67–73.


Section V. Reestablishing Populations

Reintroduction of black-footed ferrets was a primary focus of attention during the decade 

of the 1990s. This phase of recovery was envisioned in the 1988 recovery plan to encompass 

experiments with rearing of ferrets, development of monitoring techniques, testing of release 

methods, and further investigations of black-footed ferret ecology. Experiments with rearing 

and release of ferrets have been conducted. Improved rearing methods and prerelease conditioning have dramatically enhanced postrelease survival of ferrets, and a new captive breeding 

center was designed to accommodate those methods. Other data from many ferret releases have 

been evaluated to produce the papers of this section.

Abstract

Although the monitoring of black-footed ferret (Mustela 

nigripes) populations following reintroductions has not been 

haphazard, several ferret recovery groups since 1994 have 

recommended development of uniform standards prescribing 

minimum methods, intensities, and frequencies of monitoring 

that would provide data on population size, mortality rates, 

and recruitment. Such standards would promote comparability 

of data among sites, document expectations for those who will 

attempt to establish new populations, and allow the U.S. Fish 

and Wildlife Service and other responsible groups to better 

assess progress made toward achieving recovery objectives. 

Our recommendations are based on methods that have been 

successfully used to monitor natural and reintroduced populations of ferrets and are an attempt to balance needs and costs. 

We suggest a combination of marking ferrets with passive integrated transponder (PIT) tags and annual spotlight searches 

coupled with automated transponder readers to individually 

identify survivors. Unmarked ferrets should be captured and 

implanted with PIT tags whenever possible. These and other 

methods are detailed. Circumstances that may dictate other 

methods or more intensive monitoring (e.g., high rates of loss 

or low recruitment) also are discussed.

Keywords: anesthesia, black-footed ferret, monitor, 

Mustela nigripes, snow tracking, spotlight, transponder, trap

Introduction

The need to prescribe standards for monitoring blackfooted ferrets (Mustela nigripes) at reintroduction sites has 

become apparent to the U.S. Fish and Wildlife Service (FWS) 

and members of the Black-footed Ferret Interstate Coordinating Committee (ICC), who discussed formulating standards 

at the ICC annual meetings of 1994 and 1995. That need 

was reaffirmed as an action item in an American Zoo and 

Aquarium Association program review (Hutchins and others, 

1996) and at the Black-footed Ferret Conservation Subcommittee (of the Black-footed Ferret Recovery Implementation 

Team) meeting of 2001. Standards are needed in order to (1) 

accurately assess progress toward recovery goals, (2) clearly 

define monitoring expectations for future sites for black-footed 

ferret reintroduction, (3) provide guidance regarding methods 

and associated limitations, (4) assure FWS that participants 

provide consistent feedback on progress, and (5) make limited 

data comparable for broad-scale interpretations.

The need for standards does not imply that monitoring is 

presently haphazard. Indeed, several groups releasing blackfooted ferrets have used similar strategies, most commonly 

spotlighting, to evaluate ferret status and trends; however, 

standardizing would increase the opportunity for comparisons 

among sites, years, and other variables of interest. Our suggestions are an attempt to balance needs and cost. Our goal was 

to prescribe methods that maximize applicability of the most 

basic data but would not preclude any group from participation because of cost. Reviews of monitoring efforts during the 

early years of ferret reintroductions in Wyoming, Montana, 

South Dakota, Arizona, Utah, Colorado, and Mexico revealed 

strengths and weaknesses that influenced our recommendations. This prescription defines minimum levels of monitoring, 

but we encourage all working groups to consider using more 

intensive monitoring efforts, when applicable, to help address 

questions of importance to recovery goals.

We are not suggesting procedures for so-called clearances 

(U.S. Fish and Wildlife Service, 1988) related to section 7 of 

the Endangered Species Act, although some of the techniques 

we discuss are useful for those purposes. We do not exhaustively analyze or describe methods beyond the minimum 

1

U.S. Geological Survey, Fort Collins Science Center, 2150 Centre Ave., 

Bldg. C, Fort Collins, CO 80526.

2

U.S. Fish and Wildlife Service, Charles M. Russell National Wildlife 

Refuge, P.O. Box 110, Lewistown, MT 59457.

3

U.S. Fish and Wildlife Service, 2900 4th Ave. N., Suite 301, Billings, MT 

59101.

4

Prairie Wildlife Research, P.O. Box 515, Wall, SD 57790.

5

U.S. Fish and Wildlife Service, National Black-footed Ferret Conservation 

Center, P.O. Box 190, Wellington, CO 80549.

Monitoring Black-footed Ferrets During Reestablishment 

of Free-ranging Populations: Discussion of Alternative 

Methods and Recommended Minimum Standards

By Dean E. Biggins,1 Jerry L. Godbey,1 Marc R. Matchett,2 Louis R. Hanebury,3 Travis M. Livieri,4 and Paul E. 

Marinari5
156 Recovery of the Black-footed Ferret

prescription (e.g., radio telemetry) but provide references for 

more information on those topics. We describe monitoring of 

black-footed ferrets only; monitoring of prairie dog populations, associated species, and diseases at reintroduction sites 

is also important, but such topics are beyond the scope of this 

paper.

Objectives

To monitor is to watch, observe, or check, especially for a 

real purpose (Webster’s New Collegiate Dictionary). Monitoring is needed to accomplish the following objectives:

• assess progress toward site-specific population establishment and make attendant decisions related to the 

need to continue to release captive-bred animals and 

numbers that should be released,

• detect serious problems or catastrophic population 

declines (e.g., due to diseases) that might be remediated,

• assess recovery at the national level, and

• test hypotheses regarding methods involved in establishing self-sustaining ferret populations (e.g., rearing, release, translocation, disease prevention, ferret 

searches, predation reduction).

Specifically, monitoring may provide data to (1) estimate 

population size, composition, and rates of natality and mortality; (2) assess genetic representation within a population; (3) 

identify causes of mortality; (4) document spatial distribution 

of ferrets including dispersal and habitat use; and (5) assess 

condition of ferrets, exposure to diseases, and parasite loads.

Types of Data: Balancing Needs and 

Costs

Useful minimum monitoring levels must produce 

information that identifies whether or not there are serious 

problems and allows assessment of progress toward local 

and national recovery goals (the first three objectives listed 

above). If losses of ferrets are low during initial releases, and 

if later populations appear to be self-sustaining, then monitoring can be maintained at these minimum levels. If problems 

are evident (e.g., excessive losses of ferrets), then we suggest 

increased levels of monitoring to identify their causes. The 

alternatives are site abandonment or sustained augmentation of 

ferrets. Abandonment does not contribute to our understanding 

and may result in repeated mistakes. Sustained augmentation 

seems inefficient but may, in the end, be needed at some sites. 

The fourth listed objective of monitoring relates to 

experimentation and hypothesis testing to better understand 

the ecology of ferrets and improve reintroduction strategies, 

thereby enhancing the prospect for successful species 

recovery. This objective may necessitate monitoring that is 

different and sometimes more intensive than the minimum 

levels prescribed below. This learning objective is sufficiently 

important to programmatic decisions that it may at times take 

precedence over other objectives. Needs vary by site and year; 

further discussion of this objective is beyond the scope of this 

paper. 

Minimum data needed to accomplish the first three 

objectives are estimates of population size, survival rates, 

and annual recruitment. A critical review of the last four 

decades of black-footed ferret monitoring, however, reveals 

that there never have been estimates of these attributes that 

were free of known biases. Recently, we have qualified these 

estimates as “minimums,” recognizing that not all ferrets will 

be found (Biggins and others, 1998). Moreover, “survival” 

rates should really be termed “retention” rates, where failure 

to retain ferrets at a reintroduction site can be due to emigration or mortality. Retention rates are likely biased downward 

because of undetected ferrets, but actual survival rates could 

be higher than retention rates if dispersal away from the 

reintroduction sites occurred without concurrent mortality. 

Population size, survival, recruitment, and associated variances can be estimated with closed form models or iterative 

numerical optimization if unbiased surveys are repeated over 

short time spans (Otis and others, 1978; White and others, 

1982), and even more analytical tools are available if those 

multiple surveys done in short spans are replicated again over 

longer spans (robust designs: Kendall and others, 1995; Hines 

and others, 2003; program MARK: White and Burnham, 

1999). The increased effort in repeated surveys is obvious, but 

avoiding bias caused by observer familiarity gained during 

previous surveys calls for additional constraints, problematic 

logistics, and even greater costs. Thus, we believe that the 

effort required could not be sustained over multiple reintroduction sites and years; the 24-year history of rather intensive 

monitoring of black-footed ferrets provides ample evidence 

regarding how much can be accomplished with available 

resources. Realistically, the tactics that have been used over 

the past 10 years are likely to remain the ones used to monitor 

black-footed ferret populations in the future, and the measures 

of population size, survival, and recruitment obtained by those 

monitoring methods (described below) will have to serve as 

indices to population attributes.

Although those indices (e.g., population size) are biased, 

they are nearly always based on complete coverage of respective reintroduction sites during spotlight surveys. Thus, issues 

of spatial sampling are not relevant. Although coverage may 

be complete, the counts are not a census because all ferrets are 

not found. We do not regard this bias as a fatal flaw, in part 

because it is unlikely to be large and in part because the counts 

can be adjusted for effort, providing indices that are particularly useful in a comparative sense (e.g., comparisons among 

groups and years within sites). Diminishing cumulative detections of unique ferrets over several days of spotlight searches 

(discussed below) provide reassuring evidence that large 
Monitoring Black-footed Ferrets During Reestablishment 157

numbers of ferrets usually do not remain undetected during 

spotlight surveys. The standardization of search methods 

suggested below also will enhance comparability of data sets.

Data Collection Methods

Relatively few techniques have proven effective to 

“watch, observe, and check” black-footed ferrets; each method 

has its advantages, disadvantages, limitations, and risks. 

The methods currently used are snow tracking, spotlighting, 

capture-mark-recapture, and radio telemetry, but each can be 

utilized at varying levels of intensity and can be coupled with 

other strategies to increase the quality and quantity of data. 

Indeed, use of multiple methods allows cross-checking and 

verification of data.

Snow Tracking

Snow tracking involves searching from the ground or 

aircraft to locate tracks and other sign (especially diggings) of 

black-footed ferrets. Individual ferrets can sometimes be identified based on geographic location of tracks and origin and 

terminus points. Counts can be cumulative, giving an estimate 

of ferret numbers, provided that snow conditions remain optimal for at least several days. The strategy involves searching 

along ground transects (Richardson and others, 1987) or aerial 

flight lines (Biggins and Engeman, 1986; Miller and Biggins, 

1988) until tracks or diggings are encountered. Track sets then 

are individually followed from origin to terminus to determine 

individuality and gather accessory information on movement 

pattern (use of space, but only crudely related to time) and to 

opportunistically collect scat for diet information. Broad-scale 

searches for tracks have revealed the presence of ferrets on 

prairie dog colonies that would not otherwise have been monitored. Absence of tracks, however, does not prove absence 

of ferrets because ferrets may remain inactive for many days 

following a snowstorm. 

Snow tracking is least likely to adversely impact ferrets, 

requires little specialized equipment, and is relatively inexpensive. The principal disadvantage is weather dependency; 

although snow is common in the northern and western portion 

of the ferret’s original range, good tracking conditions occur 

only sporadically. Best results are attained when snow cover 

is continuous and undisturbed for several days. Warm sunny 

spells can cause patchiness, and winds can quickly erase 

evidence. Prairie dog (Cynomys spp.) tracks cause confusion 

during searches from the air and ground and may obliterate 

ferret tracks; however, white-tailed prairie dogs (C. leucurus) and Gunnison’s prairie dogs (C. gunnisoni) routinely 

hibernate, and black-tailed prairie dogs (C. ludovicianus) 

also may enter torpor (Lehmer and others, 2001), allowing 

effective midwinter ferret searches during prolonged spells 

of calm, cold weather following accumulations of snow. A 

team of searchers must respond immediately when favorable 

conditions develop. Each site should have a snow-tracking 

plan targeting priority areas for searches so that implementation can be rapid and efficient. Identification of mustelid tracks 

is not always straightforward; long-tailed weasel (Mustela 

frenata) tracks cause potential confusion (Miller and Biggins, 

1988). Individual identities of ferrets can be ascertained if they 

have been marked with passive integrated transponder (PIT) 

tags (see subsection on Capture, Handling, and Marking). If 

ferrets are not individually identified, conservative time and 

space separation criteria should be used (see subsection on 

Minimum Level of Monitoring) to determine the minimum 

number of different ferrets present because ferrets can move 

long distances each night and because several ferrets can 

reside in close proximity.

Spotlighting

Spotlighting has been the universal technique for finding 

black-footed ferrets (Campbell and others, 1985). Prairie dog 

colonies are scanned at night with high-intensity spotlights 

by individuals on foot or in vehicles (e.g., all-terrain vehicles 

or trucks). Recently, most spotlighting has been conducted 

by using continuous illumination while the observer moves 

slowly (10 km/h), but earlier workers, searching on relatively 

small prairie dog colonies, preferred a systematic schedule of 

intermittent illumination from a fixed location (Henderson and 

others, 1969; Fortenbery, 1972). Standardization to the extent 

possible is very important because variation in the manner of 

implementation can lead to erratic results, but standardization 

must be balanced with site-specific needs.

Compared to snow tracking, spotlighting gives much 

more accurate temporal data to accompany spatial data. 

The eyes of ferrets reflect an emerald green shine, but other 

animals, such as badgers (Taxidea taxus), coyotes (Canis 

latrans), weasels (Mustela spp.), deer (Odocoileus spp.), and 

pronghorns (Antilocapra americana), can cause confusion. 

Interorbital distance, distance from ground, and behaviors help 

distinguish ferrets from some other mammals, but distances 

can be deceiving at night, and experience is necessary for reliable and efficient identification. Coyotes tend to briefly look 

at the spotlight, run a short distance, stop, and then look at the 

spotlight again. Weasels dart about much more quickly than 

ferrets and have a more subdued eyeshine. Swift foxes (Vulpes 

velox) run with a rigid gait, so the eyeshine does not undulate, 

then may stop and briefly lay close to the ground. When 

ferrets are moving, their eyeshine tends to bounce because of 

their bounding gait. Deer and pronghorns have much larger 

eyes and tend to be bedded down at night in groups; their 

eyeshine rises when they stand up.

Reported detection rates range from 1.4–102.6 hours 

per black-footed ferret sighting and up to nearly 264 hours 

per unique ferret located (table 1) for surveys of reintroduced 

and wild populations. Sighting rates are influenced by ferret 

density, but topography, vegetation, and varying behaviors of 

the animals (e.g., because of weather, season, origin of stock, 

rearing method) may also contribute to variation in sightability 
158 Recovery of the Black-footed Ferret

(Marinari, 1992). The probability of detecting an individual 

free-ranging ferret with spotlights has not been estimated for 

any set of conditions. Cumulative counts over time, however, 

have been plotted and may generically illustrate probability 

of detection during short time spans, assuming no mortality occurs. Data from the Meeteetse, Wyo., population of 

ferrets on white-tailed prairie dog habitat suggest that about 

82 percent of the cumulative total number of ferrets had been 

counted after four nights of spotlight searches (Forrest and 

others, 1988). Similar data from spotlighting in 17 blacktailed prairie dog colonies in the Conata Basin of South 

Dakota (T. Livieri, unpub. data, 2002) resulted in a steeper 

curve, with 92 percent of the cumulative total counted after 

three nights and 98.5 percent counted after four nights (fig. 

1). For the South Dakota data set, the cumulative proportion 

of ferrets counted also increased as a function of cumulative time spent spotlighting adjusted by area covered during 

the search (fig. 2). Although most ferrets appear to be found 

during diligent searches, individuals can be elusive. In Utah, a 

female remained undetected for 24 months (three surveys) (B. 

Zwetzig, oral commun., 2004); in Arizona, two females were 

not located for 27 months (Hoss and others, 2004); and an 

adult male in South Dakota was first relocated 40 months after 

release (W. Perry, oral commun., 1998).

Location Time Source Hours

Number 

of hours/

ferret 

sighting

Number of 

hours/unique 

ferret sighting

Southwest South Dakota 1966–67 Hillman (1968) 462.0 4.0

Meeteetse, Wyo. Summer 1983 Forrest and others (1988) 260.0 3.0

Summer 1984 Forrest and others (1988) 554.0 4.3

Summer 1985 Forrest and others (1988) 647.0 11.2

Shirley Basin, Wyo. October 1991 Hnilicka and Luce (1992) 121.5 12.2

November 1991 Hnilicka and Luce (1992) 258.5 28.7

Summer 1992 Hnilicka and Luce (1993) 1,256.1 35.9 125.6

November 1992 Hnilicka and Luce (1993) 925.1 17.5 51.4

Summer 1993 Luce and others (1994) 675.8 35.6

October 1993 Luce and others (1994) 1,244.7 52.0

Summer 1994 Staley and Luce (1995) 570.7 95.1

October 1994 Staley and Luce (1995) 591.3 34.8 118.3

C.M. Russell NWR, Mont. 1994–96 Stoneberg (1996) 952.7 3.1 5.9

Conata Basin/Badlands, S. Dak. Fall 1994 Plumb and Marinari (1996) 247.5 7.7 35.4

Summer 1995 Plumb and Marinari (1996) 600.4 26.1 66.7

Conata Basin, S. Dak. September 16–23, 2002 T. Livieri (unpub. data) 462.0 1.4 3.1

Aubrey Valley, Ariz. June–December 2002 Winstead and others (2003) 1,847.0 102.6 263.9

Aubrey Valley, Ariz. June–November 2003 Hoss and others (2004) 2,014.0 69.4 83.9

Table 1. Examples of search efforts expended for locating black-footed ferrets (Mustela nigripes) with spotlights.

Figure 1. Black-footed ferrets (Mustela nigripes) encountered 

per night during spotlight searches on 17 black-tailed prairie 

dog (Cynomys ludovicianus) colonies, September 16-22, 2002, in 

Conata Basin, S. Dak. 
Monitoring Black-footed Ferrets During Reestablishment 159

Cumulative total spotlight counts of ferrets continue to 

increase over time spans of months, but in these longer spans 

it is not reasonable to assume that the estimates are unaffected 

by losses of animals. For a 4-year data set from South Dakota 

(T. Livieri, unpub. data, 1999–2002; data from those colonies 

that were repeatedly searched each month), monthly detection 

rates for males appeared to be lower than rates for females 

(table 2). Assuming a constant monthly survival rate of 0.9763 

(annual survival of 75 percent), the increasing cumulative 

monthly counts in table 2 can be approximated by (constant) 

monthly spotlight detection rates of 0.722 for males and 0.918 

for females. These estimates need refinement but seem to 

reflect differences in ability to detect adult males and adult 

females with spotlight searches. 

Spotlighting can alter behaviors of black-footed ferrets. 

Responses to the lights seem to vary among individual ferrets. 

Some ferrets may avoid the light by decreasing aboveground 

activity, and others may attempt to escape through increased 

movements (Campbell and others, 1985). Spotlights emitting 

white light probably should not be used for prolonged observations of a ferret (Campbell and others, 1985). More equipment (e.g., spotlights, backpack units, batteries) is needed for 

spotlighting than for snow tracking. Similar to snow tracking, 

located ferrets can be identified with remote transponder readers or through capture. 

Capture, Handling, and Marking

Whether ferrets are located by spotlighting or snow 

tracking, identification of each individual may enable (1) 

cumulative minimum counts of animals while positively 

avoiding double counting, (2) an overview of dispersal movements, (3) tests of hypotheses regarding comparisons between 

treatments (e.g., rearing conditions, sex, site, habitat use, 

release method; Biggins and others, 1998), and (4) assessment 

of likely matrilineal relationships within populations (Biggins 

and Godbey, 2003). With some monitoring designs, marking 

also may allow (1) use of mark-recapture methods for population estimation (Otis and others, 1978; White and others, 

1982; White and Burnham, 1999; Rivest and Daigle, 2004), 

(2) use of survival estimators (Lebreton and others, 1992), and 

(3) estimation of age-specific mortality rates.

Successful methods for marking ferrets are passive 

integrated transponder (PIT) implants (Fagerstone and Johns, 

1987) and ear tattoos (Fagerstone and others, 1985). Tattoos 

are usually identifiable only on ferrets that are in hand and 

sometimes become illegible or disappear entirely. Less 

commonly, transponders have ceased functioning or have been 

lost from the ferrets. Passive integrated transponder tags are 

relatively inexpensive and easy to install and have become 

the preferred technique for marking ferrets. Two transponders 

should be implanted, one on the posterior part of the head and 

the second dorsally between the hips. After a ferret has been 

located by spotlighting or snow tracking, its transponders 

can be identified with an automated reader that is left at the 

occupied burrow (Stoneberg, 1996) (fig. 3), or the ferret can 

be captured and identified with a hand-held reader. 

If an attempt at automated transponder reading fails, 

capture can be used as a backup. Capture involves additional 

stress on animals (Thorne and others, 1985) but provides an 

Figure 2. Black-footed ferrets (Mustela nigripes) encountered per 

minute per hectare during spotlight searches on 17 black-tailed 

prairie dog (Cynomys ludovicianus) colonies, September 16-22, 

2002, in Conata Basin, S. Dak. An exponential curve was fitted to 

data. 

 Cumulative counts

1999 2000 2001 2002 Mean

Males

July–August 71.4 70.0 85.2 65.4 73.0

September 92.9 76.7 92.6 96.2 89.6

October 92.9 93.3 100.0 100.0 96.6

November 100.0 96.7 100.0 100.0 99.2

December–on 100.0 100.0 100.0 100.0 100.0

Females

July–August 93.9 92.9 88.3 94.1 92.3

September 100.0 100.0 95.0 100.0 98.8

October 100.0 100.0 100.0 100.0 100.0

November 100.0 100.0 100.0 100.0 100.0

December–on 100.0 100.0 100.0 100.0 100.0

Table 2. Percent of the cumulative total number of black-footed 

ferrets (Mustela nigripes) counted during 1999–2002 at Conata 

Basin, S. Dak.
160 Recovery of the Black-footed Ferret

opportunity to assess condition and take samples of blood, 

parasites, etc. These samples could prove invaluable in disease 

evaluations and for genetic studies. Traps (see fig. A1 in 

appendix) must be in good working order. It is exasperating to 

find a malfunctioning trap at the end of an extended attempt 

to capture an animal. If a burrow system is thought to have 

multiple openings, openings lacking traps may be plugged 

with rocks, wood, or plastic cups (44 oz). When trapping is 

finished, all traps must be retrieved, and all burrows must be 

unplugged.

Dye marking captured ferrets can prevent double counting during a survey and helps identify ferrets already captured 

during a trapping session. Dye can be applied to captured 

animals without use of anesthetics. Effective dyes include 

Nyanzol D (Hoogland, 1995) and hair dyes. Dyes, however, 

are temporary compared to transponders, lasting at best until 

the next molt; PIT tags should be used whenever possible, 

whether or not fur is dyed.

Anesthesia is necessary for many of the procedures 

mentioned above. Anesthetics used in the field on black-footed 

ferrets have included ketamine, a ketamine-medetomidine 

mixture (reversed with atipamezole) (Kreeger and others, 

1998), telazol, and isoflurane. Gas anesthesia (including 

isoflurane) requires a relatively bulky and complicated apparatus, including an induction chamber, vaporizer, mask, oxygen 

bottle, and connecting tubes. Isoflurane, however, allows a 

highly controllable level of anesthesia and maintenance of 

much higher blood oxygen concentrations (Gaynor and others, 

1997). 

Field technicians who need to capture and handle blackfooted ferrets must complete a certification course. Presence 

of a veterinarian is beneficial when using anesthetics and 

handling ferrets. Ferrets should not be released until fully 

recovered from anesthesia, which may take hours with some 

injectable anesthetics.

Radio Telemetry

Radio telemetry has been used on black-footed ferrets 

since 1981 (Biggins and others, 1985, 1986). Telemetry has 

distinct advantages; animals are individually identifiable from 

remote locations with minimal human disturbance, behaviors 

can be monitored remotely (e.g., movements, home ranges, 

activity cycles, dispersal), fates can be identified, additional 

methods of survival analysis are available (Heisey and Fuller, 

1985; Pollock and others, 1989), causes of mortality can be 

identified, and habitat use can be objectively assessed (White 

and Garrott, 1990). Disadvantages include the expense and 

impact of placing transmitter packages on or in the animals. 

Ferrets are assumed to be influenced by a transmitter, whether 

external or implanted; the effect can vary from trivial to 

devastating. Discussions about whether or not to use radio 

telemetry should focus on the degree of suspected impact 

weighed against potential gains in knowledge. Neck abrasions 

have been caused by collars, and premature collar loss has 

been common. The currently recommended collar is made of 

wool and degrades within several weeks to months (Biggins, 

Godbey, Miller, and Hanebury, this volume).

Compared to spotlighting and snow tracking, radio telemetry on black-footed ferrets is expensive and relatively difficult 

to master. Use of radio triangulation during ferret reintroductions has concentrated on intensive but short-term (30–60 days 

postrelease) data collection to compare behaviors of animals 

and document their fates (Biggins and others, 1999: Biggins, 

Godbey, Livieri, and others, this volume). Less labor-intensive, 

automated signal detection was used in releases of ferrets in 

South Dakota and Montana with emphasis on determining 

fates of ferrets, but interpretation of data was problematic. 

Because of the large commitment of time and funds and the 

possibility of adverse impacts on ferrets carrying transmitters, 

we regard radio telemetry as a specialized tool that should not 

be considered for routine monitoring of black-footed ferrets 

(Biggins, Godbey, Miller, and Hanebury, this volume).

Alternative Techniques

Other techniques that have been used in attempts to locate 

ferrets include scent dogs (Reindl, 2004); scent attractants 

coupled with remote cameras or transponder readers; implantable radio transmitters; long-range transponders; night vision 

equipment, such as light amplifiers and infrared detectors; and 

track plates. To date, these techniques have not proved widely 

Figure 3. Automated passive integrated transponder readers 

in waterproof boxes may be left at burrows occupied by blackfooted ferrets (Mustela nigripes). Transponder numbers will be 

recorded as the ferret passes near (or through) the loop antenna 

placed to encircle the burrow entrance.
Monitoring Black-footed Ferrets During Reestablishment 161

applicable under field conditions, but they may become more 

useful in the future.

Recommended Standards

Minimum Level of Monitoring

Under the present circumstances and state of technology, we recommend marking all ferrets, including as many 

wild-born individuals as possible, with two transponder chips; 

spotlighting to locate black-footed ferrets; and identifying all 

ferrets located by using combinations of remote transponder 

readers and capture. Dye marking in addition to PIT tagging 

can allow the searchers to bypass ferrets, avoiding the need 

to set a reader or capture the animals to find out if they have 

already been PIT tagged. Failure to read the PIT tag each 

time a ferret is located, however, may preclude more rigorous 

assessments of population attributes and ferret movements. 

Exactly how these tools are deployed depends on the phase of 

reintroduction and the objectives for monitoring. 

For sites where ferrets are released in fall, we recommend 

a minimum of two spotlighting periods, the first beginning 

30 days after the final release (if there were several, closely 

spaced, sequential releases) and the second, postreproductive 

survey beginning in August of the following year. An existing 

ferret population that has not received additional releases of 

ferrets during the previous 12 months may be monitored with 

an August survey only. A prebreeding survey in March–April 

is highly desirable (for both recently released and established 

populations) but is not considered a requirement. If possible, 

ferret searches should be conducted during bright moonlight. 

Preliminary analyses for Siberian polecats (M. eversmannii) 

and black-footed ferrets suggest that radio-tagged individuals 

of both species were more active during bright nights (full 

moon) than during dark nights (new moon); when the moon 

was partially illuminated, they were more active during the 

part of the night when moonlight was present than when it was 

absent (Biggins, 2000).

Clark and others (1984) suggested methods for locating 

ferrets, and the FWS later recommended criteria for blackfooted ferret surveys to clear prairie dog towns for development activities, application of toxicants, or other actions that 

might be detrimental to an existing population of black-footed 

ferrets (U.S. Fish and Wildlife Service, 1988). Because the 

guidelines were developed from techniques used at Meeteetse 

to monitor a wild population, some aspects are applicable to 

the standards proposed here for monitoring released ferrets. 

The basic recommendations of the survey guidelines are 

reiterated below, and each of these is followed by suggested 

modifications (if any) applicable to the minimum standards for 

monitoring reintroduced ferret populations.

1.When monitoring existing populations, surveys should 

be conducted between August 1 and September 30. 

This is the period when young ferrets have become sufficiently active above ground that they can be captured 

for marking, and it is normally prior to dispersal so that 

litters are usually separately identifiable. Adult males 

seem to be less detectable than adult females during 

this period (table 2).

2. Prairie dog towns should be continuously surveyed 

between dusk and dawn on each of three to five 

consecutive nights to ensure systematic coverage and 

increased opportunity to discover black-footed ferrets. 

A ferret can stay inactive for days (Biggins and others, 1986; Richardson and others, 1987), presumably 

depending on weather and its food supply. We suggest 

adding more nights (if necessary) until no (or few) new 

ferrets are found. If scheduling dictates that spotlighting cannot be continuous from dusk until dawn, then 

gaps in coverage should be rotated among nights so 

that no time period is neglected.

3. Detection depends on the ferret being above ground 

and facing the observer at the time the spotlight is 

directed toward it. Pass the spotlight across the landscape, and follow with a sweep back across the same 

path. A ferret looking away from the light during the 

first pass may become curious and turn toward the light 

on the second pass. Large prairie dog towns should be 

divided into tracts, and each tract should be systematically and repeatedly searched. Each searcher should 

concentrate on an area that ensures at least one pass 

every 30–60 minutes. Rough terrain, dense vegetation, 

and lack of road access may dictate small tracts to 

result in effective coverage. On occasion, the objective may be only to document presence or absence of 

ferrets on colonies, in which case tracts could be large 

(up to 800 ha). The area should be as small as practical to increase the opportunity for detection. In some 

cases backpack spotlighting may be necessary (e.g., 

if vehicle access is impossible or legally restricted). 

If searches are done on foot, then each person should 

concentrate on about 130 ha or less. Boundaries of 

tracts should be well marked to keep searchers oriented 

at night.

4. Observations on each prairie dog town or tract searched 

should begin at a different geographic point on each 

successive night to maximize the chance of intercepting a black-footed ferret during its nighttime activities, the patterns of which tend to be somewhat animal 

specific and repetitive. Even within a night, searchers 

should consider varying their search patterns while 

ensuring even coverage (e.g., alternate traveling northsouth and east-west).
162 Recovery of the Black-footed Ferret

5. Previous guidelines suggested that survey crews consist 

of one vehicle and two observers equipped with two 

spotlights of 200,000–300,000 candle power. Teams 

searching for ferrets in areas with known populations 

have used a wider variety of equipment and organizational strategies. Single searchers on foot, in trucks, 

and with all-terrain vehicles (ATVs) have been effective, and other types of spotlight equipment also have 

been used. Because relative efficiency of various strategies is somewhat site dependent, we propose no limits. 

Use equipment that is suitable for the weather, terrain, 

and personnel.

Additional specifications include the following:

1.It is better to search each site entirely within a short 

time span by using a large number of searchers than to 

use few people over a long time span. The long-span, 

low-intensity method leads to problems in specifying the time interval for which the estimate is relevant 

(e.g., for estimating survival) and increases potential 

for confusion in counting individuals that are not 

recaptured or otherwise identified (e.g., double counting or missing ferrets that moved).

2. Use a systematic sampling scheme giving uniform coverage to the entire area, even though higher densities 

of burrows may be present in some areas than others. 

Resist the temptation to repeatedly return to places 

where ferrets have been seen. Some of the fringe areas 

of prairie dog colonies may have the largest populations of prairie dogs, and intuitive perceptions of habitat quality are not always reliable. Provide markers to 

assist with relocating ferrets and orienting the surveyor.

3. Diligently attempt to identify all ferrets. If a transponder cannot be read remotely, then try to capture the 

ferret. If some members of the team are more adept 

at capture than others, then consider using them as a 

dedicated “capture” crew whose job is to capture and 

identify ferrets rather than search for them. Occasionally, individual ferrets can be identified by unique 

physical characteristics that can be distinguished after 

capture or, even more uncommonly, without capture. 

Acceptable examples we have seen include deep scars, 

missing portions of ears, and missing toes. We do not 

consider differences in coloration and individual mask 

patterns to be sufficiently reliable for individual identification.

4.If individual ferrets are not identifiable, then we recommend a conservative approach to classifying them 

as separate individuals. Unless snow allows absolute 

separation of track sets, ferrets can be classified as 

separate individuals only if it was nearly impossible 

for an animal to have moved between the two locations during the time interval between sightings. For 

sightings separated by <500 m, the sightings must be 

simultaneous (fig. 4). For sightings separated by longer 

distances, we assumed a maximum speed of 6 km/h 

for a ferret, decreasing in a nonlinear manner with 

increasing distance. This maximum has been used to 

screen radio-telemetry data for errors (Breck and Biggins, 1997). We reduced the maximum speed to a low 

of 0.694 km/h with a separation of 50 km because the 

maximum documented movement of a ferret in a 3-day 

period was about 50 km (Biggins and others, 1999). 

Two sightings with distance and time separations that 

plot above the curves of figure 4 can be assumed to be 

separate individuals. This approach mandates substantial evidence for inclusion of animals into a population 

count. To avoid underestimation of population size for 

unmarked populations, a larger survey crew will be 

Figure 4. Minimum separations of distance and time needed to 

classify two sightings of black-footed ferrets (Mustela nigripes) 

as different individuals. Plot B is the lower portion of the curve 

in plot A, rescaled to provide better resolution. Separations 

of two sightings plotting above the curves can be considered 

separate individuals (e.g., two sightings 4 km apart separated by 

30 minutes). 
Monitoring Black-footed Ferrets During Reestablishment 163

necessary (to increase the probability of simultaneous 

sightings of ferrets living in close proximity to each 

other).

5. A brief meeting should occur the morning following each spotlight search session to discuss results 

from the previous night. One important purpose is to 

assess the number of unique individuals that are likely 

represented by ferrets seen but not identified (using the 

criteria of 4 immediately above). 

6. Use a standardized form with a map on the reverse 

side. Record all nonspotlighting periods (e.g., rest 

breaks) on the form, sketch ferret locations on the map, 

and place a marked flag at each ferret location. Use 

Global Positioning System (GPS) equipment to obtain 

coordinates of ferret locations, and record these coordinates on the data form (see appendix for examples 

of forms and checklists). Once coordinates and other 

necessary data have been collected, remove all flags.

7. Ferrets should be double marked before release with 

two PIT tags (anterior and posterior). At present, 

incompatibility among manufacturers requires that 

the ferret program adopt a single system. The tags and 

readers currently used in the ferret program are made 

by AVID® Microchip I.D. Systems (Folsom, La.). 

Transponder technology is developing rapidly, and 

other systems may be practical in the future. 

8. Unmarked ferrets that are wild caught should be 

marked or re-marked if they have lost previous markings. Field anesthesia by a veterinarian or certified 

individual is necessary.

9. An annual report to the FWS should include a table 

listing all ferrets identified in monitoring surveys. Ideally, the table should be in a commonly used computer 

spreadsheet. For each ferret, the following accessory 

information should be provided:

a. Studbook number and field identification number 

(telemetry number, site-specific wild-born animal 

number, PIT tag number, etc.)

b. Sex

c. Method of identification

d. Date(s) of capture or identification

e. Location(s) of capture or identification (Universal 

Transverse Mercator [UTM] coordinates from GPS 

receiver; include datum and grid zone)

f. Observer(s)

g. Date of original release (if applicable)

h. Specimens taken (blood, fecal, parasites, etc.)

i. Other data taken (weight, measurements, etc.)

j. If previously unmarked wild-born kit, identify litter 

size and associated dam. 

10. A standard release form (see appendix), filled out for 

 each ferret released, should also be forwarded to the

 Black-footed Ferret Recovery Program Coordinator.

 As in 9 above, the forms can be tabulated and for

 warded in spreadsheet form on a magnetic disk (see 

 Plumb and Marinari [1996] for an example table).

Recommended Precautions—Legality, 

Human Safety, and Animal Safety

1.If using all-terrain vehicles (ATVs), heavy batteries 

used to power spotlights can change weight distribution and make the vehicles unstable.

2.Riders of ATVs should be certified if required by 

the employer. Night operation and use of a spotlight 

increase the difficulty. Special training should be 

provided on ATV safety and night use. Use appropriate 

protective gear and clothing.

3. Obtain all permits and notify appropriate authorities 

regarding timing and location of spotlighting activity. 

Spotlighting is prohibited or regulated in some States. 

A Federal endangered species permit will be required.

4.Listen to weather reports and be familiar with local 

conditions. Weather can change rapidly, and impending 

changes may not be obvious at night. Hazards include 

lightning, dangerously large hail, tornadoes, and disorientation at night, especially in snowstorms. These 

phenomena are not imaginary; spotlight searchers have 

had close calls with all of them.

5. Searchers should be fully familiar with their assigned 

areas, which may require a visit during daylight. A 

compass or personal GPS unit may allow a technician to avoid becoming lost during thick fog or heavy 

snowfall. Searchers should work in pairs when there is 

a threat of adverse weather.

6.The survey crew should be as well equipped as possible 

with two-way radios. For safety and efficiency, it is 

especially important to maintain frequent communication with individuals working in remote areas. 

7.Landowners must agree (preferably in writing) to the 

activities being conducted on or around their properties 

and should be kept well informed of progress.
164 Recovery of the Black-footed Ferret

8.Respect property, whether public or private. Avoid 

rutting muddy roads, and follow applicable rules and 

procedures for off-road driving with ATVs or larger 

vehicles. If you inadvertently damage property (e.g., 

gates, fences, cattle guards), make any needed repairs 

or arrange to have them done.

9. Spotlights are disruptive, so minimize the observation 

time with intense white light. After locating a ferret, it 

should be observed in the periphery of the light beam, 

using the least illumination possible to maintain contact for necessary follow-up activities (e.g., transponder reading, trapping). Avoid repeated harassment of 

the same animal. 

10.When trapping, do not separate a mother from her kits 

for extended periods. Although unusual circumstances 

may dictate either more lenient or more restrictive 

limits, we suggest limiting such separations to <24 

hours during late July–September. Separations should 

be much shorter if it becomes necessary to trap an 

adult female (that has young kits) earlier in the season. Remember that a burrow blocked by a trap can 

separate the dam from her kits even if no ferrets are 

caught. Traps should be checked at least once per hour 

by approaching the trap and looking all the way into it. 

Closed traps should not be left in burrows (ferrets have 

been inadvertently caught in closed traps). Badgers and 

other predators can kill an entrapped ferret, and severe 

weather can cause hyperthermia or hypothermia. 

11. Use properly maintained traps. Traps that are poorly 

maintained or misused have injured ferrets. For 

example, ferrets have received abrasions and lacerations when forcing their way through gaps at the back 

door, even though the doors were secured with clips. 

We recommend clipping or otherwise fastening each 

corner of the back door. Check for treadle sensitivity, 

protruding wires, broken welds, and bent parts. Poorly 

maintained traps may increase the amount of time 

spent harassing an animal if repeated attempts become 

necessary to catch it. Wrapping traps in pieces of wool 

blanket or burlap helps protect a captured ferret from 

wind and cold and seems to create a more enticing 

tunnel that may facilitate capture and keep the animal 

calm after capture.

12. Ferrets usually should be released into the burrow 

where they were captured and during hours of darkness 

whenever possible. If necessary, a ferret may be held 

in a cool location until the following night. A portion 

of a prairie dog can be given to any ferret that must 

be captured for handling or marking to help mitigate 

the stress of the procedure. If presented at the time of 

release, ferrets often will take these offerings into the 

burrow. Prairie dog remains may attract badgers or 

other predators, so their use should be judicious and 

closely monitored. If your site is within the known 

range of plague, we suggest precautions to avoid inadvertently feeding plague-contaminated carcasses (use 

prairie dogs from plague-free zones or those that have 

been properly quarantined).

13.Contact the Black-footed Ferret Recovery Program 

Coordinator for latest developments regarding trapping 

and handling ferrets, and refer to Thorne and others 

(1985) for additional details.

The best training for monitoring black-footed ferrets is 

assisting in an effort that is already underway. Persons who 

will be responsible for monitoring at a new reintroduction site 

should participate in monitoring at an existing site well before 

the new project begins.

Expanding Beyond the Minimum Standards

1. Groups of ferrets may be released sequentially at a 

site throughout extended periods (60 days or more). 

Spotlight surveys have been conducted 30 days after 

the last release (Montana and South Dakota) and 30 

days after the midpoint of extended releases (Wyoming). For releases over relatively long spans of time, 

a solution might be to conduct more than 1 survey at 

about 30 days postrelease, treating groups of animals 

as separate releases. 

2. Prior estimates of survival of released ferrets using 

spotlighting data were treated as minimum survival 

because ferrets may have remained undetected during 

surveys. With several searches repeated over a short 

time span (e.g., 2 weeks) true survival rate or population size may be estimable. Separate estimates of the 

probability of detection and accompanying variation 

could be investigated with repeated sampling within 

short time spans. The assumption of no emigration or 

other losses is problematic, so each complete search 

should be carried out quickly (one to three nights) and 

repeated as often as expedient.

3. As conditions permit, snow tracking should be used to 

augment spotlighting. Data collected by snow tracking 

may not be directly comparable to spotlighting data. 

Because maximum comparability through standardization across sites and years is an important consideration, snow tracking may supplement spotlighting but 

cannot replace it. Ferret scats have been collected during snow tracking, providing additional opportunities 

for evaluations of food habits (Sheets and others, 1972; 

Campbell and others, 1987) and for molecular genetic 

assessments.

4.Telemetric monitoring will most likely provide con-
Monitoring Black-footed Ferrets During Reestablishment 165

structive feedback for management decisions if used 

during the first release at a new site, at sites with high 

rates of ferret disappearance, during a dramatic population decline, or in studies designed to test hypotheses 

having wide-scale implications (see also Biggins, Godbey, Miller, and Hanebury, this volume). In the interest 

of avoiding additional burden to a dwindling population, it may be tempting to reduce monitoring intensity 

(and eliminate telemetry) at a time when information 

is most desperately needed. The information gained 

through detailed studies during a crisis may be critically important for future success at that site and for 

the recovery program in general. A “failure” may be 

recharacterized as a success if enough is learned to 

avoid repetition of the event at that same site or at other 

sites. As with snow tracking, use of radio telemetry 

does not eliminate the need for the spotlight surveys.

5.The addition of a spring spotlighting survey, conducted 

as described above for the fall and summer surveys, 

provides a useful assessment of overwinter survival 

and an estimate of the breeding population of ferrets. 

These surveys are often conducted in March or April 

(Matchett, 1997).

Other Issues—Duration of Monitoring Program, 

Altering the Intensity, Monitoring and Research

If the ferret population is not yet near estimated carrying 

capacity but its growth is as expected or above, the minimum 

monitoring strategy should be adequate. Because there will be 

a need to know when a population may require augmentation, 

and when a population is doing so well that it can be a source 

of animals for other populations, annual monitoring at these 

minimum levels should be conducted for each year that ferrets 

are released and at least 2 years following the final release. A 

ferret population may be surveyed in alternate years if it has a 

positive growth rate or remains stable because of birth of kits 

at the site for 2 years following the final release and if the site 

will not be serving as a source for translocations of ferrets. 

The most intensive monitoring should be planned for the first 

few years of releases at a site when there are many questions 

and no established record of success, with decreases in intensity during subsequent years. If population growth becomes 

slow or negative, intensive monitoring again is appropriate to 

identify the problem(s). Increased spotlighting and/or radio 

telemetry may be needed in some cases. Other types of monitoring (e.g., for diseases such as plague and distemper; prairie 

dog abundance and habitat quality) are also needed, and their 

results help define the relative need for ferret monitoring. The 

situation predictably will be dynamic, calling for flexibility in 

program management. If some working groups have insufficient resources to respond rapidly to changes, the leadership in 

the national program may need to recommend reallocation of 

resources (e.g., funds authorized under section 6 of the Endangered Species Act, different priorities for research support) 

to sites in response to shifting needs. Even the minimum 

monitoring standards proposed above may need modification 

if (1) the entire program becomes dramatically more or less 

successful than at present, (2) funding radically changes, (3) 

available habitat becomes fully occupied by ferrets, and (4) 

new technology makes more efficient techniques available. We 

strongly recommend close communication between working 

groups and national program managers during the process of 

formulating site-specific monitoring plans. 

The suite of methods described for monitoring blackfooted ferrets has been used for both research and management 

applications, but the distinction between the two purposes is 

poorly defined. Many ferret releases in the near future probably will have a blend of learning objectives (implying research 

with indirect benefits to long-term recovery) and population 

establishment objectives (implying management actions with 

direct, short-term benefits). A single monitoring program often 

contributes to both purposes. For example, snow tracking in 

1982–86 at Meeteetse yielded winter population estimates 

for ferrets, helping to track the welfare of the population in 

the immediate sense, and gave information on movements of 

animals and other aspects of ecology (Richardson and others, 

1987). Used during releases of ferrets, radio telemetry has 

allowed relocation of animals that dispersed into unsuitable 

habitat and has enabled documentation of heavy losses of 

ferrets to predation, information with important short-term 

management implications. In several cases, the primary 

purpose of radio telemetry was to test hypotheses of differential survival and behavior of groups of ferrets produced and 

released under varying conditions (Biggins and others, 1999). 

The minimum spotlighting standards recommended above 

emphasize the immediate need to assess population attributes. 

Addressing other objectives probably will require a more 

intensive strategy, expanded by adding other methods and/or 

increasing the amount of spotlighting (spatially or temporally).

References Cited

Biggins, D.E., 2000, Predation on black-footed ferrets (Mustela nigripes) and Siberian polecats (M. eversmannii)—conservation and evolutionary implications: Fort Collins, 

Colorado State University, Ph.D. dissertation, 185 p.

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168 Recovery of the Black-footed Ferret

Appendix. Forms, Checklists, and Other Information that May Be Useful When 

Spotlighting, Capturing, and Handling Black-footed Ferrets
Monitoring Black-footed Ferrets During Reestablishment 169

Spotlight Schedule/Assignments

Night of:

Hours:

Name PDTs/route Vehicle No. of readers Radio Missing BFF/other
170 Recovery of the Black-footed Ferret

Start and end 

time

Map and 

observation 

number Time seen BFF ID

Transponder 

number Location

Time trap

set/checked

Predators 

observed

Black-footed Ferret (BFF) Survey Form

Mark sequential observation numbers on reverse-side map. Flag each location with BFF ID, date, and time 

for later GPS mapping.

Observer: Night of:

 (e.g., 3/19–20/98)

Transportation type:

Prairie dog colony: (Sketch area searched on map on reverse side.)

Cloud cover: Moon phase:

Snow cover %: Temp.: Wind speed/direction:

Comments:

Total search minutes: 
Monitoring Black-footed Ferrets During Reestablishment 171

ID Sex Date

Head

transponder

Pelvis

transponder Dye Location CDV

Black-footed Ferret Markings
172 Recovery of the Black-footed Ferret

Black-footed Ferret Handling Checklist

1. Verify lack of transponder, other markings, and need to handle

2. Date Time Sex Age ID

3. Dam Stud Location and plot on map

4. Trapper Team

5. Anesthetize at 3.0–4.0 ISO Time

6. DIAL ISOFLURANE TO 1.75, transfer to face mask Time

7. PATIENT NO. 

Time ISO Oxygen Pulse rate

% oxygen 

saturation Respiration rate Temperature

8. Implant transponder chips HEAD PELVIS

9. Test transponder chips

10. Collect hair and label envelope

11. Collect blood and label VACUTAINER (cc) NOBUTO (y/n)

12. Give 1 cc, SC canine distemper vaccine. If recapture, booster given at 2 weeks

13. Give penicillin injection (<1,000 g = 0.3 mL SC >1,000 g = 0.4 mL SC)

14. Apply dye mark: ADULT MALE = ---- WILD MALE = X OTHER

 ADULT FEMALE = WILD FEMALE = 0

15. Health inspection notes, read old tattoo, teeth, anomalies, etc.

16. ISOFLURANE AND OXYGEN OFF

17. Weigh 

18. Monitor recovery

19. Disinfect/clean all equipment and surfaces, prepare for next animal
Monitoring Black-footed Ferrets During Reestablishment 173

Dosages of Injectable Anesthetics for Black-footed Ferrets

KETAMINE/DIAZEPAM DOSAGES MEDETOMIDINE/KETAMINE

premixed 10 mL KET (1,000 mg) with 3.0 mg/kg KETAMINE + 0.075 mg/kg MEDETOMIDINE

2 mL DIAZEPAM (10 mg) Antagonize with 0.45 mg/kg; ATIPAMEZOLE after >30 min

Weight (g)

Light (20 mg/

kg; cc)

Medium (25 

mg/kg; cc)

T/T dose (30 

mg/kg; cc)

Heavy (35 

mg/kg; cc) KET (cc) MED (cc) TOT (cc) ATI (cc)

100 0.020 0.025 0.030 0.035 0.075 0.038 0.11 0.045

200 0.040 0.050 0.060 0.070 0.150 0.075 0.23 0.090

300 0.060 0.075 0.090 0.105 0.225 0.113 0.34 0.135

400 0.080 0.100 0.120 0.140 0.300 0.150 0.45 0.180

500 0.100 0.125 0.150 0.175 0.375 0.188 0.56 0.225

600 0.120 0.150 0.180 0.210 0.450 0.225 0.68 0.270

700 0.140 0.175 0.210 0.245 0.525 0.262 0.79 0.315

800 0.160 0.200 0.240 0.280 0.600 0.300 0.90 0.360

900 0.180 0.225 0.270 0.315 0.675 0.338 1.01 0.405

1,000 0.200 0.250 0.300 0.350 0.750 0.375 1.13 0.450

1,100 0.220 0.275 0.330 0.385 0.825 0.412 1.24 0.495

1,200 0.240 0.300 0.360 0.420 0.900 0.450 1.35 0.540

1,300 0.260 0.325 0.390 0.455 0.975 0.488 1.46 0.585

1,400 0.280 0.350 0.420 0.490 1.050 0.525 1.58 0.630

1,500 0.300 0.375 0.450 0.525 1.125 0.562 1.69 0.675

DOSAGE = BODY WEIGHT * DOSE

 CONCENTRATION

MED/KET CONCENTRATIONS: KET = 4.0 mg/mL

 MED = 0.2 mg/mL

 ATI = 1.0 mg/mL
Figure A1. Design of a trap for black-footed ferrets (Mustela nigripes). This trap is a modification of the model described by Sh
Abstract

By 1973, radio telemetry was regarded as an important 

potential tool for studying the elusive, nocturnal, and semifossorial black-footed ferret (Mustela nigripes), but fears of 

using invasive techniques on this highly endangered mammal 

caused delays. We began radio collaring ferrets in 1981. Use 

of radio telemetry on ferrets proved to be both challenging and 

rewarding. We document two decades of development and use 

that led to the present radio-tagging techniques and methods 

for radio tracking. The 7-g radio collar commonly used after 

1992 was smaller and lighter, relative to mass and size of 

subjects, than collars used in studies of other Mustela. Other 

important developments were a Teflon® coating to shed mud, 

a highly flexible stainless steel cable for whip antennas, and a 

nondurable wool collar. Although collar-caused neck abrasions 

have continued to occur sporadically, a retrospective assessment of minimum survival rates for 724 reintroduced ferrets 

(392 radio tagged), using data from spotlight surveys, failed 

to detect negative effects of radio-collars. In a South Dakota 

study, ferrets that were found to have hair loss or neck abrasions when collars were removed did not exhibit movements 

significantly different from those of radio-tagged ferrets with 

no evidence of neck problems. Prototype transmitters designed 

for surgical implantation had insufficient power output for 

effective use on ferrets. Early attempts at tracking radio-tagged 

ferrets by following the signal on foot quickly gave way to 

following movements by triangulation, which does not disturb 

the subjects. The most effective tracking stations were camper 

trailers fitted with rotatable, 11-element, dual-beam Yagi 

antennas on 6-m masts. We used radio telemetry to produce 

83,275 lines of data (44,191 indications of status and 39,084 

positional fixes via triangulation) for 340 radio-collared ferrets 

during the reintroduction program. Tracking by hand and from 

aircraft augmented triangulation, allowing us to locate animals 

that dispersed long distances and enabling us to determine 

causes of mortality. Justifying further use of radio telemetry 

on black-footed ferrets requires careful consideration of costs 

and benefits.

Key words: black-footed ferret, collar, Mustela eversmannii, Mustela nigripes, radio telemetry, radio tracking, Siberian 

polecat, survival, triangulation

Introduction

Radio telemetry has been used as a tool to study vertebrates for more than 50 years (Kimmich, 1979) and Mustela

since the mid-1970s (Erlinge, 1979). The technique is especially useful for re-locating individual animals that are highly 

mobile, secretive, and difficult to observe. Black-footed ferrets 

(M. nigripes) are among the most nocturnal of carnivores, 

and they are semifossorial, attributes that reduce our ability to monitor them with other techniques. Ferrets may be 

located with spotlights, a technique that is often employed 

for conducting annual surveys of their abundance (Campbell 

and others, 1985; Biggins and others, 1998a). Spotlighting, 

however, affects the behaviors of ferrets (Campbell and others, 

1985), making it less attractive for the intensive monitoring that may be required for behavioral studies. Techniques 

must be matched to objectives, and the relative advantages 

and disadvantages of radio telemetry, spotlighting, and snow 

tracking for studying black-footed ferrets have been summarized elsewhere (Biggins, Godbey, Matchett, and others, this 

volume). This article addresses the challenges of applying 

radio telemetry to studies of black-footed ferrets, in part to 

help a potential investigator decide whether it is the appropriate tool for the goals of the project being considered.

Because of difficulties encountered by earlier researchers 

in studying this secretive species and because technologies 

were rapidly advancing, radio telemetry was recognized as a 

“vital” tool for future ferret investigations (commentary by 

E. Brigham in Linder and Hillman, 1973, p. 162). Erickson 

(1973, p. 156) emphasized a need to use radio telemetry 

on ferrets, lamenting that “the black-footed ferret is one of 

the least well known of all of the endangered mammals of 

the United States, despite 10 years of intensive research.” 

The anticipated importance of this tool was reflected in a 

primary objective of the first captive breeding program for 

Radio Telemetry for Black-footed Ferret Research 

and Monitoring

By Dean E. Biggins,1 Jerry L. Godbey,1 Brian J. Miller,2 and Louis R. Hanebury3

1

U.S. Geological Survey, Fort Collins Science Center, 2150 Centre Ave., 

Bldg. C, Fort Collins, CO 80526. 2

Wind River Ranch Foundation, P.O. Box 27, Watrous, NM 87753.

3

U.S. Fish and Wildlife Service, 2900 4th Ave. N., Suite 301, Billings, MT 

59101.
176 Recovery of the Black-footed Ferret

black-footed ferrets (commencing in 1971), which was “not 

to produce animals for release in the wild, but to learn more 

about . . . safe marking methods” and “means of following 

their travels and home range” (commentary by R. Erickson in 

Linder and Hillman, 1973, p. 26). These experiences of the 

1970s motivated development of prototype transmitters for 

black-footed ferrets, but, by the latter years of that decade, no 

free-ranging ferrets could be found. Our use of radio telemetry 

on black-footed ferrets began in 1981 with the discovery of 

the last known extant population west of Meeteetse, Wyo. Our 

intent is to review the use of radio telemetry for black-footed 

ferret research during the subsequent two decades. There is a 

particular need to document the problems and our attempts to 

find solutions. Detailed discussions of hardware and methods 

that did not work seem as important as discussion of the 

triumphs, if only to provide a better starting point for those 

who might wish to engage in improving the techniques. We 

review the challenges of radio tagging these animals, methods 

used to gather data once they have been tagged, and methods 

for analyzing those data.

Radio Tagging Black-footed Ferrets

In a prophetic prediction of upcoming problems, 

Erickson (1973, p. 157) stated “There is no known way to 

safely develop and test methods of installing radio-transmitter harnesses on live ferrets in the wild.” Although the first 

transmitter packages intended for use on black-footed ferrets 

(fig. 1) were indeed tested on surrogate domestic ferrets (M. 

putorius furo; fig. 2) (C. Hillman and S. Martin, oral commun., 

1980), problems developed when the collars were first used on 

black-footed ferrets at Meeteetse in 1981–82. Neck abrasions 

sometimes occurred with these 15-g collars, and they had 

low power output (table 1, version A-1), in part caused by the 

inefficient brass loop antenna that also served as a collar (fig. 

1). The low power resulted in frequent loss of contact with 

subjects (Fagerstone and Biggins, 1986). Although a more 

powerful collar prototype was produced in 1982 (table 1, 

version B-1), it seemed too bulky for use on ferrets. That 

transmitter was attached to a harness, but tests on surrogate 

prairie dogs (Cynomys spp.) (fig. 3) were unsuccessful. The 

original packages were again used in 1982, but the brass loop 

collars were difficult to fit and collar loss was high (Fagerstone and Biggins, 1986). These first radio collars for ferrets 

transmitted on 164 MHz. 

We also conducted comparative experiments with reception of signals emanating from underground transmitters on 

30 MHz and 164 MHz, reasoning that the longer wavelengths 

would better penetrate soil. The lower frequencies performed 

no better than the higher frequencies during underground 

trials, but problems with transmitting and receiving antennas 

were exacerbated with the lower frequencies (lower frequencies need larger antennas for efficient transmission and reception). All subsequent transmitters were on 164–165 MHz at 

frequencies licensed to the U.S. Department of the Interior.

Early in 1983 we submitted specifications for a new 

transmitter collar to manufacturers of wildlife telemetry equipment, requesting their assistance in producing an improved 

transmitter package. Prototypes from three of the five companies that responded exceeded dimensional or weight limits. 

Two units (table 1, version D-1, fig. 4; table 1, version C-1, 

fig. 5) seemed satisfactory and were used on 10 black-footed 

ferrets in August 1983 (Fagerstone and Biggins, 1986). Reception range was several times greater with model D-1 than with 

model A-1 used in 1981–82. During 1983, however, breakage 

of the whip antenna was common, and sometimes accumulations of clay resulted in large increases in mass and dimensions of the transmitter package (fig. 6). The accumulations of 

clay likely were partially responsible for some neck injuries. 

Various treatments and coatings, including polished acrylic 

(fig. 7A), wool (fig. 7B), and Teflon® (DuPont, Wilmington, 

Del.) heat-shrink tubing (fig. 8), were used in laboratory trials 

and on prairie dogs and ferrets in the field during 1983 and 

1984 to alleviate the mud accumulation problem (Fagerstone 

and Biggins, 1986). The Teflon tubing solved the problem of 

mud accumulation; however, its slippery surface seemed to 

Figure 1. The first radio collars tested for use on black-footed 

ferrets (Mustela nigripes), ca. 1979 (version A-1 of table 1). 

A model similar to “A” was used on ferrets. Photograph by 

D. Biggins.

Figure 2. Early transmitter packages with tuned loop antennas 

(version A-1 of table 1) were tested on domestic ferrets (Mustela 

putorius furo). Photograph by U.S. Fish and Wildlife Service.
 Radio Telemetry for Black-footed Ferret Research and Monitoring 177

Pulse

Version Year Type Weight (g) Antenna

Effective 

power1 Width2 Rate3

Battery life 

(days)

A-1 1981 collar 15 14.0-cm loop -35 104 100

B-1 1982 harness 44.5-cm whip -9 14 34

C-1 1983 collar 10 8.9-cm whip -35 104 30 39

D-1 1983 collar 13 15.2-cm whip -12 25 66

E-1 1983 collar 16.5-cm whip -40 19

D-2 1989 collar 10 15.2-cm whip -18 11–40 25–67 59+

A-2 1991 collar 9 20.3-cm whip variable variable

D-3 1992 collar 7 20.3-cm whip -20 25 47 50

D-4 1985 implant 18 internal coil -41 47

D-5 1985 implant 4 whip -37 80

E-2 1985 implant 26 internal coil -39 60

Table 1. Transmitter packages tested during development of radio-telemetry applications for black-footed ferrets (Mustela nigripes).

exacerbate collar loss, and there were several instances of neck 

abrasions. 

Continued problems with collar loss in 1984 motivated 

additional investigation and development of transmitter attachment methods for ferrets. Disease outbreaks in Meeteetse prairie 

dogs and ferrets (Forrest and others, 1988; Ubico and others, 

1988) ended all hope for continued research on that free-ranging 

population of ferrets; however, the ensuing captive breeding 

program and its ultimate goal of reintroductions underscored 

the importance of improving radio telemetry for ferrets. In 

trials conducted in the spring of 1985, two of three free-ranging 

black-tailed prairie dogs (Cynomys ludovicianus) developed 

neck sores when fitted with old-style ferret collars made of 

vinyl-impregnated cloth but did not seem adversely affected by 

neckbands of wool (n = 4) or leather (n = 4). Prairie dogs gained 

40 percent in mass during a 3-month period. Wool collars sewed 

with cotton thread often wore sufficiently to be lost by prairie 

dogs in 3 to 6 months. Thus, a black-footed ferret with a wool 

Figure 3. Capsules with high power output (Telonics, Inc., Mesa, 

Ariz.) (version B-1 of table 1) were attached to harnesses and 

tested on surrogate prairie dogs (Cynomys spp.) in 1982. Photograph by D. Biggins.

Figure 4. A 13-g transmitter package (version D-1 of 

table 1) used on black-footed 

ferrets (Mustela nigripes)

during 1983–84. Photograph 

by D. Biggins.

Figure 5. A package coated with soft plastic used on black-footed 

ferrets (Mustela nigripes) in 1983 (version C-1 of table 1). Photograph by D. Biggins.

1

Decibels relative to 1 milliwatt (dBm).

2

Milliseconds duration.

3

Pulses per minute.
178 Recovery of the Black-footed Ferret

neckband would not be collared permanently if its radio failed 

prematurely and the animal could not be relocated for collar 

removal. 

One of the goals of research initiated in 1988 on Siberian 

polecats (Mustela eversmannii) and other surrogate species 

was to advance our proficiency in radio tagging and radio 

tracking Mustela before reintroductions of black-footed 

ferrets. Studies of captive Siberian polecats and of reproductively sterilized polecats released into prairie dog colonies in 

Colorado and Wyoming provided opportunities to develop 

and test equipment. Radio collars made of natural materials 

were first tested on 13 captive polecats at the National Zoo’s 

Conservation & Research Center, Front Royal, Va., during 

September 1989. Neckbands were made of leather or wool 

instead of the vinyl-coated fabric used previously. Collar 

retention was the primary reason for preliminary testing of 

radio collars on captive ferrets. Wool and leather collars are 

somewhat elastic, and the >10 percent stretch of these materials might allow animals to slip out of the collars. Overlapping 

ends of wool and leather collars were glued with contact 

cement. The transmitter package for polecats weighed about 

10 g, had a 15.2-cm whip antenna (table 1, version D-2), and 

was attached to a 1-cm-wide wool collar with vinyl tape (not 

Teflon). The 2-stage, 3-V transmitter had a mercury switch 

that triggered change in pulse rate, resulting in pulse intervals 

of about 0.9–2.4 seconds, with pulse interval inversely proportional to activity of the animal (as sensed by motion of the 

transmitter), and a pulse width inversely proportional to pulse 

interval to maintain consistent and predictable current drain. 

Battery longevity was about 59 days. 

Both wool and leather collars were removed by some 

captive animals, but in most cases the shed collars were 

in poor condition. Captive polecats were housed in family 

groups and tended to chew and pull on each other’s collars 

causing rapid wear that we did not expect to occur under field 

Figure 6. A collar from 1983 that accumulated a large buildup of 

clay while carried by a black-footed ferret (Mustela nigripes). 

Photograph by D. Biggins.

Figure 7. The acrylic potting material was polished (A) or encased 

in wool (B) in attempts to alleviate mud accumulation. Photograph 

by D. Biggins.

Figure 8. A 13-g transmitter package (version D-1 of table 1) from 

Wildlife Materials, Inc. (Murphysboro, Ill.), with Teflon tubing covering most of the acrylic potting material (used on black-footed 

ferrets [Mustela nigripes] during 1984). Photograph by D. Biggins.
 Radio Telemetry for Black-footed Ferret Research and Monitoring 179

conditions. Wool collars were no more likely to be pulled off 

than were leather collars, but wool collars wore more quickly. 

Because the “breakaway” feature of wool was desirable, the 

wool collar was selected for testing on the released animals to 

evaluate retention and irritation. Under field conditions, only 

1 of the 13 polecats removed its collar, but that animal did so 

twice. Whip antennas broke on collars worn by two polecats. 

One antenna became completely severed after 10 days on the 

animal, and a radio recovered from a dead polecat had several 

broken strands in its antenna wire. Our simple solution was 

to use slightly heavier wire and an extra layer of heat-shrink 

coating extending 1 cm above the point where the antenna 

protruded from the radio capsule. No sign of worn hair or neck 

abrasion was noted on recovered polecats; however, there were 

only a few days of wet weather during our polecat release 

study, and the soil was sandier than soil at the Meeteetse 

black-footed ferret study area. Therefore, the potential for mud 

accumulation on radio collars was not fully assessed. During 

a short wet period, a small amount of mud was found on the 

collar of one recaptured polecat, but the mud fell away easily. 

Poor survival of polecats hampered the evaluation of radiocollar performance in that study (Biggins, 2000a).

Additional polecats released in 1990 (n = 44) accumulated about 600 animal days wearing the type of radio 

collars described above (but with the modified antennas), 

combining the time that animals carried radio collars during 

arena conditioning with monitoring time after release. The 

wool collars continued to function well overall. One collar 

deteriorated rapidly and was lost from a polecat after only 2 

weeks, perhaps because that animal (no. 34, wild caught in 

China) was exceptionally active. Several other animals lost 

collars, likely in part because of rapid weight loss after release, 

particularly with obese animals (Biggins, 2000a). One instance 

of neck abrasion was noted, and again it was with animal no. 

34. That animal was recollared after losing her first transmitter 

collar; perhaps the tendency was to fit the second collar too 

tightly because of the prior loss.

The polecat from China (no. 34), radio tracked until the 

study ended, lost 50 percent of her body mass and her radio 

collar during the first several weeks postrelease. Perhaps 

that scenario helps explain the high rate of lost radio contact 

with wild-caught polecats (3/5 versus 5/39 for captive-bred 

polecats). Other factors also can cause loss of radio contact. 

Two recovered radio collars were damaged, presumably by 

the teeth of coyotes (Canis latrans). The signal from one of 

those collars was barely audible above ground, even at short 

range (<100 m), suggesting the possibility of complete radio 

failure from bites of coyotes or badgers (Taxidea taxus). Radio 

signals also can be lost when animals are in burrows >2 m 

below ground. Because loss of radio contact could have been a 

result of predation, dispersal, or premature transmitter failures, 

functional longevity for collars could not be estimated.

Similar versions of these transmitters with wool collars 

also were used to study free-ranging Siberian polecats (fig. 9) 

(Zhou and others, 1994) and alpine weasels (Mustela altaica) 

in China (fig. 10). Collars of wool functioned well generally, 

but premature collar loss and occasional neck abrasions 

continued to be problems. Collar loss was especially common 

in the alpine weasel study (Wei and others, 1996). The polecat 

transmitter packages with variable pulse rates used in China 

and the United States produced easily interpreted activity data.

Because of the effective combination of wool collars and 

activity-type transmitters used on polecats, this 10-g unit by 

Wildlife Materials, Inc., (WMI, Murphysboro, Ill.) and a similar variable-pulse rate model by AVM Instrument Company, 

Ltd., (Colfax, Calif.) (table 1, version A-2) were adopted for 

monitoring 37 of 49 black-footed ferrets released during the 

first reintroductions in 1991 at Shirley Basin, Wyo. (U.S. Fish 

and Wildlife Service, 1992). Collars were worn by ferrets for 

2–4 weeks before they were released, allowing prerelease 

observation of animals but also expending 40–68 percent of 

Figure 9. Additional collar tests (version D-2 of table 1) were 

conducted on a subspecies of free-ranging Siberian polecat 

(Mustela eversmannii dauricus) in Inner Mongolia, China. 

Photograph by D. Biggins.

Figure 10. We used smaller transmitters with wool collars to study 

alpine weasels (Mustela altaica) in Qinghai, China. Photograph by 

D. Biggins.
180 Recovery of the Black-footed Ferret

the expected battery life and producing noticeable wear to the 

wool collars.

Fluctuations in mass of animals appeared to create 

problems. Ferrets from the Henry Doorly Zoo (Omaha, Nebr.) 

were 26 percent heavier when collared than were their wild 

counterparts at Meeteetse (P < 0.001) and were 37 percent 

heavier at 5 days postcollaring. Seven of the zoo ferrets 

developed neck sores while being held in cages, perhaps 

because of the increasingly snug fit of the collars as the ferrets 

gained mass. Ferrets raised at the Wyoming Game and Fish 

Department facility were not heavier than Meeteetse ferrets 

prerelease. We accumulated 460 animal days of postrelease 

telemetric monitoring on the 37 ferrets. Loss of mass postrelease (Biggins, 2000a) likely loosened the fit of collars and 

may have exacerbated premature collar loss that occurred in 

nine known instances involving six individual ferrets (ferrets 

were sometimes recollared). One free-ranging ferret had a 

neck laceration when recaptured, but other injuries confused 

determination of the cause(s). There were two known cases of 

mud adhering to the vinyl tape used to attach the transmitter 

package to the collar.

Continued problems with collars in 1991 resulted in a 

more conservative approach to collar configurations used for 

ferret studies in subsequent years. Beginning in 1992, ferrets 

were collared with smaller transmitter packages, and Teflon 

heat-shrink tubing became the standard method for attaching 

the transmitter to the double- or triple-layered wool collar 

(fig. 11). The WMI transmitter had a 20.3-cm flexible steel 

whip antenna (table 1, version D-3). We located an improved 

type of stainless steel wire for whip antennas (designed for 

operating prosthetic limbs) that further reduced the problem of 

breakage resulting from metal fatigue. The 1.5-V battery gave 

an estimated 45–150 days of transmitting, depending on power 

output desired. The completed package weighed 6.0–7.2 g 

(usually <1 percent of the subject’s mass). We believe that 

miniaturization of the transmitter-collar assembly reduced 

problems of collar loss and neck abrasion in black-footed 

ferrets. See appendix for instructions on final assembly of 

these collars and the procedure for fitting them to ferrets.

Serious neck injuries may be caused by improper fit of 

radio collars; abrasions on radio-collared black-footed ferrets 

in 1991 fueled controversy over effects of collars on survival 

of ferrets. Oakleaf and others (1993), using data generated 

from spotlight searches after the second ferret release in 1992, 

stated that “survival indices are significantly (P = 0.002–

0.055) greater for black-footed ferrets released without telemetry compared to ferrets released with telemetry collars.” These 

authors presented four criteria that should be met to enhance 

comparability of collared and noncollared groups in future 

studies. Data for their analyses were generated under conditions that violated two of their criteria, similarity in habitat 

quality and equal accessibility for spotlight searches in areas 

where radio-collared and noncollared ferrets are released. 

Radio-collared ferrets were released on lower quality habitat, 

as measured by densities of prairie dog burrows, than were 

noncollared ferrets, and the areas with collared ferrets were 

less easily searched via spotlighting. Prior recognition of the 

possibility of confounding can be inferred from the hypothesis 

generated before the 1992 release of ferrets, which stated that 

“survival of ferrets released in best habitat, without telemetry 

and with good logistics for spotlight surveys is higher than 

survival in habitat that is possibly less than the best, with 

telemetry, and possibly poorer conditions for spotlighting” (B. 

Oakleaf, quoted in Miller and others, 1996, p. 129). Regarding 

habitat quality, mounting evidence demonstrates a negative 

correlation between ferret dispersal and density of prairie 

dog burrows (Biggins and others, 1999; Biggins, 2000b), 

and ferrets prefer areas with high burrow density (Biggins, 

Godbey, Matchett, and Livieri, this volume).

Confounding of collar effect and other variables was 

problematic in the 1992 sample involving 89 ferrets but 

became less troublesome as sample size increased because the 

potentially confounding variables were not consistently associated with the same primary treatment groups. Thus, it may be 

revealing to examine a much larger data set of reencounters, 

resulting from spotlight surveys about 1 month postrelease, for 

724 ferrets released in four States during 12 years (table 2). 

For all States except Wyoming, cage-reared ferrets were 

excluded from the analysis because ferrets that lack preconditioning in outdoor pens have relatively poor survival rates 

(Biggins and others, 1998a). We could not categorize rearing 

status for some of the ferrets released in Wyoming; thus, we 

pooled rearing categories in Wyoming (similar to the analysis 

of Oakleaf and others, 1993). A multivariate general model 

(with site-year and mark category) and competing nested 

submodels were evaluated with program SURVIV (White, 

1983). Comparisons of Akaike’s Information Criteria (AIC) 

associated with these models (table 3) favored either the 

submodel that pooled collared and noncollared ferrets (AIC = 

52.86) or the general model (AIC = 51.14). Not surprisingly, 

reencounter rates (the product of probabilities of survival 

and capture) for sites-years were likely different. Although 

Figure 11. Fitting a lightweight (6–7 g) transmitter collar (version 

D-3 of table 1) to a black-footed ferret (Mustela nigripes). This 

style of collar has been used since 1991. Photograph by R. 

Reading.
 Radio Telemetry for Black-footed Ferret Research and Monitoring 181

evidence was somewhat equivocal regarding collars, the most 

parsimonious model of the two with low AIC values suggested 

no effect of collars (fig. 12). Regardless of improvements in 

sample size and reduced confounding potential, this remains 

a post hoc analysis of data from experiments designed to test 

other hypotheses. Interactions are probable (fig. 12) and the 

unbalanced design (table 2) allows numerous possible explanations to account for the disparate results for different sites 

and years. Nevertheless, these data do not support the contention that radio collars negatively affect reencounter rates of 

released black-footed ferrets. Perhaps cases of management 

intervention enabled by radio telemetry help compensate for 

potentially negative influences of collars. On a few occasions, 

ferrets that dispersed from suitable habitat were captured and 

translocated; other interventions (also rare) included capture, 

rehabilitation, and rerelease of ferrets that were injured or in 

poor condition.

In a study of translocated ferrets conducted in South 

Dakota in 1999 (Biggins and others, 2000a), neck abrasions 

that ranged from minor hair loss to a case of severe ulceration 

were noted on 10 radio-tagged black-footed ferrets (of 36 

released) when animals were reobserved during the study or 

recaptured for collar removal at the end of the study. A categorical variable (abrasion, no abrasion) for neck condition was 

evaluated during statistical modeling to assess movements and 

dispersal of the primary treatment groups (released captivereared versus wild-born ferrets). There was no evidence that 

Year No radio Radio No radio Radio No radio Radio No radio Radio Total

1991 12 37 49

1992 52 37 89

1993 48 48

1994 17 7 6 37 67

1995 35 37 72

1996 28 39 67

1997 2 57 59

1998 41 42 83

1999 30 35 18 83

2000 27 29 56

2001 35 8 43

2002 8 8

Total 28 52 50 211 105 55 149 74 724

Montana South Dakota Utah Wyoming

Table 2. Numbers of black-footed ferret (Mustela nigripes) kits released with and without radio collars. Assessment included only 

preconditioned kits (except in Wyoming).

Table 3. Modeling minimum short-term (1 month) survival rates 

of 392 radio-collared and 332 noncollared black-footed ferrets (Mustela nigripes) released in Montana, South Dakota, Utah, 

and Wyoming.

Figure 12. Minimum survival rates of preconditioned black-footed 

ferret (Mustela nigripes) kits at about 1 month postrelease.

Model Log-likelihood npa AICb

General -17.534357 8 51.06871

All same -92.542614 1 187.08523

Collaring same -22.228649 4 52.45729

Sites-years same -85.786658 2 175.57332

a

np = number of parameters.

b

AIC = Akaike’s Information Criterion.
182 Recovery of the Black-footed Ferret

neck abrasions affected any of the attributes of movements 

examined (P > 0.19 for all models), even though the experimental design and statistical analyses were sufficiently powerful to detect significant effects of several other variables.

In summary, collar-caused mortality of ferrets has not 

been documented, and there is no evidence of negative effects 

of radio telemetry on ferret populations or average behaviors 

within groups of ferrets. Nevertheless, collaring can at times 

negatively impact individual ferrets. Moreover, it seems best 

to assume, even without the latter evidence, that an unnatural 

protuberance of any sort will influence a free-ranging animal’s 

behavior to some degree, even if that influence is not detectable statistically. Such influences may be acceptable, particularly if it can be reasonably assumed that they equally affect 

all treatment groups of an experiment. Decisions on whether 

or not to use this monitoring tool may rest with cost/benefit 

analyses. If information potentially gained could enhance 

success of future conservation of the ferret, risk to individuals 

may be warranted. The arguments, however, appear similar 

to those discussed with reference to releasing adult ferrets 

(Biggins, Godbey, Livieri, and others, this volume), wherein 

“some conservationists and ethicists may justify extreme means 

to achieve the goal of preservation and recovery” of a species, 

while “others may set inviolate moral standards regarding the 

welfare of individuals.”

Compared to other recent studies of Mustela that have 

involved radio telemetry, our present collars have rather 

conservative dimensions and mass. Considering Mustela of 

sizes similar to black-footed ferrets, 27-g and 25-g collars 

were fitted to feral domestic ferrets in New Zealand (Moller 

and Alterio, 1999; Byrom, 2002), collars of 25–35 g were 

placed on European polecats (M. putorius) in Italy (Marcelli 

and others, 2003), and endangered European mink (M. 

lutreola) were tagged with collars of about 13 g in Spain 

(Zabala and others, 2003). Collars weighing 10 g (likely 4–6 

percent of body mass) were placed on stoats (M. erminea) in 

New Zealand (Moller and Alterio, 1999). Although Jedrzejewski and others (2000) tagged least weasels (M. nivalis) in 

Poland with collars of only 3.5–4.5 g, that mass was about 

4 percent of the body mass of their subjects. Realizing the 

sensitivity of these animals to handling and collaring, the 

latter investigators placed the weasels into an enclosure for 

several days of observation before final release at the location 

of capture. We are aware of problems of collar loss and neck 

abrasion caused by radio collars in other studies of radiotagged Mustela, although discussions of such difficulties are 

seldom published. 

Problems with collars precipitated evaluations of intraperitoneal and subcutaneous implants for black-footed ferrets. 

Surgically implanted transmitters have been used effectively in 

several other mustelids such as river otters (Lutra canadensis; 

Hoover, 1984), badgers (Minta, 1993; Goodrich and Buskirk, 

1998), and American mink (Mustela vison; Stevens and others, 

1997). In 1985, we solicited prototype implantable transmitters suitable for ferrets from radio-telemetry equipment 

suppliers. Two of these units were designed for intraperitoneal use (table 1, versions D-5 and E-2), and a smaller unit 

(table 1, version D-4) was to be used subcutaneously. All had 

disappointingly low power output, leading us to believe that 

the problems we had in 1991 with loss of contact with ferrets 

would be worse with the implanted transmitters. Power output 

of the implants was initially lower than even that of the first 

radio collars used (table 1) and could be expected to be further 

degraded after implanting by signal attenuation caused by the 

ferret’s body. Thus, we did not proceed to the next planned 

step in tests, which was to surgically implant the transmitters 

into surrogate Siberian polecats.

We did, however, use intraperitoneal and subcutaneous 

implants in American badgers at the Meeteetse study area in 

1984. The dorsally implanted subcutaneous units with 15.2-cm 

implanted whip antennas radiated more powerful signals than 

did intraperitoneal units in the same animals, but abscesses 

that developed around the subcutaneous transmitters resulted 

in their premature loss. Compared to signals from the radiotagged ferrets, which were then carrying relatively powerful 

transmitters (table 1, version D-1), signals from the subcutaneous implants in badgers were about as easily received from our 

fixed stations, but the intraperitoneal implants in badgers were 

much more difficult to track. Allowing that technology might 

have improved during the subsequent decade, we repeated 

the process of acquiring prototype implantable transmitters 

for ferrets in 1997, with generally similar results. Relatively 

poor reception range is a well-known attribute of implantable 

transmitters, in part because of the compromises necessary 

with transmitter antennas, which can translate into reduced 

precision and accuracy of data (Koehler and others, 2001). In 

our case, low power output resulted in rejection of implant 

technology before it was necessary to weigh the additional 

risks and costs of the surgeries needed for implanting and 

removing the transmitter. It also would have been necessary to 

consider the possible impact of implants on fertility of females 

and the possibility that implanted ferrets might not be locatable when it was time to remove the transmitter.

Radio-tracking Strategies

We quickly realized after radio tagging the first blackfooted ferret in 1981 that signal-following techniques using 

hand-held tracking equipment were unlikely to generate the 

type, quality, and volume of data we were seeking. Much time 

was wasted searching for the subjects given the combination 

of relatively inefficient receiving antennas and low power 

output from the transmitters. Aside from the partial solution of developing more powerful transmitters (discussed 

above), it also was necessary to use much more directional 

and sensitive receiving antennas in order to maintain contact 

with the ferrets. Also, our signal-following attempts at night 

often appeared to disturb the ferrets. Thus, we decided to 

develop several stations of varying mobility equipped with 

larger antenna arrays from which tracking could be remotely 
 Radio Telemetry for Black-footed Ferret Research and Monitoring 183

accomplished via triangulation. Camper trailers with dualbeam 11- or 12-element, rotatable Yagi antennas (fig. 13) 

became the mainstay of the tracking system, augmented by 

more mobile truck-mounted receiving equipment (fig. 14). The 

relatively high receiving efficiency of these stations resulting 

from the larger antennas was further enhanced by increasing the heights of the arrays with masts of 4.5–6.0 m and by 

placing the stations on hilltops whenever possible. Although 

reception range was highly variable for these stations and the 

transmitters that were developed later (table 1, version D-3), 

we commonly radio-tracked ferrets at distances of 0.5–2.0 

km and received signals from as far as 26.0 km on occasion 

(Biggins and others, 1999).

Knowing the exact locations of stations is a prerequisite for accurate triangulation. These data were produced 

(in Universal Transverse Mercator coordinates) by using 

traditional survey techniques (transit and chain) in the 1980s, 

followed by location data from a differentially corrected 

Global Positioning System in later years. Meticulous accuracy 

testing of each station improved the data in two ways. First, 

such tests allowed assessment of bias patterns inherent in each 

station and development of correcting algorithms to improve 

accuracy of data during processing. Second, the residual 

variation in bearings from stations, after bias was corrected, 

allowed estimates of accuracy to be associated with each estiFigure 13. Camper trailer fitted with rotatable, 11-element, 

dual-beam Yagi array, used to radio track black-footed ferrets (Mustela nigripes) at the Meeteetse, Wyo., study area in 

1983–84. Photograph by D. Biggins.

Figure 14. Truck-mounted, collapsible, 5-element Yagi array used 

to radio track black-footed ferrets (Mustela nigripes) at Meeteetse, Wyo., during 1982–84. Photograph by D. Biggins.

Figure 15. Accuracy testing and referencing involve comparison 

of true azimuths and radio bearings to beacon transmitters.

mated location for a ferret. Tests were conducted by contrasting telemetric bearings to 60–100 beacon transmitter locations 

surrounding the tested station with a set of known bearings 

to those beacon locations measured with a surveyor’s transit 

(fig. 15). We employed a split sample technique to analyze 

test data, using half of the sample to derive the bias corrections and the second half to assess residual variation after the 

corrections were applied (fig. 16).

A second prerequisite for accurate triangulation is the 

ability to reference bearings from the antenna. Bearings can 

be usefully processed only when they are relative to a known 
184 Recovery of the Black-footed Ferret

entity, such as grid north. One could simply align the main 

beam of the antenna to north with a compass and set the 

compass rose to zero. This method is rather crude (White and 

Garrott, 1990); at least two problems cause variable results. 

First, the physical and electronic alignment of antennas is 

seldom absolutely parallel. Second, there is considerable 

variation in the electronic aiming (fig. 16). If one could 

successfully get the aim exactly right at one particular point on 

the compass rose, then it would still not be correct for many 

other points around the compass rose. Some sort of averaging is needed. To solve these problems, we used reference 

transmitters placed at known points in the study area. Actual 

azimuths to the beacons were known for each station and 

were compared to the telemetric bearings to those transmitters

(fig. 15), taken at the beginning of each tracking session. The 

compass rose inside a station was set so that zero was approximately at grid north (e.g., using a compass), and then readings 

to multiple beacons were used to provide an average correction that was applied to each subsequent bearing on an animal. 

Bias adjustment was applied before the referencing correction 

was made, the same as the process used when animals were 

tracked. Because the accuracy of this procedure affects all 

subsequent data, we cannot overemphasize the care needed in 

referencing. It would be nice to have many beacon transmitters 

(e.g., 50)! In practice, we used three to six beacons to avoid 

allowing referencing to become the dominant feature of a 

tracking session.

Although it is possible to plot triangulation data from 

pairs of these stations directly on maps to ascertain the 

whereabouts of the ferrets being tracked, it is more accurate 

and faster to process these data via computer. Advantages of 

conducting at least some of this processing while radio tracking include the following: (1) station selection can be adjusted 

as animals being tracked move about; (2) radio-tracking 

errors can be detected in time to correct them; (3) instances of 

mortality can be recognized quickly, resulting in better diagnoses of causes; (4) ferret dispersal can be detected in time 

to allow remedial action, if desired; and (5) in the case of lost 

radio contact, the last location calculated gives a starting point 

for searches. A computer program written by one of us (DEB) 

to accomplish these field processing tasks assisted the technicians with radio tracking ferrets at Meeteetse. The program 

was used on a programmable calculator in 1982 and was 

adapted to the first laptop computers that became widely available in 1983. That program evolved into TRITEL (Biggins 

and others, 2000b), which has been repeatedly modified since 

1983 to accomplish referencing and bias corrections, convert 

azimuth data into coordinates, calculate error estimates for 

each telemetric fix (fig. 17), and store resulting data.

Procedures for radio tracking and processing data are 

detailed in a separate report (Biggins and others, 2000b). We 

have relied on intensive triangulation from these kinds of 

stations to produce large volumes of data. Although we have 

at times recollared ferrets to extend data gathering over several 

months, all telemetric studies were relatively short term. To 

monitor reintroductions, ferrets often were radio tracked for 

just 2–4 weeks postrelease, but stations were usually occupied 

during all hours of the day or during all hours of darkness, 

with fixes generated by occupants at two or more stations 

coordinating their tracking with two-way radio communication. Intensity of re-location for individual ferrets varied 

(3–60 minutes between consecutive fixes on an individual), 

depending mostly on how many individual animals were being 

monitored. During the reintroduction phase of black-footed 

ferret recovery (1989–2000), we used this tracking strategy 

Figure 16. An accuracy test done at station 4, UL Bend National 

Wildlife Refuge, Mont., 1994. In this case, bias was corrected by 

using a formula defining the sine curve plotted. Residual variation produced a bias-adjusted accuracy estimate of ±0.63o (90% 

confidence) for future bearings from this station.

Figure 17. An example of a telemetric fix and error quadrangle 

(black-footed ferret [Mustela nigripes] no. 26, South Dakota, 

10/23/97, 0148 h) produced by intersecting bearings and their associated error arcs from two tracking stations.
 Radio Telemetry for Black-footed Ferret Research and Monitoring 185

to monitor 340 radio-tagged ferrets and polecats, accumulating 83,275 lines of data that included 44,191 indications of 

status and 39,084 estimates of location (fixes). Data on status 

demarked beginning and ending points of tracking sessions, 

activity of animal (active, inactive) as determined by variation 

in signal strength, and pulse interval records when transmitters 

with variable pulse rates were used. Status data were recorded 

with fixes but were the only data recorded when triangulation 

was not possible (e.g., when only a single station received an 

adequate signal).

Radio-telemetry data from triangulation allows many 

options for analyses (summarized by White and Garrott, 

1990). For black-footed ferrets, we have used radio telemetry 

to examine survival rates (Biggins, 2000a), linear movements 

(Biggins and others, 1999), dispersal (Biggins, Godbey, 

Livieri, and others, this volume), habitat preferences (Biggins 

and others, 1985; Biggins, Godbey, Matchett, and Livieri, this 

volume), indices of spatial use (Biggins and others, 1998b), 

and activity cycles (Biggins and others, 1986; Biggins, 2000a). 

The examples noted above are not exhaustive, and other 

options for use of the large data sets generated during multiple 

studies are currently being pursued. We believe that several 

features of analyzing telemetric data for ferrets, however, are 

worthy of emphasis here.

First, the inevitable errors that occur during triangulation 

must be detected and eliminated to the extent possible. Our 

system for handling data from triangulation, consistent with 

a pattern noted by Kenward (1987), has resulted in a series of 

custom computer programs for manipulating the output from 

TRITEL and screening for errors (Breck and Biggins, 1997). 

Similar to the BIOCHECK routine of White and Garrott 

(1990), our error screening involves searches for nonsensical 

data entries (e.g., unreasonable dates or times) and for data 

that fall outside limits set by a priori knowledge of ferret 

behaviors (e.g., maximum speed of movement). Errors are 

either corrected by referral to original data sheets, or offending 

lines are removed.

Second, estimates of ferret locations derived from 

triangulation are subject to direction-finding variation, as 

noted above. Estimates of such error associated with each fix 

(“error quadrangles” when two stations are used) are stored 

with each fix when TRITEL is used to process bearings. Our 

error screening process removes data lines with error estimates 

exceeding specified limits for lengths of diagonals or area 

of the quadrangle. Just as importantly, we have used these 

attributes of error as covariates in multivariate statistical analyses and often retain them in statistical models as “control” 

variables even if their estimated effect is small or not statistically significant. Although tracking error is nuisance variation 

when one is attempting to assess other treatments, it often 

accounts for significant variation (Biggins and others, 1998b, 

2000a; Biggins, 2000a). If, however, a response variable is 

already known to be positively correlated to tracking error, 

then the use of tracking error as a covariate is not warranted. 

An example is dispersal. Because error is in part a function 

of distance separating station and subject, sizes of the error 

quadrangles increase as ferrets disperse away from tracking 

stations. Unlike other movements within the monitored area, 

radio-tracking error should not be used to explain variation 

in dispersal by ferrets because increased tracking error is an 

expected consequence of dispersal.

Third, the ferret data we have generated are serially 

correlated because of short interfix intervals; each telemetric 

fix cannot be considered independent (Swihart and Slade, 

1985). The level of detail present in our data sets allows 

powerful behavioral comparisons (see examples cited above), 

but caution must be exercised in analyzing these data when 

independent observations are required (e.g., home range 

estimation; see White and Garrott, 1990). 

The close association between black-footed ferrets and 

prairie dog colonies facilitates the radio tracking of ferrets 

from fixed tracking stations. Ferrets often remain within 

predictable boundaries where radio tracking coverage was 

nearly complete with careful placement of multiple stations 

(e.g., the Montana study of Biggins, Godbey, Matchett, 

and Livieri, this volume). Nevertheless, if we would like to 

monitor every animal in our sample with equal intensity and 

accuracy, triangulation from fixed stations is problematic (not 

unlike data from any other method of radio tracking or monitoring). Signal quality and accuracy of fixes vary with range 

and topography, and positioning of stations interacts with these 

factors to create uneven trackability of animals throughout any 

study area. The movements of some animals may be described 

more accurately and completely than the movements of 

others, and probabilities of detecting mortality cases may vary 

accordingly. Animals that disperse away from fixed stations 

may be tracked with lower intensity, lower accuracy, or not 

at all. Attributes such as cumulative movements are affected 

by frequency and accuracy of relocations. The consequences 

can be serious if the goal is to characterize the behaviors of 

the species. When comparing treatment groups (e.g., sexes, 

ages, rearing treatments), the consequences are more benign 

if we can reasonably assume that animals are distributed in 

the study area in such a way that members of each group are 

about equally trackable on average. The possibility of groupspecific biases should be carefully considered for each case. 

For example, if dispersal is the attribute of interest, it may or 

may not be logical to rely on data from fixed-station triangulation. If dispersal distances have been artificially truncated by 

reception range of the tracking system, power of a comparative experiment may be reduced and dispersal distances will 

be underestimated to the greatest degree for groups whose 

members tend to disperse most frequently and farthest. 

Nevertheless, radio tracking from fixed stations has enabled 

us to detect significant between-group differences in dispersal 

(Biggins and others, 1998b, 1999). A germane statistical adage 

might be “if the tree falls, the axe was sharp enough” (Martin 

and Bateson, 1990, p. 126). 

We have augmented triangulation with hand tracking, 

automated signal monitoring and data logging, and tracking 

from aircraft. Hand tracking, usually with a hand-held receiver 

and a 3-element Yagi antenna, was often used to investigate 
186 Recovery of the Black-footed Ferret

ferrets whose transmitters (a) were in unusual locations, 

(b) had moved rapidly, (c) had not been detected for long 

periods, (d) were stationary above ground at night, or (e) were 

above ground during daytime. These circumstances often led 

to re-location of ferrets that had dispersed (fig. 18) or to ferrets 

that had been killed by predators (fig. 19; Biggins, Godbey, 

Livieri, and others, this volume). We attempted to visit the 

location of the last fix if contact with a transmitter was lost 

for 2 or more days; listening for a radio signal while walking 

a narrowly spaced grid (ca. 2-m spacing) sometimes allowed 

detection of the transmitter belowground to depths of >4 m. 

Signal strength was correlated with depth of the transmitter; 

weakest signals could be received only when the operator was 

almost directly above the transmitter with the Yagi antenna 

pointing vertically downward (Biggins, 2000a). Signals 

seldom emanated from burrow entrances (contrary to the 

predictions of some electronic engineers). Remains of badgerkilled ferrets were located by careful searches and excavated 

(fig. 20). Lost contact with transmitters also precipitated aerial 

searches at some sites. Each aircraft was equipped with a 

pair of 4-element Yagi antennas (affixed to each wing strut) 

and a switch to allow the operator to listen to the signal from 

each antenna separately. Homing on the source of a signal 

was accomplished by equalizing the null from each antenna 

(Gilmer and others, 1981). Radio-tracking flights helped 

locate ferrets that dispersed to different prairie dog colonies, 

especially when the flights were at night when ferrets are 

Figure 18. Hand tracking enabled us to locate black-footed ferrets (Mustela nigripes) that had dispersed into unusual habitats. 

Photograph by M. Albee.

Figure 19. Siberian polecat (Mustela eversmannii) killed by a 

predator. Photograph by D. Biggins.

Figure 20. Hand-held tracking equipment enabled location of 

transmitters below ground, necessitating excavation to determine 

fate of animals such as this Siberian polecat (Mustela eversmannii) killed by a badger (Taxidea taxus). Photographs by D. Biggins.
 Radio Telemetry for Black-footed Ferret Research and Monitoring 187

most active above ground (Biggins and others, 1986; Biggins, 

2000a). The most common product of flights, however, was 

detection of lost collars and cases of aboveground predation 

on ferrets that had dispersed (or their transmitters had been 

dispersed by the predator) beyond signal reception range 

of tracking stations. In short, these follow-up techniques, 

although arguably less technologically demanding than the 

radio tracking by triangulation, have provided the critically 

important details on fates of animals that other strategies 

cannot produce.

We used signal monitoring both with automated chart 

recorders and with computer loggers in attempts to collect 

information on aboveground activity of ferrets and polecats 

(Biggins, 2000a). The technique was useful to supplement data 

from triangulation, particularly on animals that were beyond 

the boundaries of the area that could be effectively monitored 

by tracking stations; however, the relative insensitivity of 

automated systems to detection of weak signals, coupled with 

the large activity areas of black-footed ferrets, limits the utility 

of automated tracking for ferrets.

Summary

The wide range of problems and accomplishments 

accompanying the use of radio telemetry on ferrets provides 

an opportunity for both detractors and proponents to present 

powerful arguments. Although success was never close to 

total, failures were not devastating to data or the ferrets. We 

would like to reemphasize that radio telemetry is an expensive and labor-intensive method for monitoring black-footed 

ferrets and that attaching radio transmitters to ferrets poses 

risks to the animals. It is essential, therefore, to carefully 

consider the objectives of a study to ascertain whether other 

tools would suffice. Justifications for use of radio telemetry 

on ferrets include unexplained lack of success in establishing a ferret population and tests of hypotheses that have 

large-scale management implications and require behavioral 

information. Cost/benefit analyses regarding use of telemetry 

should include as costs the potential future losses of ferrets 

if a perceived need for information remains unfulfilled. In some 

cases, short-term recovery objectives may become subordinate to 

learning objectives that could advance long-term recovery goals.

Acknowledgments

We thank the electronics laboratory of L. Kolz, of the 

former Denver Wildlife Research Center (U.S. Fish and 

Wildlife Service), for testing power output and other attributes 

of prototype implant and collar-mounted transmitters. Evaluations of prototype transmitters would not have been possible 

without sincere efforts and special attention of engineers and 

technicians at Advanced Telemetry Systems (Isanti, Minn.), 

AVM Instrument Company, Ltd., Custom Telemetry and 

Consulting (Watkinsville, Ga.), Telonics, Inc., and Wildlife 

Materials, Inc. We appreciate the dedication of the many 

technicians over a 20-year period that assisted with setting up, 

testing, and operating tracking stations.

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190 Recovery of the Black-footed Ferret

Collars that we are presently using are considerably more 

fragile than their predecessors and are intentionally designed 

to lack durability. Most black-footed ferrets (Mustela nigripes) 

shed the collars within 2 months (often much sooner). Use of 

more durable collars seems to increase the risk of neck sores. 

Presently, collars 1 cm wide are made of 100 percent wool, 

folded into three layers and sewn with 100 percent cotton 

thread. The edges are not bound, so the wool will fray rapidly. 

After sewing the wool into long strips of uncut collars, we 

prestretch the material. It is soaked in water and hung to dry 

with a 200-g weight clamped to the lower end. Collars are then 

cut to 15–18 cm lengths. To attach a collar to the transmitter 

unit, both are inserted into a 2.5-cm length of Teflon® heatshrink tubing (1.25 cm diameter), and a heat gun (or other 

heat source such as a gas stove or propane torch) is used to 

shrink the tubing. Overheating the transmitter packages can 

cause malfunctions. High temperature for a short duration 

works better than less heat applied for longer times. The object 

is to heat the tubing without overheating the transmitter and 

battery. After shrinking the tubing, the package is cooled 

rapidly by wrapping it in a cool, wet sponge. Equipment and 

supplies needed to attach these collars to ferrets include scissors, a hemostat clamp, contact cement, a telemetry receiver, 

and a hair dryer. Mustelids characteristically have little neck 

constriction, making exact collar fit important. The attachment 

procedure for black-footed ferrets may be accomplished in the 

following steps:

1. Remove the magnet and check transmitter operation.

2. Restrain ferrets with a light dose of ketamine/diazepam 

(about 17–20 mg per kg of body weight) for this noninvasive procedure (Thorne and others, 1985). Recently, 

we have been using isoflurane gas anesthesia, which 

is more controllable (Biggins, Godbey, Matchett, and 

others, this volume). New innovations in gas anesthesia 

(e.g., sevoflurane; Gaynor and others, 1997) have additional advantages but require different vaporizers. The 

U.S. Fish and Wildlife Service requires ferret handlers 

to be trained in anesthesia and handling procedures.

3. As soon as the animal is tranquil enough to handle, 

make a trial fit of the collar and mark the length 

needed, allowing about 1-cm overlap of ends. Mark 

the area of overlap that will be glued, but do not trim 

excess from the long end of the collar until later. The 

extra length makes it easier to fit on the animal and can 

be trimmed at the end of the process.

4. Coat the inside of one end and the outside of the other 

end with contact cement. We use the Weldwood®

(DAP® Products, Inc., Baltimore, Md.) version that has 

a toluene solvent, which seems to work better than the 

versions with other solvents. The glue-drying process 

takes 3–10 minutes. A hair dryer speeds drying. The 

first coat of cement normally penetrates the wool. 

Unless the glue is quite thick, the first coat must be 

dried completely and a second coat applied and dried 

until tacky.

5. Wrap the collar around the animal’s neck and press 

a tiny portion of the glued strip together lightly. This 

process allows a final check for snugness before the 

final gluing is done. Collar fit is critical; it should be 

snug but not tight. The collar should rotate fairly easily around the neck. Also, a small closed hemostat or 

small scissors should slide easily between the neck and 

collar, but if you can insert your little finger, the collar 

is probably too loose.

6. If the fit seems satisfactory, press the glued ends 

together firmly. Use the hemostat to clamp the ends, 

repeatedly clamping and releasing until the entire overlap area has been pressed together firmly. Trim excess 

wool from the long end of the collar. We know of only 

one occasion when the glue joint failed, and that was 

when a technician did not realize that he had to let the 

glue dry before pressing the ends together. In fact, we 

have not been able to separate the final joint by pulling 

the ends apart—the material always tears. It may even 

be difficult to separate the ends during the trial fitting 

if they have made too much contact.

Appendix. Notes on Radio Collaring Black-footed Ferrets
Abstract

A successful captive breeding program for highly endangered black-footed ferrets (Mustela nigripes) has resulted in 

surplus animals that have been released at multiple sites since 

1991. Because reproductive output of captive ferrets declines 

after several years, many adult ferrets must be removed from 

captive breeding facilities annually to keep total production 

high. Adults are routinely released, with young-of-the-year, on 

prairie dog (Cynomys spp.) colonies. We evaluated postrelease 

movements and survival rates for 94 radio-tagged young and 

adult ferrets. Radio-tagged adult ferrets made longer movements than young ferrets during the night of release and had 

significantly lower survival rates for the first 14 days. Coyotes 

(Canis latrans) caused the largest number of ferret losses. A 

larger data set of 623 ferrets represented adults and young that 

were individually marked with passive integrated transponders 

but were not radio tagged. Minimum survival rates, calculated 

primarily from ferrets detected during spotlight searches and 

identified with tag readers, again were significantly lower 

for adults than for young ferrets at 30 days postrelease (10.1 

percent and 45.5 percent survival, respectively) and at 150 

days postrelease (5.7 percent and 25.9 percent). Assessment of 

known survival time by using linear modeling demonstrated 

a significant interaction between age and sex, with greater 

disparity between adults and kits for females than for males. 

Postrelease survival of adult ferrets might be increased if 

animals were given earlier and longer exposure to the quasinatural environments of preconditioning pens.

Keywords: age, behavior, mortality, Mustela nigripes, 

predation, radio telemetry

Introduction

Black-footed ferrets (Mustela nigripes) nearly became 

extinct when diseases invaded the last known free-ranging 

population near Meeteetse, Wyo., in 1985 (Lockhart and 

others, this volume). A rescue effort resulted in a captive 

population that has provided ferrets for reintroduction since 

1991. The mean life expectancy of free-ranging black-footed 

ferrets in the ancestral Meeteetse population was about 0.9 

years (calculated by using the negative reciprocal of the natural log of 0.34, an annual survival rate estimated by Forrest 

and others, 1988). With such a short average life expectancy, 

natural selection may have applied little pressure for sustained 

productivity in older age classes of ferrets. In captivity, 

productivity declines rapidly after ferrets are only a few years 

old (Williams and others, 1991). Efficient management of 

the captive breeding program thus involves relatively rapid 

rotation of animals (Marinari and Kreeger, this volume). Older 

animals are placed in zoos for exhibit and used for research, 

but the supply of such animals exceeds the demand. Adult 

ferrets are routinely released at reintroduction sites, a practice that has been criticized. Although both young and adult 

ferrets have been released at several sites, their postrelease 

movements and survival have not been compared. Marking 

of animals, spotlight searches, and identification of surviving ferrets are tools routinely used for monitoring at release 

sites (Biggins, Godbey, Matchett, and others, this volume), 

providing useful multiyear data sets. In certain years, more 

intensive radio-telemetry studies (Biggins, Godbey, Miller, and 

Hanebury, this volume) were directed at testing hypotheses 

regarding prerelease experience and rearing methods. Cumulative data from these former efforts provide the opportunity 

to contrast the movements and survival of released adult and 

young ferrets.

Methods

Stratification Based on Rearing and Prerelease 

Experience

Rearing conditions and prerelease experience have 

profound effects on behaviors of young ferrets (Miller and 

Postrelease Movements and Survival of Adult and Young 

Black-footed Ferrets

By Dean E. Biggins,1 Jerry L. Godbey,1 Travis M. Livieri,2 Marc R. Matchett,3 and Brent D. Bibles4

1

U.S. Geological Survey, Fort Collins Science Center, 2150 Centre Ave., 

Bldg. C, Fort Collins, CO 80526.

2

Prairie Wildlife Research, P.O. Box 515, Wall, SD 57790.

3

U.S. Fish and Wildlife Service, Charles M. Russell National Wildlife 

Refuge, P.O. Box 110, Lewistown, MT 59457.

4

Colorado Division of Wildlife, 317 W. Prospect Rd., Fort Collins, CO 

80526.
192 Recovery of the Black-footed Ferret

others, 1990a,b; Vargas, 1994; Biggins and others, 1999; 

Biggins, 2000), ultimately influencing postrelease survival 

rates (Biggins and others, 1998). Released adult (age at release 

>1 year) black-footed ferrets reported herein were given 

experience in outdoor pens, including living in natural burrows 

and killing prairie dogs, for 1–4 months prior to release. 

Former experimental designs that focused on testing effects 

of rearing on young ferrets (kits), however, involved more 

categories of preconditioning and more carefully controlled 

environments (Biggins and others, 1998). Those experimental 

designs encompassed most of the radio-tagged kits used in the 

following analyses but only a portion of the released kits that 

were not telemetrically monitored. Because early experiments 

indicated that cage-reared kits were dramatically different in 

several respects from their counterparts with experience in 

pens (Biggins and others, 1998, 1999), we did not include 

cage-reared kits in any of our analyses (telemetry or recapture). We also excluded kits that were born in pens or transferred into pens at the natal facility at an early age (<60 days) 

with their dams (the PENRES category of Biggins and others, 

1998) from the telemetric data set. For a large number of kits 

that were not part of the early experiments, preconditioning 

was much more variable. Thus, our capture-recapture analyses 

encompassed a more broadly defined “preconditioned” group 

of kits that ranged from those placed in pens prior to 60 days 

of age with dams to those shipped after 90 days of age, without accompanying adults, from their original breeding facility 

to pens at other facilities or to remote pens near reintroduction 

sites.

In summary, we used two types of data to examine the 

influence of age of ferrets on their movements and survival. 

Radio telemetry provided information on cumulative movements, dispersal, minimum survival rates, and causes of 

mortality. A larger sample of ferrets that were individually 

marked (including those that were radio tagged) allowed additional estimates of survival via mark-recapture methods.

Radio Telemetry

We radio collared 137 black-footed ferrets with 5-g 

transmitter packages attached to 100 percent wool collars 

with Teflon® (DuPont, Wilmington, Del.) heat-shrink tubing 

(the latter to resist mud accumulation). Radio-tagged ferrets 

were released on Gunnison’s prairie dog (Cynomys gunnisoni) habitat in the Aubrey Valley of northern Arizona and on 

black-tailed prairie dog (C. ludovicianus) habitat at UL Bend 

National Wildlife Refuge in Montana and the Burns Basin 

portion of Badlands National Park, S. Dak.

Transmitters, with their 20-cm whip antennas, provided 

a pulsed signal (pulse interval = 1.5 seconds; pulse width = 20 

milliseconds) of about -14 dB, with battery life of about 45 

days. Radio location was accomplished via triangulation from 

fixed stations fitted with paired, 11-element Yagi antennas on 

rotating masts (Biggins and others, 1999; Biggins, Godbey, 

Miller, and Hanebury, this volume). We tested accuracy of 

stations by comparing station-derived azimuths with true 

azimuths to beacon transmitters. We used standard deviations 

of the differences between such pairs in confidence intervals 

to predict the accuracy of future azimuths and the areas and 

diagonals of error quadrangles associated with positional 

“fixes” (White and Garrot, 1990). An initial test for each 

station provided data for evaluating bias patterns and developing formulas for adjustment, and a second set of readings 

was used to calculate residual variation after bearings were 

adjusted (Biggins and others, 1999). We referenced stations 

prior to each tracking session (Biggins, Godbey, Miller, and 

Hanebury, this volume) using from 2 to 5 beacon transmitters. 

For these analyses, we used radio-telemetry data for the first 

14 days postrelease (although tracking extended over a longer 

period at some sites). We used the computer program TRITEL 

to process azimuth data (Biggins, Godbey, Miller, and Hanebury, this volume); processing included adjustments for 

referencing and bias and calculation of coordinates and error 

estimates for each fix. Hand-held tracking equipment assisted 

us in recovery of lost collars and dead ferrets. 

For comparisons of age groups, we used the subset of the 

137 instrumented animals (excluding 20 PENRES kits and 23 

cage-reared kits as defined above) that included 38 adults and 

56 “preconditioned” kits (table 1). We screened data for gross 

radio-tracking and data entry errors by using the systematic 

approach of Breck and Biggins (1997). We then summarized 

cumulative movements between consecutive fixes and dispersal from the release site for each ferret and night. We analyzed 

cumulative movements by using a repeated measures multivariate general linear model (MGLM) with average area of error 

quadrangle, sex, and site as covariates. We used square root 

transformations of the response variables to improve normality 

and homoscedasticity of residual variation. We assumed that 

the area of an error quadrangle would account for a portion 

of the variation in the cumulative movement of a ferret and 

retained this measure of tracking error as a control variable 

in statistical models regardless of its significance. Because 

dispersal is defined as movement away from the release site 

and increased distance from tracking stations causes larger 

error quadrangles, tracking error was not considered in statistical evaluations of dispersal, but sex and site were included as 

covariates.

Causes of mortality were determined by evidence at 

recovery sites (e.g., tracks, scat, fur, feathers, digging), condition of carcass (e.g., hemorrhage, bite wounds, saliva), and 

radio-tracking data (patterns of fixes and activity, timing of 

death). We assessed risk-adjusted survival rates by relating 

deaths (table 1) to days of telemetric monitoring (Heisey and 

Fuller, 1985). An estimate of maximum survival resulted from 

considering only known deaths. Counts of animals known 

dead underestimate mortality rates because not all dead 

animals are detectable (underground deaths due to badgers, for 

example, may be underestimated) and because some proportion of loss of telemetric contact with animals is due to transmitter damage inflicted during predation. (We have recovered 

a few badly damaged transmitters that were barely functional 

and assume that others became nonfunctional.) We thus 
Postrelease Movements and Survival 193

estimated a minimum survival rate by summing the number of 

ferrets that were lost from telemetric contact and the number 

known to be dead. If an animal was found alive at a later date 

during spotlight surveys, it was considered alive for the first 

14 days (even if telemetric contact was lost and it would have 

been listed as missing). If an animal died or became missing 

after 14 days, it was treated as alive for the first 14 days. Our 

multivariate general model had eight parameters (two ages, 

four site/year combinations). In this survival analysis, we 

compared models and their nested submodels using likelihood 

ratio tests.

Mark-recapture

The data set for this portion of the study (table 2) 

included all black-footed ferrets released during 1994–2000 

at Badlands National Park and Conata Basin in South Dakota, 

ferrets released during 1994–97 at UL Bend in Montana 

(including the radio-tagged individuals mentioned above), and 

ferrets released at the Coyote Basin site of Utah and adjacent 

Colorado. Of the 623 ferrets released (table 2), 325 were males 

and 298 were females. All released ferrets were individually 

marked, mostly with passive integrated transponder (PIT) 

tags subcutaneously implanted over the shoulder. A second 

PIT tag often was implanted over the hip (Biggins, Godbey, 

Matchett, and others, this volume). “Recapture” (in this 

case, mostly reading the transponder) was accomplished via 

spotlight surveys to locate the ferrets (Campbell and others, 

1985; Biggins and others, 1998) followed by placement of a 

transponder reader at the occupied burrow to automatically 

read and retain the chip number (Biggins, Godbey, Matchett, 

and others, this volume). The first survey at each site usually 

was conducted about 1 month postrelease, with additional 

surveys conducted prior to the breeding season (in some cases) 

and postwhelping (most sites). Intensity of these survey efforts 

varied among sites and years due to availability of resources.

Counts of surviving animals at 30 and 150 days postrelease were based on the same released ferrets and thus 

cannot be considered statistically independent. Also, the 

71 radio-tagged ferrets in South Dakota and Montana are a 

subset of the 623 animals considered in the capture-recapture 

analyses. We chose to maintain separate 30-day and 150-day 

mark-recapture analyses (rather than a more complex single 

model) because of sample size differences and unequal time 

intervals between surveys and because survival estimates for 

these time periods can be compared with similar estimates 

reported elsewhere for ferrets. Survival was considered cumulatively; ferrets found alive at 150 days (or later) were counted 

as alive on day 30 even if they were not found in the earlier 

period. Because spotlight sessions of equal intensity were 

Arizona

 (1996)

South Dakota 

(1995–96)

Montana

 (1994)

Montana

 (1995) Total

Ferrets

Adult 15 14 5 4 38

Kit 8 11 10 27 56

Total 23 25 15 31 94

Ferret-days of monitoring

Adult 90.7 21.3 25.0 29.7 166.7

Kit 43.0 93.2 54.0 291.0 481.2

Total 133.7 114.5 79.0 320.7 647.9

Deaths

Adult 4 11 3 1 19

Kit 0 5 5 1 11

Total 4 16 8 2 30

Table 1. Numbers of preconditioned adult and young radio-tagged black-footed ferrets (Mustela nigripes) released in Arizona, South 

Dakota, and Montana during 1994–96.

South 

Dakota 

(1994–2000)

Montana 

(1994–97)

UtahColorado 

(1999–2001) Total

Adult 49 13 60 122

Kit 261 80 160 501

Total 310 93 220 623

Table 2. Numbers of preconditioned adult and young black-footed 

ferrets (Mustela nigripes) marked and released at sites in Montana, South Dakota, and Utah-Colorado during 1994–2001.
194 Recovery of the Black-footed Ferret

not replicated systematically at all sites (or even among days 

within sites), we did not attempt traditional capture-recapture 

modeling where capture rates and survival rates could be 

estimated separately. Our rates, therefore, must be considered 

as minimum survival (the products of capture rate and survival 

rate), recognizing that not all ferrets were likely to have been 

located at any site. Interpretation of the comparisons between 

adults and kits thus requires the assumption that each age 

class (within each site) was equally detectable by spotlighting, 

an assumption that we believe is reasonable. We estimated 

survival rates from spotlight searches by using an iterative 

numerical optimization procedure (program SURVIV; White, 

1983). The general model included 12 parameters (3 sites, 2 

sexes, 2 ages). 

Elapsed time from release until the last detection for each 

ferret also was calculated. Time intervals between releases and 

the first spotlight survey and between subsequent spotlight 

surveys varied considerably among sites and years, from a 

single survey per year to nearly continuous surveys. Variability in survey timing tended to distribute this measure of 

survival in a continuous (but skewed) form, and square root 

transformation improved its suitability for use as a continuous 

response variable in a MGLM analysis, allowing additional 

assessment of the potential interaction between age and sex. 

We recognize that detectability of ferrets via spotlight 

searches is likely to differ among sites due to differences in 

access, vegetative cover, topography, intensity of effort, and 

other variables. Thus, we consider multivariate modeling, 

with a site variable included, as critically important. Potential 

differences in search efficiency also preclude any conclusions 

regarding differences in survival among sites.

An important consideration in our experimental design, 

for both telemetric and capture-recapture data, was to maintain 

a reasonable balance of treatments within sites (and within 

years, with one exception). Other priorities always affected 

allocations of animals, but, to avoid serious confounding 

during interpretation of results, we did not allow any cell of 

the design to be empty. Thus, groups of released ferrets that 

did not contain adults and kits of both sexes were excluded 

from analyses. The exception to this general rule occurred 

within the telemetry data set, where adult ferrets were released 

in Badlands National Park in spring, and kits were released at 

that site during fall of the following year.

We followed the principle of parsimony in evaluating 

competing statistical models (Lebreton and others, 1992), 

attempting to reduce general models to simpler submodels 

by eliminating variables that appeared to have low explanatory power. For capture-recapture analysis within program 

SURVIV, reduced models were evaluated by likelihood ratio 

tests and Akaike’s Information Criterion (AIC) (Anderson and 

Burnham, 1994). For MGLM evaluations and likelihood ratio 

testing, P values >0.10 were deemed sufficient for eliminating 

variables from models.

Results

Radio Telemetry

Of the 137 radio-tagged ferrets that were released, 59 

were considered lost to the population, mostly as a result of 

predation (fig. 1). Coyotes (Canis latrans) caused the most 

losses, but prior to its removal a great horned owl (Bubo 

virginianus) had a substantial impact at one site in South 

Dakota. American badgers (Taxidea taxus) were common 

on prairie dog colonies where ferrets were released, but 

they killed ferrets only occasionally. The species of predator 

responsible for ferret deaths could not always be determined, 

however, resulting in some classifications of “unknown predator” or “unknown raptor” (fig. 1).

Multivariate repeated measures analysis of square root 

transformed cumulative movements for ferrets that were 

monitored for at least three nights yielded a significant interaction between night postrelease and age group (F2,59 = 7.407, 

P = 0.001) with a model that included age, site, and mean area 

of error quadrangle (per animal over three nights). Thus, the 

pattern of change in nightly movements of kits and adults was 

significantly different over the first three nights postrelease 

(fig. 2; nontransformed data). Tracking error (area of error 

quadrangle) contributed significantly to the variation in movements (F1,60 = 5.620, P = 0.021), underscoring the importance 

of a variable to account for this source of “nuisance” variation 

Figure 1. Causes of loss for 59 of 137 radio-tagged black-footed 

ferrets (Mustela nigripes) released in Montana, South Dakota, 

and Arizona during 1994–96. “Rescued” ferrets are those that we 

assume would have been lost without our intervention (translocation or treatment for injuries). “Unknown predator” and “Unknown 

raptor” are general categories for which the species of predator 

could not be identified.
Postrelease Movements and Survival 195

in models of movement. Nightly movements also appeared to 

be different at different sites (F3,60 = 3.693, P = 0.017).

The relatively long movements of adult ferrets on the 

night of release suggested that they might have “bolted” from 

the release site (i.e., dispersed rapidly away from the point of 

release), but a repeated measures analysis of dispersal during 

the first three nights did not support that contention (fig. 3). 

Although there was a significant tendency for ferrets to drift 

away from their release sites over the first several nights 

(F2,58 = 8.860, P < 0.001), the pattern of dispersal was not 

significantly different for kits and adults (F2,58 = 1.107, P = 

0.337). Thus, “bolting” is not an appropriate description of the 

behaviors of adults. They simply moved more than kits during 

their first night but did not tend to leave the area of release any 

differently than did kits. In this analysis of dispersal, there was 

no evidence of differences among sites (F3,59 = 1.209, P = 0.315).

Survival of radio-tagged adults appeared to differ significantly from survival of radio-tagged kits. For the estimates 

of maximum survival, generated by considering only known 

Figure 2. Mean cumulative nightly movements for adult and young 

radio-tagged black-footed ferrets (Mustela nigripes) during the 

first three nights postrelease (mean + SE).

Figure 4. Postrelease survival curves for preconditioned adult 

and young radio-tagged black-footed ferrets (Mustela nigripes)

extrapolated from daily survival rates (assuming a constant hazard 

rate for the 14-day period of the experiment and using only known 

deaths).

deaths (fig. 4), likelihood ratio testing did not support reduction in number of parameters by pooling sites (X2 = 45.4, df = 

6, P < 0.001) or ages (X2 = 29.3, df = 4, P < 0.001). The same 

was true for the estimates of minimum survival, using ferrets 

known to be dead plus those with whom radio contact was 

lost during the 14-day tracking period (sites, X2 = 38.6, df = 

6, P < 0.001; ages, X2 = 38.7, df = 4, P < 0.001). There was 

thus a similar overall pattern of differences between survival 

rates of adults and kits, regardless of the method of categorizing mortalities (fig. 5). If about one-third of the missing 

animals actually died when their signals were lost, the overall 

projected survival rates for 30 days postrelease would have 

been 42 percent for kits and 11 percent for adults. The curves 

vary among sites and with methods, but kit survival was higher 

than adult survival within each comparison. 

Mark-recapture

The preferred model of minimum survival from the spotlight search data was the general model for both the short-term 

(30-day) (table 3) and the long-term (150-day) assessment 

(table 4), although the evidence for distinction between the 

sexes was strongest in the long-term evaluation. Minimum 

survival rates were higher for kits than for adults in 11 of the 

12 pairs of estimates for the two time periods, averaging 45.5 

percent (kits) and 9.8 percent (adults) at 30 days (fig. 6) and 

25.9 percent (kits) and 5.7 percent (adults) at 150 days (fig. 

7). Minimum survival rates tended to be higher for females 

Figure 3. Nightly maximum displacement from release sites 

for young and adult radio-tagged black-footed ferrets (Mustela 

nigripes) during the first three nights postrelease (mean ± SE).
196 Recovery of the Black-footed Ferret

than for males (figs. 6 and 7). For kits, the disparity between 

sex-specific survival rates was proportionately greater for 

the long-term estimates (males, 18.9 percent; females, 35.7 

percent) than for the short-term estimates (males, 39.9 percent; 

females, 53.4 percent). Moreover, there seemed to be different 

patterns for adults and kits within the two genders for both the 

short-term and long-term data sets. That potential interaction 

warranted closer examination.

General linear modeling of elapsed time between release 

and the last detection demonstrated a significant interaction between sex and age (F1,617 = 5.522, P = 0.011); known 

survival times tended to be shorter for adults than for kits 

(fig. 8). We retested the sexes separately because of the interaction. As implied by the pairs of graphs, female kits survived 

significantly longer than did adult females (F1,294 = 40.250, P 

< 0.001), but the difference between the age groups was only 

marginally significant for males (F1,294 = 3.387, P = 0.067).

Discussion

Radio Telemetry

There is a potential bias built into assessments of ferret 

movements. Repeated measures analyses, particularly, require 

complete sets of multiple measures on single animals; any 

ferret that lacked a measure of movement for any of the first 

three nights postrelease (fig. 2), for example, was excluded 

from our analysis. Thus, ferrets that tend to engage in risky 

behaviors tend to be removed (by death) at higher rates from 

the sample, likely causing movements to be generally underestimated, and (more seriously for this kind of experiment) the 

effect may be greater on some treatment groups than on others. 

If we assume that there is a positive correlation between 

movement and mortality rate (Biggins and others, 1998), we 

likely underestimate movement differences between groups. 

We have been able to detect such differences, but more subtle 

disparities between treatment groups may remain unnoticed. 

Statistical models that are not based on repeated measures also 

would be affected, but more flexible rules for handling those 

data should result in a less dramatic influence. Although early 

deaths of individuals having presumably lower fitness may 

cause a shift in representation of animals, their movements 

before they were killed remain in data sets used for statistical 

analyses other than repeated measures.

Survival of radio-tagged kits differed more dramatically 

from adults at the Burns Basin, S. Dak., release site than at 

any other site (fig. 4). Although the same release site was used 

for both kits and adults, and they were radio tracked from the 

same system, Burns Basin was the only site where adults and 

kits were not released at the same time. It is possible that the 

differences there were due to year or season. 

The different appearance of survival curves among 

sites generated from telemetric data (fig. 4) should not be 

construed as being linked to the species of prairie dog or other 

site-specific conditions. Efficiency of radio tracking is likely 

responsible for much of the variation. The Aubrey Valley site 

Figure 5. Bracketed high and low survival of adult and young 

radio-tagged black-footed ferrets (Mustela nigripes) for the first 

30 days postrelease, generated by using only ferrets known to be 

dead (high) and known deaths plus ferrets lost to radio contact 

(low). Curves were extrapolated from daily survival rate estimates 

assuming a constant hazard rate for the period.

Model ln(L)a npb AICc

Versus 

modeld X 2 P

1. General -19.842 12 63.685

2. Sites same -65.391 4 138.783 1 91.10 <0.001

3. Ages same -53.722 6 119.443 1 67.76 <0.001

4. Sexes same -28.060 6 68.120 1 16.44 0.012

Table 3. Modeling of short-term (30-day) return rates for preconditioned black-footed ferrets (Mustela nigripes) released at three sites, 

with parameter estimates for sites, sexes, and ages.

a

ln(L) = log-likelihood.

b

np = number of parameters.

c

AIC = Akaike’s Information Criterion.

d

The model identified in this column was compared via a likelihood ratio test to the model in the first column (same row), resulting in the Chisquare value and corresponding probability given in the last columns.
Postrelease Movements and Survival 197

in Arizona, in particular, presented a challenge. Wide spacing between stations was necessary to achieve appropriate 

coverage of the site, but contact was lost with many animals 

because of the long distances over which they were tracked. 

The result may have been a reduced probability of finding 

dead ferrets, and such known mortalities were the basis for the 

curves generated. In contrast, Burns Basin in South Dakota 

provided much better radio-tracking conditions that favored 

finding cases of mortality (stations were on high points). 

Overall, highest rates of survival for kits have been in South 

Dakota as exemplified by our mark-recapture data set. Site 

characteristics have influenced the efficiency of both spotlight 

searches and radio telemetry, causing us to adopt experimental 

designs that compare two or more treatments within sites, to 

replicate the design over multiple years and sites to achieve 

adequate sample sizes, and to exercise caution in interpreting 

results from multiple sites. We might have remained more 

suspicious about the possible ramifications of our design and 

Model ln(L)a npb AICc

Versus 

modeld X 2 P

1. General -16.687 12 57.374

2. Sites same -34.662 4 77.324 1 35.95 <0.001

3. Ages same -35.060 6 82.121 1 36.75 <0.001

4. Sexes same -28.609 6 69.219 1 23.84 0.001

Table 4. Modeling of long-term (150-day) return rates for preconditioned black-footed ferrets (Mustela nigripes) released at three sites, 

with parameter estimates for sites, sexes, and ages.

a

ln(L) = log-likelihood.

b

np = number of parameters.

c

AIC = Akaike’s Information Criterion.

d

The model identified in this column was compared via a likelihood ratio test to the model in the first column (same row), resulting in the Chisquare value and corresponding probability given in the last columns.

Figure 6. Minimum short-term (30-day) survival of adult and young 

preconditioned black-footed ferrets (Mustela nigripes) released 

onto prairie dog colonies (Cynomys spp.) in Montana (MT), South 

Dakota (SD), and Utah (UT) during 1994-2001 (mean ± SE).

Figure 7. Minimum long-term (150-day) survival of adult and young 

preconditioned black-footed ferrets (Mustela nigripes) released 

onto prairie dog (Cynomys spp.) colonies in Montana (MT), South 

Dakota (SD), and Utah (UT) during 1994–2001 (mean ± SE).

potential for confounding without the corroborating results 

produced by the much larger sample sizes of released ferrets in 

the mark-recapture portion of the study.

Mark-recapture

Differences between survival rates of males and females 

were not detected previously (Biggins and others, 1998) in a 

much smaller data set of 262 ferrets (64 of those animals were 

included in our present data), although there was speculation 

that the expected trend toward lower male survival in longerterm data was developing and would be validated with larger 

sample sizes. Failure to detect such a difference in our data 

would indeed have been troubling given the male bias in the 

numbers of animals released (325 males:298 females) and 

the female-biased composition of free-ranging ferret populations (Forrest and others, 1988). For kits only (comparable 

to the analysis of Biggins and others, 1998), a gender effect 
198 Recovery of the Black-footed Ferret

in our data was detectable for both the short-term and longterm evaluations, but the effect became more dramatic over 

time. Perhaps males became increasingly territorial during the 

approach of breeding season.

The significant interaction between age and sex classes 

has potentially nontrivial management ramifications. The 

interaction may be explained if older age tends to confer 

greater social status to males than to females, creating a potential problem if adult males have poor long-term survival skills 

compared to preconditioned male kits. Some older males may 

be able to competitively exclude younger males, subjecting the 

latter to temporarily higher mortality. The competition could 

result in lower overall survival rates for young males without 

any compensatory increase in long-term survival rates for 

adult males. If younger males are lost during transitory social 

interactions with dominant older males, and the older males 

tend to be lost before breeding season, a shortage of males 

could result. This phenomenon could occur with concurrent releases of adult and young males, or during releases to 

augment populations. 

General Considerations

Daily survival rates estimated from the 14-day radiotelemetry data set for 94 radio-tagged ferrets seemed reasonably predictive of the 30-day survival rates derived from 

mark-recapture data on the larger data set of 623 animals. 

The telemetrically originated survival rates of 42 percent 

for kits and 11 percent for adults are similar to the overall 

capture-recapture survival rates of 46 percent for kits and 10 

percent for adults. Such comparisons are speculative, however, 

because of the differing methods and attendant assumptions. 

First, it is only a reasonable guess to assume that one-third 

of cases of lost radio contact were due to death of the ferret. 

Second, the 14-day survival rates were produced under the 

assumption of a constant hazard rate, an assumption that 

was repeated to produce the 30-day extrapolation. Hazard 

rates likely decline postrelease as ferrets with lower fitness 

are culled and as ferrets learn about their new environment. 

Over short time spans, the flat hazard rate seems reasonable. 

Applying a rate generated during the first 2 weeks postrelease 

to long time spans would be ill-advised. Indeed, our spotlight 

detections at 150 days (25.9 percent of kits released and 5.7 

percent of adults released) were much higher than the respective 3.0 percent and 0.0 percent expectations of the extrapolated daily rates from the first 14 days of radio-tracking data. 

Third, the mark-recapture estimates are for minimum survival; 

the actual rate must be somewhat higher assuming we do not 

count all ferrets present. Finally, the average rates discussed 

here ignore the implications of statistical modeling, which 

suggested that rates should be separately estimated for sexes 

and sites.

Postrelease survival of adult black-footed ferrets might be 

improved if all young were reared in pens whether they were 

immediately destined for release or for the captive breeding 

program. A type of phase-specific learning (Davey, 1989) 

in which an animal may “imprint” on features of its habitat 

during a critical period of development has not been investigated for ferrets, but differences in postrelease survival and 

movements of ferrets as a result of rearing history (Vargas, 

1994; Biggins and others, 1998, 1999) arouse suspicion. 

Even if imprinting is not involved, cultural transmission of 

important behaviors may be enhanced by a natural environment (Biggins, 2000). Ensuring that each generation has 

early learning experience in a quasi-natural environment has 

several potential benefits. Whether or not all kits are raised 

in pens, increasing the amount of time they spend in outdoor 

pens could be advantageous. Females that have spent three 

summers rearing young in the burrows of outdoor pens may 

make better candidates for release than females without such 

experience. Perhaps males could be kept in the outdoor pens 

during much of the remainder of the year, a practice that may 

accrue additional benefits in reproductive performance (D. 

Kwiatkowski, oral commun., 1991). Additional investigations 

of these types of variables might lead to enhanced postrelease 

survival of captive-reared ferrets.

At this point in the recovery program, black-footed ferret 

kits seem to have short-term and long-term survival rates 

at least fourfold higher than those of adult ferrets. On the 

other hand, ferrets released at age 3 or 4 likely have already 

exceeded the mean life spans of their wild-born counterparts 

in established populations. Some female ferrets released as 

adults have produced litters (in South Dakota and Arizona), 

and a male released at age 5 in South Dakota survived at least 

3.5 years longer, becoming the oldest known ferret in the wild 

at age 8.5. Such anecdotal information representing extreme 

cases should carry little weight in decisionmaking, but neither 

do we presume that data on survival rates for hundreds of 

ferrets can lead to unequivocal recommendations regarding the 

advisability of releasing adults. Decisions will need to depend 

Figure 8. Number of days of known survival for marked adult and 

young black-footed ferrets (Mustela nigripes) calculated using 

detections from spotlight searches, snow-track searches, and 

radio telemetry (mean ± SE).
Postrelease Movements and Survival 199

partly on interpreting survival rates of released adults relative 

to other groups of wild and released ferrets, but philosophical 

views will continue to exert an influence. 

Some conservationists and ethicists may justify extreme 

means to achieve the goal of preservation and recovery of a 

species, assuming that the importance of a species is greater 

than the sum of the rights of its individual constituents (Gunn, 

1980). In the words of Rolston (2006, p. 116), “Extinction 

shuts down the generative processes in a kind of superkilling. 

It kills forms (species) beyond individuals.” Others may set 

inviolate moral standards regarding the welfare of individuals 

wherein the “mere size of the relative population of the species 

to which a given animal belongs makes no moral difference 

to the grounds for attributing rights to that individual animal 

or to the basis for determining when that animal’s rights 

may be justifiably overridden or protected” (Regan, 2004, p. 

360). Even when thinking is focused on the individual ferret, 

however, opinions differ. Some emphasize the relative safety 

of a captive ferret; there is little danger it will miss a meal or, 

worse yet, become one. This line of ethical reasoning could 

lead to removing each individual “from its predator-filled 

natural habitat and providing it with a safe, food-rich environment . . . while exterminating the species” (Agar, 1995, p. 

403). The controversy over releasing adult ferrets, however, 

has a narrower focus and seems to stem mostly from differences of opinion over the relative values of longevity and 

freedom. Remaining in captivity may allow a zoo animal to 

avoid an “untimely death” (Regan, 2004, p. 396) but prolongs 

the “harm” (in the form of “deprivation”) that the animal may 

“suffer as a result of being caged” (Regan, 2004, p. 99). The 

relative impacts of these “inflictions” and “deprivations” have 

been contrasted (Regan, 2004, p. 303). Although these philosophical issues may be suitable topics for debate in appropriate forums, extensive discussion of them is beyond the scope 

of this paper.

Acknowledgments

Funding for this study was provided in part by the 

following Federal agencies: U.S. Geological Survey, U.S. 

Fish and Wildlife Service, U.S. Department of Agriculture 

Forest Service, U.S. Bureau of Land Management, National 

Park Service, U.S. Army, and U.S. Air Force. The Utah 

Division of Wildlife Resources, the Colorado Division of 

Wildlife, the Montana Department of Fish, Wildlife and 

Parks, and the South Dakota Department of Game, Fish and 

Parks also consistently provided funds and direct assistance 

from field personnel. Wildlife Trust and Project Lighthawk 

contributed financial and in-kind support during some years. 

We are indebted to the dedicated personnel at the National 

Black-footed Ferret Conservation Center, outdoor pen facilities (Vermejo Park Ranch, Conata Basin, Bowdoin National 

Wildlife Refuge, Browns Park National Wildlife Refuge, 

Aubrey Valley, and Pueblo Army Depot), and participating 

zoos (Cheyenne Mountain, Henry Doorly, Louisville, Metro 

Toronto, National, and Phoenix) for producing and preconditioning black-footed ferrets for release. Space does not permit 

proper individual recognition of the >150 technicians and 

volunteers who spent long hours radio tracking and searching 

for ferrets with spotlights over the 8-year period and were thus 

crucial to the accumulation of this large data set.

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Abstract

Predation on black-footed ferrets (Mustela nigripes) is 

a potential problem at reintroduction sites, causing up to 95 

percent of the documented mortality of ferrets. Strategies to 

reduce mortality due to predation can focus on preconditioning 

ferrets prior to reintroduction and/or managing predators of 

ferrets. Biologists have tried three general strategies to control 

predators at reintroduction sites: (1) selective removal of individual predators, (2) nonselective removal of coyotes (Canis 

latrans), and (3) electric fences to exclude coyotes from 

release sites. We conducted a post hoc review of data from 

releases during 1994–2003 at 11 sites in South Dakota and 

Montana to address whether or not predator management has 

benefited reintroduced black-footed ferrets. Limited evidence 

indicates that (1) individual great horned owls (Bubo virginianus) can cause significant ferret mortality and that identifying 

and removing these individuals can be beneficial, (2) lethal 

control of coyotes may have inverse effects on ferret survival, 

and (3) electric fencing does not enhance short- or long-term 

survival of reintroduced ferrets. The data are confounded by a 

variety of factors, making conclusions tenuous. Well designed 

studies are needed to properly address the effectiveness of 

predator management for enhancing ferret survival.

Keywords: black-footed ferret, Bubo virginianus, Canis 

latrans, coyote, electric fencing, great horned owl, Mustela 

nigripes, predator control

Introduction

Successful recovery of black-footed ferrets (Mustela 

nigripes) will ultimately depend upon our ability to understand 

and manage a number of ecological factors (e.g., genetic 

inbreeding, disease, habitat, and predation) that influence 

survival, reproduction, and recruitment of ferrets in recovering populations. The role of predators in ecology, conservation biology, and wildlife management has gained increasing 

recognition as a factor to understand and potentially manage 

(Estes and others, 2001; Terborgh and others, 1999). For 

ferrets, mammalian and avian predation has been identified as 

a critical ecological component in both established populations 

(Forrest and others, 1988) and reintroduction efforts (Biggins 

and others, 1998; Biggins, 2000; Biggins, Godbey, Livieri, and 

others, this volume).

For example, at Meeteetse, Wyo., where the ancestral 

free-ranging population of ferrets was studied, 57 percent of 

known mortality of wild ferrets was due to predation (Forrest 

and others, 1988). Predation by great horned owls (Bubo 

virginianus), golden eagles (Aquila chryseatos), and coyotes 

(Canis latrans) was recorded, leading Forrest and others 

(1988) to conclude that in the Meeteetse ferret population: 

(1) annual mortality was high, (2) few if any ferrets lived to 3+ 

years, (3) 59 percent to 77 percent of all juveniles disappeared 

each year (when disease was not present), (4) adults disappeared at a rate about 80 percent of that seen in juveniles, and 

(5) predation was the most significant cause of ferret mortality 

(when disease was not present). For reintroduced animals, 

predation is equally if not more important, accounting for over 

95 percent of the ferrets lost from reintroductions (Biggins, 

2000; Biggins, Godbey, Livieri, and others, this volume). For 

those ferrets killed by predators, coyotes accounted for over 60 

percent of the mortality and may have accounted for another 

20–30 percent of unconfirmed predation. Badgers (Taxidea 

taxus), great horned owls, and other raptors accounted for a 

small portion of the predation.

A number of factors likely contribute to the dynamics 

of predator-ferret interactions, including predator density and 

behavior, availability of alternative prey, habitat conditions, 

and, for reintroduced animals, the level of preconditioning 

individuals receive before being released to the wild. Preconditioning enhanced survival of reintroduced ferrets and Sibe1

U.S. Department of Agriculture, National Wildlife Research Center, 4101 

LaPorte Ave., Fort Collins, CO 80521.

2

U.S. Geological Survey, Fort Collins Science Center, 2150 Centre Ave., 

Bldg. C, Fort Collins, CO 80526.

3

Prairie Wildlife Research, P.O. Box 515, Wall, SD 57790. 

4

U.S. Fish and Wildlife Service, Charles M. Russell National Wildlife

Refuge, P.O. Box 110, Lewistown, MT 59457.

5

U.S. Bureau of Land Management, Malta Field Office, HC 65, Box 5000, 

Malta, MT 59538.

Does Predator Management Enhance Survival of 

Reintroduced Black-footed Ferrets?

By Stewart W. Breck,1 Dean E. Biggins,2 Travis M. Livieri,3 Marc R. Matchett,4 and Valerie Kopcso5
rian polecats (Mustela eversmannii; Biggins and others, 1991, 

1998, 1999). The foregoing research helped lead to establishment of a general preconditioning program for all ferrets 

released into the wild. Concurrent with the preconditioning 

research, biologists and managers from different release 

sites also tried techniques for managing predators to enhance 

survival of newly released ferrets. Early studies indicated that 

mortality of surrogate Siberian polecats was higher in areas 

with more predators (Biggins and others, 1991). Predator 

management primarily focused on coyotes and included both 

lethal and nonlethal techniques. Lethal management primarily 

involved removing coyotes in and around release areas prior to 

release of ferrets. To a lesser extent badgers and great horned 

owls were occasionally killed, mostly in attempts to stop individuals that apparently developed a search image for ferrets. 

In addition to lethal control, many release sites used electrified 

fencing to exclude terrestrial predators (primarily coyotes 

and badgers) for short periods (30–60 days postrelease). The 

results of these management actions have not been synthesized 

and published outside of internal reports. Our objective here is 

to explore existing data to determine if lethal coyote control, 

electric fencing, or selective removal of individual predators 

enhanced short-term and/or long-term survival of reintroduced 

black-footed ferrets.

Study Area and Methods

We synthesized data from black-footed ferret reintroduction 

sites in Montana and South Dakota and only used data on ferrets 

that had been preconditioned. Although other data were available from releases in Wyoming, Arizona, and Colorado/Utah, 

differences in prairie dog (Cynomys) species, preconditioning of 

ferrets, detectability of ferrets, and monitoring methodology from 

these sites precluded their inclusion in this analysis. In Montana 

a total of 10 releases occurred from 1994 to 2003, and in South 

Dakota, 10 releases occurred from 1994 to 1999 (table 1). All 

releases occurred on black-tailed prairie dog (C. ludovicianus) 

colonies, with higher densities of prairie dogs occurring on the 

South Dakota sites.

For each release, both short-term (30 days postrelease) 

and long-term (6–8 months postrelease) estimates of survival 

were determined by spotlighting ferrets (Campbell and 

others, 1985). Each survival estimate was based on a multiple 

night effort in which personnel in vehicles and on foot 

surveyed release areas with spotlights to detect ferrets. Any 

ferret detected was identified by using an automatic passive 

integrated transponder (PIT) reader placed at the burrow 

containing the animal (Biggins, Godbey, Matchett, and others, 

this volume). Transponders (i.e., PIT tags) were implanted 

subcutaneously in each individual prior to release. Survival 

rates were calculated as the percent of ferrets found alive and 

thus represent minimum survival estimates. Lack of replication 

in spotlight surveys over short time spans prevented separate 

estimation of detection rates and survival rates, precluding the 

use of more sophisticated methods of survival analysis.

We used short- and long-term minimum survival estimates to evaluate whether lethal coyote control and/or electric 

fencing increased ferret survival. Lethal coyote control was 

carried out in a variety of ways and intensities across release 

sites and years. Some release sites were subjected to extensive 

coyote removal in and around release areas. At other sites 

smaller numbers of coyotes were removed in conjunction with 

disease monitoring, and at some sites no coyote removal was 

performed (table 1). We categorized the level of coyote control 

as high, medium, or low. High intensity control combined 

aerial gunning, opportunistic removal onsite, and disease 

sampling. Medium intensity control combined opportunistic 

removal onsite and disease sampling in and around the release 

area. Low intensity effort involved just disease sampling or no 

lethal control. 

Electric fencing (ElectroNetTM; Premier1Supplies, Washington, Iowa) was used in attempts to exclude coyotes from 

some release sites during some years. ElectroNet is 107 cm in 

height, powered by 12-V deep cycle batteries, and constructed 

with 10 alternately charged conductors supported with vertical 

plastic stays every 30 cm. ElectroNet is designed to exclude 

mammalian species the size of coyotes and badgers while 

allowing ferrets and other smaller mammals to move through 

the fence. Experimental trials of ElectroNet excluded coyotes 

from bait stations for up to 2 weeks (Matchett, 1995), and 

telemetry data from ferret reintroduction sites indicated that 

ElectroNet may have enhanced short-term survival of ferrets 

within fenced enclosures (Matchett, 1999). We tried to extend 

knowledge of the utility of ElectroNet by testing for differences 

in both short- and long-term minimum survival between those 

reintroduction sites that used ElectroNet and those that did not 

(table 1). The perimeter of fencing used at reintroduction sites 

varied from 3.5 km to 13 km and was maintained for a minimum of 30 days postrelease. 

We hypothesized that ferrets in areas with higher densities of prairie dogs (i.e., South Dakota), higher levels of coyote 

control, and electric fencing would have higher estimates 

of both short- and long-term survival. We generated linear 

models to evaluate this prediction; competing models included 

interaction terms and combinations of four explanatory 

variables (see tables 2 and 3 for a complete list of models). 

We used likelihood-based methods (Buckland and others, 

1997; Burnham and Anderson, 1998) to quantify strength of 

evidence for alternative models explaining patterns of ferret 

survival. Estimating the “weight,” or probability that a given 

model is the best approximation to truth among the models 

considered, is a means for reporting the relative support for 

alternative models where the weights from the candidate 

list of models sum to 1. Thus a model with a weight of 1 

has complete support and a model with a weight of 0 has no 

support (Burnham and Anderson, 1998). 

We used Proc GenMod with the logit link option, 

which assumes a binomial distribution (SAS Institute Inc., 

1999) to analyze each model and create output required to 

204 Recovery of the Black-footed Ferret
calculate Akaike’s Information Criterion (AIC) values. We 

used ferrets as replicates (n = 489) and performed a separate 

analysis for short- and long-term survival data. For each 

analysis we assessed the goodness-of-fit by calculating the 

deviance on the global (fully parameterized) model. We used 

cˆ (deviance/df) to adjust for overdispersion (i.e., lack of fit) 

and used the small-sample correction of AIC (QAICc; Lebreton and others, 1992; Burnham and Anderson, 1998) to rank 

the models and generate an estimate of the weight. We based 

inferences of survival on the top model.

Results 

General patterns in the data show that: (1) both shortand long-term minimum survival estimates have increased in 

latter years of reintroduction efforts (this was especially true 

in South Dakota; see table 1); and (2) there was a great deal 

of variation in estimates of survival across sites and years 

(short-term low = 20 percent, short-term high = 88 percent; 

long-term low = 4 percent, long-term high = 69 percent). 

Deviance for both global models (short- and long-term 

analyses) was large (35.5 and 32.7, respectively; P < 0.001) 

indicating that overdispersion was problematic (i.e., fit of 

model was not good). Based on QAICc weights (tables 2 and 

3), both short- and long-term minimum survival of reintroduced ferrets were supported by models showing a difference 

primarily between levels of coyote control and fencing. Ferret 

survival was inversely related to coyote control with releases 

that had the highest levels of control showing approximately 

12 percent lower minimum survival compared to the lowest 

levels of control for both short- and long-term analyses (figs. 1 

and 2). Evidence of the effectiveness of electric fencing was 

opposite of what we predicted; ferrets released in areas with 

fencing showed lower short- and long-term minimum survival 

than ferrets released in areas without fencing, 3 percent and 5 

percent, respectively (figs. 1 and 2). The variable site was not 

a factor in either analysis, indicating no detectable differences 

in minimum survival between release sites. There was only 

weak evidence that survival of ferrets differed between States 

(i.e., the variable State was part of the 2nd ranked model in 

the long-term analysis; table 3), indicating differences in 

prairie dog density between States did not appear to influence 

survival.

Release area and year

Number of ferrets 

released

Short-term 

survival

Long-term 

survival

Number of

coyotes removed Electric fence used?

MT 94 17 0.47 0.41 Medium No

MT 95 33 0.61 0.33 High Yes

 MT 96 39 0.56 0.15 High Yes

MT 97 20 0.55 0.20 Medium Yes

MT 98 21 0.43 0.14 Medium Yes

MT 99 23 0.35 0.04 Medium Yes

MT 01 (BLM 40) 20 0.40 0.15 Low Yes

MT 02 (BLM 40) 25 0.32 0.16 Low No

MT 03 37 0.76 0.38 Low No

MT 03 (BLM 40) 20 0.20 missing Low No

SD 94 13 0.38 0.23 Medium No

SD 95 37 0.30 0.08 Medium No

SD 96 (Agate) 15 0.53 0.07 High Yes

SD 96 (Burns) 24 0.29 0.13 High Yes

SD 97 (Kosher) 21 0.76 0.24 Medium Yes

SD 97 (Sage) 36 0.86 0.69 Medium Yes

SD 98 (Agate) 25 0.88 0.28 Low No

SD 98 (Sage) 15 0.73 0.33 Low No

SD 99 (Hecktable) 36 0.86 0.44 Low No

SD 99 (Sage) 12 0.75 0.50 Medium No

Table 1. Descriptive data on black-footed ferret (Mustela nigripes) survival (short-term = 30 days, long-term = 6–8 months) and predator 

control efforts (high, medium, or low) from 20 release sites in Montana and South Dakota.

Predator Management 205
Table 3. Results of the Akaike’s Information Criterion (AIC) model selection procedure to determine the model that best explains longterm (6–8 months) survival patterns of reintroduced black-footed ferrets (Mustela nigripes), 1994–2003. NPAR is the number of parameters, QAICc

 is a version of AIC adjusted for overdispersion, DELQAICc is the difference in QAIC relative to the smallest value in the set, 

and Weight is an estimate of the likelihood of each model (Burnham and Anderson, 1998). Variables in the models are: fence (present 

or not), coyote (level of lethal coyote control: low, medium, high), State (Montana or South Dakota), and site (eight different release 

sites). Dot indicates a model with only an intercept (i.e., no explanatory variables). The symbol * indicates an interaction between two 

variables, and | indicates all possible combinations of the variables.

Model NPAR QAICc DELQAICc Weight

Fence coyote 4 130.67 0.00 0.484

State fence coyote 5 132.98 2.30 0.153

Fence coyote fence*coyote 5 133.16 2.48 0.140

Fence 2 134.31 3.64 0.078

Coyote 3 135.39 4.72 0.046

State fence 3 135.47 4.80 0.044

Dot 1 136.68 6.00 0.024

State coyote 4 138.84 6.17 0.022

State 2 139.60 8.93 0.006

State fence State*fence 4 140.79 10.12 0.003

State coyote State*coyote 6 143.15 12.47 0.001

State|fence State|coyote fence|coyote 9 193.08 62.41 0.000

Site 8 227.04 96.37 0.000

Model NPAR QAICc DELQAICc Weight

Fence coyote 4 123.41 0.00 0.51

Fence 2 124.93 1.52 0.24

Coyote 3 125.55 2.14 0.18

Dot 1 128.85 5.44 0.03

State fence State*fence 4 129.00 5.59 0.03

Fence coyote fence*coyote 5 132.18 8.77 0.01

State fence 3 133.20 9.79 0.00

State coyote 4 136.25 12.84 0.00

State 2 137.76 14.34 0.00

State fence coyote 5 139.17 15.76 0.00

State coyote State*coyote 6 166.83 43.42 0.00

State|fence State|coyote fence|coyote 9 222.85 99.44 0.00

Site 8 346.79 223.38 0.00

Table 2. Results of the Akaike’s Information Criterion (AIC) model selection procedure to determine the model that best explains 1-

month survival patterns of reintroduced black-footed ferrets (Mustela nigripes), 1994–2003. NPAR is the number of parameters, QAICc is 

a version of AIC adjusted for overdispersion, DELQAICc is the difference in QAIC relative to the smallest value in the set, and Weight is 

an estimate of the likelihood of each model (Burnham and Anderson, 1998). Variables in the models are: fence (present or not), coyote 

(level of lethal coyote control: low, medium, high), State (Montana or South Dakota), and site (eight different release sites). Dot indicates 

a model that only includes an intercept (i.e., no explanatory variables). The symbol * indicates an interaction between two variables, and 

| indicates all possible combinations of the variables.

206 Recovery of the Black-footed Ferret
Discussion

A general pattern that emerged from the data was that 

estimates of both short- and long-term survival were highly 

variable even in later years of releases. Variation in survival 

could be due to a number of factors, including differences 

in habitat quality, random variation, measurement error, and 

differences in predation pressure. One factor relating to predators that may have contributed to variation in survival estimates is the role of one or a few problem individuals. Here we 

define problem individuals as predators that seem to develop 

a search image for ferrets, consequently becoming disproportionately more successful than other predators at finding and 

killing ferrets. Critical to the discussion of problem individuals 

is the realization that mortality of single animals has a larger 

effect in small populations than in larger populations (Krebs 

and others, 1995; Krebs, 1996). Thus, it is possible for one 

or a few individual predators to have a large overall effect on 

a small population of reintroduced ferrets. A likely example 

of problem individuals was seen in South Dakota during the 

1996 releases (table 1). Nearly half (11 of 24) of the known 

mortalities that occurred during that release season were 

caused by one to three great horned owls. In response to the 

identified problem, three great horned owls were killed on and 

around the release site, and no further known mortalities were 

caused by owls. Problem individuals could explain the pattern 

observed in Montana in 2003 where one release site had a 

high short-term survival rate of 76 percent while the other had 

short-term survival of 20 percent, even though no predation by 

owls was observed.

Our analyses indicated that the relationship between the 

level of lethal coyote control and ferret survival was opposite 

of what we hypothesized; that is, more intensive efforts to 

remove coyotes related to poorer survival for ferrets. This 

relationship was apparent for both short- and long-term data 

(figs. 1 and 2). However, several factors are important to 

consider before drawing any conclusions regarding these 

patterns. First, most of the high-level efforts for controlling coyotes occurred in earlier years of releases. Thus, the 

general increase in estimates of survival over time could 

reflect improvements in preconditioning of ferrets rather than 

changes in coyote control. Although no data exist to quantify 

the “quality” of ferrets released over time, it seems possible 

that preconditioning programs could have improved as the 

programs were optimized. Second, our method for categorizing levels of coyote control was not ideal. If future research 

addresses this question, then quantifying density of coyotes 

pre- and postremoval would be paramount for relating coyote 

control to ferret survival. Third, increasing survival over 

successive years may be an artifact of increasing observer 

efficiency at detecting ferrets or other factors related to estimating survival. The fundamental problem that gives rise to 

interpretative difficulties mentioned in factors 1–3 previously 

(and others) is the unbalanced design. All treatments were not 

replicated at all sites and certainly not in all years at all sites. 

For example, the BLM 40 complex had only “low” predator 

control for all 3 years that ferrets were released. Site-specific 

impacts of unmeasured factors (e.g., disease) may be misinterpreted as treatment effects. Finally, some of the confusion 

regarding the effectiveness of predator management stems 

from poor understanding of coyote ecology and behavior in 

and around release sites. Almost no reliable information exists 

on activity patterns, use of prairie dog habitat by coyotes, and 

response of coyotes to control efforts as it relates to blackfooted ferrets. 

Nevertheless, it is interesting to speculate on how higher 

levels of coyote control could cause a decrease in ferret 

survival. Assuming that killing coyotes creates voids filled by 

coyotes from surrounding territories, one possibility is that 

Figure 2. Estimates of long-term (6–8 months) minimum survival of 

reintroduced black-footed ferrets (Mustela nigripes) for two explanatory variables: fencing (present or not) and lethal coyote control (low, 

medium, and high). In total, 489 ferrets were released from different 

sites in Montana and South Dakota from 1994 to 2003. Error bars represent 95% confidence intervals.

Figure 1. Estimates of short-term (1 month) minimum survival of 

reintroduced black-footed ferrets (Mustela nigripes) for two explanatory variables: fencing (present or not) and lethal coyote control (low, 

medium, and high). In total, 489 ferrets were released from different 

sites in Montana and South Dakota from 1994 to 2003. Error bars represent 95% confidence intervals.

Predator Management 207
as new individuals begin to establish territories, their movements and behavior enhance the probability of encountering 

ferrets. Many of the ferrets that have been found killed by 

coyotes were not eaten, indicating that the interaction between 

coyotes and ferrets may more accurately be described as a 

form of competition (i.e., intraguild predation; Holt and Polis, 

1997; Palomares and Caro, 1999). In competitive interactions, 

individual animals may not develop specific search images for 

competitors but rather respond to competitors in an opportunistic fashion. Creating situations in which coyotes are more 

active (i.e., filling voids) may enhance encounter rates and 

create greater threat for ferrets.

Of the tools used to control coyote predation, electric 

fencing offered the most potential to completely eliminate 

coyote predation on ferrets. The general impression from 

biologists working at release sites was that fencing did exclude 

coyotes. At minimum we expected to see higher short-term 

survival rates for ferrets at sites that used fencing. We found 

no evidence, however, that fencing enhanced ferret survival 

for the short- or long-term; in fact, we detected slightly 

lower survival rates (figs. 1 and 2) at sites that used fencing. 

Again we caution against strong interpretation of these data 

for reasons already mentioned, but a couple of factors may 

explain this pattern. 

First, fencing was only used during earlier years of reintroductions (table 1). Though we tried to control for preconditioning in this analysis, it is possible that ferrets released 

in later years had better preconditioning that enhanced their 

survival. Second, we know great horned owl predation had 

a large effect on survival of ferrets at two sites (Agate and 

Burns) in South Dakota in 1996, both sites that used fencing. 

Fencing does not deter avian predation, and in this analysis we 

were unable to control for owl or other avian predation. If we 

could have controlled for avian predation, it is possible that we 

would have detected higher survival of ferrets released in areas 

with fencing, at least for the short-term. Finally, in years when 

fencing was used, anecdotal observations indicate that many 

of the ferrets killed by coyotes occurred when ferrets moved 

outside the fence boundary. Again we were unable to control 

for this confounding factor in this analysis. 

Our results highlight the need to perform well designed 

experiments to better elucidate the possible benefit of predator management to enhance black-footed ferret survival at 

reintroduction sites. The fact that survival of reintroduced 

ferrets remains highly variable indicates that factors other than 

preconditioning are important. Based on our understanding 

of ferret ecology, it is likely that predation is responsible for 

most of the mortality. Understanding whether or not we can 

manage this predation pressure remains an important goal for 

ferret recovery. Equally important to recovery efforts is the 

need to understand the role that predation plays in established 

populations of black-footed ferrets. Such data would not only 

provide direct benefits to ferret conservation by potentially 

increasing the number of ferrets that could be translocated but 

would also provide better parameter estimates for modeling 

exercises that depend upon understanding the role of important 

ecological forces. The most effective means for determining 

the role of predation in ferret demography and ecology would 

be to manipulate predator populations and compare responses 

to unmanipulated populations. Because coyotes are the most 

important predator of ferrets, we suggest using electric fencing 

to exclude coyotes as it offers the most potential to control 

coyote predation.

For the manager who must decide whether or not to 

manage predators in and around reintroduction sites, we offer 

the following recommendations. First, great horned owls 

view ferrets as prey and probably can develop a search image 

for ferrets. Problem individuals may have large impacts on a 

population of reintroduced ferrets. If great horned owls are 

present in the immediate vicinity of a release area, it may be 

wise to remove individual owls, and, if possible, remove perch 

sites as well. Second, there is no evidence that lethal removal 

of coyotes at the levels of control implemented in previous 

releases enhances short- or long-term survival of ferrets. 

Extensive control efforts may eliminate coyotes from release 

sites, temporarily reducing predation pressure on ferrets. 

However, rates of recolonization by coyotes after such removal 

are poorly understood and may have important implications 

for ferrets. Lethal removal of a few individual coyotes probably will not enhance ferret survival because coyotes are 

often abundant and possibly because of the way coyotes and 

ferrets interact. Last, electric fencing appears to be an effective 

method for excluding coyotes and may offer benefits for reintroduced ferrets as long as the fencing is up and functioning. 

However, maintaining fencing over the long-term is difficult 

and expensive; thus, fencing is generally only used for short 

periods (1–2 months). Once fencing is removed, there is no 

evidence to suggest that the short-term benefits translate into 

enhanced long-term survival. Thus, for future reintroductions 

we do not recommend fencing unless the manager can maintain it for long periods or identify how short-term protection 

may aid long-term survival of ferrets. 

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Colorado State University, Ph.D. dissertation, 184 p.

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Predator Management 209

Section VI. Managing Diseases

Although this section does not correspond to a single specific part of the 1988 recovery 

plan, diseases were recognized as important. When the plan was written, canine distemper was 

thought to have been responsible for the sudden collapse of the Meeteetse population of ferrets. 

Thus, canine distemper was the disease receiving most discussion in that plan. Plague was 

assumed to influence ferrets only indirectly through its impact on prairie dogs; it thus received 

emphasis in habitat and population management sections of the plan. Events of the 1990s 

(including one described in this section) documented the ferret’s direct and extreme susceptibility to plague. Plague may be the overwhelmingly important determinant in the list of factors 

potentially influencing successful establishment of viable populations of black-footed ferrets.

Abstract

Plague (Yersinia pestis infection) presents serious risks 

not only to humans but also to wildlife species such as prairie 

dogs (Cynomys spp.) and the critically endangered blackfooted ferret (Mustela nigripes). The effects of plague are 

sufficiently serious to hamper recovery of ferrets and prairie 

dogs in areas that experience repeated epizootic activity. In 

order to more effectively manage and reduce plague risks 

for both wildlife and humans, we must improve our understanding of what factors influence the distribution of plague, 

the transmission and spread of epizootics, and the ability of 

the plague bacterium to maintain itself indefinitely in some 

populations of rodent hosts and their flea (Insecta: Siphonaptera) vectors. This article provides a review of our current 

knowledge of plague ecology. We also describe how recent 

research advances are providing significant new knowledge 

and methodologies that can help us better manage plague risks 

and reduce the impact of the disease on mammalian populations, including those of conservation interest.

Keywords: disease ecology, flea, plague, rodent, Yersinia 

pestis, zoonosis

Introduction

Plague is a flea-borne zoonotic disease caused by the 

bacterium Yersinia pestis (Gage, 1998). The disease is best 

known as the cause of devastating pandemics, including the 

Black Death of the Middle Ages. These same pandemics, as 

well as other more regional outbreaks, also provide striking 

demonstrations of plague’s ability to spread rapidly across 

vast geographic areas, a process that occasionally results in the 

establishment of long-term foci of infection among suitable 

populations of susceptible mammalian hosts and competent 

flea vectors. At present, active plague foci are found in many 

countries in Asia, Africa, and the Americas (Gage, 1998; 

Tikhomirov, 1999; World Health Organization, 2004). In the 

United States, evidence of plague infection has been identified during recent decades in mammals or fleas in 17 western 

States (fig. 1).

Although most evidence suggests that virtually any 

mammal exposed to Y. pestis is likely to become infected, the 

true vertebrate hosts are certain species of rodents (Pollitzer 

and Meyer, 1961; Gage and Kosoy, 2005). Plague-related 

mortality can vary greatly between rodent species and even 

among populations within the same species. In some rodent 

species mortality approaches 100 percent (Poland and Barnes, 

1979). Although certain other rodents appear to be more 

resistant to plague, even supposedly resistant populations can 

experience mortality rates in excess of 40 percent (Rivkus 

and others, 1973). Mortality can also be high among various 

nonrodent species found naturally infected with Y. pestis. 

Wild and domestic felines, as well as some lagomorphs 

(hares, rabbits, and pikas), are extremely susceptible (Gage 

and others, 1994; Gage and Kosoy, 2005). Identification of 

high seropositivity rates among other nonrodent species, 

such as coyotes (Canis latrans), badgers (Taxidea taxus), 

and feral hogs (Sus scrofa), suggests that these species are at 

least moderately resistant to plague-related mortality (Gage 

and others, 1994). While most nonrodent species, with the 

exception of a few lagomorphs and the house shrew (Suncus 

murinus) of southeastern Asia and Madagascar, are not significant hosts of plague, certain mammalian predators and birds of 

prey probably play important ecological roles by transporting 

infected fleas from one region to another (Gage and others, 

1994). 

Elton’s (1958) classic book on the ecology of invasions 

mentions plague as an example of an agent that can spread 

explosively across vast areas, infecting not only commensal 

rats (Rattus spp.) and “wild” rodents but also other mammals, 

including humans (Gage and others, 1995; Gage and Kosoy, 

2005). Within the past two decades, an increasing number 

of biologists have become aware of the devastating effects 

plague has on certain mammal species of conservation interest 

(Biggins and Kosoy, 2001a,b). Mortality among infected 

black-tailed prairie dogs (Cynomys ludovicianus) reportedly 

approaches 100 percent during epizootics, and other prairie 

dog species (Cynomys spp.) also are quite vulnerable to the 

disease (Kartman and others, 1962; Lechleitner and others, 

1962, 1968; Rayor, 1985; Ubico and others, 1988; Anderson 

and Williams, 1997; Cully, 1997; Cully and others, 1997, 

2000; Girard and others, 2004; Stapp and others, 2004). 

Recent evidence also indicates that plague epizootics can 

cause significant reductions in genetic diversity among prairie 

dog populations (Trudeau and others, 2004). In some situations plague has both direct and indirect impacts on wildlife 

populations. Prairie dogs and their endangered predator, the 

1

Bacterial Zoonoses Branch, Division of Vector-Borne Infectious Disease, 

Centers for Disease Control and Prevention, P.O. Box 2087, Fort Collins, CO 

80522.

Recent Trends in Plague Ecology

By Kenneth L. Gage1 and Michael Y. Kosoy1
214 Recovery of the Black-footed Ferret

black-footed ferret (Mustela nigripes), are both severely 

affected by plague, and recovery efforts for black-footed 

ferrets are hampered not only by the fact that plague outbreaks 

eliminate the ferrets’ prey but also because the ferrets themselves are extremely susceptible to the disease (Williams and 

others, 1994; Biggins and Kosoy, 2001b; Biggins and Godbey, 

2003). The devastating impact of plague on these and other 

mammalian species of conservation interest has resulted in a 

renewed emphasis on identifying means for managing plague, 

including techniques as diverse as insecticidal control of 

vector fleas and immunization of animals with recombinant 

vaccines (Creekmore and others, 2002; Seery and others, 

2003; Mencher and others, 2004; Rocke and others, 2004). 

Biggins and Godbey (2003) also discuss partial solutions to 

the problem of black-footed ferret recovery, including means 

for increasing breeding in captive populations, increasing 

survival of released animals, and taking advantage of South 

Dakota sites that are located slightly east of the known distribution of plague.

In order to more effectively manage and reduce human 

and wildlife risks associated with plague, we must improve 

our understanding of the factors that influence transmission, 

the occurrence and spread of epizootics, and the ability of 

plague to maintain itself in natural foci. This article provides 

a brief update on our current knowledge of plague ecology 

and describes how recent research has contributed to a better 

understanding of the topic and improved methodologies for 

studying plague. Also discussed are the many gaps in our 

knowledge of how plague is maintained in natural foci, what 

roles certain rodent and vector species play in transmission 

dynamics, and how environmental factors influence the occurrence, spread, and persistence of epizootics.

The Plague Bacterium and Its Origins

Yersinia pestis is a gram-negative bacterium belonging 

to the family Enterobacteriaceae. Unlike other members of 

this group, which are transmitted through fecal-contaminated 

food and water and live in the guts of their hosts, Y. pestis is 

typically spread from host to host through the bites of infectious fleas and inhabits the blood, as well as lymphatic and 

reticuloendothelial systems, of its hosts. This dramatic shift 

in mode of transmission and vertebrate host habitat appears to 

have been associated, at least in part, with the acquisition of 

genes that encode virulence and transmission factors. Homologous genes for some Y. pestis virulence factors can be found 

in other species of Yersinia, including Y. pseudotuberculosis. 

The origin of genes encoding other virulence or transmission 

Figure 1. Counties with plague-positive mammals or fleas (1970–present). Figure courtesy of Centers for Disease Control and 

Prevention.
Recent Trends in Plague Ecology 215

factors is not always clear, but most evidence suggests they 

were acquired through horizontal transfer of genetic material 

from other enteric bacteria (Prentice and others, 2001; Gage 

and Kosoy, 2005). The virulence factors of Y. pestis play 

important roles in enabling host invasion, dispersal within the 

host, or development of high level bacteremias that greatly 

increase the likelihood that blood-feeding fleas will imbibe 

sufficient Y. pestis to become infected and later transmit the 

plague bacterium to other hosts. Yersinia pestis transmission factors promote survival of the plague bacterium in the 

guts of vector fleas and its transmission by these insects. 

For a more thorough review of virulence and transmission 

factors and their role in maintaining the natural transmission 

cycle of plague, see reviews by Perry and Fetherston (1997), 

Hinnebusch (2003), and Gage and Kosoy (2005).

Until relatively recently, it was believed that the plague 

bacterium first appeared many millions of years ago, perhaps 

as early as the upper Oligocene or lower Miocene (Kucheruk, 

1965). According to Kucheruk (1965), plague initially arose in 

cricetid rodents living in semidesert and desert environments. 

He based these conclusions on an analysis that indicated that 

the predominant plague hosts in Asia, Africa, and the Americas belonged to the Cricetidae, a family that at the time of 

Kucheruk’s publication included gerbillines, cricetines, arvicolines, and sigmodontines. While this suggestion is still generally accepted, other former Soviet researchers have recently 

proposed that Y. pestis first evolved in marmots (Marmota

spp.) and their fleas (Suntsov and Suntsova, 2000).

Recent molecular studies clearly indicate that Y. pestis is 

very closely related to the gut microbe Y. pseudotuberculosis

(Bercovier and others, 1980; Trebesius and others, 1998). The 

high degree of relatedness between these two bacteria strongly 

suggests that they have diverged only recently, as suggested 

by Achtman and others (1999), who proposed that Y. pestis

might have arisen as a clone of Y. pseudotuberculosis only 

1,500–20,000 years ago (Achtman and others, 1999; Wren, 

2003). This last finding is particularly interesting because of 

its implications for the degree of coadaptation or coevolution 

that might have occurred between Y. pestis and its hosts and 

vectors. The recently reported genomic sequences of three 

Y. pestis strains also reveal many interesting features of this 

bacterium and support the contention that the Y. pestis genome 

is still in a state of rapid flux and might be undergoing reductive evolution as it loses the ability to express certain genes 

that remain active in Y. pseudotuberculosis but are not required 

for Y. pestis to be maintained in a vector-borne transmission 

cycle (Wren, 2003). Indeed, it has been suggested that the 

disruption of genes still expressed in Y. pseudotuberculosis

might be essential for Y. pestis to survive in a vector-vertebrate 

host life cycle (Wren, 2003).

The actual geographic origin of the plague bacterium 

was a subject of considerable speculation during much of the 

20th century. Based on the analysis of plague hosts cited in 

the previous paragraph, Kucheruk (1965) felt that Y. pestis

probably appeared in either North American or Asian cricetids. More recent lines of reasoning, however, suggest that a 

North American origin is highly unlikely. First, epidemiologic 

evidence strongly indicates that plague did not exist in the 

United States prior to the last pandemic when rat-infested 

ships introduced Y. pestis to the San Francisco area around 

1900 (Link, 1955; Barnes, 1982). Second, microbiological evidence indicates that North American isolates almost 

invariably reduce nitrates to nitrites but fail to acidify glycerol, 

which identifies them as belonging to the orientalis biovar that 

was involved in the late 19th and early 20th century pandemic 

mentioned above (Devignat, 1951; Guiyoule and others, 

1994). Even more convincing results have been provided by 

recent molecular investigations, including ribotyping and 

single nucleotide polymorphism analyses, which indicate that 

United States strains are genetically similar to other orientalis 

biovar strains collected from areas in other continents that 

also experienced rat-associated outbreaks during the last 

pandemic (Guiyoule and others, 1994; Achtman and others, 

2004). In general, most lines of evidence, including levels of 

strain diversity within particular geographic regions, suggest 

an Asian origin for Y. pestis, although the plague bacterium 

clearly has existed in Africa for more than a millennium and 

probably considerably longer.

The availability of appropriate methodologies for detecting and analyzing variations among plague strains will have a 

significant impact on our ability to understand the evolution of 

plague and how strain differences influence various aspects of 

Y. pestis biology, including its ecology, virulence, and modes 

of transmission. Early attempts to analyze variation among 

plague strains relied primarily on phenotypic characteristics, 

such as reactivities in various biochemical tests, virulence for 

different types of laboratory animals, production of selected 

virulence factors, or apparent host associations (Devignat, 

1951; Tumanskii, 1957, 1958; Levi, 1962; Stepanov, 1975; 

Kozlov, 1979). More recently, investigators have analyzed 

variation among Y. pestis strains by using DNA probes, ribotyping, multiple locus variable number tandem repeat assays 

(MLVA), and analyses of IS100 elements and single nucleotide polymorphisms (Guiyoule and others, 1994; Gorshkov 

and others, 2000; Klevytska and others, 2001; Motin and 

others, 2002; Achtman and others, 2004; Girard and others, 

2004). Many of the above studies were intended primarily to 

demonstrate the feasibility of using a particular system for 

analyzing variation and, thus, examined mostly strains from 

established reference collections. By contrast, Girard and 

others (2004) used MLVA to track the spread of plague during 

an actual epizootic in prairie dogs in northern Arizona. These 

authors also used their MLVA results, in conjunction with 

other field and laboratory data, to construct a mutation-rate 

model that suggested that plague dynamics in their systems 

consisted of a rapid expansion phase, which was associated 

with population growth and dispersal, followed by a persistent 

phase characterized by lower reproduction and dispersal rates. 

The identification of additional markers should be favored by 

the recent publication of the complete genomic sequences of 

three Y. pestis strains (Parkhill and others, 2001; Deng and 

others, 2002; Song and others, 2004).
216 Recovery of the Black-footed Ferret

The phenotypic and genetic studies cited in the previous paragraph identified differences among strains from 

different foci and host sources, but they fail to answer the 

question of whether the observed differences among Y. pestis

strains simply reflect geographic variation or actually provide 

evidence that regional variants of Y. pestis are indeed adapted 

to a particular host species. Fortunately, the new molecular 

methodologies described earlier should provide researchers 

with valuable tools for answering this question as well as other 

important ecological and evolutionary questions. Analyses 

of North American strains should be particularly interesting because, as indicated previously, Y. pestis apparently has 

existed in this continent for only a little over 100 years, and 

the few orientalis biovar strains that were introduced at that 

time probably were highly similar, having originated in the 

same region of southwest China. Because the diversity among 

these invading strains of Y. pestis was very low, researchers have an interesting opportunity to examine how Y. pestis

changes over time and whether this bacterium is likely to 

exhibit different characteristics, such as increased or decreased 

virulence, when it is associated with a particular host or vector 

species.

Plague Transmission Cycles and 

Maintenance of Plague in Natural Foci

Figure 2 presents a generalized illustration of the natural 

transmission cycle of plague. In order for flea-borne transmission of plague bacteria to occur, a flea must take a blood 

meal from a rodent with a heavy Y. pestis bacteremia, become 

infected with plague bacteria, and later transmit this bacterial 

infection to another susceptible rodent host. Some researchers assume that rodent-to-flea-to-rodent transmission can 

occur indefinitely in so-called enzootic cycles that cause few 

apparent deaths among the purportedly resistant rodent hosts 

(enzootic or maintenance hosts) of these cycles. According to 

this same concept of plague maintenance and transmission, the 

disease occasionally spills over to other much more susceptible hosts (epizootic or amplification hosts) that often die in 

rapidly spreading epizootics, thereby posing increased plague 

risks for other mammals, including humans (Poland and 

Barnes, 1979; Poland and others, 1994). As indicated in fig. 2, 

Y. pestis occasionally is transmitted through consumption of 

infected prey or, perhaps, as a result of inhaling infectious 

respiratory droplets from animals with pneumonic plague 

and cough. The roles of these last two modes of transmission 

in maintaining natural foci have not been determined but are 

generally assumed to be less important than flea-borne transmission. A few researchers also have proposed that hosts can 

acquire plague as a result of digging in soil containing viable 

Y. pestis (Mollaret, 1963). Such infections presumably would 

be acquired through breaks in the skin or inhalation of Y. pestis

stirred up by an animal’s digging activities.

Rodent Hosts of Plague

Based on early observations in India and elsewhere 

(Pollitzer, 1954), plague initially was believed to exist in 

Figure 2. Generalized plague transmission cycle for the United States. Figure courtesy of Centers for Disease Control and Prevention.
Recent Trends in Plague Ecology 217

nature almost exclusively in commensal rats (primarily 

Rattus rattus and R. norvegicus) and rat fleas (primarily 

Xenopsylla cheopis), but it soon became clear that Y. pestis

also could be found in a variety of wild (noncommensal) 

rodents and their fleas. In the first decade of the 20th century, 

McCoy (1908) reported plague among California ground 

squirrels (Spermophilus beecheyi), and others noted soon 

thereafter that although epizootic activity among rats had 

largely disappeared, the disease continued to persist in other 

small mammals around the San Francisco Bay area (Link, 

1955). In Asia, Zabolotny (1915) suggested the possibility 

of wild rodent foci, noting that pneumonic plague outbreaks 

in Manchuria probably originated from hunters handling 

tarbagans (Marmota sibirica) rather than as a result of human 

exposure to infectious rat fleas. Later studies confirmed that Y. 

pestis could persist among a variety of rodent species and their 

fleas without the involvement of commensal rats (Pollitzer, 

1954; Pollitzer and Meyer, 1961).

Following the recognition that certain wild rodents are the 

major hosts of plague, researchers began to ask what characteristics allow particular rodent species to play important roles 

in the ecology of plague while others play little or no role. At 

first glance the number of potential rodent hosts is surprisingly 

high. Pollitzer (1960) identified 203 rodent species or subspecies reported to be naturally infected with Y. pestis, a list that 

could now be slightly extended. However, only a few of these 

species can be considered truly important hosts of plague, 

primarily those belonging to the families Sciuridae and Muridae. Among the sciurids, the predominant plague hosts include 

members of certain genera of burrow-dwelling squirrels (Spermophilus [formerly Citellus], Cynomys, Ammospermophilus) 

and chipmunks (Tamias, including Eutamias and Neotamias). 

Within the Muridae a number of species in the subfamilies 

Murinae, Gerbillinae, Arvicolinae, and Sigmodontinae are 

considered to be important hosts in various regions (Kucheruk, 

1965; Gage, 1998; Gratz, 1999). 

Among the topics discussed in this paper, probably the 

most neglected by recent researchers has been the response of 

native rodent species to Y. pestis infection and the roles these 

animals play in the long-term maintenance of plague foci in 

different regions. Although many rodents are mentioned in the 

literature as major plague hosts, the actual evidence to support 

these claims is often weak, particularly for those putative host 

species found in certain regions where relatively little research 

has been done (Gage, 1998; Gratz, 1999). Factors believed to 

influence the suitability of a particular rodent host for plague 

include the degree of its population-level resistance to Y. 

pestis-related mortality, its ability to serve as a source of infection for suitable flea vectors, the presence of large numbers 

of fleas on many members of the host population throughout 

much of the year, and occupation of burrows or nests that 

support development and maintenance of high flea populations 

(Pollitzer and Meyer, 1961; Gage and Kosoy, 2005).

Among these factors, one of the most contentious has 

been the degree to which population-level resistance to 

Y. pestis-related mortality is essential for the maintenance of 

plague by one or more rodent species in a particular focus 

(Pollitzer and Meyer, 1961; Gage and Kosoy, 2005). Host 

resistance to plague is undoubtedly influenced by many 

factors, including species, genetic factors within and among 

populations of a particular species, age, breeding status, prior 

immunity, physiologic condition, and probably other considerations. When assessing the importance of resistance, it is clear 

that its presence could favor the survival of host populations 

in plague-affected areas, although other factors also could 

operate to reduce mortality and prevent total die-offs among 

these animals, including the presence of patchy environments 

that could provide refuges for subpopulations within a larger 

metapopulation. Seasonal changes in the activities of susceptible hosts or competent flea vectors also might temporarily 

interrupt or slow down transmission to the point where host 

populations could be sustained from year to year by recruitment of new individuals (Pollitzer and Meyer, 1961).

One problem encountered in discussing resistance among 

plague hosts is the somewhat confusing use of the term itself. 

Host populations that are considered resistant rarely, if ever, are 

uniformly resistant to Y. pestis-related mortality but typically 

consist of a mixture of somewhat resistant individuals that 

become infected but recover and other animals that are more 

susceptible and succumb to plague. For example, mortality 

rates among great gerbils (Rhombomys opimus), which 

are considered resistant hosts, typically are 40–60 percent 

(Petrunina, 1951; Rivkus and others, 1973). Although this 

figure appears high, it is significantly lower than the mortality 

rates experienced by many other rodents, including other 

sympatric species of gerbils in the genus Meriones. Others 

have demonstrated that resistance can be associated with past 

exposure to plague (Birukova, 1960; Thomas and others, 1988; 

Levi, 1994). Several experiments demonstrated differences 

in plague resistance between populations of midday gerbils 

(Meriones meridianus) from different sides of the Volga River 

(Birukova, 1960; Levi, 1994). Levi (1994) compared median 

lethal doses (LD50) of Y. pestis for live-caught gerbils from 

a population on the west side of the river and another from 

the east side and found that in three trials, the LD50 values 

for populations on the west side were 2, 4, and 216 colony 

forming units (CFUs) while those on the east side of the Volga 

exhibited LD50 values of 3.397 x 106, 1.000 x 106, and 39.400 x 

106 CFUs. Captive-born hybrids of representative individuals 

(F1 generation) from both populations exhibited intermediate 

levels of resistance, as did the offspring of these individuals 

(F2 generation), suggesting that the observed resistance had 

a genetic basis. According to Levi (1994), these experiments 

helped explain how midday gerbils are able to serve as primary 

hosts for plague on the east side of the Volga but have a lesser 

role west of the river. Interestingly, these same populations of 

midday gerbils did not differ in their sensitivities to infection 

with the agents of tularemia and brucellosis. These authors 

also noted that two populations of another gerbil species, the 

tamarisk gerbil (Meriones tamariscinus), from the western and 
218 Recovery of the Black-footed Ferret

eastern sides of the Volga were found to be highly sensitive to 

plague infection (LD50 values of 6.800 x 102 and 5.000 x 102

CFUs, respectively).

In North America, Thomas and others (1988) demonstrated that captive-born northern grasshopper mice (Onychomys leucogaster) from a plague-free region of Oklahoma 

were much less resistant to plague than were mice of the same 

species from a north-central Colorado population that had 

been exposed to plague. In another North American paper, 

Quan and Kartman (1956) demonstrated that different populations of deer mice (Peromyscus maniculatus) and California 

voles (Microtus californicus) varied in their susceptibility to Y. 

pestis. Differences in susceptibility have been demonstrated to 

have a genetic basis in California voles (Hubbert and Goldenberg, 1970). Although the above data indicate that populations 

of some rodent species are highly resistant to Y. pestis, others, 

such as those of the black-tailed prairie dog, nearly always 

succumb to infection whenever they are struck by plague 

epizootics (Poland and Barnes, 1979; Biggins and Kosoy, 

2001a,b).

Regardless of whether resistant hosts must be present 

in order for plague foci to persist, flea-borne transmission of 

plague bacteria among rodents depends on the presence of 

animals that are capable of serving as sources of infection 

for feeding fleas. Experimental results indicate that fleas are 

likely to become infected with Y. pestis only after feeding on 

animals that have very high bacteremias (>106 Y. pestis/mL 

blood) (Burroughs, 1947; Engelthaler and others, 2000). In 

general, animals that have such high bacteremias often appear 

moribund, and few, if any, survive their infections. Thus, 

resistant animals that develop little or no bacteremia following 

infection probably are unlikely to serve as significant hosts 

for infecting fleas. Resistant individuals that survive infection 

could, however, still play important ecological roles by serving 

as hosts for maintaining flea populations and contributing 

offspring to the next generation of hosts. While many of the 

offspring of these animals also might be resistant, it is possible 

that at least some of their littermates will be susceptible.

Some animals might not be completely resistant, at least 

in the sense of being able to rapidly clear themselves of infection, but rather survive their initial bout of illness and go on to 

develop a chronic infection with Y. pestis. While evidence for 

chronic infections among North American species is almost 

nonexistent, the phenomenon has been observed in laboratory 

rats infected with nonencapsulated plague (F1-minus) strains 

(Williams and others, 1975; Williams and Cavanaugh, 1983). 

If wild rodents were chronically infected with fully virulent 

Y. pestis and later experienced a recrudescence of infection, 

perhaps as a result of breeding stress or decreased immune 

function in older individuals, they could develop a fatal bacteremia of sufficient magnitude to infect feeding fleas. 

As noted above, differences of opinion exist about the 

importance of resistance among host populations. Some 

researchers have suggested that the role a particular host 

population plays in plague maintenance can be inferred 

largely from its level of resistance (Pollitzer, 1954; Pollitzer 

and Meyer, 1961; Rall, 1965). For example, great gerbils are 

believed to be the major hosts of plague in certain central 

Asian desert foci. The percentage of resistant animals among 

great gerbil populations in these foci has been reported to be 

40–60 percent, a level that is higher than that found in gerbils 

of the genus Meriones, which occur in the same foci (Rivkus 

and others, 1973). In other situations resistance does not 

appear to differ greatly among various potential host species, 

making it difficult to assert that one host is more important 

than another based strictly on the observed levels of host resistance. For example, resistance was similar among great gerbils 

(50–80 percent), little susliks (Spermophilus pygmaeus) 

(50–70 percent), and midday gerbils (44–60 percent) in a 

Kazakh steppe focus (Atshabar, 1999).

Others have argued that the importance of resistance can 

be overemphasized and that other mechanisms can lead to the 

persistence of plague among highly susceptible host species 

(Pollitzer, 1954). While plague might kill most animals in a 

highly susceptible population, survival can be influenced by 

age, season, or physical condition, thus allowing some hosts to 

survive and reproduce. Rodents also might be able to become 

infected shortly before entering hibernation, develop a latent 

infection as their body temperatures drop, and then not experience severe illness or die of plague until they reawaken in the 

spring (Gayskii, 1944; Pollitzer, 1954; Pollitzer and Meyer, 

1961). Maevskii and others (1999) also reported that Y. pestis

could be isolated from the “mummified” carcasses of longtailed susliks (S. undulatus) for 7.5 months after these animals 

first entered hibernation. Spatial isolation among colonies or 

subpopulations of highly susceptible hosts also could allow 

plague to be maintained in metapopulations of these animals 

(Pollitzer and Meyer, 1961; Gage and Kosoy, 2005). In those 

foci where highly susceptible hosts live in a mosaic of distinct 

habitat patches, plague is unlikely to kill all of the susceptible 

animals in each patch or go from patch to patch without at 

least some delay, thereby allowing the disease to persist by 

spreading from patch to patch at a rate that is low enough 

to allow host populations in previously affected patches to 

recover before once again being exposed to Y. pestis infection. 

Types of Plague Hosts

Another unresolved question about the role of different 

rodent hosts in the natural cycle of plague is whether a single 

host or multiple hosts are required for long-term maintenance 

of natural transmission cycles. Fenyuk (1940, 1948) believed 

that certain rodent species and their fleas could maintain 

plague in the absence of other rodent species and referred to 

such animals as primary hosts. Secondary hosts were those 

species that routinely become infected but are incapable of 

supporting long-term maintenance of Y. pestis in a particular 

focus. Although secondary hosts are by definition incapable 

of maintaining plague foci in the absence of primary hosts, 

some proponents of this concept believe they are important in 

spreading the disease during epizootics. 
Recent Trends in Plague Ecology 219

Expanding on the primary host hypothesis, Rall (1965) 

proposed the concept of monohostality wherein maintenance 

of plague in a particular focus depends on the presence 

of a single rodent species and its fleas. Probably the most 

commonly cited examples of monohostal foci are those involving great gerbils in central Asia (Petrov, 1959). Acceptance 

of this proposal has not been universal, and maintenance 

of plague in other Asian foci has been suggested by other 

investigators to involve multiple host species (polyhostal foci) 

(Kalabukhov, 1965). The “Daurian enzootic area” of central 

Asia represents a proposed polyhostal focus, with Siberian 

marmots (M. sibirica), Daurian susliks (S. dauricus), pikas 

(Ochotona spp.), and voles (Microtus spp.) all presumedly 

playing important roles in maintaining this plague focus (Kalabukhov, 1965). The question of whether various United States 

foci are monohostal or polyhostal has received little attention. 

Although existing evidence does not allow firm conclusions 

to be made, Y. pestis infections are frequently identified in 

multiple rodent species in the western United States, particularly in certain southwestern States (New Mexico, Colorado, 

Arizona) and some mountainous regions of California and 

nearby areas (Barnes, 1982; Gage and others, 1995), suggesting that at least some of these foci are polyhostal. 

American workers have rarely used the terms primary 

and secondary hosts or monohostality and polyhostality. 

Instead, the most commonly cited concept categorizes rodent 

hosts as either enzootic or epizootic (Poland and Barnes, 1979; 

Poland and others, 1994). Supporters of this concept suggest 

that enzootic hosts and their fleas maintain plague during 

interepizootic periods and share certain features, including 

heterogeneous population responses to Y. pestis infection, 

low mortality following infection, long multiestrous breeding 

seasons with high reproductive potential, short life expectancies, flea infestations during all seasons, and a relatively high 

likelihood that antibody will be detected within the population. The most commonly proposed enzootic hosts are various 

species of Peromyscus and Microtus. By contrast, epizootic 

hosts are considered to have low to moderate resistance to Y. 

pestis infection, often experience high morbidity and mortality 

when infected, exhibit relatively little population-level heterogeneity to infection, and often experience heavy infestations 

with one or more species of vector flea that are likely to peak 

in abundance during the warmer months of the year, which 

is the time when transmission rates also appear to be highest. 

Proposed epizootic hosts include various species of Cynomys, 

Spermophilus, Ammospermophilus, Tamias, and Neotoma

(Barnes, 1982; Gage and others, 1995).

In reality, evidence to support the enzootic-epizootic host 

concept is often lacking or questionable. Obviously, epizootics with dramatic die-offs do occur among proposed epizootic hosts, but corresponding data to indicate that supposed 

enzootic hosts, such as deer mice or voles, are essential for the 

maintenance of plague during interepizootic periods is largely 

lacking. Another plausible alternative is that plague does not 

rely on any one host for its maintenance in a particular focus 

during the intervals between epizootics, but rather circulates at 

much reduced rates among most, if not all, of the same hosts 

that commonly become infected during epizootics. Under such 

circumstances, a fair amount of mortality could occur among 

these hosts during interepizootic periods but go virtually 

undetected because of the lack of routine rodent surveillance 

in most plague-enzootic areas.

The Role of Fleas in Transmitting 

Yersinia pestis

Because of its obvious role in rat-associated bubonic 

plague outbreaks during the last pandemic, many early studies 

concentrated on the role of the Oriental rat flea (Xenopsylla 

cheopis) as a vector of plague. Within two decades after 

Yersin’s 1894 discovery of the plague bacterium, Bacot and 

Martin (1914) demonstrated that Y. pestis proliferates in the 

midgut and proventriculus of an infected flea, forming recognizable colonies within a few days after the fleas ingest an 

infectious blood meal. They also showed that Y. pestis colonies 

can proliferate in an infected flea to such an extent that its 

proventriculus, a globular spine-filled structure at the end of 

the foregut, becomes blocked by a mass of bacteria and blood 

cell remnants. Once blockage of the proventriculus occurs, 

blood is no longer able to pass through the foregut to the 

midgut or “stomach” of the flea, resulting in its eventual starvation. Because the blocked rat flea is starving, it will repeatedly attempt to feed on almost any available mammalian host, 

including humans. As the flea repeatedly fails in its efforts to 

ingest blood, it attempts to clear the proventricular blockage 

by regurgitating, a process that does not clear the block but can 

dislodge plague bacteria from it. These dislodged bacteria and 

a small amount of ingested blood are then flushed back into 

the bite wound, resulting in infection of the host. Fleas that 

fail to become blocked were found to transmit at much lower 

rates or not at all, which led to the currently accepted dogma 

that the only efficient plague vectors are those that become 

blocked. 

Within the past decade the molecular basis by which Y. 

pestis promotes blocking in infected X. cheopis has become 

clear (Hinnebusch, 1997, 2005). Hinnebusch and others (1996) 

demonstrated that Y. pestis strains containing mutations in 

certain genes (hmsR and hmsH ) found in the hemin storage 

(hms) locus were incapable of forming blockages in infected 

X. cheopis fleas. The hemin storage locus derives its name 

from the ability of strains that possess a functional hms locus 

to bind hemin to their surfaces. In general, hemin-binding 

strains appear to be more “sticky” than strains that cannot 

bind hemin and are, thus, more likely to form clumps of Y. 

pestis in the flea’s gut or adhere to the cuticular spines in 

its proventriculus (Bibikova, 1977; Hinnebusch and others, 

1996). Other investigators have demonstrated that blocking 

depends on temperature, with fleas rarely becoming blocked, 
220 Recovery of the Black-footed Ferret

or actually clearing themselves of blockages, when maintained 

at temperatures above 27.5°C (Cavanaugh, 1971; Hinnebusch 

and others, 1998).

Additional studies have demonstrated that survival of 

plague bacteria in flea midguts depends on the expression of a 

gene (ymt) found on the largest of the three Y. pestis plasmids 

(approximately 110 kb) (Hinnebusch and others, 2002). The 

product of this gene (Ymt), which is a phospholipase D, has 

been referred to as murine toxin because of its high toxicity 

for murines (rats and mice) but not other types of rodents or 

mammals belonging to other orders. The study by Hinnebusch 

and others (2002), however, suggests that the true function of 

Ymt is to promote the survival of Y. pestis in the flea vector 

and that its toxicity for murines is merely coincidental. Even 

more recent studies have suggested that colonization of flea 

guts by Y. pestis might depend on biofilm formation by the 

plague bacterium (Darby and others, 2002; Jarrett and others, 

2004).

This research has greatly improved our understanding 

of how Y. pestis promotes its transmission by flea vectors, 

but we still have little knowledge of why some flea species, 

including those found on wild rodents and presumed to be 

important vectors, vary so greatly in their ability to transmit 

plague (Gage and Kosoy, 2005). Taxonomic affinities appear 

to provide little guidance, as demonstrated by the pulicid fleas 

of the genus Xenopsylla. The Oriental rat flea (X. cheopis) 

and a less widely distributed African rat flea (X. brasiliensis) 

are both highly efficient vectors, but their congener X. astia, 

which is common on rats in the Indian subcontinent and 

southeastern Asia, is a very poor vector (Pollitzer, 1954). 

Many decades ago, it was hypothesized that the structure or 

arrangement of the proventricular spines might be important 

determinants of a flea’s ability to transmit Y. pestis (Eskey and 

Haas, 1940). In support of this contention, Korzun and Nikitin 

(1997) reported that blocking in a ground squirrel flea, Citellophilus tesquorum, was positively associated with high levels 

of fluctuating asymmetry among the proventricular spines of 

these fleas.

Although the structure of the proventricular spines might 

very well influence the blocking process, it does not explain 

why Y. pestis appears to be unable to survive and develop in 

the guts of certain fleas. Among the poorest plague vectors 

are a number of flea species commonly associated with man 

and his domestic animals, including the so-called human 

flea (Pulex irritans), the cat flea (Ctenocephalides felis), 

the dog flea (C. canis), and sticktight fleas (Echidnophaga 

gallinacea) (Pollitzer, 1954). For example, P. irritans often 

clear themselves of infection within days after ingesting an 

infectious blood meal and rarely become blocked. Although 

these insects can transmit plague, they appear to do so only 

when large numbers of fleas are placed on susceptible hosts 

within a few hours after being allowed to feed on a Y. pestisinfected animal, suggesting that hosts are infected through the 

introduction of plague bacteria on contaminated flea mouthparts (mechanical transmission) rather than by the feeding 

of blocked fleas (Pollitzer, 1954; Blanc, 1956). It should be 

noted that despite its poor vector competency, some authorities believe that P. irritans is a significant vector of plague to 

humans in those situations where people live in unsanitary, 

heavily flea-infested homes that are often shared with domestic animals (Pollitzer, 1954; Blanc, 1956). These findings 

raise the possibility that infected but unblocked fleas on wild 

rodents also might transmit plague bacteria under certain 

circumstances.

Fleas found on wild rodent hosts also vary considerably 

in their ability to support Y. pestis infections and transmit 

plague bacteria (Eskey and Haas, 1940; Douglas and Wheeler, 

1943; Burroughs, 1944, 1947; Holdenried, 1952; Pollitzer, 

1954; Kartman and Prince, 1956; Kartman, 1957; Kartman 

and others, 1958a,b; Pollitzer, 1960; Pollitzer and Meyer, 

1961; Engelthaler and others, 2000). While some wild rodent 

fleas appear to block at high rates and become infectious 

soon after ingesting a Y. pestis-containing blood meal, other 

species require considerably longer periods of time to become 

blocked. The time required for blocking to occur in some 

species is sufficiently long that most of the infected fleas 

are likely to die before block formation actually occurs. A 

recent comparison of the development of Y. pestis infections 

in X. cheopis and Oropsylla montana, a ground squirrel flea, 

demonstrated that Y. pestis colonies became established very 

early in the course of infection in both the proventriculus and 

the midgut of infected X. cheopis (Engelthaler and others, 

2000). In O. montana, however, Y. pestis colonies initially 

appeared only in the midguts of infected fleas, which meant 

that the midgut infection had to proliferate and spread considerably before colonization of the proventriculus could occur. 

Because colonization of the proventriculus is delayed, the 

average time required for blocking to occur in O. montana

is much longer than in X. cheopis. The failure of many O. 

montana to become blocked and the fact that these fleas 

transmit at much lower rates than X. cheopis are particularly 

interesting because O. montana is considered to be the primary 

vector of plague to humans in the United States. Published 

results of experimental infection and transmission studies (see 

citations at the beginning of this paragraph) done with other 

species of wild rodent fleas suggest that the situation observed 

for O. montana is more typical than that seen with X. cheopis. 

Of particular interest are the limited studies done with ground 

squirrel, prairie dog, and woodrat (Neotoma spp.) fleas, which 

typically indicate that most of these fleas are relatively poor 

plague vectors compared to X. cheopis (see earlier citations 

in this paragraph). While many wild rodent fleas reportedly 

block and transmit at low rates, a few, such as Hystrichopsylla 

dippei, appear to be quite efficient vectors (Kartman and 

others, 1958b). Although the studies cited earlier in this paragraph make it obvious that X. cheopis is an exceptional plague 

vector, this does not mean that Y. pestis is always successful 

in its attempts to colonize and establish a stable infection in 

this flea. Engelthaler and others (2000) found that by 6 weeks 

after ingesting a Y. pestis-infected blood meal, 60 percent of 

all X. cheopis had cleared themselves of infection. Despite 

this fact, however, the infection rates observed in X. cheopis
Recent Trends in Plague Ecology 221

6 weeks after taking an infectious blood meal were still much 

higher than those observed in O. montana (60 percent versus 

15 percent, respectively).

Many of these studies raise questions about whether 

transmission by blocked fleas is actually essential for the rapid 

spread of Y. pestis during epizootics or for the interepizootic 

maintenance of plague. One possibility is that in some situations partially blocked fleas could transmit at sufficiently high 

rates to be important vectors. Burroughs (1947) and Engelthaler and others (2000) demonstrated that O. montana fleas were 

capable of transmitting within 4 days after feeding on an infectious host, a much shorter time than that required for blockage 

in these species, but perhaps too long for strictly mechanical 

transmission of viable Y. pestis on contaminated mouthparts to 

occur. Burroughs (1947) and others (Voronova, 1989; Degtyareva and others, 1990; Gan and others, 1990; Bazanova and 

others, 1991) list additional examples of the transmission of Y. 

pestis by partially blocked or apparently block-free fleas.

The role that mechanical transmission might play in 

natural foci also should be reexamined. As noted previously, 

early studies of potential plague vectors indicated that some 

fleas, such as the human flea, rarely became blocked but 

occasionally transmitted plague when fleas that had fed on 

an infected host were quickly transferred in large numbers 

to susceptible hosts, a finding that is typically interpreted 

as evidence for mechanical transmission (Pollitzer, 1954; 

Blanc, 1956). Later studies, particularly those of Burroughs 

(1944, 1947) and Kartman and others (1958a,b) also provided 

evidence that common North American rodent fleas are 

capable of transmitting Y. pestis by mechanical means. Quan 

and others (1953) provided interesting evidence that even 

X. cheopis is capable of mechanically transmitting plague 

bacteria. Based on the results of the studies noted earlier and 

others, Burroughs (1947) and Kartman and others (1958a,b) 

suggested that mechanical transmission might be important, 

particularly during epizootics when host densities are high and 

the likelihood that fleas will rapidly transfer from dead hosts 

to susceptible ones is also high. Kartman and others (1958a,b) 

further suggested that the bulk of transmission during epizootics occurs through mechanical means while transmission of 

plague during interepizootic periods is accomplished by those 

rodent fleas that are capable of becoming blocked and transmitting at high efficiencies. In particular, he cited Malaraeus 

telchinum, a flea that is extremely abundant on mice and 

voles in some regions of the West, as a likely mechanical 

vector during epizootics and Hystrichopsylla dippei, a far 

less abundant but much more efficient plague vector, as an 

important vector during interepizootic periods. Unfortunately, 

others have not pursued this hypothesis, and it would be very 

interesting to know whether other “pairs” or groups of fleas 

play similarly complimentary roles during epizootic and 

interepizootic periods. It also would be worthwhile to determine whether the rapid rates of transmission observed during 

plague epizootics in prairie dogs or other highly susceptible 

hosts are due to mechanical transmission or transmission by 

blocked fleas. The former can take place virtually immediately 

after a flea has fed on a heavily bacteremic host, but the latter 

typically requires an extrinsic incubation period of 2 or more 

weeks before fleas can become blocked and, therefore, capable 

of efficiently transmitting. Alternatively, hosts might become 

infected by consuming other animals that have died of plague 

or through respiratory contact with hosts that have pneumonic 

plague.

Although laboratory experiments can help determine 

whether a particular flea species is likely to be an important 

vector, other factors also need to be considered in determining 

the actual role a potential vector will play in nature (Gage, 

1998; Gage and Kosoy, 2005). Obviously, fleas that feed on 

hosts that are seldom infected with plague, or live only in 

plague-free areas, are unlikely to be important. Fleas that are 

highly host-specific might be very important for transmitting 

plague among members of a particular host species but would 

rarely spread the disease to other hosts. The seasonality and 

abundance of the flea’s hematophagous adult stage also are 

likely to be important. Many important vectors occur most 

abundantly on their hosts during those warm months when 

plague transmission also peaks. Another potentially important 

factor is the ability of fleas to survive in off-host environments 

while waiting for an alternative host to appear. 

Maintenance of Plague Between 

Transmission Seasons and Between 

Epizootics

Figure 2 provides a basic overview of the plague 

transmission cycle but unfortunately conveys almost no 

information on the relative roles different components play in 

maintaining plague between transmission seasons or during 

interepizootic periods when little or no Y. pestis-related illness 

is apparent among the normal hosts of the disease. At least 

four different hypotheses can be advanced for long-term 

maintenance of plague (Gage and Kosoy, 2005): continuous 

enzootic transmission among rodent hosts and their fleas at 

more or less steady rates except during irregularly occurring 

epizootics; chronic infection of rodents with eventual relapses 

of the disease in these animals and subsequent infection of 

vector fleas following these relapses; prolonged survival of 

infected fleas in host nests or burrows; and indefinite survival 

of Y. pestis in soil, soil protozoa, or perhaps even plant tissues. 

The following sections discuss the above hypotheses of plague 

maintenance.

Are Rodents Merely Amplifying Hosts 

or True Reservoirs of Infection?

In order for plague to be maintained through continuous 

enzootic transmission, the rodent hosts and flea vectors must 
222 Recovery of the Black-footed Ferret

both be present and active throughout the year. In temperate 

regions some plague hosts enter hibernation or become much 

less active during winter months, which could interrupt the Y. 

pestis transmission cycle. For example, marmots (M. sibirica

and certain other Marmota spp.), which are thought to be critically important plague hosts in some Asian foci, hibernate for 

many months and, thus, are unlikely to become infected after 

entering hibernation or support ongoing transmission during 

this period. If their fleas also become inactive during winter 

months or lack the opportunity to acquire new infections from 

hibernating hosts, transmission could be interrupted. One 

possible solution to this dilemma could be the survival of Y. 

pestis in hibernating animals (Gayskii, 1944; Pollitzer, 1954). 

According to this hypothesis, a Y. pestis-infected animal might 

enter hibernation prior to becoming ill, thus slowing or temporarily halting the progression of Y. pestis infection as a result 

of the effects of low host body temperature on the growth of 

the pathogen or its virulence. Upon reawakening in the spring, 

the infection could reactivate, causing the animal to become ill 

and develop a Y. pestis bacteremia of sufficient magnitude to 

infect feeding fleas, thereby continuing the cycle of rodentto-flea-to-rodent transmission for another year. While this 

explanation seems plausible and does have some experimental 

support, little is known about its importance in natural foci. 

Also, such an explanation is unlikely to be important in tropical or subtropical foci. If hibernating animals die of plague 

before reawakening in the spring, it is also possible that plague 

bacteria could survive in their dried tissues for many months 

after the animals have died (Maevskii and others, 1999).

According to some investigators, rodents that do not 

hibernate might develop chronic infections and act as reservoirs for maintaining plague from one transmission season 

to the next (Pollitzer, 1954; Pollitzer and Meyer, 1961). 

Experimental evidence suggests that individual great gerbils in 

central Asia survive infection and then develop granuloma-like 

lesions in their livers and perhaps other tissues that contain 

viable Y. pestis (Suleimenov, 2004). These plague bacteriacontaining lesions can reportedly persist for many months, 

thereby allowing latent infections to become reactivated during 

the spring as adult hosts experience increased stress due to 

breeding or decreased immune system function due to old age. 

Great gerbils that experience reactivation of their infections 

are believed to circulate sufficient Y. pestis in their bloodstream to infect feeding fleas. One of the practical problems 

encountered in evaluating the importance of presumed chronic 

infections in rodents under field conditions is whether lesions 

observed in the tissues of suspected carrier hosts are really 

indicative of chronic infection or simply a sign of resolving 

infections.

Some researchers have argued that plague could be maintained through the winter months by continuous transmission 

between certain hosts and their fleas. Such a pattern of transmission has been proposed for deer mice (P. maniculatus) and 

their allies (other Peromyscus spp.) or various species of voles 

(Microtus spp. and others) (Poland and Barnes, 1979; Poland 

and others, 1994). Deer mice and other mice of the genus 

Peromyscus remain active in all seasons, are often infested 

with fleas during the winter months, and reproduce throughout 

much of the year, which results in the ongoing introduction 

of susceptible animals into local mouse populations. Whether 

populations of Peromyscus or voles can indeed maintain 

plague through continuous rodent to flea to rodent transmission is at present uncertain. In a 13-month study (March 

1954–April 1955) of 1,458 Microtus californicus found dead 

in a San Mateo County plague focus, Y. pestis was identified 

in the tissues of these animals during 10 of the 13 months. 

The only months when positive animals were not identified 

were December 1954 (n = 52), March 1955 (n = 33) and April 

1955 (n = 27) (Kartman and others, 1962). Considering the 

relatively low number of dead animals examined during those 

3 months, Y. pestis might have indeed been present all year in 

at least some voles within this focus.

Fleas as Reservoirs of Plague

While no one disputes that fleas are the only significant 

vectors of plague, they also could act as long-term reservoirs 

by maintaining Y. pestis in off-host environments during the 

intervals between transmission seasons or during periods 

of host hibernation (Gage and Kosoy, 2005). Many studies 

indicate that infected but unblocked, and even blocked, fleas 

can survive for many months in off-host environments. In one 

study, infected Ctenopthalmus breviatus survived for up to 396 

days when held on wet sand at temperatures of 0–15°C (Golov 

and Ioff, 1926, 1928). Other studies indicated that Oropsylla 

silantiewei could survive for as long as 558 days without 

feeding while Citellophilus tesquorum and Neopsylla setosa

did so for 275 and 180 days, respectively (cited by Kozlov, 

1979). Sharets and others (1958) reported that Rhadinopsylla 

ventricosa fleas remained infected with Y. pestis for at least 

420 days. Bazanova and Maevskii (1996) succeeded in 

maintaining more than half of all C. tesquorum altaicus fed on 

infected susliks (Spermophilus undulatus) over a period from 

mid-September to mid-June, which provided sufficient time 

for these fleas to survive through the hibernation period of 

their hosts. One female in their experiments survived through 

two winters, living for a total of 411 days after being fed on 

an infected suslik. Even more importantly, when infected C. 

tesquorum altaicus that had been starved through the hibernation period of their hosts were later allowed to feed, they 

succeeded in transferring plague to these animals, thus demonstrating that these fleas could act as both vectors and reservoirs 

of infection. In North America, Kartman and others (1962) 

reported the recovery of infected Oropsylla labis (syn. Opisocrostis labis) and O. tuberculata cynomuris (syn. Opisocrostis 

tuberculatus cynomuris) from abandoned prairie dog burrows 

for more than a year after their hosts had died of plague.
Recent Trends in Plague Ecology 223

Survival of Plague in Soils, Soil Protozoa, 

Plant Tissues, or Other Unusual Sites

Some investigators have proposed that plague might 

survive during interepizootic periods in the soil of burrows 

(Mollaret, 1963). In one experiment, four species of gerbils 

(Meriones libycus, M. persicus, M. tristrami, and M. vinogradovi) developed plague after being allowed to dig burrows in 

laboratory enclosures containing soils contaminated on the 

previous day with Y. pestis in a broth culture (Mollaret, 1963). 

In other experiments, it was claimed that plague survived 

many months in both sterilized and nonsterilized soils 

(Mollaret, 1963; Baltazard, 1964). According to supporters 

of this hypothesis, rodents can become infected by burrowing 

in soils that are contaminated with the remains or excreta of 

infected mammals or fleas. Other researchers have expressed 

skepticism about this hypothesis, noting methodological 

concerns about the few studies that have been advanced in its 

support or that the observed patterns of disease spread and host 

population recovery often fail to agree with the suggestion 

that new epizootics are initiated through contact of animals 

with contaminated soils (Gage and Kosoy, 2005). Also, unlike 

anthrax or certain other soil-dwelling organisms, Y. pestis does 

not form a sporelike structure, and most evidence suggests 

that plague bacteria die relatively quickly outside their hosts 

or vectors (Brubaker, 1991; Perry and Fetherston, 1997). 

Recently, some have presented evidence that plague might be 

able to survive in soil protozoa rather than in a free state in 

soils (Nikul’shin and others, 1992; Nersesov and Tsikhistavi, 

1997; Domaradsky, 1999; Pushkareva, 2003). Recent studies 

also have shown that plague bacteria can form biolfilms on a 

nematode species (Caenorhabditis elegans) commonly used 

in laboratory studies (Darby and others, 2002), but we know 

of no evidence indicating that soil nematodes become infected 

under natural conditions. Others have suggested that Y. pestis

might survive in plant tissues (Rivkus and others, 1993; Litvin, 

1997) or in a latent nonculturable state in soils (Suchkov and 

others, 1997). Although none of these hypotheses has received 

strong support, they cannot be completely rejected on the 

basis of currently available data and are worthy of additional 

research.

Factors Affecting Rates of Plague 

Transmission and Incidence of

Epizootics

One of the most striking aspects of plague is its ability 

to spread explosively among susceptible animal populations 

and across landscapes during epizootics. Almost equally 

striking is the fact that these relatively brief periods of intense 

transmission are followed by much longer intervals when 

the lack of obvious mortality among highly susceptible hosts 

makes it seem as if the disease has completely disappeared 

from a particular focus. In most instances, however, at least 

some Y. pestis transmission can still be identified in the 

suspect area through use of a sensitive monitoring technique, 

such as serosurveys of coyotes or other rodent-consuming 

carnivores (Gage and others, 1994). While much remains to 

be learned about the conditions that lead to plague epizootics 

or halt their progress, transmission rates can be affected by 

such factors as host resistance, densities of host and vector 

populations, the length of time that Y. pestis can persist in 

off-host flea populations, the vector competency of local flea 

species, the length of the extrinsic incubation period before 

fleas become infective for hosts, the likelihood that rodents 

will become chronically infected, periods of host inactivity 

(hibernation or aestivation), and seasonal changes and other 

climatic factors that influence the timing of host and vector 

life cycles as well as their survival and reproduction (Pollitzer, 

1954; Pollitzer and Meyer, 1961; Poland and Barnes, 1979; 

Poland and others, 1994; Gage, 1998; Gage and Kosoy, 2005).

One of the most important questions in plague ecology 

is what conditions lead to the onset of epizootics. Modeling 

studies of human rat-associated plague suggest that if plague 

can persist in small rat subpopulations, it will spill over at 

irregular intervals to other susceptible rat subpopulations, 

causing epizootics and increased risks of flea-transmitted 

bubonic plague in humans (Keeling and Gilligan, 2000a,b). In 

these studies, persistence was favored by a high proportion of 

resistant individuals, and short-lived epizootics occurred when 

plague was introduced into subpopulations composed primarily (>80 percent) of susceptible individuals. In a more recent 

modeling study using rodent plague surveillance data from 

Kazakhstan, Davis and others (2004) reported that the invasion and persistence of plague in great gerbil populations was 

related to rodent density. They also found that as populations 

fell below certain thresholds, plague was likely to disappear 

from an area that had been invaded earlier in the course of an 

epizootic.

The suggestion that rodent population densities affect the 

invasion and persistence of plague in host populations is not 

surprising but still leaves open the question of what factors 

initially cause rodent populations in plague foci to increase 

and epizootic activity to become likely. Human plague risks 

typically increase greatly during epizootics, and the occurrence of increased numbers of human cases is generally 

believed to reflect increased epizootic activity. Parmenter and 

others (1999) analyzed human plague in New Mexico and 

found that human risks were correlated with increases in cool 

season precipitation from the previous year. They attributed 

this increase in human risk to a trophic cascade effect where 

increased cool season precipitation led to increased food 

availability for rodents. It was hypothesized that as food 

availability increases, so do survival and reproduction of 
224 Recovery of the Black-footed Ferret

rodent hosts and perhaps flea vectors of plague. In agreement 

with the results of Davis and others (2004), they postulated 

that increased rodent numbers increase the risk of epizootics, 

as well as human cases. In a later study, Enscore and others 

(2002) demonstrated that both late winter precipitation and 

threshold temperatures were associated with human plague 

risks in the Four Corners region of the American Southwest. 

These last authors suggested that the trophic cascade model of 

Parmenter and others (1999) be modified to include threshold 

temperature effects that might affect not only rodent populations but also flea survival and reproduction. In particular, they 

suggested that years with exceptionally high numbers of days 

above certain threshold temperatures were likely to be those 

with low flea populations because of the negative effects of 

hot summer temperatures on flea survival and reproduction, or 

perhaps the ability of these insects to transmit plague (Cavanaugh, 1971; Cavanaugh and Marshall, 1972; Enscore and 

others, 2002). Collinge and others (2005b) attempted to test 

the generality of the trophic cascade model (Parmenter and 

others, 1999) as modified by Enscore and others (2002) and 

found that the occurrences of reported plague events in prairie 

dogs were not associated with certain climatic variables in 

Boulder County, Colo., but were associated with precipitation 

and temperature effects in a Phillips County, Mont., site. The 

authors concluded that the timing and magnitude of precipitation and temperature might influence the occurrence of plague 

in some but not all areas. They also reported that the best 

climatic predictors in the Montana site corresponded well with 

those noted in the above studies of human plague cases in the 

southwestern United States. In another Colorado study, Stapp 

and others (2004) demonstrated that epizootics in prairie dogs 

living on grasslands in north-central Colorado were associated 

with El Niño events.

Landscape Ecology of Plague

The influence of landscape structure on plague distribution and dynamics has been investigated in only a few of the 

world’s plague foci. Bibikov and others (1963) stated that 

localities where plague infection can be maintained for a 

long period of time occupy relatively small portions of the 

territories that are endemic for plague, and speculated that, for 

unknown reasons, these sites present more auspicious conditions for the circulation of Y. pestis than other sites that are 

only affected sporadically. In other studies, Alexeev (1991) 

and Karimova (2002) used landscape characteristics for typing 

plague foci in desert zones of Kazakhstan and central Asia. 

Medzykhovsky and others (2001) demonstrated an association 

between the distribution of plague epizootics in the transUralian steppe regions of eastern Kazakhstan and certain soil 

and grass characteristics. Serzhanov and others (1982a) found 

that places where plague persists over long periods of time in 

central Asian deserts are closely associated with landscapes 

characterized by abundant underground water lying near the 

surface (hydrologic lenses). These authors also demonstrated 

a correlation between the dynamics of plague epizootics and 

groundwater characteristics in nine different landscapes in 

Turkmenia. Based on these observations, Serzhanov and others 

(1982b) proposed the use of hydrothermal indices for the 

ecological typing of plague foci. In another interesting study, 

Rotshild (2001) hypothesized that levels of trace metals in 

natural environments influence the distribution and occurrence 

of plague. His hypothesis was based on multiple observations 

in the Altai Mountains, Tuva (eastern Siberia), the Kyzyl 

Kum desert in Uzbekistan, and a sandy semidesert area of the 

Caspian lowlands where he found correlations between epizootic plague activity and decreased or increased concentrations 

of Fe, Co, and Ti and low concentrations of Cu, Ni, and V. 

In the United States, plague foci are known to occur in a 

variety of landscapes in numerous western mountain ranges, 

the High Plains, and intermountain grasslands (Barnes, 1982). 

Although plague might make brief epizootic intrusions into 

some areas, it remains conspicuously absent from certain 

extremely hot desert regions of the southwestern States, 

including the Sonoran Desert in southern Arizona. Although 

the reasons for plague’s absence in these areas are unknown, it 

is tempting to speculate that the extremely hot, dry conditions 

in these desert areas are likely to limit transmission by fleas 

because these insects probably face severe desiccation when 

they are not closely associated with a host or protected burrow 

system or when they attempt to quest at burrow entrances in 

such exceptionally hot and dry environments. 

A so-called “plague line” appears to exist at about the 

100th meridian of longitude, a line that along much of its 

length marks the zone of transition from the tall grass prairies to the short grass habitats of the High Plains (Barnes, 

1982). Among the factors that might influence the location of 

this “plague line” are rodent and flea diversity and changes 

in burrow microclimates or other features of burrow ecology. Although some recognized plague hosts occur on the 

plains, including black-tailed prairie dogs, thirteen-lined 

ground squirrels (Spermophilus tridecemlineatus), spotted 

ground squirrels (S. spilosoma), and southern plains woodrats (Neotoma micropus), the diversity of important plague 

hosts clearly decreases as one moves away from the Rocky 

Mountains onto the High Plains. By contrast, numerous rodent 

hosts of plague occur in relatively close proximity to each 

other in the lower elevation coniferous woodlands, foothills, 

and nearby plains. Many of these species, including woodrats, 

prairie dogs, ground squirrels, and chipmunks, live in burrows 

or complex nests that are often heavily infested with fleas. 

Another factor that might be important is the habitat complexity found near the Rockies and on High Plains sites nearest to 

these mountains. The more varied and patchy habitats around 

the Rockies could provide partial barriers and slow the movement of plague from one habitat to another, thus providing 

a limited refuge for some rodent populations and increasing 
Recent Trends in Plague Ecology 225

the likelihood that sufficient hosts will survive epizootics and 

keep transmission going from one season the next. As one 

moves on to the High Plains, however, the habitats appear to 

be more homogeneous with fewer barriers to the spread of 

plague, which could result in rapidly spreading epizootics that 

kill nearly all susceptible rodents and leave few individuals 

to support ongoing transmission. Plague probably is unlikely 

to persist in areas with such relatively homogeneous habitats 

but could, perhaps, repeatedly invade them when widespread 

epizootics sweep across the landscape. 

Regional or local landscape ecology studies are almost 

nonexistent in the plague foci of the western United States. A 

single recent study by Collinge and others (2005a) used logistic regression to analyze two long-term data sets on plague 

occurrence in prairie dogs. The first of their two study sites 

was located in Boulder County, Colo., a region subject to rapid 

human development, and the second was in Phillips County, 

Mont. Associations were found at both sites between plague 

occurrence, landscape parameters, and colony characteristics. 

The best models from both sites predicted positive effects on 

plague occurrence of proximity to colonies that experienced 

plague and negative effects of road, stream, and lake cover.

Conclusions

Although some important findings, such as those describing how Y. pestis promotes its transmission by flea vectors, 

have occurred in recent years, many aspects of our understanding of plague ecology have progressed little since the 

mid-20th century. This is surprising when one considers the 

exciting new advances in many relevant fields or technologies, including molecular biology, immunology, population 

genetics, microbiology, geographic information systems, 

remote sensing, and mathematical modeling. Among the many 

interesting issues that have yet to be addressed satisfactorily 

are the degree to which Y. pestis exhibits adaptations to major 

hosts and vectors or vice versa, the relative roles of various 

factors in determining levels of host resistance, the roles 

many rodent species play in plague maintenance, the structure of plague foci in North America and elsewhere, the true 

significance of mechanical transmission or the transmission 

of plague bacteria by partially blocked fleas, the reasons why 

different flea species vary so greatly in vector competency, and 

the roles that climatic variables, landscape features, host and 

vector densities, or other factors play in influencing the spread 

of plague or the occurrence of epizootics. Fortunately, many 

of these questions can now be addressed, as interest in plague 

and funding for its study have increased as a result of recent 

concerns about the use of plague as a weapon of bioterrorism 

and the recognition that Y. pestis can adversely impact many 

wildlife species.

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Abstract

Plague, a disease caused by the bacterium Yersinia pestis, 

was introduced into North America ca. 1900 and is now 

common within the ranges of three species of prairie dogs 

(Cynomys spp.) that collectively composed the former range of 

the highly endangered black-footed ferret (Mustela nigripes). 

An experimental population of black-footed ferrets living 

in quasi-natural outdoor pens suffered 90 percent mortality 

after they ate prairie dogs infected with Y. pestis. Lethal and 

sublethal exposure of Siberian polecats (Mustela eversmannii) subsequently released into those pens suggested that live 

Y. pestis can be maintained in animal tissues within burrow 

systems for at least 2 months. A combination of low levels 

of prairie dog mortality and persistence of Y. pestis in dead 

hosts may pose a chronic hazard for free-ranging black-footed 

ferrets in areas where plague is enzootic.

Keywords: black-footed ferret, disease, introduced 

disease, invasive species, Mustela eversmannii, Mustela 

nigripes, plague, Siberian polecat, Yersinia pestis.

Background

Plague was once believed to be millions of years old, but 

recent genetic evidence suggests that the causative bacterium,

Yersinia pestis, may have evolved from Y. pseudotuberculosis 

only 1,500–20,000 years ago (Achtman and others, 1999). The 

disease has caused devastating epidemics in humans. Plaguelike symptoms were recorded in human populations of Asia 

and Africa as early as 541 A.D. Most scientists believe that 

plague was introduced into North America from Asia in the 

late 19th century via rats (Rattus spp.) transported by ships 

(Biggins and Kosoy, 2001). There is now evidence of plague 

infection in wild mammals or fleas (Insecta: Siphonaptera) 

from 17 western States in the United States (Gage and Kosoy, 

this volume).

There are multiple transmission modes for plague, including vector transport (flea bites), aerosol, and consumption 

of contaminated food items (Gage and Kosoy, this volume). 

Early cases of plague were linked with rodent infestations and 

assumed to be from rodent bites, but it was soon recognized 

that fleas could spread the disease among hosts (Gage, 1998). 

Aerosol transmission involves expulsion of contaminated 

droplets of fluid from the lungs of infected animals as they 

cough; the droplets containing Y. pestis may be ingested or 

inhaled by another potential host. Transmission has also been 

documented through consumption of infected animals (Gage 

and Kosoy, this volume). Although some carnivores become 

infected and do not survive, other species seem quite resistant 

(Barnes, 1982; Gage and others, 1995).

Plague is common within the ranges of three species of 

prairie dogs (Cynomys spp.) that collectively composed the 

former range of the highly endangered black-footed ferret 

(Mustela nigripes). The black-footed ferret is extremely 

dependent on prairie dogs and their colonies (Biggins and 

Godbey, 2003). Plague causes periodic and sometimes 

dramatic die-off of prairie dogs, indirectly affecting ferret 

survival through reduction of prey biomass (Oldemeyer and 

others, 1993). In 1985, discovery of plague in the white-tailed 

prairie dogs (C. leucurus) supporting the last known population of wild ferrets in Meeteetse, Wyo. (Ubico and others, 

1988), caused great concern about the future of ferret habitat. 

White-tailed prairie dogs were found to be highly susceptible 

to the disease, but susceptibility of the black-footed ferret was 

unknown (Williams, 1986). The fears of habitat loss and an 

unstable prey base proved well founded. A 10-year decline in 

prairie dogs at Meeteetse left only a remnant population. The 

initial steep decline of prairie dogs at Meeteetse (fig. 1) was 

accompanied by a decline in ferrets, which may have been 

exacerbated by a second disease, canine distemper (Forrest 

and others, 1988). The dramatic ferret population decline 

Exposure of Captive Black-footed Ferrets to Plague and 

Implications for Species Recovery

By Jerry L. Godbey,1 Dean E. Biggins,1 and Della Garelle2

1

U.S. Geological Survey, Fort Collins Science Center, 2150 Centre Ave., 

Bldg. C, Fort Collins, CO 80526.

2

Cheyenne Mountain Zoo, 4250 Cheyenne Mountain Zoo Rd., Colorado 

Springs, CO 80906.
prompted the capture of remaining ferrets for captive breeding 

(Biggins and others, 1997).

The captive breeding program to produce animals for 

reintroduction into native habitat (i.e., complexes of prairie 

dog colonies) was ultimately successful (Biggins and Godbey, 

2003). Reintroductions of ferrets were begun in 1991 into 

Wyoming white-tailed prairie dog colonies at Shirley Basin 

where plague was known to be established. The Shirley Basin 

population of prairie dogs also declined (fig. 1), but more 

recently the population has shown some signs of recovery. 

In 1994, releases of ferrets began in a Montana black-tailed 

prairie dog (C. ludovicianus) complex also known to have 

plague. Plague has been documented at most reintroduction 

or potential reintroduction sites, with the exception of those in 

South Dakota, throughout the ferret range.

Plague was not believed to be a direct hazard to ferrets 

at the time of the first reintroductions. Williams and others 

(1991) initially reported that domestic ferrets (Mustela 

putorius furo) and Siberian polecats (Mustela eversmannii) 

were resistant to plague and suggested that “concern about 

black-footed ferret mortality directly due to Y. pestis infection is probably not warranted.” It was therefore surprising 

to hear of the death of a black-footed ferret due to plague 

infection (Williams and others, 1994). Williams’s further work 

with black-footed ferret × Siberian polecat hybrids provided 

additional evidence on the direct hazard of plague. Nine of 

12 hybrids tested became infected and died from ingestion of 

plague-killed mice; the three survivors failed to show an antibody response (E. Williams, oral commun., 1996). A subsequent trial resulted in 100 percent mortality of four blackfooted ferrets exposed to about 800 organisms (equivalent to 

one flea bite dose) of Y. pestis by subcutaneous injection (E. 

Williams, oral commun., 1999).

Plague Exposure of Captive Blackfooted Ferrets at Pueblo

On November 19, 1995, an experimental colony of 

black-footed ferrets was inadvertently exposed to plague at 

a research facility housed at the U.S. Army’s Pueblo Chemical Depot, Pueblo, Colo. The facility consisted of modified 

buildings and enclosures that provided quasi-natural environments for rearing and conditioning black-footed ferrets 

prior to release. Indoor cages and outdoor pens of various 

sizes were also used. Outdoor pens consisted of earth-filled 

structures (fig. 2) with combinations of natural burrows dug 

by prairie dogs, seminatural burrows constructed of 10.2-cm 

corrugated plastic drain pipe buried to a depth of about 1 

m, and nest boxes. Studies on ferret behaviors were being 

conducted by using Siberian polecats, black-footed ferrets, 

and domestic ferrets reared in various environments. There 

were 64 resident black-footed ferrets in three categories at the 

time of the exposure. Twenty-three ferrets were assigned to the 

behavioral studies. Twenty-six ferrets had just been received 

and were being conditioned as experimental groups for release 

in Arizona and Montana. Fifteen ferrets 4−7 years old were 

being held awaiting transfer to zoos as display animals. Most 

of the black-footed ferrets were provided a diet of prairie dog 

portions on alternating days; Siberian polecats and domestic 

ferrets were fed commercial mink chow. The prairie dogs were 

live-trapped from various sources, quarantined for 10 days, 

sacrificed, and then frozen until used.

Thirty ferrets were fed on November 19, 1995. The food 

included portions from five quarantined black-tailed prairie 

dogs originating in Montana that were removed from one 

freezer and two nonquarantined Gunnison’s prairie dogs (C. 

gunnisoni) captured from a site near Cortez, Colo., in August 

Figure 1. Changes in Wyoming white-tailed prairie dog (Cynomys 

leucurus) populations in areas with known plague. (Adapted from 

Biggins and Kosoy, 2001. Reprinted with permission of the Journal 

of the Idaho Academy of Science, Pocatello, Idaho.)

Figure 2. Black-footed ferrets (Mustela nigripes) occupied complex burrow systems dug by prairie dogs (Cynomys spp.), making 

them difficult or impossible to locate during and after the outbreak 

of plague.

234 Recovery of the Black-footed Ferret
1994 and stored in a second freezer. Only the Montana prairie 

dogs were to be fed, but new animal care personnel were 

unaware of the distinction. All seven prairie dogs were cut 

into large pieces on a common cutting board and placed into a 

bowl for transport to the pens.

Two days after feeding (November 21, 1995), the crew 

discovered the first obviously ill black-footed ferret in an 

outdoor pen. The ferret died soon after it was captured. Foodborne disease or poisoning was immediately suspected, so the 

remaining food was removed, the facilities were quarantined 

for 10 days, and vitamin K was administered to counteract 

possible rodenticide poisoning. Ten uneaten or partially eaten 

pieces of prairie dog were found. Black-tailed and Gunnison’s 

prairie dog parts could not be distinguished because the 

skin had been removed. The recovered food and the bowl 

were sent to the Centers for Disease Control (CDC) in Fort 

Collins, Colo., for testing. Several whole prairie dogs from the 

Montana shipment and two Gunnison’s prairie dogs remaining in the second freezer were also sent to the CDC. Repeated 

searches of the pens over the next 2 days disclosed other sick 

and dead ferrets. Clinical signs included lethargy and bloody 

stools. Of the 30 animals possibly exposed, 19 died and 8 were 

missing and presumed dead in underground burrows. Blackfooted ferret remains were sent to Colorado State University 

for necropsy, and tissue samples were forwarded to the CDC 

for plague testing. The three surviving animals were quarantined, and blood was drawn and sent to the CDC.

Three of the 10 recovered prairie dog pieces, the two 

remaining Gunnison’s prairie dogs, and a swab taken from the 

inner surface of the transport bowl tested positive for plague. 

There was no evidence of plague in the tested Montana blacktailed prairie dogs. All dead ferrets were positive for plague 

in one or more tissue samples. Internal organs showed various 

stages of infection, but all included intestinal hemorrhaging and congested lungs. Clinical signs were consistent with 

advanced stages of plague. 

Labels on recovered freezer bags indicated that the two 

Gunnison’s prairie dogs fed to the ferrets had died during 

capture or shipment. During 1994 and 1995, former technicians working at the Pueblo facility received several shipments 

of Gunnison’s prairie dogs from Mr. Gay Balfour of Dog 

Gone, Inc., Cortez, Colo. Mr. Balfour used a modified industrial street cleaning machine with a large vacuum to extract 

live prairie dogs from their burrows. A small percentage of his 

catch was injured or killed during capture, and a few prairie 

dogs may have been dead in the burrow when extracted by the 

vacuum. These nonquarantined prairie dogs were to be tested 

later for plague and stored separately from quarantined prairie 

dogs. One or both of the Gunnison’s prairie dogs fed to the 

ferrets was likely infected with Y. pestis. It is unlikely that all 

30 black-footed ferrets received Gunnison’s prairie dog pieces. 

We believe the infected portions of Gunnison’s prairie dog 

cross-contaminated the rest of the prairie dog pieces during 

processing on the cutting board and/or while being carried in 

the transport bowl. 

Surviving ferret #1148 shared a pen with another 

black-footed ferret (#268) that died from plague. Initial 

serum samples from #1148 (December 14, 1995) showed no 

evidence of plague exposure as judged by passive hemagglutination assay; however; surviving ferrets #565 (titer 1:128) 

and #1508 (titer 1:256) did show evidence of exposure (fig. 

3). Ferrets #1508 and #1148 were transferred to reintroduction 

sites (Montana and Arizona, respectively) before additional 

blood samples could be taken. Ferret #565 remained at Pueblo, 

and blood samples were taken at 2-week intervals to follow 

the immune response. The titer level for ferret #565 increased 

to 1:2,048 and then diminished to 1:64 over the next 5 months 

(fig. 3).

Questions arose regarding the persistence of plague 

underground, and we elected to move some of the resident 

Siberian polecats from cages to the outdoor pens for exposure 

testing. On January 23, 1996, 11 male-female pairs of polecats 

were transferred into pens that had held ferrets that either died 

or disappeared. We radio tagged the polecats and took baseline 

serum samples prior to the transfer. Polecats were located each 

day visually or via radio telemetry. Additional blood samples 

were taken approximately monthly for 5 months, and irregularly thereafter.

On January 28, 1996, polecat #889 was found dead 

underground via radio telemetry. Necropsy and tests of tissues 

indicated plague as the cause of death. The pen had previously housed a black-footed ferret (#1410) whose body was 

not recovered. On February 13, 1996, polecat #800 carried the 

partially mummified remains of a formerly missing blackfooted ferret (#1471) into a nest box. Subsequent tests of the 

polecat’s blood indicated no evidence of exposure to plague; 

however, the remains of ferret #1471 were positive for plague. 

Figure 3. Antibody responses (as determined by passive hemagglutination) of black-footed ferrets (Mustela nigripes) #1508 and 

#565. Estimated date of exposure was 11/19/1995.

Exposure of Captive Black-footed Ferrets to Plague 235
On February 23, 1996, polecat #094 recovered the remains of 

black-footed ferret #636. Serum samples indicated that polecat 

#094 was positive for plague and remained so for more than 

3 months without clinical symptoms (fig. 4). The recovered 

body of black-footed ferret #636 also tested positive for 

plague. Siberian polecat #293, housed in a pen where ferret 

#526 had disappeared, also tested positive for plague. Thus, of 

the 22 polecats moved to the black-footed ferret pens, 3 tested 

positive for plague, 1 of which died. One of the seropositive 

surviving polecats was likely exposed when it recovered the 

remains of a plague-positive ferret. However, an additional 

polecat that recovered a plague-positive ferret tested negative. 

The remaining 18 polecats, including the pen mates of the 

three that were seropositive for plague, tested negative.

Black-footed ferrets are known to scavenge opportunistically; that habit, combined with the fact that plague has been 

repeatedly detected at most of the black-footed ferret reintroduction sites, suggests that plague-killed rodents constitute a 

real and eminent hazard for free-ranging black-footed ferrets. 

Because of the persistence of live Y. pestis in carcasses for 

more than 2 months in relatively cool and humid prairie dog 

burrows, the hazard may linger long after an epizootic has 

killed the rodents. If Y. pestis resides in prairie dog colonies, 

occasionally causing disease in individual prairie dogs or other 

rodents, the risk posed by even widely spaced carcasses could 

be serious for the relatively mobile foraging ferrets. 

Although titers of the Siberian polecats declined, they 

remained sufficiently high during the course of monitoring 

(ca. 1 year) to suggest immunity to plague (fig. 4). Because 

the native habitats of Siberian polecats are centered on Asian 

foci of plague, these polecats were hypothesized to be more 

resistant than black-footed ferrets to the disease. Nevertheless, 

plague killed 88 percent of 33 polecats exposed to Y. pestis

through subcutaneous injections and consumption of plaguekilled mice (Castle and others, 2001), a loss rate similar to the 

suspected mortality rate for black-footed ferrets (90 percent) 

in the Pueblo incident.

The initial plague exposure of ferret #565 was more than 

3 weeks before the first blood sample was taken on December 

14, 1995. The greatest measured antibody response (1:2,048) 

was on January 11, 1996 (fig. 3), followed by a decline. 

The relatively low titers after just 3 months may have been 

insufficient to confer protection against subsequent exposure 

to plague (fig. 3). Consequently, long-term protection against 

plague via vaccination may be problematic in black-footed 

ferrets. More research is clearly needed.

It is unlikely that plague can be eliminated from the wild 

in North America. Protection of the black-footed ferret from 

this disease may depend in part on the ability to reduce its 

spread among and within prairie dog colonies and complexes. 

The use of pesticides to reduce flea populations provides 

some hope of reducing plague outbreaks and stabilizing 

treated areas (Durbian and others, 1997; Karhu and Anderson, 

2000; Seery and others, 2003). Repeated dusting of burrows 

with pesticides, however, is labor intensive and perhaps not 

practical for large colonies and complexes. If a management 

tool (e.g., insecticide) can eliminate plague from a prairie dog 

colony, both ferrets and prey will be afforded some protection. 

Initial results suggest that flea control may reduce or eliminate epizootics of plague on prairie dog colonies but may not 

eliminate Y. pestis completely. If low levels of enzootic plague 

remain on such colonies, the threat to ferrets may be substantial, and additional management intervention (e.g., vaccination 

of ferrets) may be necessary. 

Plague is currently common throughout the majority of 

the black-footed ferret’s historical range. Remaining plaguefree areas have become vital to reestablishment of the ferret. 

Why some prairie dog complexes are plague free and how 

long they will remain so are unknowns. The few remaining 

Figure 4. Antibody responses (as determined by passive hemagglutination) of two Siberian polecats (Mustela eversmannii) 

exposed to plague. Earliest potential date of exposure was 

1/23/1996, when polecats were moved into pens.

Discussion

Black-footed ferrets may die within 48 hours of consuming plague-infected meat. Of the 30 animals in the group 

potentially exposed, 27 likely died (some were missing), and 

3 survived (2 with antibody responses and 1 with no seroconversion even though its pen mate died of plague). This high 

rate of mortality was surprising given the circumstances of 

exposure. Some ferrets apparently ate prairie dog pieces that 

were surface-contaminated (by mixing with other pieces from 

infected prairie dogs) and probably received a fairly low dose 

of Y. pestis. Perhaps the two ferrets that survived exposure and 

showed antibody response consumed very low numbers of 

bacteria.

236 Recovery of the Black-footed Ferret
plague-free areas provide a unique opportunity to learn about 

black-footed ferret habitat before plague becomes endemic. 

As experimental reintroductions and plague research continue, 

special consideration should be given to existing plague-free 

areas for recovering the black-footed ferret and for increasing 

our knowledge of plague dynamics through comparisons of 

areas with and without the disease.

References Cited

Achtman, M., Zurth, K., Morelli, G., Torrea, G., Guiyoule, A., 

and Carniel, E., 1999, Yersinia pestis, the cause of plague, 

is a recently emerged clone of Yersinia pseudotuberculosis: 

Proceedings of the National Academy of Sciences, v. 96, 

p. 14043–14048.

Barnes, A.M., 1982, Surveillance and control of bubonic 

plague in the United States: Symposia of the Zoological 

Society of London, v. 50, p. 237–270.

Biggins, D.E., and Godbey, J.L., 2003, Challenges to reestablishment of free-ranging populations of black-footed ferrets: 

Comptes Rendus Biologies, v. 326 (Suppl. 1), p. S104–S111.

Biggins, D.E., and Kosoy, M.Y., 2001, Influences of introduced plague on North American mammals—implications 

from ecology of plague in Asia: Journal of Mammalogy, v. 

82, p. 906–916.

Biggins, D.E., Miller, B.J., Clark, T.W., and Reading, R.P., 

1997, Management of an endangered species: the blackfooted ferret, in Meffe, G., and Carroll, R., eds., An 

introduction to conservation biology (2d ed.): Sunderland, 

Mass., Sinauer Associates, Inc., p. 420–426.

Castle, K.T., Biggins, D.E., Carter, L.G., Chu, M., Innes, K., 

and Wimsatt, J., 2001, Susceptibility of the Siberian polecat 

to subcutaneous and oral Yersinia pestis exposure: Journal 

of Wildlife Diseases, v. 37, p. 746–754.

Durbian, F.E., III, Dullum, J., and Matchett, R., 1997, Summary of the 1997 prairie dog flea dusting effort on the 

Charles M. Russell and UL Bend National Wildlife Refuges 

and adjacent Bureau of Land Management lands: Lewistown, Mont., U.S. Fish and Wildlife Service, Charles M. 

Russell National Wildlife Refuge, 6 p.

Forrest, S.C., Biggins, D.E., Richardson, L., Clark, T.W., 

Campbell, T.M., III, Fagerstone, K.A., and Thorne, E.T., 

1988, Population attributes for the black-footed ferret (Mustela nigripes) at Meeteetse, Wyoming, 1981–1985: Journal 

of Mammalogy, v. 69, p. 261–273.

Gage, K.L., 1998, Plague, in Collier, L., Balows, A., Sussman, 

M., and Hausler, W.J., Jr., eds., Topley and Wilson’s microbiology and microbial infections (9th ed.), vol. 3, Bacterial 

infections: London, Arnold, p. 885–904.

Gage, K.L., Ostfeld, R.S., and Olson, J.G., 1995, Nonviral 

vector-borne zoonoses associated with mammals in the 

United States: Journal of Mammalogy, v. 76, p. 695–715.

Karhu, R.R., and Anderson, S.H., 2000, Effects of pyriproxyfen spray, powder, and oral bait treatments on the relative 

abundance of fleas (Siphonaptera: Ceratophyllidae) in 

black-tailed prairie dog (Rodentia: Sciuridae) towns: Journal of Medical Entomology, v. 37, p. 864–871.

Oldemeyer, J.L., Biggins, D.E., Miller, B.J., and Crete, R., 

eds., 1993, Management of prairie dog complexes for the 

reintroduction of the black-footed ferret: Washington, D.C., 

U.S. Fish and Wildlife Service, Biological Report 13, 96 p.

Seery, D.B., Biggins, D.E., Montenieri, J.A., Enscore, R.E., 

Tanda, D.T., and Gage, K.L., 2003, Treatment of blacktailed prairie dog burrows with deltamethrin to control fleas 

(Insecta: Siphonaptera) and plague: Journal of Medical 

Entomology, v. 40, p. 718–722.

Ubico, S.R., Maupin, G.O., Fagerstone, K.A., and McLean, 

R.G., 1988, A plague epizootic in the white-tailed prairie 

dogs (Cynomys leucurus) of Meeteetse, Wyoming: Journal 

of Wildlife Diseases, v. 24, p. 399–406.

Williams, E.S., 1986, Experimental infection of white-tailed 

prairie dogs (Cynomys leucurus) with plague (Yersinia 

pestis): Preliminary report to the U.S. Fish and Wildlife 

Service, Fort Collins, Colo., 11 p.

Williams, E.S., Mills, K., Kwiatkowski, D.R., Thorne, E.T., 

and Boerger-Fields, A., 1994, Plague in a black-footed ferret (Mustela nigripes): Journal of Wildlife Diseases, v. 30, 

p. 581–585.

Williams, E.S., Thorne, E.T., Quan, T.J., and Anderson, S.L., 

1991, Experimental infection of domestic ferrets (Mustela putorius furo) and Siberian polecats (M. eversmanni) 

with Yersinia pestis: Journal of Wildlife Diseases, v. 27, 

p. 441–445.

Exposure of Captive Black-footed Ferrets to Plague 237
Abstract

On several occasions from 1989 to 2002, burrows within 

black-tailed prairie dog (Cynomys ludovicianus) towns on the 

Rocky Mountain Arsenal National Wildlife Refuge, Colo., 

were dusted with a variety of insecticides (carbaryl, permethrin, and deltamethrin) to reduce flea (Insecta: Siphonaptera) 

abundance in attempts to control plague epizootics. Prairie dog 

populations were monitored with a combination of various 

mapping techniques and population abundance indices (visual 

counts). A single application of deltamethrin significantly 

reduced populations of the plague vector Oropsylla hirsuta

and other flea species on prairie dogs and in prairie dog 

burrows for at least 84 days. A plague epizootic on the Rocky 

Mountain Arsenal National Wildlife Refuge caused high 

mortality of prairie dogs on some untreated towns but did not 

appear to affect nearby towns dusted with deltamethrin. Largescale relocation efforts followed plague epizootics during the 

late 1980s and 1990s in an effort to rebuild populations to 

support wintering eagles and hawks. Between 1989 and 2002, 

the U.S. Fish and Wildlife Service relocated 12,692 prairie 

dogs at the Refuge. A comprehensive population monitoring 

program was instituted in the early 1990s to gage the effectiveness of plague control and relocation. This paper presents 

a summary of the plague control, relocation, and population 

monitoring program.

Keywords: black-tailed prairie dog, Cynomys ludovicianus, deltamethrin, permethrin, pesticide, plague, Yersinia 

pestis

Introduction

Black-tailed prairie dogs (Cynomys ludovicianus) have 

been intensively managed at the U.S. Fish and Wildlife 

Service’s (FWS) Rocky Mountain Arsenal National Wildlife 

Refuge (Refuge) since 1986, when a significant bald eagle 

(Haliaeetus leucocephalus) communal winter roost was 

discovered. Wintering bald eagles feed largely on prairie dogs, 

through kleptoparasitism of ferruginous hawk (Buteo regalis) 

predations (U.S. Fish and Wildlife Service, 1990). At the 

Refuge, prairie dogs also provide habitat for burrowing owls 

(Athene cunicularia) and grassland birds, and prey for coyotes 

(Canis latrans), badgers (Taxidea taxus), and several species 

of hawks.

The Refuge is located approximately 16 km northeast 

of downtown Denver, Colo. Beginning in 1942, the U.S. 

Army used the site to manufacture chemical and incendiary 

weapons. After World War II, private companies leased the 

industrial site for the manufacture of pesticides and herbicides. 

The Refuge was designated as a Superfund site in 1986 and 

is currently undergoing environmental remediation. Congress 

passed legislation in 1992 that established the Refuge upon 

completion of environmental cleanup. The legislation also 

states that the Refuge will be managed as if it were a national 

wildlife refuge during the cleanup. The data collected and 

analyzed here are part of a larger-scale effort to characterize 

populations of wildlife at the Refuge. The Refuge currently 

covers over 6,900 ha in a mosaic of habitat types, including 

wetland, riparian, and various types and successional stages of 

grasslands.

Plague (Yersinia pestis infection) has periodically 

affected prairie dog towns on the Refuge since the 1970s. 

Efforts to control plague during past epizootics involved insecticides, including carbaryl and permethrin dust to control fleas 

(Insecta: Siphonaptera). Most early efforts were conducted 

primarily in the interest of public health rather than specifically to protect prairie dogs as important habitat for wildlife 

species. 

Large-scale relocation efforts followed plague epizootics 

during the late 1980s and 1990s in an effort to rebuild populations to support wintering eagles and hawks. A comprehensive 

population monitoring program was instituted in the early 

1990s to gage the effectiveness of the plague control and relocation program. This paper presents a summary of the plague 

control, relocation, and population monitoring program, which 

included mapping active prairie dog colony distribution and 

visual counts.

Monitoring Distribution and Abundance 

of Prairie Dogs

Prairie dog colony distribution was determined by using 

aerial photo interpretation and field verification in all surveys 

conducted from 1988 to 1993 (Ebasco Services, Inc., 1989; 

U.S. Fish and Wildlife Service, 1994). Black and white section 

Use of Pesticides To Mitigate the Effects of Plague

By David B. Seery1,2

1

U.S. Fish and Wildlife Service, Rocky Mountain Arsenal National Wildlife 

Refuge, Commerce City, CO 80022.

2

Current address: U.S. Department of Agriculture, 664 E. Broadway, 

Bloomfield, NM 87413.
Use of Pesticides To Mitigate the Effects of Plague 239

photographs with a scale of approximately 1 cm = 79.2 m 

were used as field reference guides. Frosted mylar was placed 

over each section photo, and the boundaries of prairie dogs 

towns were delineated in the field. Only active prairie dog 

towns were included in the survey. Mapping was usually 

conducted in the spring after emergence of prairie dog litters, 

with some additional mapping at other times of the year to 

document changes due to plague. Upon completion of the field 

mapping, the area of each town was determined by using a 

Radian’s Contour Plotting System-1 (CPS-1) for the 1988–90 

surveys and an electronic planimeter for the 1991–93 surveys.

Prairie dog town distribution was mapped from 1994 to 

2002 by using a TDC1 Global Positioning System (GPS) unit 

and Pathfinder® software (Trimble Navigation, Ltd., Sunnyvale, Calif.). GPS positions were collected by walking the 

perimeters of active prairie dog towns and recording positions 

at 10 to 15 second intervals. The perimeters were determined 

by outermost active prairie dog burrows or by vegetation clipping (where obvious). GPS data files collected and stored in 

the rover unit were then downloaded to a computer for subsequent differential analysis. Differential correction (to increase 

accuracy to 2–5 m) was completed by using community 

base station files downloaded locally or from the U.S. Forest 

Service in Fort Collins, Colo. Areas of prairie dog towns were 

then determined by using Pathfinder software. Final maps 

were developed with ArcView® software (Environmental 

Systems Research Institute, Inc., Redlands, Calif.).

Visual counts have been used to estimate populations of 

white-tailed prairie dogs (Cynomys leucurus) and Richardson’s 

ground squirrels (Spermophilus richardsonii) and have been 

shown to correlate well with estimates obtained from markrecapture data from the same sites (Fagerstone, 1983; Fagerstone and Biggins, 1986; Menkins and others, 1990). Visual 

counts were chosen as the primary method to estimate population density of black-tailed prairie dogs on the Refuge. This 

methodology was developed by the FWS’s National Ecology 

Research Center (now the Fort Collins Science Center, U.S. 

Geological Survey) in Fort Collins, Colo., to evaluate blackfooted ferret habitat (Biggins and others, 1993). Study plots of 

various sizes were established (depending on time, personnel, 

and suitability of habitat constraints), and in some cases entire 

prairie dog towns were counted. Visual counts were conducted 

for 3 consecutive days on each plot, starting approximately 30 

minutes after sunrise and continuing (with 15 minutes between 

counts) until prairie dog numbers began to decrease, usually 

midmorning. The highest individual count of prairie dogs 

recorded during the 3 days was then used to determine the 

density of each plot (highest count/area). Densities were then 

summed and divided by the number of plots to determine the 

mean density for each year.

History of Plague at the Refuge

Early efforts to control plague at the Refuge began in the 

mid-1970s. Carbaryl insecticide was infused into prairie dog 

burrows in an effort to control fleas and the spread of plague in 

the central portion of the Refuge. Primary attention was given 

to prairie dog towns located close to areas of human activity. 

Other than a few general statements and a hand-drawn map of 

the area, this plague epizootic was not well documented. The 

duration and extent of the epizootic and the effectiveness of 

the treatment are unknown.

The next recorded plague event started in November 1988 

in the northeast corner of the Refuge. Mapping of prairie dog 

towns on the Refuge had just been completed (October 1988) 

in response to the discovery of a communal bald eagle winter 

roost in 1986. Due to the importance of prairie dogs as a food 

source for wintering bald eagles, increased attention was given 

to controlling this epizootic. Plague rapidly spread through a 

large (>600 ha) prairie dog town on the eastern portion of the 

Refuge and reached the southeast corner of the refuge in 20 

days (J. Harrison, oral commun., 1994).

Previous studies have indicated that permethrin dust 

was effective at reducing fleas in burrows and on prairie dogs 

(Beard and others 1992; Barnes, 1993). A large quantity of 

permethrin powder was obtained and applied in prairie dog 

burrows in attempts to control plague, mostly in the interest of public health. The plague epizootic continued through 

September 1989, reducing prairie dog towns on the Refuge 

by 95 percent. Although the effects of plague on prairie dog 

towns was well documented during this event, the techniques 

involved with application of permethrin powder were not. It is 

believed that application rates were as suggested on the product label (1–2 oz/burrow) and that pressurized applicators were 

used. It is unknown, however, when and where (before, during, 

or after the passage of plague, or in active or inactive towns) 

the powder was applied. This lack of information hindered 

development of strategies to control future epizootics.

A prairie dog relocation program began in August 1989 

(table 1). A standardized approach to prairie dog relocation 

techniques was developed in order to maximize efficiency and 

success of the relocation efforts. The FWS developed cooperative agreements with several private relocator groups from the 

Denver area. These groups were composed of private citizens 

who advocated saving prairie dog towns from destruction 

caused by the rapid growth of urban development. Prairie dogs 

were collected for relocation by a number of methods, including water flushing, vacuum truck, and live trapping.

Following the large-scale and successful relocation 

program (tables 1 and 2; 6,842 prairie dogs relocated through 

1993, yielding >980 ha of active prairie dog towns in 1994), 

plague once again hit in May 1994, starting in the northeastern portion of the Refuge. The progression of this epizootic 

followed the same path as the one in 1988–89, proceeding 

south and east through a large (>400 ha) town on the east 

side of the Refuge. Several attempts were made to halt the 

advance of plague by dusting with permethrin powder on 

active portions of prairie dog towns in advance of the epizootic. Visual observation of prairie dogs above ground was used 

to determine where to begin dusting and to map the extent 

of plague. Plague continued to advance (much as wildfire 

spreads), slowing in its progress for several days to a few 
240 Recovery of the Black-footed Ferret

weeks as it encountered dusted areas, but eventually continuing to spread to all areas of the prairie dog town. By the fall 

of 1994, plague had decimated approximately 695 ha (or 

about 70 percent of the area occupied in May 1994) and had 

moved to the western portion of the Refuge. Plague continued 

throughout the winter. By the following spring, only about 73 

ha of active prairie dog towns remained, and by September 

1995, only 9 ha of active prairie dog towns remained (table 

2). The second large-scale relocation program began shortly 

thereafter, and by the time this effort was winding down in 

1998, over 4,000 prairie dogs had been relocated onto the 

Refuge from outside sources. The population rebounded 

quickly, reaching over 350 ha of active prairie dog towns after 

relocation of 5,072 prairie dogs (table 1), and continuing to 

grow to over 660 ha by 2000 (table 2), when the next epizootic 

arrived.

In January 2000, inspection of a prairie dog town at the 

northeast corner of the Refuge revealed no living prairie dogs. 

A plague control program was instituted immediately, with 

crews applying 35.9 kg of permethrin powder on about 40 ha of 

prairie dog towns within 1.6 km of the plague site by February 

2, 2000. No other plague activity was observed on the Refuge 

until April 18, 2000, when a contractor working on a nearby 

remediation project found a dead prairie dog approximately 

1.6 km from the earlier outbreak of plague. The carcass was 

sent to the Centers for Disease Control (CDC), and plague was 

confirmed on April 19, 2000. The plague control program was 

started again on April 20, 2000, concentrating on towns within 

1.6 km of where the carcass had been found. Additional dead 

prairie dogs were found during May 2000 (plague positive), and 

the scope of plague control was expanded. By the end of June 

2000 all active prairie dog towns in adjoining sections (about 51 

ha) were dusted.

During the course of this epizootic, a product evaluation study on Deltadust® (Aventis Environmental Health, 

Montvale, N.J.) began. Deltadust (a powdered formulation of 

deltamethrin) is a relatively new product containing a synthetic 

pyrethroid similar to permethrin and is reportedly waterproof, 

providing insecticidal action for up to 8 months. From July 

to October 2000, the FWS, assisted by plague lab personnel 

from the CDC in Fort Collins, Colo., evaluated Deltadust at 

the Refuge. The results of the study (Seery and others, 2003) 

indicated Deltadust was effective at reducing flea populations 

within prairie dog burrows and had a residual effect over 84 

days posttreatment. No toxic effects were noted in the prairie 

dog population from application of Deltadust into the burrows. 

After initial results were obtained from this study, additional 

quantities of deltamethrin were obtained and used immediately 

in attempts to control the continued spread of the epizootic. 

From July to October 2000, approximately 46 ha of high 

priority prairie dog towns (important to wintering bald eagles, 

public use areas, burrowing owl breeding areas, etc.) were 

treated with deltamethrin. All of these sites were monitored 

over the winter (2000–01). The treated sites survived without 

any sign of plague whereas most of the areas dusted with 

permethrin had succumbed to plague. However, plague activity 

was observed again in the spring of 2001 (based on plaguepositive carcasses) in limited, widely dispersed areas across 

the Refuge. Populations of prairie dogs on several, but not all, 

of the towns dusted with deltamethrin in the summer and fall 

of 2000 were eventually decimated from plague during 2001, 

6 to 10 months after towns were treated.

Discussion of Plague Management

From 1988 to 2001, a variety of insecticides (carbaryl, 

permethrin, and deltamethrin) were used at the Refuge in 

attempts to control plague in prairie dogs. Early attempts were 

aimed mostly at providing protection in areas heavily used by 

humans and were generally ineffective at controlling plague in 

prairie dogs. By 1994, more emphasis was given to providing 

protection to the prairie dogs themselves and even attempting to stop the spread of plague. In some cases, when applied 

early, these actions were successful in halting the spread 

of plague. On the Refuge, studies also indicated significant 

flea reductions after burrows were dusted with permethrin, 

although flea numbers on prairie dogs returned to previous 

levels 10 to 18 days posttreatment (Karhu and Anderson, 

Source

Year On refuge Off refuge Total

1989 132 579 711

1990 447 2,525 2,972

1991 252 2,125 2,377

1992 229 438 667

1993 48 67 115

1994 175 0 175

1995 276 140 416

1996 43 1,711 1,754

1997 207 1,659 1,866

1998 269 502 771

1999 90 0 90

2000 208 0 208

2001 261 0 261

2002 309 0 309

Total 2,946 9,746 12,692

Table 1. Summary of black-tailed prairie dog (Cynomys ludovicianus) relocations at Rocky Mountain Arsenal National Wildlife 

Refuge from 1989 to 2002.
Use of Pesticides To Mitigate the Effects of Plague 241

2000). At the “ball field” prairie dog town, for example, the 

application of permethrin dust in a timely manner saved a 

prairie dog town used for environmental education programs. 

This town, located west of the administration complex on 

the Refuge, was a popular stop for school groups on tours of 

the Refuge. On July 8, 1995, Refuge personnel found a dead 

prairie dog at the site. The carcass was sent to the CDC in Fort 

Collins, Colo., for testing. After receiving confirmation of 

plague, the entire prairie dog town was dusted (0.68 ha) with 

permethrin on July 12, 1995. No other prairie dog carcasses 

were found during that time. That town was used during 

visual counts (June) to estimate abundance, so the population was known (n = 77). Over the course of the next month, 

periodic visual counts were made to monitor effectiveness of 

the treatment. After a month the population stabilized at about 

25 prairie dogs, a loss of about two-thirds of the population. 

There were no other signs of mortality from plague at the site 

over the next several years, and the population began a gradual 

rebound, growing to cover 7.3 ha by 2001.

It appears that Deltadust, when applied in the manner 

described by Seery and others (2003), significantly reduces 

flea populations within prairie dog burrow systems and on 

prairie dogs. Deltamethrin has a significant residual effect, 

with flea populations still at nondetectable levels by day 84. 

Cessation of mortality of prairie dogs following application of 

deltamethrin accompanied flea reductions caused by the treatment. By comparison, previous studies evaluating permethrin 

dust have reported low numbers of fleas after 84 days (Beard 

and others, 1992).

Deltamethrin represents an effective alternative to 

permethrin dust for controlling flea populations in prairie 

dog towns. Its relative effectiveness, ease of application, and 

safety should make it an important tool for managing plague 

epizootics in these animals. The long residual activity of 

deltamethrin suggests that single applications may reduce 

fleas throughout most of the season of plague activity, which 

typically occurs during the warmest 4 to 5 months of the year. 

These advantages also suggest that deltamethrin can be useful 

a r = per capita growth rate, ln[N(t+1)/N(t)].

b

1988–90 data from Stollar and Associates (1992).

c

No data available, density estimated.

Table 2. Black-tailed prairie dog (Cynomys ludovicianus) population estimates at Rocky Mountain Arsenal National Wildlife Refuge, 

1988–2002. Three major plague epizootics occurred in prairie dogs on the Refuge: 1988–89, 1994–95, and 2000–02.

Year

Prairie dogs/ha

(mean + SE) n (plots) Area occupied (ha) Estimated population r

a

1988b 20.2 + 1.6 24 1,850.8 37,406 ----

1989b 20.2c --- 99.8 2,017 -2.92

1990b 12.2 + 2.0 6 232.9 2,842 0.343

1991 14.6 + 1.08 10 555.56 8,134 1.05

1992 17.8 + 1.79 12 663.27 11,793 0.371

1993 22.57 + 1.77 12 737.05 16,636 0.344

1994 23.47 + 1.31 10 982.75 23,065 0.327

1995 (May) 50.86 + 9.49 9 72.86 3,708 -1.83

1995 (Sept.) 50.86 + 9.49 9 9.0 458 -2.09

1996 41.16 + 5.6 8 35.9 1,478 1.17

1997 54.8 + 10.8 6 139.77 7,640 1.64

1998 32.8 + 3.78 10 357.77 11,735 0.429

1999 24.5 + 4.41 10 533.74 13,076 0.108

2000 ---- ---- 666.75 ---- ----

2001 (May) ---- ---- 250.43 ---- ----

2001 (Oct.) ---- ---- 105.0 ---- ----

2002 28.4 + 4.31 15 127.02 3,607 ----
242 Recovery of the Black-footed Ferret

for protecting prairie dogs as an important habitat component 

for raptors and other carnivores, such as black-footed ferrets, 

at wildlife conservation locations (for wintering, breeding, and 

translocation).

References Cited

Barnes, A.M., 1993, A review of plague and its relevance to 

prairie dog populations and the black-footed ferret, in Oldemeyer, J.L., Biggins, D.E., Miller, B.J., and Crete, R., eds., 

Management of prairie dog complexes for the reintroduction 

of the black-footed ferret: Washington, D.C., U.S. Fish and 

Wildlife Service, Biological Report 13, p. 28–37.

Beard, M.L., Rose, S.T., Barnes, A., and Montenieri, J., 1992, 

Control of Oropsylla hirsuta, a plague vector, by treatment 

of prairie dog burrows with 0.5% permethrin dust: Journal 

of Medical Entomology, v. 29, p. 25–29.

Biggins, D.E., Miller, B.J., Hanebury, L.R., Oakleaf, B., 

Farmer, A.H., Crete, R., and Dood, A., 1993, A technique 

for evaluating black-footed ferret habitat, in Oldemeyer, 

J.L., Biggins, D.E., Miller, B.J., and Crete, R., eds., Management of prairie dog complexes for the reintroduction of 

the black-footed ferret: Washington, D.C., U.S. Fish and 

Wildlife Service, Biological Report 13, p. 73–88.

Ebasco Services, Inc., 1989, Black-tailed prairie dog—activity 

survey interim report: Prepared for the U.S. Army Program 

Manager’s Office for the Rocky Mountain Arsenal Contamination Cleanup.

Fagerstone, K.A., 1983, An evaluation of visual counts 

for censusing ground squirrels, in Kaukeinen, D.E., ed., 

Vertebrate pest control and management materials, fourth 

symposium: Philadelphia, Pa., American Society for Testing 

and Materials, ASTM ATP 817, p. 239–246.

Fagerstone, K.A., and Biggins, D.E., 1986, Comparison of 

capture-recapture and visual count indices of prairie dog 

densities in black-footed ferret habitat: Great Basin Naturalist, v. 8, p. 94–98.

Karhu, R.R., and Anderson, S., 2000, Effects of pyriproxyfen 

spray, powder, and oral bait treatments on the relative abundance of fleas (Siphonaptera: Ceratophyllidae) in blacktailed prairie dog (Rodentia: Sciuridae) towns: Journal of 

Medical Entomology, v. 37, p. 864–871.

Menkins, G.E., Biggins, D., and Anderson, S., 1990, Visual 

counts as an index of white-tailed prairie dog density: Wildlife Society Bulletin, v. 18, p. 290–296.

Seery, D.B., Biggins, D.E., Montenieri, J.A., Enscore, R.E., 

Tanda, D.T., and Gage, K.L., 2003, Treatment of blacktailed prairie dog burrows with deltamethrin to control fleas 

(Insecta: Siphonaptera) and plague: Journal of Medical 

Entomology, v. 40, p. 718–722.

Stollar and Associates, 1992, Biota annual monitoring report 

for 1990 and summary report for 1988 to 1990, comprehensive monitoring program, final report: Prepared for Rocky 

Mountain Arsenal Field Office, U.S. Fish and Wildlife 

Service, 16 p.

U.S. Fish and Wildlife Service, 1990, Annual Progress Report: 

Commerce City, Colo., Rocky Mountain Arsenal Field 

Office, 177 p.

U.S. Fish and Wildlife Service, 1994, Annual Progress Report: 

Commerce City, Colo., Rocky Mountain Arsenal Field 

Office, 182 p.
Abstract

This study was conducted to further assess the feasibility 

of vaccinating black-footed ferrets (Mustela nigripes) against 

plague (caused by the bacterium Yersinia pestis). On days 0 

and 28, 17 postreproductive ferrets were immunized by subcutaneous injection with a recombinant fusion protein containing F1 and V antigens from Y. pestis. Another 17 animals 

received a placebo by the same route. Two weeks after the 

second immunization, mean antibody titers to Y. pestis F1 and 

V antigens were measured and found to be significantly higher 

in vaccinates than their preimmunization values (P < 0.0001) 

and significantly higher than the control values (P < 0.0001). 

Six months postimmunization, 16 vaccinates and eight 

controls were challenged with approximately 8,000 colony 

forming units of virulent plague by subcutaneous inoculation. Eleven of 16 vaccinates (69 percent) survived with no ill 

effects whereas all eight control animals died within 3–6 days. 

Two months later, the 11 surviving vaccinates were challenged 

again by ingestion of a plague-infected mouse. None of the 

animals showed any ill effects and all survived. In contrast, 

seven control ferrets fed infected mice died within 2–4 days, 

including one animal that did not actually ingest the mouse but 

was likely exposed to it. This study demonstrates that immunization of ferrets with the recombinant F1-V fusion protein 

can induce significant antibody responses and reduce their 

susceptibility to plague infection.

Keywords: black-footed ferrets, immunization, Mustela 

nigripes, sylvatic plague, vaccine, Yersinia pestis

Introduction

Sylvatic plague, caused by the bacterium Yersinia pestis, is 

primarily a disease of wild rodents that is transmitted between 

mammals via flea (Insecta: Siphonaptera) bite, direct contact, 

ingestion, or inhalation. Since its introduction into the United 

States in the early 1900s, plague has become firmly established 

in native rodent populations throughout the West, causing 

frequent epizootics (Barnes, 1993). For many species of wildlife, plague mortality has become a serious conservation issue. 

Over half of the North American rodent species of conservation 

concern (Hafner and others, 1998), including several species of 

prairie dogs (Cynomys spp.), reside within the range of plague 

in western North America (Barnes, 1982). In addition, the 

endangered black-footed ferret (Mustela nigripes), which relies 

almost exclusively on prairie dogs for food and shelter, is highly 

susceptible to plague and suffers high mortality upon infection 

(Williams and others, 1994; Rocke and others, 2004).

Current methods to control plague in prairie dog colonies 

include dusting burrows with insecticides after the onset of an 

epizootic and population reduction. Although these methods 

have limited success in controlling outbreaks in rodents, they 

may be applied too late to be effective for ferrets, and population reduction is inappropriate for an endangered species. 

Recent studies have shown that multiple doses of a recombinant vaccine, consisting of two fused plague antigens, F1 and 

V (F1-V protein), protect laboratory mice against the bubonic 

or pneumonic form of plague (Heath and others, 1998). In a 

pilot study conducted at the U.S. Geological Survey’s National 

Wildlife Health Center (NWHC) in Madison, Wis., six of 

seven ferrets that received a three-dose regimen of F1-V 

protein via subcutaneous injection survived challenge with 

7,800 colony forming units (CFU) of Y. pestis 3 weeks after 

their last booster dose (Rocke and others, 2004). The objectives of the study described herein were to assess vaccine efficacy with a larger group of animals and with a longer duration 

between vaccination and challenge (6 months).

Methods

Thirty-four ferrets (23 females and 11 males) were 

selected for this study at the U.S. Fish and Wildlife Service, 

Vaccination as a Potential Means To Prevent Plague in 

Black-footed Ferrets

By Tonie E. Rocke,1 Pauline Nol,1,4 Paul E. Marinari,2 Julie S. Kreeger,2 Susan R. Smith,1 Gerard P. Andrews,3

and Arthur W. Friedlander3

1

U.S. Geological Survey, National Wildlife Health Center, 6006 Schroeder 

Rd., Madison, WI 53711.

2

U.S. Fish and Wildlife Service, National Black-footed Ferret Conservation 

Center, P.O. Box 190, Wellington, CO 80549.

3

U.S. Army Medical Research Institute of Infectious Diseases, Bacteriology 

Division, Fort Detrick, Frederick, MD 21702.

4

Current address: U.S. Department of Agriculture, Animal and Plant Health 

Inspection Service, National Wildlife Research Center, 4101 LaPorte Ave., 

Fort Collins, CO 80521.
244 Recovery of the Black-footed Ferret

National Black-footed Ferret Conservation Center (NBFFCC), 

Wheatland, Wyo. (now located near Wellington, Colo.), where 

the initial immunization and collection of baseline blood 

samples took place. All animals were 3–4 years of age and 

had been vaccinated previously against rabies and canine 

distemper. At the NBFFCC, animals were marked individually 

with subcutaneous embedded microchips (AVID® Microchip 

I.D. Systems, Folsom, La.) and housed individually in 2.5-cm 

wire-mesh cages (61 x 61 cm) with vinyl floors. Wooden nest 

boxes (45 x 22 x 28 cm) were attached to the exterior of the 

cages via 30-cm corrugated drain pipe. Bedding consisted 

of absorbent cellulose (ALPHA-driTM; Shepherd Specialty 

Papers, Watertown, Tenn.). The animals were fed 60–70 g 

of a raw horsemeat diet (Toronto Zoo Small Carnivore Diet; 

Milliken Meat Products, Ltd., Scarborough, Ontario, Canada) 

once daily. Water was provided ad libitum in ceramic bowls or 

sipper bottles. 

For challenge experiments, all ferrets were transported 

to the NWHC where they were placed in a Biosafety Level 3 

animal holding facility. Upon arrival, the animals were treated 

prophylactically for coccidiosis and housed individually 

in stainless steel cages as described previously (Rocke and 

others, 2004). The animals were fed Toronto Zoo Small Carnivore Diet or Dallas Crown Carnivore Diet (Dallas Crown, Inc., 

Kaufman, Tex.) when the Toronto Zoo Small Carnivore Diet 

was unavailable. Methods of anesthesia and blood sampling 

were described in Rocke and others (2004).

This study was reviewed and approved by NWHC’s 

Animal Care and Use Committee and Biosafety Committee. 

All personnel handling plague-infected animals or carcasses 

were required to wear powered, air-purifying (Hepa-filtered) 

respirators with fullface shields, rubber aprons and boots, and 

double surgical gloves. In addition, personnel collecting and 

handling animals and conducting necropsies were required to 

take prophylactic antibiotics (as prescribed by occupational 

health physicians).

On days 0 and 28, 17 ferrets at NBFFCC received 0.5 

mL F1-V vaccine-adjuvant preparation (100 μg of antigen) 

by subcutaneous injection between the scapulae. The F1-V 

fusion protein and our methods of preparing the vaccine have 

been described previously (Heath and others, 1998; Rocke 

and others, 2004). Seventeen control animals received a 

placebo of 0.5 mL of Dulbecco’s Medium (Sigma Chemical 

Co., St. Louis, Mo.). One control animal was euthanized due 

to disease unrelated to vaccination; the rest were transported 

to NWHC the 12th week postvaccination where they were 

held in isolation for several months prior to plague challenge. 

During this period, two other animals (one vaccinate and 

one control) were euthanized due to disease issues unrelated 

to vaccination. The control animal had severe abscessation 

and edema of the neck region from which Streptococcus 

zooepidemicus was isolated. The vaccinate experienced acute, 

medically nonresponsive hind limb paresis. Upon histological 

examination, both animals were found to have kidney lesions 

(tubular nephrosis and glomerulopathy). 

Six months postvaccination (day 178), six vaccinates and 

eight controls were challenged with 7,800 CFU of our Y. pestis

challenge stock (CO92) described previously (Rocke and 

others, 2004); the bacteria were administered in 0.2 mL sterile 

saline by subcutaneous injection in the scapular region. Blood 

samples were taken from animals prior to first vaccination and 

on days 28, 42, and 167. Animals were monitored daily for 

signs of illness, and day of death was noted; severely debilitated animals were euthanized by CO2

 asphyxiation. 

To determine if survivors were protected from further 

plague infection, the 11 vaccinated ferrets surviving 2 months 

after the initial subcutaneous challenge were bled to determine 

titers to plague antigens, and each was then orally challenged 

with a single plague-infected mouse; seven unvaccinated 

ferrets each fed a single infected mouse served as controls. 

For the oral challenge, 6-week-old mice were inoculated with 

a 0.1-mL volume of >4,000 CFU Y. pestis by intradermal 

injection. Upon death within 3 days after challenge, the mice 

were placed in the cage of each ferret. Any carcasses or parts 

of carcasses not ingested by ferrets within 3–4 hours were 

removed and discarded. Any ferrets surviving the second 

challenge were bled to determine antibody titers after 4 weeks 

and then euthanized by intracardiac injection of euthanasia 

solution (Euthasol; Delmarva Laboratories, Midlothian, Va.). 

In both experiments, dead or euthanized ferrets were immediately necropsied. Selected tissues were collected for bacterial 

isolation (Rocke and others, 2004) and histology.

Serology

Blood samples were collected in sterile glass serum 

separator tubes from all animals prior to immunization, boost, 

and challenge. Survivors were also bled after challenge. After 

centrifugation of blood samples, the serum was transferred 

to 2-mL polypropylene tubes and frozen at -20oC for future 

analyses. Antibodies against F1 and V antigens were measured 

by using an enzyme-linked immunosorbent assay (ELISA) as 

previously described (Rocke and others, 2004). 

Statistical Analysis

Antibody titers were transformed by calculating the 

log10 of the reciprocal titer value. Change in titer was then 

calculated by subtracting an individual animal’s transformed 

preinoculation anti-F1 or anti-V titer from the transformed 

titer of each of that same animal’s subsequent blood samples. 

Statistical difference in change of titer between groups was 

tested separately at each blood sampling period by using a 

one-tailed Mann-Whitney test at P = 0.05 (Zar, 1999). Difference in survivorship between groups was tested at P = 0.05 

by using the Fisher Exact test (Zar, 1999), and days to death 

were compared by using a one-tailed Mann-Whitney test at P 

= 0.10.
Vaccination as a Potential Means To Prevent Plague 245

Results

All 17 F1-V vaccinated ferrets developed significant 

antibody titers to both F1 and V antigens after immunization. In contrast, antibody titers of control animals remained 

negative. Geometric mean titers in anti-F1 and anti-V antibody 

increased significantly after the initial dose of vaccine was 

administered (P < 0.0001) and increased to even higher levels 

(means of 1:25,000 and 1:40,000, respectively) after the 

second dose, or boost (P < 0.0001) (fig. 1). Within 6 months, 

the mean anti-F1 and anti-V titers of vaccinates declined 

significantly (P = 0.0004 and P < 0.0001, respectively), 

although they were still significantly higher than their prevaccination titers (P < 0.0001) and the unvaccinated controls prior 

to challenge (P < 0.0001).

Eleven of the 16 vaccinated ferrets that were inoculated 

with Y. pestis survived the subcutaneous challenge and showed 

no signs of illness. The other five vaccinates became sick 

and died with an average time to death of 9.4 days. The first 

vaccinate died on day 4 with unusual gross lesions, including bloody diarrhea, multifocal hemorrhage throughout the 

intestines, and swollen kidneys. Yersinia pestis was isolated 

in low numbers from the spleen, and S. zooepidemicus was 

also isolated from the retropharyngeal lymph node. The three 

vaccinates that died on days 7 and 9 had gross lesions more 

consistent with unvaccinated controls (enlarged and slightly 

hemorrhagic lymph nodes, enlarged spleen, mottled lungs), 

and Y. pestis was isolated from numerous tissues from all three 

carcasses. The last vaccinate died on day 18 postchallenge. 

No Y. pestis was isolated from any tissue, but S. zooepidemicus was found in the spleen, lymph nodes, liver, lungs, 

heart, esophagus, and an abscessed region on the neck. In 

comparison, all eight unvaccinated controls inoculated with 

Y. pestis died within 3–6 days of challenge, with an average 

time to death of 4.3 days. All had gross lesions consistent 

with plague infection, and large numbers of Y. pestis were 

isolated from the tissues of all animals. Including the animals 

that had S. zooepidemicus, the survival rate of vaccinates was 

significantly higher than that of controls (P = 0.02), and time 

to death was significantly longer (P = 0.02). At the time of 

subcutaneous challenge, the mean anti-F1 titer of vaccinates 

that survived (9,030) was not significantly higher (P = 0.165) 

than that of vaccinates that died (5,580). The mean anti-V titer 

was significantly higher (P = 0.035), however, in surviving 

vaccinates (16,950) compared to those that died (9,030). 

Two months after the subcutaneous challenge, the 11 

surviving vaccinates received a second plague challenge 

via consumption of a plague-infected mouse. Each of them 

consumed an entire infected mouse, and all survived with no 

apparent clinical signs. In contrast, the seven control animals 

presented with infected mice all died within 2–4 days, including one animal that did not ingest its mouse but presumably 

licked or sniffed it; this animal died on day 4. Yersinia pestis

was isolated from most of the controls, with the exception of 

one that died on day 2 that had an overwhelming infection of 

S. zooepidemicus.

Discussion

In this study, the majority (69 percent) of vaccinated ferrets 

survived subcutaneous plague challenge 6 months post-immunization in contrast to the unvaccinated controls that all died of 

the infection. These results are similar to those of our previous pilot study in which six of seven (86 percent) vaccinated 

ferrets survived subcutaneous challenge with the same dose 

of Y. pestis (Rocke and others, 2004). In that study, however, 

ferrets received an extra boost of F1-V just 3 weeks prior to 

challenge in a three-dose regimen whereas in the present study, 

the animals received only two doses and were not challenged 

with the bacteria until 6 months later. Mean anti-F1 and anti-V 

antibody titers of immunized animals increased significantly 

after vaccination, particularly after the boost; however, they 

decreased over the next several months to nearly preboost titers. 

Vaccinates that survived subcutaneous challenge had a slightly 

higher mean anti-V titer than those vaccinates that succumbed 

to the same challenge.

In nature, ferrets are likely exposed to plague by several 

means. They may be bitten by infected fleas as they navigate 

through burrows or as they feed on prairie dogs. It is also 

highly likely that ferrets contract plague while feeding on 

infected prairie dogs through either direct contact or inhalation of the bacteria. The one unvaccinated ferret in our study 

that contracted plague and died within 4 days even though it 

declined to consume the infected mouse is evidence of their 

extreme susceptibility to the bacteria via this route. Interestingly, in this study vaccinated ferrets that survived an initial 

subcutaneous challenge with Y. pestis all survived ingestion 

of an infected mouse 2 months later. This result suggests 

Figure 1. Geometric mean anti-F1 and anti-V antibody titers in 

black-footed ferrets (Mustela nigripes) immunized with F1-V 

protein. The dates of the first and second vaccinations (prime 

and boost), first subcutaneous challenge with Yersinia pestis, and 

second challenge via ingestion of infected mice are indicated with 

1 arrow, 2 arrows, 3 arrows, and 4 arrows, respectively.
246 Recovery of the Black-footed Ferret

that flea-bite exposure of vaccinated ferrets in nature could 

potentially boost their immune response enough to ward off 

further plague infection via consumption of infected prey. We 

suspect that some vaccinated ferrets would also survive an 

initial oral challenge with infected mice. In a previous pilot 

study, two of five vaccinated ferrets survived after ingestion of 

infected mice as an initial challenge (T. Rocke, unpub. data, 

2001). These results are promising but insufficient, so we are 

currently exploring methods for boosting mucosal immunity in 

vaccinates.

At least four ferrets in this study were found to have S. 

zooepidemicus infections, one prior to challenge and three 

after challenge. In addition, three other ferrets had kidney 

lesions (glomerulonephritis) visible upon histologic examination of tissues that may have resulted from a previous infection 

(T. Rocke, unpub. data, 2003). Kidney damage is a reported 

sequela to S. zooepidemicus infection in humans (Barnham 

and others, 1983; Francis and others, 1993; Pinto and others, 

2001) and horses (Divers and others, 1992). Raw horsemeat 

has been a documented source of S. zooepidemicus for other 

small carnivorous mammals, including short-nosed bandicoots 

(Isoodon macrourus) and shrews (Tupaia glis and Elephantulus rufescens) (Shaw and others, 1984) and several primate 

species (Schiller and others, 1989). In our study, ferrets were 

fed raw horsemeat diets from two different sources, both at 

NBFFCC and NWHC. Samples of the meat were cultured 

after the infection was diagnosed, but the bacterium was not 

isolated. Even though the source of infection is still unknown, 

we believe many of our study animals may have had underlying S. zooepidemicus infections or were recovering from an 

infection. This bacterium may have significantly impacted the 

ability of vaccinated ferrets to withstand challenge to Y. pestis.

Summary

The results of this study suggest that two doses of the 

F1-V protein are sufficient to reduce ferret mortality from 

subcutaneous injection of plague for at least 6 months postimmunization, even in the face of a chronic, underlying Streptococcus infection. We suspect that vaccination of younger 

animals (<1 year old) and animals that are less stressed 

would result in even higher antibody titers, better resistance 

to the disease, and longer duration of immunity. Until other 

methods of plague control are developed, the F1-V vaccine 

could protect ferrets in captive breeding facilities and animals 

intended for release programs. Black-footed ferret kits and 

dams in captive breeding programs are fed wild prairie dogs 

that are captured, quarantined, and killed for that purpose. 

However, the loss of numerous captive ferrets at one facility 

from ingestion of plague-infected prairie dog meat demonstrated the potential hazard of this practice (Castle and others, 

2001) even with disease precautions and quarantine of the 

prairie dogs. Vaccination of captive ferrets against plague 

could reduce this risk. Ferrets intended for release into the 

wild could be immunized with F1-V antigen several times 

prior to release and reimmunized upon recapture, preferably 

within 6 months to 1 year postrelease. This might reduce 

mortality rates of ferrets during plague outbreaks. However, 

because black-footed ferrets are completely dependent on 

prairie dogs for their survival and prairie dogs are likewise 

highly susceptible to plague, the ultimate recovery of ferrets 

will require maintenance of stable prey populations and thus 

prevention of plague in prairie dogs.

Acknowledgments

We thank C. Quist and L. Sileo for assistance with 

necropsies and histologic examination of tissues. We would 

also like to thank B. Buehl, T. Hoffman, C. Pauly, A. Randel, 

and P. Vertz for animal care and technical assistance; A. 

Bersch for statistical analyses; and J. Osorio for critical review 

of the manuscript. Funding was provided by the U.S. Fish and 

Wildlife Service and U.S. Geological Survey. 

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Vaccination as a Potential Means To Prevent Plague 247

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Andrews, G.P., and Baeten, L.A., 2004, Recombinant 

F1-V fusion protein protects black-footed ferrets (Mustela 

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Abstract

Canine distemper virus (CDV) causes a systemic disease 

that is highly virulent to mustelids and other carnivore (Order 

Carnivora) species and is found worldwide. Endemic canine 

distemper in wild and domestic carnivores in the United States 

has made reintroduction of endangered black-footed ferrets 

(Mustela nigripes) difficult in the absence of safe and effective CDV vaccines and vaccination practices. Toward this 

end, researchers have explored appropriate animal models and 

vaccine preparations in highly susceptible species. Published 

studies involving domestic ferrets (M. putorius furo) using 

Galaxy-D® and evaluating a recombinant canarypox-vectored 

vaccine for oral administration are reviewed. In addition, we 

present new findings in domestic and black-footed ferrets and 

Siberian polecats (M. eversmannii) that have extended our 

understanding of CDV in the black-footed ferret and other 

at-risk carnivore species. Original research presented here 

includes trials that determined an effective challenge dose 

(by route) of virulent CDV in domestic ferrets and Siberian 

polecats; the low likelihood of collateral vaccination with 

Galaxy-D; the adverse effect of modified-live virus boostering in black-footed ferrets receiving killed vaccine previously 

and the response of Siberian polecats receiving canarypoxvectored recombinant CDV vaccine (reCDV); the absence of 

an effect of reCDV vaccination on conception, pregnancy, and 

neonatal growth in Siberian polecats; and the apparent inefficacy of active reCDV vaccination during the period of passive 

immunity in young Siberian polecats. In the final section, we 

discuss emerging concerns and avenues for disease intervention that may present new opportunities to solve problems in 

vaccine safety, vaccine availability, field vaccine delivery, and 

other therapeutic modalities.

Keywords: black-footed ferret, canarypox, canine 

distemper, ferret, morbillivirus, oral vaccine, paramyxovirus, 

recombinant, Siberian polecat

Introduction

Canine distemper virus (CDV; family Paramyxoviridae, 

genus Morbillivirus) is a single-stranded, negative sense, 

16-kilobase RNA virus encoding six genes (designated N, P, 

M, F, H, L) and eight protein products. The N gene has been 

used for diagnostic CDV identification (Wimsatt and others, 

2001; Rzezutka and Mizak, 2002) while the M and P genes 

have been used in phylogenetic analyses (Barrett and others, 

1993; Saliki and others, 2002) and subtype identification 

(Roelke-Parker and others, 1996; Carpenter and others, 1998; 

van de Bildt and others, 2002; Bronson and others, 2003), 

respectively. Phylogenetic analysis using other genes has 

repositioned CDV within the paramyxoviridae (Westover and 

Hughes, 2001). Vaccine developers have focused on hemagglutinin (HA) and fusion (F) gene product antigens, which 

appear to confer highly protective immunity when antibodies 

are successfully raised in response to vaccination. 

Canine distemper virus is found worldwide. The hallmarks of CDV-induced disease are the result of primary 

host tissue tropisms for the cutaneous (maculopapular rash, 

erythema), respiratory (increased respiratory rate or labored 

respirations, dyspnea, cyanosis), gastrointestinal (diarrhea), 

and central and peripheral nervous systems. While respiratory 

and gastrointestinal manifestations of this disease can cause 

considerable morbidity and mortality, it is often the central 

nervous system manifestations that portend death during its 

clinical expression (Leisewitz and others, 2001). Nervous 

signs attributed to CDV include seizures, tremors, depression, and myoclonia (peripheral nervous signs). While some 

tissue tropism differences in CDV are expected, the Center 

for Veterinary Biologics (CVB; Veterinary Services, Animal 

and Plant Health Inspection Service, U.S. Department of 

Agriculture [USDA]) virulent challenge strain ultimately leads 

to neurological disease; nervous signs can also dominate in 

The Quest for a Safe and Effective Canine Distemper Virus 

Vaccine for Black-footed Ferrets

By Jeffrey Wimsatt,1 Dean E. Biggins,2 Elizabeth S. Williams,3 and Victor M. Becerra4

1

Center for Comparative Medicine, Department of Medicine, University of 

Virginia Health System and Department of Biology, University of Virginia, 

Charlottesville, VA 22904.

2

U.S. Geological Survey, Fort Collins Science Center, 2150 Centre Ave., 

Bldg. C, Fort Collins, CO 80526.

3

Deceased; Department of Veterinary Sciences, University of Wyoming, 

Laramie, WY 82070.

4

Virology Section, Center for Veterinary Biologics, Policy, Evaluation, and 

Licensing, Veterinary Services, Animal and Plant Health Inspection Service, 

U.S. Department of Agriculture, Ames, IA 50010.
The Quest for a Safe and Effective Canine Distemper Virus Vaccine 249

previously vaccinated mustelids that ultimately succumb to 

CDV infection (J. Wimsatt, unpub. data, 1996–98).

Canine distemper primarily affects carnivores (Order 

Carnivora), but may opportunistically infect other taxa 

(Appel and others, 1991; Svansson and others, 1993; Appel 

and Montali, 1994; Appel and Summers, 1995; Kennedy 

and others, 2000; Pollack, 2001; Noon and others, 2003). In 

terms of its risk to endangered carnivores, CDV is the most 

significant pathogenic virus known, and the black-footed ferret 

(Mustela nigripes) reintroduction program must address this 

ongoing threat to captive breeding and wild population stability (Williams and Thorne, 1996). 

It is the general intent of this paper to accomplish two 

somewhat disparate goals. First, we chronicle what research 

on canine distemper virus prophylaxis in mustelids has 

revealed, the roles of various animal models and vaccine 

preparations in the quest, and where new discoveries could 

likely lead these pursuits in the future. Second, we present 

new findings of black-footed ferret responses to CDV vaccination and studies using CDV vaccines in surrogate animals to 

find a practical approach for CDV prophylaxis in susceptible 

Mustela species.

The Ecology of Canine Distemper Virus 

and the Risk It Presents to the Blackfooted Ferret

Canine distemper virus is enzootic in urban and rural 

settings (Grinder and Krausman, 2001). Canine distemper 

virus becomes rapidly inactivated once in the environment 

(Fox and others, 1998) but is readily spread by aerosol, even 

under dry, hostile conditions (Williams and others, 1988, 

1997). In the wild, transfer can occur at carnivore food (e.g., 

burrow entrances) and water sources. Wildlife epizootics may 

emerge as a consequence (Noon and others, 2003).

Traditionally, the primary reservoir and ultimate source 

of CDV outbreaks in the wild is assumed to be unvaccinated 

domestic dogs that infect wildlife with CDV during chance 

encounters. The potential role of wild carnivores (especially 

young) as primary reservoirs of CDV is difficult to discount 

(Guo and others, 1986; Gese and others, 1991, 1997; Williams 

and Thorne, 1996; Williams and others, 1997; Cypher and 

others, 1998; Grinder and Krausman, 2001; Arjo and others, 

2003) since high CDV seroprevalence rates, suggestive of 

high levels of exposure, are found in several wild species (Guo 

and others, 1986; Gese and others, 1991, 1997; Williams and 

others, 1997; Cypher and others, 1998; Dunbar and others, 

1998; Truyen and others, 1998; Grinder and Krausman, 2001; 

Ikeda and others, 2001). During a recent outbreak of CDV at 

an urban zoo, wild raccoons (Procyon lotor) were found to 

harbor a unique CDV variant (Lednicky and others, 2004), 

and they appeared to serve as a distinct reservoir. Most dogs 

are vaccinated for CDV (Greene and Appel, 1998); as a result, 

wild carnivores may be of greater infective potential to highrisk species, such as the black-footed ferret, than are domestic 

dogs. However, resident CDV in domestic dogs is under strong 

vaccine-induced selection pressure (Mochizuki and others, 

1999; Hashimoto and others, 2001; Lednicky and others, 

2004) and thus cannot be discounted as an emergent source in 

the future.

One area of growing relevance to captive and exotic 

carnivores is the possibility of CDV persistence and later viral 

shedding (elaboration and release of virus by renewed replication from the host at a later date) after the primary infection 

has subsided. This issue is of great concern where modifiedlive virus (MLV) vaccines are used in nontarget species. 

Persistence of morbillivirus infections has led to such 

diseases as subsclerosing panencephalitis in humans (Dyken, 

2001; Garg, 2002; Schneider-Schaulies and others, 2003), 

Paget’s disease (Cartwright and others, 1993; Fraser, 1997; 

Mee and others, 1998; Friedrichs and others, 2002; Hoyland 

and others, 2003), and canine orthopedic conditions (Mee and 

others, 1993; Harrus and others, 2002). Autoimmune-mediated demyelination associated with measles or CDV infection 

has been studied in relation to its possible association with 

multiple sclerosis (Anonymous, 1978; Appel and others, 1981; 

Cook and others, 1986; De Keyser and others, 2001; Hernan 

and others, 2001). A link between infectious obesity and CDV 

has been proposed as well (Dhurandhar, 2001; Verlaeten and 

others, 2001).

Recently, evidence of CDV persistence has been documented in domestic dogs in which selected strains of the 

virus survived without detection by the host immune system 

(Lincoln and others, 1971; Povey, 1986; Leisewitz and others, 

2001). A major requirement for chronically persistent CDV 

infection involves the selection of a cell-associated strain 

with limited capability for antigen presentation (Vandevelde 

and Zurbriggen, 1995) and conferring only limited antibody 

diversity (Rima and others, 1987); this latter strain differs 

in its pathogenesis from more virulent forms causing acute 

disease (Vandevelde and others, 1980). One key site of CDV 

persistence may be dendritic cells, reflecting a change in CDV 

cell tropism (Wunschmann and others, 2000). The condition 

“old dog encephalitis” is one presentation of chronic CDV 

infection (Lincoln and others, 1971; Hall and others, 1979; 

Tobler and Imagawa, 1984; Evans and others, 1991; Axthelm 

and Krakowka, 1998). Moreover, a tropism for epithelial cells 

(in addition to the typical tropism for macrophages) in culture 

suggests that persistent strains behave more akin to vaccine 

strains (Evans and others, 1991). A recent case report highlighted the risk of CDV persistence from vaccine strains when 

a red panda (Ailurus fulgens) vaccinated 3 years earlier with a 

commercial MLV CDV vaccine developed progressive CDVinduced neurological disease and subsequently died (Bronson 

and others, 2003). Gene typing (P gene) demonstrated that 

the offending CDV isolate was actually the original vaccine 

strain. Another recent paper suggested that incomplete CDV 

expression of fusion (F) protein may facilitate persistent viral 

infection; likewise, hemagglutinin (HA) heterogeneity of new 
250 Recovery of the Black-footed Ferret

emerging strains could lead to more widespread CDV persistence if F protein immunity becomes the primary source of 

protection following vaccination (Meertens and others, 2003).

Animal Models for Testing CDV Vaccines Destined for the Black-footed 

Ferret

Historically, guidelines for vaccinating free-ranging 

and captive wild carnivores were derived from those used for 

vaccines in domestic dogs, mink (Hagen and others, 1970), 

and domestic ferrets (M. putorius furo) (Hagen and others, 

1970; Farrell and others, 1971). Interestingly, while domestic 

dogs are commonly vaccinated, they are not among the most 

CDV-susceptible carnivore species. One study estimated that 

up to 70 percent of urban dogs that were exposed to natural 

CDV infection never developed overt disease signs although 

they seroconverted, suggesting occult infection (Rockborn, 

1957). Likewise, experience has shown that vaccines developed for high efficacy in dogs (and also sometimes used 

safely in some wild canids) may be too virulent for more 

susceptible species (Fox and others, 1998) such as red pandas 

(Bush and others, 1976; Itakura and others, 1979; Montali 

and others, 1983; Appel and Summers, 1995), gray foxes 

(Urocyon cinereoargenteus) (Halbrooks and others, 1981), 

and selected Mustela species (Carpenter and others, 1976; 

Montali and others, 1983, 1994; Sutherland-Smith and others, 

1997). Canine cell line origin passaged vaccines were quickly 

realized to be pathogenic to domestic ferrets, commonly 

vaccinated as pets against CDV (Fox and others, 1998). Early 

MLV CDV vaccines intended for ferrets utilized primary 

chick embryo passage. These procedures were expensive, and 

assuring product uniformity was an ongoing concern (Fox and 

others, 1998).

An immune deficiency in black-footed ferrets that may be 

of prime importance in explaining the unique, extreme susceptibility of this species to CDV and other infectious diseases is 

the diminished production of the proimmune cytokine interleukin-6 (Stoskopf-Kennedy and others, 1997). In contrast, 

Siberian polecats (M. eversmannii) appear to produce greater 

amounts of interleukin-6 (S. Wisely, oral commun., 2004). 

Homozygosity among Wyoming black-footed ferrets is recognized from genetic comparisons to historical populations from 

Kansas and to Siberian polecats (Wisely and others, 2002); 

this limited diversity may have contributed to the unique 

susceptibility of black-footed ferrets to natural and vaccine 

strains of CDV. Further investigations will reveal whether 

other highly susceptible species exhibit the same predisposition to diminished interleukin-6 production. Other cytokines 

need to be explored in this light as well (Bencsik and others, 

1996; Grone and others, 2002).

A recent refinement in the production of one widely used 

CDV vaccine strain involved serial passage of the virus on an 

immortal primate Vero cell line (rather than chick embryo) 

and a more controlled process of vaccine attenuation. These 

procedures appear to improve product reliability, but highly 

susceptible species still succumb to vaccine-induced viral 

disease (Sutherland-Smith and others, 1997). 

The characterization of appropriate models for the study 

of CDV vaccines in susceptible species has been a high priority. Based on taxonomy, domestic ferrets appeared to provide 

a close model for interpreting the likely CDV responses of 

black-footed ferrets as compared with other carnivores; more 

closely related Siberian polecats (O’Brien and others, 1989) 

and black-footed ferret × polecat hybrids helped to further 

define the likely impact and efficacy of existing vaccine strategies destined for the black-footed ferret (Williams and others, 

1996). Recently, surplus black-footed ferrets have sometimes 

been available for CDV vaccine studies (J. Kreeger, oral 

commun., 2004), but definitive challenge studies may still rely 

heavily on other mustelid models.

Vaccines: the Past, Present, and Future

Traditionally, killed virus (KV) vaccines were reserved 

for species and situations where MLV vaccines were considered unsafe. Potential disadvantages of KV vaccines include: 

unreliable inactivation; short-lived immunity (in addition, 

adjuvants that may cause some side effects may be required); 

the need for high antigenic doses (possible side effects if 

redosed); variable protection in poor responders; and finally, 

the induction of humoral (antibody production) rather than 

cell-mediated (i.e., T cell-mediated cellular) immunity 

(Schultz and Zuba, 2003). Thus, KV vaccines may not protect 

against overwhelming exposures to wild-type CDV; protection 

in such instances likely requires both robust humoral and cellmediated immune responses. A nonadjuvanted KV vaccine 

was produced for use in highly susceptible species such as 

the black-footed ferret and red panda by Dr. Max Appel, of 

the Baker Institute at Cornell University; this vaccine was 

provided until a more favorable vaccination strategy became 

available.

Commercial CDV vaccines are primarily modified-live 

products incorporating carefully selected wild strains that 

respond favorably to serial passage and graded attenuation. Of 

these, the Onderstepoort strain has been most extensively used 

for vaccination in the domestic ferret and exotic carnivores 

in zoological collections, first as the chick-embryo product 

Fromm-D (Solvay Co., Mendota Heights, Minn.; no longer 

produced) and later as the primate Vero cell line attenuated 

vaccine, Galaxy-D® (Schering-Plough Animal Health, Inc., 

Union, N.J.). As a rule, modified-live products do not supply 

sufficient antigenic load to confer immunity unless active 

infection is engendered by vaccination (Schultz and Zuba, 

2003). A recent study on the efficacy of Galaxy-D in domestic 

ferrets demonstrated, by polymerase chain reaction (PCR) 

amplification, the presence of CDV vaccine virus in the blood 
The Quest for a Safe and Effective Canine Distemper Virus Vaccine 251

5 days following the first of two inoculations. A primary 

vaccination series led to protective immunity as defined by 

virulent strain challenge (Wimsatt and others, 2001). Modified-live CDV vaccines have been shown to provide substantial and long-lived immunity following a primary vaccination 

series that invokes both cell-mediated and humoral immunity 

in dogs and domestic ferrets (Gorham, 1966, 1999). In the 

past, Fervac-D® (United Vaccines, Inc., Madison, Wis.) and 

other modified-live CDV vaccines (Fromm-D and Galaxy-D) 

routinely used in domestic ferrets were tested in surrogate 

species and were found unsuitable for black-footed ferrets. 

Either primary (CDV-induced) or secondary immunosuppression-related disease ensued when black-footed ferrets 

and black-footed ferret hybrids were vaccinated with these 

formulations (E. Williams, oral commun., 1995). Lymphocyte 

apoptosis accompanies CDV infection leading to its immunosuppressive effects (Moro and others, 2003a,b). As with 

natural infection, the immunosuppressive fallout of CDV 

infection from modified-live vaccination can lead to significant secondary morbidity and mortality in stressed or particularly susceptible individuals. The closely related measles 

and CDV viruses directly inactivate lymphocytes by virusdependent and independent means (Krakowka, 1982) whereas 

more “adapted” strains do not inhibit lymphocyte proliferation 

(Schultz, 1976; Schlender and others, 1996) or T cell-mediated 

cytotoxicity (Tipold and others, 1999), and lead to the elaboration of immune-modulatory substances (Krakowka and others, 

1987; Tipold and others, 1999).

Our interest in modified-live CDV vaccination in the 

black-footed ferret arose in exploring the possibility that a 

reliable, less virulent, modified-live vaccine might be used to 

booster black-footed ferrets that had been vaccinated previously with a KV vaccine. A modified-live CDV booster would 

be expected to last for the reproductive life of the animal, thus 

obviating the need for vaccination in the wild after reintroduction. Experimental KV vaccine (inactivated Onderstepoort 

strain) was widely used by zoos to protect high-risk species 

such as lesser pandas and black-footed ferrets (R. Montali, 

oral commun., 1996), but a vaccinated cohort had never been 

extensively challenged under controlled conditions to determine efficacy because of the scarcity and inherent value of 

these species. Use of a CDV modified-live booster following 

repeated KV vaccination served as a mild challenge. Boostering efficacy was further tested by subsequent virulent strain 

challenge. Based on experience gleaned from studies on 

surrogate species and hybrids with various candidate vaccines, 

current vaccine trials now focus primarily on safer subunit 

vaccines for genetically “bottlenecked” or exquisitely susceptible species.

More recently, the advent of vectored vaccines employing 

a wide range of different vectors and supplying antigens for 

many diseases affecting many species (Tartaglia and others, 

1990, 1992, 1993; Paoletti and others, 1993, 1994, 1995; 

Taylor and others, 1994; Pincus and others, 1995) has fostered 

new optimism about the potential to find a safe and effective 

CDV vaccine for use in highly susceptible species.

Recent Studies Guiding Use of CDV 

Vaccine in Mustelids

All animals undergoing vaccine and challenge trials 

described below were housed in a biosafety level-2 room 

in modified rabbit cages and fed a high quality cat (Siberian polecats or domestic ferrets) or mink (black-footed 

ferret) chow; water was provided free choice. Animals were 

randomly assigned to treatment groups unless otherwise 

specified and grouped in cage racks by treatment. All animals 

were supplied with 40.6-cm (10.2-cm diameter) PVC hide 

tubes with fixed end caps. Animals were anesthetized without 

restraint by placing a second end cap with an inhalant anesthetic delivery port over the opposite end while the animal was 

inside.

Anesthesia was induced using 5 percent isoflurane in 3 

L/min oxygen. After approximately 2 minutes, the animal was 

transferred from the PVC chamber to a face mask, and anesthesia was maintained at 1–2 percent isoflurane in 1.5 L/min 

oxygen. Care was taken to anesthetize the controls before the 

vaccinates in all cases. Blood samples (1 mL) were collected 

from the cranial vena cava or from an external jugular vein 

into serum tubes, and serum was frozen until assayed. Under 

anesthesia, vaccination was accomplished by subcutaneous 

injection (Galaxy-D, following the manufacturer’s instructions, and canarypox-vectored recombinant canine distemper 

virus [reCDV] vaccine), or by the oral route (reCDV), spraying the reconstituted vaccine in the back of the mouth.

Serology and Challenge Strain Dose Validations

An adapted standard serum microneutralization test was 

used to assess CDV titers (Appel and Robson, 1973). All 

virulent CDV challenge studies employed the CVB USDA 

Snyder Hill virulent challenge strain (Lot # 90-18). This same 

strain is used for vaccine challenge studies required for USDA 

licensing of commercial CDV vaccines. Dose selection for 

these studies was validated as described below.

Initial challenge dose-response studies using six domestic 

ferrets per group and five dose groups (J. Wimsatt, unpub. 

data, 1996) established a minimal 100 percent lethal intraperitoneal dose of CVB Lot # 90-18 challenge strain ferret spleen 

suspension in domestic ferrets as a dilution of 1:1,000 (pH 

7.0, delivered in 1 mL total volume). Thus, for all subsequent 

challenge studies, regardless of the Mustela species tested, a 

1-mL volume of challenge strain diluted to 1:250 in phosphate 

buffered saline (same pH and total volume) was used. This 

final lethal dose selected for challenge studies was confirmed 

in four Siberian polecats (J. Wimsatt, unpub. data, 1996) and 

was also found to be 100 percent effective (lethal) when used 

in challenge controls in subsequent studies. Later investigations extended these initial determinations to suggest that 

combined oral/intranasal instillation yielded the same results 

as intraperitoneal administration in Siberian polecats (J. 

Wimsatt, unpub. data, 1997) and domestic ferrets (Wimsatt 
252 Recovery of the Black-footed Ferret

and others, 2001). The only exception was that CDV-induced 

skin erythema or maculopapular rash usually occurred first at 

the site of challenge strain inoculation.

Challenge studies still remain the best available means 

to test vaccine efficacy. The significance of different routes 

of challenge, like those influencing vaccination, may be of 

considerable importance and requires careful study (Schultz 

and Zuba, 2003). While intracerebral and intraperitoneal 

challenge are commonly used, mucosal (intranasal/oral) 

challenge more closely mimics natural infection. Mucosal 

immunity is often considered the first line of defense against 

infectious agents (Ogra and others, 1980). In our studies, using 

survival as the endpoint, the intraperitoneal and oral/intranasal 

routes yielded similar results. This is of interest since CDV 

has a tropism for mucosal tissue (Jozwik and Frymus, 2002), 

and mucosal presentation to dendritic cells may stimulate 

cell-toxic lymphocytes (Etchart and others, 2001) early in 

the disease pathogenesis. Likewise, active CDV mucosal 

immunization may minimize disease-induced immunosuppression (Liashenko and others, 1999) or bypass maternal passive 

immunity (Fischer and others, 2002), leading to qualitatively 

different outcomes during challenge and vaccination. During 

challenge, such differences were not evident.

Modified-live Vaccine Studies in Domestic 

Ferrets 

A chick embryo origin product (Fromm-D) using an 

attenuated Onderstepoort strain was found to be safe and 

effective when tested in black-footed ferret × Siberian polecat 

hybrids (Williams and others, 1996) and domestic ferrets (Fox 

and others, 1998). Galaxy-D was tested in male domestic 

ferrets vaccinated and challenged as described previously 

(Wimsatt and others, 2001). Briefly, eight randomly selected 

CDV-seronegative male domestic ferrets (Marshall Farms, 

Rose, N.Y.) were subcutaneously vaccinated twice 4 weeks 

apart with Galaxy-D according to the manufacturer’s instructions. Eight control animals received saline injections. Challenge followed 21 days after the last vaccination (Wimsatt and 

others, 2001).

Virulent virus challenge produced 100 percent mortality 

in the controls, with prolonged presence of virus nucleoprotein 

in the blood detected by CDV-specific nucleoprotein reverse 

transcriptase PCR (RT-PCR). All Galaxy-D vaccinates (n = 

8) survived following a primary two vaccine series although 

one first-time and two second-time vaccinates expressed viral 

nucleoprotein in their blood following challenge (Wimsatt and 

others, 2001). After active infection, this MLV vaccine induced 

a robust immune response protective against lethal CDV challenge, indicating that domestic ferrets responded with protective adaptive immunity to this same CDV strain, originally 

packaged in the avian embryo passaged Fromm-D vaccine.

Domestic Ferret Collateral Vaccination of Cage 

Mates 

In a second study, randomly selected pair-housed male 

CDV-seronegative domestic ferrets were subcutaneously 

vaccinated with a single dose of Galaxy-D. Blood sampling for 

serology and challenge were performed as indicated in fig. 1. 

Unvaccinated CDV-naïve cage mates were blood-sampled for 

seroconversion to assess for collateral vaccination.

None of the six male co-housed domestic ferrets seroconverted in response to a single Galaxy-D delivered to their 

(CDV-naïve) cage mate up to 25 days after vaccination. All 

vaccinated ferrets (six of six) survived challenge following 

the single Galaxy-D dose. Serology values for unvaccinated 

cage mates, vaccinates, and unvaccinated controls are shown 

in fig. 2; titers for unvaccinated cage mates housed contemporaneously with Galaxy-D vaccinates remained low and 

indistinguishable from those of seronegative controls (fig. 2), 

suggesting that if primary vaccine shedding or contamination 

following vaccination occurred, it was insufficient to produce a 

MLV-induced immune response in the CDV-naïve cage mates. 

Subcutaneous vaccination of CDV-naïve domestic ferrets 

with Galaxy-D did not appear to present a sufficient antigenic 

dose for collateral vaccination of co-housed cage mates and 

thus did not lead to seroconversion. This is not surprising 

since modified-live virus load is typically too low to induce an 

immune response in the absence of a host infection (i.e., host 

infection replicates more virus, thus increasing its antigenic 

load) caused by the vaccine strain (Schultz and Zuba, 2003). 

However, the timeframe was not sufficient to conclude that 

shedding of the Galaxy-D CDV virus from vaccinates would 

not have occurred eventually from virus replication in the host.

Figure 1. Timeline for the black-footed ferret (Mustela nigripes) 

Galaxy-D booster and virulent canine distemper virus challenge 

study. Seronegative domestic ferrets (M. putorius furo) in the 

same room served as challenge strain controls, and another 

cohort of pair-housed domestic ferrets had one member of the 

pair randomly selected for Galaxy-D vaccination at the same time; 

vaccinates were later challenged with the others while the unvaccinated member of the pair was removed just prior to challenge. 

Triangles indicate days of vaccination. Arrows indicate days when 

blood samples were drawn. 
The Quest for a Safe and Effective Canine Distemper Virus Vaccine 253

Black-footed Ferrets 

Nonreproductive, older (6–8 years), mixed-sex blackfooted ferrets (culled from the breeding program) that had 

previously received one or more experimental KV vaccinations (an Onderstepoort strain-origin experimental vaccine 

produced by M. Appel, Baker Institute, Cornell University) 

were randomly assigned to one of two treatment groups after 

being matched for CDV serum neutralization titer across 

groups prior to study. At the beginning of the study, the first 

group (n = 8) received a single dose of Galaxy-D subcutaneously while the second group (n = 7) served as controls. 

Surviving vaccinates (n = 6) and controls (n = 5) were 

challenged 61 days later. The timeline for the experiment is 

shown in fig. 1. The primary endpoint of interest was survival 

although necropsies were performed to determine pathological 

changes following challenge as well as the cause of death. 

Serum neutralization titers in surviving black-footed 

ferrets revaccinated with Galaxy-D and previously vaccinated (with the M. Appel killed CDV vaccine) black-footed 

ferret controls were comparable to those observed in newly 

vaccinated domestic ferrets receiving Galaxy-D for the first 

time. As expected, these titers contrasted sharply with those 

of unvaccinated seronegative domestic ferret controls (fig. 2). 

Prior to challenge, one black-footed ferret with a titer of 1:8 

from prior vaccination succumbed (one of eight) to vaccine 

strain CDV 15 days after vaccination, and another died from 

a secondary infection, likely related to CDV-induced immunosuppression (Clostridium sp. was isolated from this case 

of vascular sepsis). In addition, a control black-footed ferret 

(unvaccinated during the present trial) succumbed to CDV 

(one of seven; it succumbed 32 days after vaccine delivery and 

had an initial titer of 1:64) although it was housed in a separate 

rack of cages adjacent to the black-footed ferret vaccinates. 

Following challenge, three of six vaccinates died, one 17 

days after challenge (1:512). Of black-footed ferret controls, 

when they were finally challenged, one died 11 days later, and 

another died in response to a secondary infection (Enterobacter faecalis-induced sepsis). All black-footed ferret challenge 

survivors developed elevated CDV titers.

Previously, CDV-naïve black-footed ferrets were shown 

to be highly susceptible to the development of canine distemper even when the virus (canine passaged) was supplied by 

vaccination as a modified-live CDV strain (Carpenter and 

others, 1976). The presence of high titers from the KV vaccine 

appeared protective for black-footed ferrets exposed to live 

attenuated CDV in vaccine (Galaxy-D) or to the challenge 

strain; nevertheless, high titers alone were not always indicative of protection, as illustrated by one animal with a high 

titer (1:512) that still succumbed to CDV. From this series, 

MLV boostering of black-footed ferrets with high circulating CDV titers was of marginal value, most likely due to the 

blocking effect of these antibodies on the vaccine strain. There 

is no evidence that cell-mediated immunity was enhanced 

from boostering. Even so, overall, titers above 1:64 in this 

series appeared to confer protection against CDV challenge. 

Perhaps more important was the observation that protection 

against CDV did not necessarily ameliorate the likelihood 

of immunosuppression and death from secondary invaders. 

Finally, of those succumbing to CDV, the precipitous onset 

of neurological signs, without other prodromal signs, was the 

hallmark of disease development in prior vaccinates. This has 

been explained as a persistence of F protein-directed immunity 

with waning HA protection and is qualitatively similar to the 

outcome observed when CDV-infected vaccinates encounter 

novel CDV strains where HA antigenicity has shifted (Stern 

and others, 1995).

Canarypox-vectored Vaccination and the Potential for Oral Vaccine Delivery

A dose-response study was performed to define the minimum protective dose and chronicle possible side effects of an 

experimental canarypox-vectored recombinant CDV vaccine 

(reCDV) in Siberian polecats, as described in detail elsewhere 

(Wimsatt and others, 2003). Briefly, subcutaneous dose groups 

received 105.5, 105.0, or 104.5 plaque-forming units (PFU, a 

measure of vector and therefore vaccine concentration), and 

oral dose groups received 108.0 and 105.5 PFU. The timeline 

used for vaccination, blood sampling, and challenge is 

shown in fig. 3; challenge was performed 61 days after the 

Figure 2. Serum neutralization titers for domestic ferret (Mustela 

putorius furo) controls (c), vaccinated domestic ferrets (vxpr), 

pair-housed unvaccinated domestic ferrets (uvpr), previously vaccinated older black-footed ferrets (M. nigripes) receiving GalaxyD boostering prior to challenge (bffv), and previously vaccinated 

older black-footed ferrets challenged with canine distemper virus 

(bffc). Controls and unvaccinated pair-housed domestic ferrets did 

not exhibit significant titer increases. Black-footed ferrets started 

with high median titers from previous vaccinations, but boostering 

with Galaxy-D had no significant effect on their titers. Challenge 

caused elevated titers in the survivors.
254 Recovery of the Black-footed Ferret

Figure 3. Timeline for canarypox-vectored recombinant canine 

distemper virus vaccine (reCDV) trials in Siberian polecats (Mustela eversmannii) and in the reCDV-Galaxy-D boostering study 

where the same timeline was used except that Galaxy-D was 

substituted for the second reCDV vaccination. Triangles indicate 

days of vaccination. Arrows indicate days when blood samples 

were drawn.

Figure 5. Serology results from the canarypox-vectored recombinant canine distemper virus (CDV) vaccine dose-response 

study where varied doses were administered subcutaneously or 

orally under isoflurane anesthesia. These results indicated that 

survivors mounted CDV serum neutralization titers above those 

of nonsurvivors. An exception was noted in the case of the oral 

105.5 group, where nonsurvivors mounted elevated titers, but these 

titers were insufficient for protection against challenge. Thus, a 

cell-mediated component of immunity, mounted at higher protective vaccine doses, must be important for vaccine efficacy with 

vectored subunit vaccines against CDV. V1 = first vaccination, V2 

= second vaccination, Ch = challenge. (Adapted from Wimsatt and 

others, 2003. Reprinted with permission of the Journal of Zoo and 

Wildlife Medicine.)

Figure 4. A survival curve is shown for canarypox-vectored 

recombinant canine distemper virus vaccine trials with Siberian 

polecats (Mustela eversmannii). Animals receiving two 108.0

PFU vaccinations orally (8.0or) survived. Those receiving lower 

subcutaneous doses (e.g., 5.0sq) had lower survival as did those 

receiving lower oral doses (e.g., 5.5or), which fared even worse. 

(From Wimsatt and others, 2003. Reprinted with permission of the 

Journal of Zoo and Wildlife Medicine.)

first vaccination. For standardization purposes, only vaccine 

expressing >95 percent expression-capable canarypox vaccine 

vector was used. Outcomes included CDV-associated clinical 

sign development, survival of virulent challenge postvaccination, and antibody development; only the latter two outcomes 

will be recounted here.

As previously reported, oral reCDV vaccination of Siberian polecats with 108.0 PFU vaccine was protective for five of 

six vaccinates, or 83.3 percent effective in protecting Siberian 

polecats against lethal CDV challenge (Wimsatt and others, 

2003). A difference in survival following challenge was noted 

in groups receiving the same vaccine dose (105.5 PFU) by 

different routes (oral vaccine, none of six survived challenge; 

subcutaneous vaccine, three of six survived) indicating that 

the parenteral route was superior to oral delivery. The difference in challenge survival between the 105.5 PFU (three of six 

survived) and 105.0 PFU (three of five survived) subcutaneous dose groups was not significant, suggesting the minimal 

protective CDV PFU dose is higher than 105.5.

A Kaplan-Meier survival analysis was performed with 

dose and route of reCDV administration as predictors (fig. 4) 

(Wimsatt and others, 2003). Protective titers in response to 

reCDV were typically lower than those measured following 

vaccination with Galaxy-D in naïve animals; higher relative 

titers in response to reCDV were associated with greater 

protective value of the vaccine, and generally predictive of 

vaccine efficacy overall, as was the case for the modified-live 

vaccine. Even so, some challenge survivors that received 

reCDV had titers low enough that they would have been 

predicted to succumb to the challenge if modified-live vaccine 

protective titers were used as a guideline (e.g., 1:50–100; see 

fig. 5). It seems plausible that the protective titer differential 

between reCDV and modified-live vaccines in challenge 

survivors reveals that cell-mediated immunity conferred by the 

reCDV vaccine is a major aspect of its protective effect.

Starting in the early 1990s, interest was developing 

among black-footed ferret conservationists for the identification of a safe and effective CDV vaccine to use in this endangered species. The potential to safeguard the black-footed 

ferret using a canarypox-vectored subunit vaccine led to a 

series of studies in Siberian polecats with the ultimate goal 

of applying this vaccine to the black-footed ferret; this work 

became a major focus starting in 1996. At the same time, it 

was recognized that this work could serve as a guide for other 

highly CDV-susceptible species. This vectored vaccine type, 
The Quest for a Safe and Effective Canine Distemper Virus Vaccine 255

on vaccination and challenge by enteric instillation (Welter 

and others, 1999). However, the risk of human infection when 

encountering the vaccinia vector remains of potential concern, 

particularly for immunocompromised individuals; a vectoredvaccine, bait-induced vaccinia infection was documented in 

a pet owner when she tried to remove a bait from her dog’s 

mouth and was bitten in the process (Rupprecht and others, 

2001). The appearance of a vaccinia strain from Brazil pathogenic to cattle and humans (Palca, 2005) may ignite a debate 

about the persistence of this virus, or of genetic constructs of 

this virus when used as a vector in the future. 

Vaccination Effect on Humoral Immunity

In this study, pokeweed blastogenesis (pokeweed is a 

nonspecific B lymphocyte mitogen) was performed on blood 

samples from Siberian polecats collected immediately prior to 

and 14 days after a single reCDV vaccination (105.5 PFU) and 

coincidentally from unvaccinated saline control polecats.

Changes in blastogenesis responses of B lymphocytes 

in primary culture between vaccinates and controls were not 

statistically different (fig. 6). Hence, reCDV vaccination did 

not appear to cause significant suppression of B cell lines 

(immunosuppression) expected during sequelae of CDV modified-live vaccination and natural CDV infection.

In this study, we hypothesized that the immunosuppression associated with modified-live vaccination would 

not occur when using vectored CDV vaccines, a major 

Figure 6. Pokeweed blastogenesis was performed on two 

samples of peripheral lymphocytes before and 14 days after 

canarypox-vectored recombinant canine distemper virus (reCDV) 

vaccination or saline control injections. The change in lymphocyte 

blastogenesis between controls and vaccinates was not significantly different and suggests that reCDV was not immunosuppressive as compared to live CDV exposure or modified-live virus 

vaccination.

sometimes referred to as a type III recombinant vaccine (Van 

Kampen, 2001), used a canarypox vector to infect local (at 

the site of delivery) host cells, which then present HA and F 

antigens to T cells and macrophages, initiating cell-mediated 

and humoral responses (Schultz and Zuba, 2003). The 

canarypox vector was chosen because pox viruses do not use 

cell receptors for cell uptake during cellular endocytosis, the 

avian virus is avirulent at mammalian body temperatures, the 

pox genome is large enough to allow sizable vaccine-related 

gene substitutions, and pox vectors potentially reduce the 

risk of host genomic splicing (Tartaglia and others, 1992, 

1993; Perkus and others, 1995a,b; Adams and others, 1997). 

Optimal recombinant vaccines are constructed to obtain high 

gene expression rates in host cells. Ideally, the immune system 

recognizes these cells and presents them to the humoral 

and cell-mediated arms of the immune system to develop a 

broad immune response with protective attributes somewhere 

between those of a modified-live vaccine and a KV vaccine 

(Schultz and Zuba, 2003). Advantages of this approach 

are that (1) no intact infectious agent is used, (2) pox virus 

products are more durable than modified-live CDV, and (3) 

adjuvants are not required. Vaccinated domestic cats (Felis 

silvestris) (Macy and Couto, 2001) appear to be at risk of 

developing injection site-associated sarcomas; this issue has 

also been raised with domestic ferrets, which appear at lower 

risk with recombinant vaccines (Merial Technical Services, 

oral commun., 2000). Another concern seen in domestic 

ferrets following repeated vaccination with approved modified-live products has been the increased risk of anaphylaxis 

(Fox and others, 1998). In one study surveying the risk of sideeffects of vaccination in domestic ferrets, adverse reactions 

were reported approximately 5 percent of the time, particularly 

in older, previously vaccinated ferrets (Greenacre, 2003). This 

appears to be rarer with some products than others (Fox and 

others, 1998) and may be less likely with vectored vaccines 

although they have not been evaluated long enough to answer 

this question conclusively at this time. Repeated vaccination 

increased glomerular immune-complex deposition in mink 

receiving a multivalent vaccine that included CDV; unfortunately, the potential risk of glomerular disease was not studied 

(Newman and others, 2002). Recent anecdotal reports suggest 

that even the commercially available vectored CDV vaccine 

(PureVax® Ferret Distemper Vaccine; Merial, Inc., Athens, 

Ga.) is not without some risk in black-footed ferrets. Recently, 

several deaths in black-footed ferrets have been linked to its 

use in zoos (D. Garelle, oral commun., 2004). 

Another important objective was to determine the efficacy of reCDV vaccine when delivered orally, so it could ultimately be used for wild black-footed ferrets in baits. Raboral 

V-RG® (Merial, Inc., Athens, Ga.), a vaccinia-vectored 

rabies subunit vaccine had been successfully packaged and 

broadcasted in baits to curtail fulminant rabies outbreaks in 

several wild carnivore populations (Fearneyhough and others, 

1998; Hanlon and others, 1998; Olson and Werner, 1999). As 

demonstrated in domestic ferrets, vaccinia likely represents 

a better vector for oral administration than canarypox based 
256 Recovery of the Black-footed Ferret

advantage of the latter type. These results confirmed that the 

reCDV vaccine did not appear to cause a blunted B lymphocyte blastogenic response to pokeweed mitogen, typical of 

immunosuppression seen with modified-live CDV vaccines.

MLV Vaccine Boostering Following Vectored 

Vaccine

Onderstepoort strain origin genes for F and HA were 

used during construction of the reCDV vaccine and are 

expressed in Galaxy-D. To assess the potential for interference or synergy expected from use of reCDV followed by 

modified-live (Galaxy-D) vaccination, Siberian polecats that 

received a single reCDV dose (105.5 PFU) were subsequently 

boostered with Galaxy-D subcutaneously. These animals were 

challenged 61 days later. The timeline employed for blood 

sampling, vaccination, and challenge is depicted in fig. 3. 

Five of five mixed sex Siberian polecats that received a 

single reCDV dose boostered with Galaxy-D survived challenge whereas six of six seronegative challenged controls 

succumbed.

This study in Siberian polecats showed that a single 

reCDV vaccination using the F and HA proteins from the 

Onderstepoort strain did not interfere with a single Galaxy-D 

vaccination that followed, in effect using the same antigens 

from this strain in both cases; likewise, during the challenge 

that followed, this combination provided 100 percent survival, 

and, in our hands, provided protection equivalent to that of a 

single Galaxy-D vaccination in domestic ferrets, as mentioned 

previously. The use of a MLV vaccine to booster the commercial reCDV vaccine (PureVax) is of interest to domestic 

ferret owners, and this practice has been shown to be effective in pet ferrets when using the currently USDA approved 

MLV (Fervac-D) vaccine (Merial Technical Services, oral 

commun., 2001). The production of low (blocking) titers and 

immune priming conferred by recombinant vectored vaccines 

may make them ideal candidates for MLV boostering that is 

expected to confer long-term immunity. 

While not specifically tested, modified-live CDV boostering in black-footed ferrets suggests that modified-live vaccination following limited reCDV vaccination may be quite risky. 

Studies are in progress to establish the duration of titered 

immunity expected in black-footed ferrets over time following 

a primary two-vaccination series with PureVax (J. Kreeger, 

oral commun., 2004). Some investigators believe that three 

primary vaccinations will be warranted to provide a longer 

duration of immunity and higher protective titers (R. Montali, 

oral commun., 2003). Whether boostering with Galaxy-D or 

another relatively safe modified-live vaccine following some 

type of primary recombinant vaccination in black-footed 

ferrets will ever be worth the risk remains unclear. One important aspect of modified-live vaccination remains attractive; that 

is, the likely provision of life-long immunity in modified-live 

vaccinates. Immunity following a primary modified-live vaccination series with chick embryo attenuation was protective 

against lethal challenge at 6 years of age in mink and domestic 

ferrets (Burger and Gorham, 1964), and 5.5 years after similar 

vaccination in another domestic ferret study (Cabasso and 

Cox, 1953); this same result was reported in dogs 6.5 years 

after vaccination (L. Carmichael, personal commun., 1997, 

as reported by Gorham, 1999, p. 559). If repeated recombinant vectored vaccine vaccination does not confer life-long 

immunity, a trial to determine if MLV boostering following a 

full reCDV primary series may be warranted in black-footed 

ferrets destined for release, since it is highly unlikely they can 

be caught again for revaccination once in the wild. Alternatively, an effective oral baiting program with recombinant 

vaccine may be developed.

Vectored Vaccine Safety During 

Pregnancy

The timeline for vaccination, blood sampling, and challenge for evaluation of vectored vaccine safety in pregnant 

Siberian polecat females is shown in fig. 7 (upper timeline). 

Twelve treatment-randomized, unvaccinated Siberian polecat 

jills were compared to 12 reCDV vaccinates. Vaccination of 

CDV-naïve, reproductively intact polecat jills with a moderate 

reCDV dose (105.5 PFU subcutaneously) immediately prior to 

conception was followed by a second vaccine dose during the 

last 10 days of pregnancy. 

Initial vectored vaccination had no significant effect on 

conception rates. Following a second vaccination at 29 days 

of gestation, birth outcomes such as litter size and kit rate of 

weight gain (measured from 17 to 35 days of age) were not 

significantly different from those in unvaccinated controls.

Canine distemper virus has been demonstrated to be 

capable of crossing the placental barrier of infected pregnant 

bitches and infecting their unborn puppies (Krakowka and 

others, 1974, 1977). Most reproductive-age bitches are either 

Figure 7. Timeline for the canarypox-vectored recombinant 

canine distemper virus (reCDV) immunization of Siberian polecat 

(Mustela eversmannii) dams to assess conception and pregnancy 

safety. Also shown is the timeline for vaccination of their kits in 

the passive immunity study. Triangles indicate days of vaccination. 

Arrows indicate days when blood samples were drawn.
The Quest for a Safe and Effective Canine Distemper Virus Vaccine 257

vaccinated or exposed to CDV prior to pregnancy, conferring 

immunity; thus, it is likely that the potential for naïve dams of 

wild species or domestic canids to pass CDV transplacentally 

is underestimated (Krakowka and others, 1974), and the potential impact of CDV on reduced fecundity has not been well 

characterized in wild carnivores. Gorham (1999) conducted 

studies exploring the potential ill effects of vaccination before 

conception and during pregnancy employing a modified-live 

vaccine in mustelids. In those studies, modified-live vaccination influenced neither litter size nor apparent fertility; these 

results are similar to ours employing reCDV and suggest that 

high virus loads may be required to see transplacental disease.

Because the reCDV vaccine uses a novel vector, we 

tested the safety of this vaccine on reproductive polecat jills 

before conception, during pregnancy, and on kit growth 17–35 

days postpartum as a prelude to vaccine use in reproductive 

black-footed ferrets. For 3 years, the National Black-footed 

Ferret Conservation Center has been vaccinating reproductive 

black-footed ferrets with PureVax starting several months prior 

to the breeding season. This practice has not caused any identifiable adverse effects on fecundity and overall production (P. 

Marinari, oral commun., 2004). 

Vectored Vaccine Use in the Face of Passive 

Immunity

In 1997, 12 randomly selected Siberian polecat kits from 

mothers vaccinated twice with reCDV before conception and 

delivery (fig. 7, lower timeline) were themselves vaccinated at 

4 and 6 weeks of age; kits received a standard challenge at 19 

weeks of age.

All kits challenged at 19 weeks of age died with characteristic signs of CDV postchallenge. At this age, maternal 

protective immunity has disappeared in domestic ferrets 

(Gorham, 1999; Welter and others, 2000), suggesting that 

active immunization for CDV with reCDV (at 105.5 PFU 

subcutaneously) in the presence of passive immunity, as tested 

in the present series, was without benefit.

Indirect evidence has suggested that antigen presentation to the cell-mediated arm of the immune system and 

particularly to T lymphocyte-induced cytotoxicity can lead to 

cell-mediated immunity independent of humoral responses 

(Siegrist and others, 1998a,b). It has been demonstrated in 

puppies (Taylor and others, 1994) that vectored vaccination 

with rabies glycoprotein results in active immunization in 

the face of blocking passive maternal antibodies. Here, we 

hypothesized that reCDV vaccine might actively protect young 

Siberian polecats postnatally even though they carried passive 

immune protection from circulating maternal antibodies 

generated against the same vaccine. According to this line of 

reasoning, active immunity would develop during postnatal 

vaccination with reCDV by independently augmenting active 

(mostly T cell-mediated) immunity. This possible application 

was attractive because maternal immunity typically blocks 

conventional vaccines during this period, and the actual trajectory of waning maternal immunity is unpredictable in mustelid 

kits (Gorham, 1999), leaving susceptible young unprotected. 

Welter and others (2000) challenged domestic ferrets at 12 

weeks of age after parenteral vaccination with canarypox 

and vaccinia-vectored CDV vaccines for F and HA. In their 

study, vector-origin antigens had little effect on survival in 

early vaccinates, which was not significantly different from 

that of CDV-naïve controls. These results are similar to ours 

for the Siberian polecat, where early vaccinates, like CDVnaïve controls, succumbed to CDV during challenge. In their 

study, Welter and others (2000) attributed this vaccination 

failure to immaturity and nonresponsiveness of the immune 

system of the domestic ferret, a relatively altricial species. Our 

results support their observation; however, a lower dose of a 

canarypox-vectored vaccine was used in our study in Siberian 

polecats, complicating the final interpretation.

Canarypox cross-vaccination was not observed in unvaccinated Siberian polecats housed in adjacent cages. Thus, 

reCDV does not appear to be prone to cross-vaccination in this 

species. Similarly, reCDV vaccinated pregnant Siberian polecat jills adjacently caged with CDV and reCDV vector-naïve 

polecat jills never seroconverted following reCDV vaccination 

(J. Wimsatt, unpub. data., 1997).

Discussion

The ability of a vaccine to protect against differing CDV 

strains depends on how close the HA and F proteins are to 

the vaccine’s Onderstepoort-origin proteins expressed by the 

vector. In this regard, Galaxy-D and the vectored (reCDV) 

vaccine are similar in the qualitative aspects of their protection. For the vectored vaccine, it is too early to assess the 

long-term effects of injecting canarypox into foreign species. 

In theory, the nature of recombinant vaccines and the limited 

antigens they express may require that they be updated more 

frequently to keep pace with strain changes, if other antigens 

can contribute to immune protection during modified-live 

infection and immunity development. If so, verified failure 

of antigenic protection with reCDV vaccines may potentially 

serve as a more exacting measure of evolving antigenic shifts 

in wild strains in the future.

In contrast to modified-live vaccination, vectored vaccine 

presentation to the mucosal membranes may yield different results from parenteral administration, reflecting limited 

vector invasiveness of mucosal surfaces, particularly in regards 

to the canarypox vector (Welter and others, 1999). Whether 

this will have a practical outcome, say in the heterogeneity 

of host responses across species following oral administration, remains to be determined. The long-term impact of live 

virus vectors and their potential to revert to virulence remains 

a matter of speculation, but careful monitoring is warranted, 

since poxviruses generally have the potential to mutate and 

adapt to new species. While replication of the canarypox virus 

in hosts appears to be minimal, the period of retention of the 
258 Recovery of the Black-footed Ferret

virus has not been as well characterized in varied species, and 

the large number of species receiving this vaccine leaves open 

the possibility of specific species predispositions and alterations in strain virulence over time, if persistence occurs. The 

recent emergence of a pathogenic variant of vaccinia virus 

may exemplify this concern (Palca, 2005).

What the Future May Hold

Considering the wide range of related morbilliviruses 

affecting diverse orders and classes of animals, and the 

demonstrated transfer of distemper and other morbilliviruses 

to bystander species (Stallknecht and others, 1991; Jacobson 

and others, 1992, 1997, 2001; Visser and others, 1993; Appel 

and Montali, 1994; Duignan and others, 1995; Richter and 

others, 1996; Karesh and others, 1997, 1999; Longbottom, 

1997; Barrett, 1999; Jauniaux and others, 2000; Bossart 

and others, 2001; Lam and Chua, 2002; Johnson, 2003), the 

potential for cross-species movement and de novo creation 

of mutated variants of CDV seems high. For example, recent 

focus on HA variability among sympatric CDV strains 

(Gemma and others, 1996) suggests that commercial vaccine 

preparations may become inadequate for protection against 

CDV in the future (Mochizuki and others, 1999). However, 

caution is always warranted when documenting a vaccine 

failure because of the possibility of other causes. These other 

causes include incomplete dosing, genetic or ill-defined causes 

of host nonresponse (Leisewitz and others, 2001), administration during occult periods of host immunosuppression, and 

suboptimal product handling prior to use. Vaccine nonresponders have been documented for more than one canine 

disease (R. Schultz, oral commun., 2003). 

A recent canine distemper outbreak at a zoo was associated with exposure to wild raccoons in the Chicago area 

(Lednicky and others, 2004). The appearance of this distinct 

strain has introduced some uncertainty about the ability of 

current commercial CDV vaccines to protect against new or 

emerging wildlife strains (Lednicky and others, 2004). Recent 

CDV disease outbreaks involving novel strains have raised the 

suspicion of vaccine failures although without controlled challenge studies these suspicions are difficult to prove (Bohm and 

others, 1989; Maes and others, 2003). Even so, this proposed 

causal relationship between novel strains, possibly from 

wildlife reservoirs, and the potential for vaccine failures has 

not been investigated adequately, employing careful ecological 

study techniques, modern molecular tools, and strain-specific 

challenge studies in vaccine-protected animals. An outbreak of 

naturally occurring CDV in black-footed ferrets highlights the 

need for safe and effective vaccines to protect them following reintroduction and as the threat continues into the future 

(Williams and others, 1988). Large cats and other carnivores 

would likely benefit as well (Blythe and others, 1983; Davidson and others, 1992; Appel and others, 1994; Harder and 

others, 1995; Roelke-Parker and others, 1996; Leisewitz and 

others, 2001). 

The emergence of vaccine-resistant virus variants, like 

the analogous emergence of antibiotic-resistant bacteria, may 

be facilitated when vaccination is widely used and selection 

pressure is high. Even so, CDV vaccines have been surprisingly reliable over the last 50 years; this may relate to the 

observation that negative sense RNA viruses are less prone to 

recombine than other viruses (Chare and others, 2003).

Outbreaks of canine distemper in distant parts of the 

world have highlighted the significance of domestic and 

wildlife reservoirs as purveyors of distemper-induced disease 

worldwide (Bohm and others, 1989). Recent investigations 

surrounding CDV outbreaks in Japan (Mochizuki and others, 

1999), Denmark (Blixenkrone-Moller and others, 1993), 

Poland (Jozwik and Frymus, 2002), and the United States 

(Lednicky and others, 2004) have brought into the focus 

the possible emergence of CDV strains no longer optimally 

immunized with commercial vaccine products. For the most 

part, such strains have shown characteristic heterogeneity in 

the HA gene, while the F component of current wild strains 

has remained surprisingly uniform across strains. This situation is analogous to using measles vaccination to cross-protect 

against CDV (Chalmers and Baxendale, 1994). When CDV 

passes across species, the possibility of variability at all sites, 

including the F protein gene, seems highly likely as new hosts 

tend to cause selection for greater virus diversity (Woolhouse 

and others, 2001). In related paramyxoviruses affecting other 

species, F gene heterogeneity has been noted and may influence species predilections, disease phenotypy, and vaccine 

efficacy in the future, especially under strong selection pressure (Collins and others, 1998; Ning and others, 2002; Ujvári 

and others, 2003). 

The Promise of New Vaccine Strategies

A recent efficacy study using an adenovirus-vectored 

vaccine demonstrated the development of significant active 

immunity against CDV with the absence of mucosal immunity 

against the adenovirus vector in domestic puppies (Fischer 

and others, 2002). None of the other available vectored CDV 

vaccines are satisfactory for immunization of very young 

carnivores, and the adenovirus vector appears superior in this 

regard.

DNA vaccines are relatively safe, simple, and cheap 

to produce. They consist of DNA-encoding genes capable 

of producing vaccine antigens in host cells and mammalian 

promoters leading to selected gene expression (Liu, 2003). 

Recently, new DNA vaccines administered intramuscularly 

were shown to be highly effective against severe CDV challenge in mice (Sixt and others, 1998) and dogs (Fischer and 

others, 2003). 

Unfortunately, nonparenteral methods of DNA vaccine 

and vectored vaccine delivery have low efficiency in producing a protective immune response. The low oral efficiency 

of the canarypox vector (Wimsatt and others, 2003) limits 

the potential use of commercial products now available 
The Quest for a Safe and Effective Canine Distemper Virus Vaccine 259

(Merial Technical Services, oral commun., 2004). Even so, a 

significant serological response was observed following oral 

administration (T. Vickers, oral commun., 2005) of two PureVax vaccine doses at once in a recent study of Channel Island 

gray foxes (Urocyon littoralis) (Vickers and others, 2004). 

Vaccinia-vectored CDV constructs exist for research use (J. 

Taylor, oral commun., 1998). Vaccinia constructs appear to 

have greater enteric efficiency for bait delivery, as has been 

demonstrated during the use of Raboral V-RG in public health 

programs to vaccinate wild carnivores against rabies and 

experimentally with a vaccinia-vectored CDV vaccine (Welter 

and others, 1999). Mucosal delivery of DNA vaccines via new 

designer carriers will likely provide new opportunities for 

oral DNA vaccine delivery in the future (Hobson and others, 

2003). With the advent of antiviral drugs, viral inhibitors of 

virus-host cell F are being developed to moderate paramyxovirus-induced disease progression, providing a new therapeutic 

approach (De Clercq, 2002).

The relatively homozygous (genetically depauperate) 

black-footed ferret is at risk of CDV-induced disease with the 

use of any currently available modified-live products. With 

the advent of designer vaccines for the concurrent delivery of 

immunostimulatory genes in concert with immunogens, the 

ability to stimulate the immune system (e.g., to express immunostimulatory levels of interleukin-6) while vaccinating will 

offer new possibilities in the future. Even the ability to correct 

an identified interleukin-6 deficiency in the black-footed ferret 

may be on the horizon through the use of gene therapy via 

vectored vaccine or naked DNA approaches. Such methods 

could eventually serve to enhance the resistance of this and 

other sensitive species to the ravages of infectious diseases, if 

germ line incorporation becomes practical.

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Williams, E.S., and Thorne, E.T., 1996, Infectious and parasitic diseases of captive carnivores, with special emphasis 
266 Recovery of the Black-footed Ferret

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Section VII. Extended Abstracts

The contributions contained in this section were presented at an evening poster session of 

the symposium. 

Introduction

Restoring viable populations of black-footed ferrets 

(Mustela nigripes) requires first restoring large complexes of 

prairie dog (Cynomys spp.) colonies. Ferret restoration within 

the former range of the black-tailed prairie dog (C. ludovicianus) requires prairie dog complexes of 4,000 ha or more 

(Conservation Breeding Specialist Group, 2004). Areas large 

enough to accommodate prairie dog complexes of this size 

may not be readily apparent from mapping the current distribution of prairie dogs alone. Remaining prairie dog colony 

distribution today is in large part an artifact of historical land 

use rather than habitat suitability or other biotic factors (e.g., 

Lomolino and Smith, 2003). Moreover, as a result of intensive 

control efforts, disease, and other management activities, few 

prairie dog complexes of this size exist in situ today. Necessarily, black-footed ferret recovery will therefore require 

expansion of prairie dogs into potential habitat, prairie dog 

translocations, and other “habitat-building” activities. Locating 

priority opportunities for ferret recovery will involve looking at both biogeographic criteria as well as socioeconomic 

constraints and concerns. Locating a suite of “restorable” 

landscapes, based on a coarse set of criteria that account for 

biological habitat suitability, land tenure, land management, 

contiguous size, and geographic representation, is a first step 

in this process (Forrest and others, 2004; Proctor and others, 

2006). We used a geographic information system (GIS) to 

identify areas with restoration potential for the black-footed 

ferret within the former range of the black-tailed prairie dog, a 

species for which there are existing models of habitat suitability (e.g., Proctor, 1998).

Methods

We identified potential black-tailed prairie dog habitat 

within its former range by overlaying a grid of 90 × 90-m 

pixels on current U.S. Geological Survey vegetation maps 

(U.S. Geological Survey, 2000). Pixels were considered 

potential habitat if they contained vegetation classified as 

grasslands, grassland/herbaceous, shrublands, or transitional 

areas, removing from consideration all pixels with slopes >10 

percent, in cropland, or in the Sandhills region of Nebraska 

(Proctor and others, 2006).

From this potential prairie dog habitat subset, we selected 

focal areas for conservation of black-footed ferrets in the 

following manner. On public lands, we identified as focal 

areas all contiguous potential prairie dog habitat of 20,000 ha 

or more. On tribal lands where ownership data were available 

(Montana and New Mexico), we identified the largest block 

of potential prairie dog habitat on each reservation with a 

minimum area of potential habitat >4,000 ha. Where ownership data for tribal lands were unavailable (e.g., the Dakotas 

and Nebraska), we simply indicated the existence of known 

large prairie dog complexes (e.g., the Rosebud, Pine Ridge, 

and Cheyenne River Reservations in South Dakota). We then 

identified all blocks of potential habitat >4,000 ha located 

on private reserves (e.g., owned by the Turner Endangered 

Species Fund or The Nature Conservancy). We placed circles 

roughly equal to the size of the potential habitat block over the 

midpoint of each focal area. Circles are meant to identify the 

general location as opposed to exact boundaries.

The results provided good geographic representation 

for most states and provinces. However, six States (Kansas, 

Nebraska, North Dakota, Oklahoma, Texas, Wyoming) 

contained fewer than five focal areas. In these States, we went 

below the 20,000-ha cutoff to identify the next largest focal 

areas on public lands so that each State had at least five focal 

areas. Finally, because Kansas still had only three focal areas 

due to limited ownership in public lands or key private lands, 

we identified an additional two blocks of potential habitat 

from private lands by iteratively removing the smaller aggregations of potential habitat until only the largest two blocks 

remained.

In areas where accurate prairie dog location data are 

available, it may also be possible to develop more robust 

Identifying Focal Areas for Conservation of Black-footed 

Ferrets and Prairie Dog Associates

By Jonathan Proctor,1,2 Bill Haskins,3 and Steve Forrest4

1

Predator Conservation Alliance, P.O. Box 6733, Bozeman, MT 59771.

2

Current address: Defenders of Wildlife, 1425 Market St., Suite 225, Denver, CO 80202.

3

Big Sky Conservation Institute, 131 S. Higgins Ave., Suite 201, Missoula, 

MT 59802.

4

Northern Great Plains Ecoregion Program, World Wildlife Fund–U.S., P.O. 

Box 7276, Bozeman, MT 59771.
272 Recovery of the Black-footed Ferret

conservation strategies based on “reserve design” concepts 

(Noss and others, 1999). Using Montana as an example, we 

identified core conservation areas for prairie dogs and associates, linkage habitat, and matrix habitat by scoring each 90 

× 90-m pixel in Montana as follows: 1 if it was classified as 

potential black-tailed prairie dog habitat, else 0; 1 if it was 

within a current prairie dog colony (as mapped between 1988 

and 2003 and defined by a 0.75-km buffer), else 0; 1 if it was 

within a block of potential habitat >4,000 ha on public land, 

tribal land, or a private reserve, else 0; and proximity to other 

prairie dog colonies, expressed as a value between 0 (at >24 

km from a colony) and 1 (on a colony). The final score for 

each pixel was the total of the individual scores for these four 

criteria. 

Results and Discussion

Using the methodology described, 92 focal areas for 

potential black-footed ferret recovery were identified (fig. 1). 

Of the areas identified, five contain seven separate ongoing 

ferret reintroduction programs; one had ferrets reintroduced, 

but they no longer survive; and eight have been identified 

through other processes as having reintroduction potential 

or are in the process of developing reintroduction programs. 

Thus, all current or proposed ferret reintroduction sites in the 

black-tailed prairie dog range were captured by this methodology. While inclusion of all of the current or proposed reintroduction sites provides some validation of the model used 

here, it also suggests that the most obvious or most readily 

restorable sites have already been identified and/or are in use. 

The remaining sites may be progressively more challenging in 

terms of their restoration potential either because of the lack 

of currently existing large prairie dog complexes or because of 

other factors such as resistance to endangered species reintroduction programs.

For Montana, development of a reserve design for ferrets 

based on multiple attributes is feasible (fig. 2) but also points 

to limited opportunities for large-scale restoration at multiple 

sites given current black-tailed prairie dog distribution. 

While we limited this analysis to black-tailed prairie 

dog habitat, data exist to identify black-footed ferret recovery 

focal areas for both white-tailed (C. leucurus) and Gunnison’s 

(C. gunnisoni) prairie dogs as well. Recent assessments of 

those two species’ ranges suggest that good opportunities 

may exist in several locations beyond the four sites where 

recovery activities for ferrets are underway on white-tailed and 

Gunnison’s prairie dog range (Seglund and others, 2005a,b). A 

comprehensive, rangewide strategy for ferret recovery should 

incorporate these and other data to map out a path for the 

future of the species.

References Cited

Conservation Breeding Specialist Group, 2004, Black-footed 

ferret population management planning workshop, final 

report: Apple Valley, Minn., IUCN/SSC Conservation 

Breeding Specialist Group, 129 p.

Forrest, S.C., Strand, H., Haskins, W.H., Reese, C., Proctor, J., 

and Dinerstein, E., 2004, Ocean of grass—a conservation 

assessment for the Northern Great Plains: Bozeman, Mont., 

Northern Plains Conservation Network and Northern Great 

Plains Ecoregion Program, World Wildlife Fund–U.S., 

http://worldwildlife.org/wildplaces/ngp/pubs/ocean-ofgrass.cfm.

Lomolino, M.V., and Smith, G.A., 2003, Prairie dog towns as 

islands—applications of island biogeography and landscape 

ecology for conserving nonvolant terrestrial vertebrates: 

Global Ecology and Biogeography, v. 12, p. 275–286.

Noss, R.F., Dinerstein, E., Gilbert, B., Gilpin, M., Miller, G.J., 

Terborgh, J., and Trombulak, S., 1999, Core areas—where 

nature begins, in Terborgh, J., and Soulé, M., eds., Continental conservation—scientific foundations of regional 

reserve networks: Washington, D.C., Island Press, p. 

99–128.

Proctor, J.D., 1998, A GIS model for identifying potential 

black-tailed prairie dog habitat in the Northern Great Plains 

shortgrass prairie: Missoula, University of Montana, M.S. 

thesis, 54 p.

Proctor, J., Haskins, B., and Forrest, S.C., 2006, Focal areas 

for conservation of prairie dogs and the grassland ecosystem, in Hoogland, J.L., ed., Conservation of the black-tailed 

prairie dog: Washington, D.C., Island Press, p. 232–247. 

Seglund, A.E., Ernst, A.E., Grenier, M., Luce, B., Puchniak, 

A., and Schnurr, P., 2005a, White-tailed prairie dog conservation assessment: Laramie, Wyo., Western Association of 

Fish and Wildlife Agencies, 138 p.

Seglund, A.E., Ernst, A.E., and O’Neill, D.M., 2005b, Gunnison’s prairie dog conservation assessment: Laramie, Wyo., 

Western Association of Fish and Wildlife Agencies, 87 p.

U.S. Geological Survey, 2000, National land cover data: EROS 

Data Center, Sioux Falls, South Dakota.
Identifying Focal Areas for Conservation 273

Figure 1. Focal areas for black-footed ferret (Mustela nigripes) recovery based on habitat suitability, land tenure, 

and public land contiguity.
Figure 2. Relative potential for black-footed ferret (Mustela nigripes) recovery and restoration in Montana based on scores derived from habitat suitaexisting prairie dog (Cynomys spp.) distribution and abundance (moderate score = 1.0–2.0, high = 2.0–3.0, very high = 3.0–4.0).
Introduction

This study was conducted in the Pinedale Anticline Lease 

Area (PALA) of the Pinedale Field Office administrative unit 

of the U.S. Bureau of Land Management, Wyoming. The 

PALA and the adjacent Jonah II Lease Area (J2LA) contain 

large reserves of natural gas (Lyon and Anderson, 2003) and 

are sites of rapid energy exploration and extraction activities. 

The objectives of the study were to assess a variety of demographic variables and to provide a basis for linking prairie 

dog (Cynomys spp.) colonies to land features that might allow 

modeling of potential habitat. Recently compiled Global 

Positioning System (GPS) maps of white-tailed prairie dog (C. 

leucurus) colonies within the two mineral leases were available. These maps have been generated and updated annually 

for several years by a private contractor as part of an ongoing 

environmental assessment of the mineral leases. Approximately 30 colonies had been identified and mapped within 

the PALA in 2001. Twenty-nine of these colonies (hereafter 

referred to as the “base map colonies”) were selected to study 

the demographics and habitat characteristics of white-tailed 

prairie dogs.

Study Area

The area consists of plains interspersed with isolated 

hills, plateaus, and low mountains. Elevation ranges from 

1,800 to 2,400 m. Winters are cold and summers are short 

and hot. Average annual precipitation ranges from 130 to 360 

mm and is fairly evenly distributed throughout the year. The 

vegetative community is classified as sagebrush steppe (Reid 

and others, 2002). 

Methods

All burrows on 29 colonies were mapped by GPS in 2003 

and classified as active or inactive. Active status was determined by the presence of fresh prairie dog scat in the opening 

or within 0.5 m of the opening. Fresh scat was defined as 

greenish, black, or dark brown in color and not dried hard or 

bleached white (Biggins and others, 1993). 

Our survey concentrated on site factors that may influence the selection of white-tailed prairie dog burrows and 

colonies. Habitat characteristics were compared between the 

original colonies and nearby “ghost” polygons. The ghost 

polygons were computer-generated replicas of the actual 

colonies that were superimposed on the landscape at randomly 

chosen locations near each actual colony and within a range 

that was accessible to the prairie dogs (fig. 1).

Soil texture, soil depth, and ground cover were assessed 

on the colonies surveyed in 2001 and compared with samples 

from ghost colonies. Soil texture was assessed at a depth of 

White-tailed Prairie Dog Population Survey and Habitat 

Evaluation in Western Wyoming

By John A. Baroch1,2 and David A. Plume3 

1

Genesis Laboratories, Inc., P.O. Box 1195, Wellington, CO 80549.

2

Current address: 343 WCR 66E, Fort Collins, CO 80524.

3

Symmetry Corporation, P.O. Box 411, Niwot, CO 80544.

Figure 1. Generating ghost polygons and soil sample sites. Base 

map white-tailed prairie dog colony (Cynomys leucurus) PDT 6A is 

shown with the corresponding ghost polygon. The four soil survey 

sites, numbered 281–284, were selected using a randomization 

procedure applied to numbered cells overlaid on the source 

polygon. A second randomization algorithm selected the length 

and direction of the vector connecting the centroids. Soil survey 

points were projected along the same vector to locate ghost survey points that maintained the geometry of the source polygon.
276 Recovery of the Black-footed Ferret

0.5 m. Samples were dried and sifted by particle size. Soil 

depth was defined as the maximum depth (up to 1 m) that 

could be reached with a hand driven, 8-cm diameter bucket 

auger. Ground cover and surface soil texture were assessed by 

the point-intercept survey method. The point-intercept method 

employs a sighting device or pin/point frame at selected 

sites to estimate the cover by type. Optical sighting devices 

eliminate observer bias when used properly since the sampling 

points are selected entirely by procedure. We fabricated an 

optical sighting device consisting of a vertical sighting tube 

with a peephole sight at the top end and an 8-cm diameter 

glass magnifying lens at the lower end. The sighting tube was 

attached to the end of a horizontal beam. When rotated 360° 

in a horizontal plane on a tripod, the optical sight described a 

circle 1 m in diameter. At each randomly selected sampling 

location, readings were made at 30° intervals. Cross hairs at 

the center of the magnifying lens pinpointed each sampling 

point.

Slope, aspect, and solar gain were derived from 10-m 

digital elevation model data and intersected with both colony 

areas and burrow point data. Colony slope and aspect were 

analyzed with circular statistical methods. The aspect of the 10 

× 10-m cell containing each burrow recorded by the GPS was 

determined, and the results were sorted by active or inactive 

status.

Results and Discussion

The 2003 survey showed a dramatic reduction in the 

number of colonies since the earlier survey in 2001, with 

only 15 of 29 colonies surveyed still active. In terms of area, 

the active colonies in the vicinity of the 29 original colonies 

totaled just 71 ha. The original colonies composed 1,407 ha 

in 2001. Figure 2 illustrates one of the larger colonies that 

diminished in extent and population between 2001 and 2003. 

Of the 37 colonies mapped in 2003, 25 had what is considered 

a favorable or healthy ratio of active to inactive burrows (>1.0) 

(Biggins and others, 1993). Twelve colonies had ratios below 

1.0. The mortality factors that threaten prairie dogs on a large 

scale include loss of habitat, urbanization, resource development, poisoning, recreational shooting, and sylvatic plague 

(caused by the bacterium Yersinia pestis) (Knowles, 2002). 

Information on the incidence and impact of plague in the study 

area is lacking.

Our comparison between the 2001 colonies and the 

randomly located ghost polygons did not find significant 

differences in soil depth (to 1 m) or in percent rocks in the soil 

profile at a depth of 0.5 m. We had hypothesized that variations in soil depth might affect site selection with regard to the 

ability to establish hibernacula below the frost line. We found 

soil depth to be at least 1 m throughout the surveyed areas. 

Since hibernacula are probably always deeper than 1 m, and 

we were not able to evaluate greater depths, we cannot draw 

any conclusions as to soil depth being a limiting factor.

The frequency distribution of mineral particle sizes on the 

surface was found to be almost identical between colonies and 

ghost polygons. However, there was nearly twice the amount 

of vegetative cover on colonies as opposed to ghost polygons. 

The significance of this observation in relation to prairie dog 

occupancy is unknown.

With respect to slope, aspect, and solar gain, we found 

that the mean angle and mean vectors were similar for all 

three sample sets (the base map colonies, ghost polygons, and 

currently active colonies). Mean angle of all polygons in the 

three groups was 160° on the base map colonies, 129° among 

the ghost polygons, and 121° among the currently active 

colonies. Mean vectors, which express the “evenness” of the 

dispersion of points around the compass, were 0.556, 0.446 

and 0.492, for the base map colonies, ghost polygons, and 

active colonies, respectively. A value of 1.0 represents uniform 

dispersion of the vectors on a 360° circle.

The orientation of active and former colonies was found 

to be exclusively to the northeast, southeast, and southwest 

quarters of the compass. This orientation avoids the prevailing 

winds but may also be related to solar flux. Plots of the energy 

distribution of the entire landscape in the study area, when 

compared to the energy distribution of active burrow sites, 

provide evidence that the prairie dogs may be selecting sites 

within a range of solar gain that differs from expected levels.

We found the slope variation to be very similar in the 

2001 colonies and the ghost polygons, but the slope variation 

on the currently occupied colonies was on average about half 

that of the other areas. This supports the hypothesis that evenness of slope may facilitate improved communications and 

predator detection (Wagner, 2002). It is unknown if the prairie 

Figure 2. Trend toward fewer active burrows and contracting colony boundaries is illustrated by white-tailed prairie dog 

(Cynomys leucurus) colony PDT 6A. Active burrows recorded by 

GPS in early summer 2003, overlaid on the colony PDT 6A, mapped 

in 2001.
White-tailed Prairie Dog Population Survey and Habitat Evaluation 277

dogs preferentially select more even terrain, or if those occupying such terrain are more successful at avoiding predation.

A similar study of the Gunnison’s prairie dog (C. gunnisoni) in Arizona was used to model habitat associations with 

the intent that the results might assist managers in estimating 

the suitability of unoccupied sites for possible prairie dog 

reintroduction (Wagner 2002). The results of our study may 

have similar applications for the white-tailed prairie dog.

References Cited

Biggins, D.E., Miller, B.J., Hanebury, L.R., Oakleaf, B., 

Farmer, A.H., Crete, R., and Dood., A., 1993, A technique 

for evaluating black-footed ferret habitat, in Oldemeyer, 

J.L., Biggins, D.E., Miller, B.J., and Crete, R., eds., Management of prairie dog complexes for the reintroduction of 

the black-footed ferret: Washington, D.C., U.S. Fish and 

Wildlife Service, Biological Report 13, p. 73–88.

Knowles, C., 2002, Status of white-tailed and Gunnison’s 

prairie dogs: Missoula, Mont., National Wildlife Federation, 

and Washington, D.C., Environmental Defense Fund, 30 p.

Lyon, A.G., and Anderson, S.H., 2003, Potential gas development impacts on sage grouse nest initiation and movement: 

Wildlife Society Bulletin, v. 31, p. 486–491.

Reid, M., Comer, P., Barrett, H., Caicco, S., Crawford, R., 

Jean, C., Jones, G., Kagan, J., Karl, M., Kittel, G., Lyon, P., 

Manning, M., Peterson, E., Rosentreter, R., Rust, S., Tart, 

D., Williams, C., and Winward, A., 2002, International classification of ecological communities—terrestrial vegetation 

of the United States. Sagebrush vegetation of the Western 

United States: Arlington, Va., NatureServe, Final report for 

the U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, Boise, Idaho, 23 p.

Wagner, D.M., 2002, Current status and habitat use of Gunnison’s prairie dogs (Cynomys gunnisoni) in Arizona: Flagstaff, Northern Arizona University, Ph.D. dissertation, 77 p.
Introduction

Plague has the potential to decimate prairie dog (Cynomys spp.) populations and is a threat to reintroduction of the 

black-footed ferret (Mustela nigripes), which requires large 

colonies of prairie dogs for food (Biggins and Godbey, 2003). 

Controlling flea (Insecta: Siphonaptera) populations on prairie 

dogs at reintroduction sites could decrease the risk of plague. 

Currently, fleas in prairie dog towns are controlled by using 

permethrin or deltamethrin dust (Beard and others, 1992; 

Seery, 2003). However, these compounds may be detrimental 

to nontarget arthropods.

Systemic insecticides, commonly used for flea control in 

veterinary medicine, might be useful in controlling flea populations on prairie dogs, but little use has been made of such 

compounds added to rodent bait for control of plague (Gratz, 

1999). Nitenpyram is an insecticide that has been used for the 

control of aphids, thrips, leafhoppers, whiteflies, and other 

suckling insects of rice and has also demonstrated effectiveness against the cat flea (Ctenocephalides felis) as a topical 

treatment (Tomlin, 2000; Moyses and Gfeller, 2001). Nitenpyram is also effective for systemic control of fleas on pets 

and is currently used in a commercial systemic flea treatment 

for dogs and cats called Capstar® (Novartis Animal Health, 

Greensboro, N.C.) (Rust and others, 2003). We investigated 

the efficacy of systemically delivered nitenpyram formulated 

at 560 mg/kg in rodent bait against a ground squirrel flea 

(Oropsylla montana), which readily transmits plague, on 

black-tailed prairie dogs (C. ludovicianus). 

Methods

Prairie dogs were captured in Larimer County, Colo., 

transported to Genesis Laboratories, Inc., and acclimated to 

laboratory conditions for 13 days. Medicated prairie dog bait 

was prepared by mixing 20 Capstar tablets (56 mg nitenpyram/

tablet) in palatable grains and powdered sugar (2 kg total). 

The medicated diet was presented to the prairie dogs for 48 

hours. Food consumption was measured daily. Artificial flea 

feeding apparatuses were constructed with centrifuge tubes, 

300 μm nylon mesh, and plumber’s glue to allow fleas to feed 

without being lost. Four to six fleas (O. montana) obtained 

from the Genesis Laboratories breeding colony were added to 

each flea feeder. The apparatuses were secured to prairie dogs 

for 24 hours with athletic tape on an area of shaved fur. The 

apparatuses were then removed, and the fleas were immediately evaluated for mortality and morbidity by placing them 

in a large white plastic container. By blowing on the fleas, 

we could ascertain mortality or morbidity by the presence or 

absence of movement. The process was repeated on day 3 after 

exposure for evaluation of residual effect.

Results

Bait containing 560 mg/kg nitenpyram was effective at 

producing mortality and morbidity in fleas at day 1 (table 1) 

and minimally at day 3. Day 1 flea mortality was 76.9 percent. 

Of the 11 fleas surviving, 5 (45.5 percent) were observed to 

be moribund. Day 3 flea mortality was 23.1 percent. Of the 

10 fleas surviving at day 3, 0 (0 percent) were observed to be 

moribund.

Discussion and Management 

Implications

Nitenpyram was initially effective at controlling O. 

montana fleas on black-tailed prairie dogs, causing greater 

than 70 percent mortality at day 1. Mortality of fleas declined 

to 23.1 percent by day 3. Observations in other ground 

squirrels (Spermophilus spp.) indicate that recolonization of 

hosts by fleas is very rapid, within 3 days (Ryckman, 1971). 

Because of the lack of residual action of systemically delivered nitenpyram, this compound would likely not be effective 

as a method of preventative plague control in prairie dogs. A 

systemically delivered insecticide with longer residual effect 

might be beneficial in a plague control program of this type.

In addition to their potential utility in controlling fleas on 

prairie dogs at black-footed ferret recovery sites, rodent baits 

containing insecticide might also be effective for preventative 

flea control in situations where rodents live close to humans, 

Can the Systemic Insecticide Nitenpyram Be Used for Flea 

Control on Black-tailed Prairie Dogs?

By Jeff N. Borchert1 and Jeff J. Mach1

1

Genesis Laboratories, Inc., P.O. Box 1195, Wellington, CO 80549.
Nitenpyram for Flea Control 279

but other rodent control methods are not acceptable to the 

public. For example, the City of Fort Collins, Colo., maintains 

approximately 2,000 acres of active black-tailed prairie dog 

colonies within the Urban Growth Area. Lethal control of 

these prairie dogs has been a contentious issue among Fort 

Collins residents (City of Fort Collins, 1998). Baits incorporating insecticides might be ideal in such a situation as potential 

risk to humans would likely decrease and the prairie dogs 

themselves would not be harmed.

References Cited

Beard, M.L., Rose, S.T., Barnes, A.M., and Montenieri, J.A., 

1992, Control of Oropsylla hirsute, a plague vector, by 

treatment of prairie dog burrows with 0.5% permethrin dust: 

Journal of Medical Entomology, v. 29, p. 25–29.

Biggins, D.E., and Godbey, J.L., 2003, Challenges to reestablishment of free-ranging populations of black-footed 

ferrets: Comptes Rendus Biologies, v. 326 (Suppl. 1), p. 

S104–S111.

Prairie dog 

number Sex

Active ingredient 

consumed (mg) Body weight (kg) Dose (mg/kg)

Number of 

fleas applied

Fleas dead/ fleas 

recovered 

Fleas moribund/

fleas recovered 

alive

Treatment

1 M 25.9 0.827 31.3 5 Not recovered

3 M 35.3 0.475 74.3 4 3/4 0/1

4 F 4.0 0.798 5.0 4 1/4 1/3

6 M 29.3 0.924 31.7 5 5/5 -

7 M 12.4 0.895 13.9 5 5/5 -

8 M 21.8 1.109 19.7 5 4/5 1/1

9 M 38.9 1.175 33.1 6 6/6 -

11 F 2.3 0.710 3.2 5 2/5 0/3

12 M 42.0 1.111 37.8 5 5/5 -

14 F 24.7 0.773 32.0 4 2/4 1/1

15 M 23.4 1.225 19.1 4 3/4 1/1

16 F 27.9 0.964 28.9 5 4/5 1/1

Mean 24.0 0.916 27.5 Total 40/52

(76.9%)

5/11

(45.5%)

Control

1 M - 0.775 - 4 0/4 -

2 M - 1.159 - 5 0/5 -

3 M - 0.655 - 5 1/5 -

4 M - 0.872 - 5 0/5 -

Mean - 0.865 - Total 1/19

(5.3%)

-

Table 1. The 24-hour efficacy of systemic nitenpyram on Oropsylla montana fed on black-tailed prairie dogs (Cynomys ludovicianus).

City of Fort Collins, 1998, Prairie dog policies for natural 

areas: Fort Collins, Colo., Prairie Dog Review Committee, 

Natural Resources Department, 24 p.

Gratz, N.G., 1999, Control of plague transmission, in Plague 

manual: Geneva, Switzerland, World Health Organization, 

p. 97–134.

Moyses, E.W., and Gfeller, F.J., 2001, Topical application as 

a method for comparing the effectiveness of insecticides 

against cat flea (Siphonaptera: Pulicidae): Journal of Medical Entomology, v. 38, p. 193–196.

Rust, M.K., Waggoner, M.M., Hinkle, N.C., Stansfield, D., 

and Barnett, S., 2003, Efficacy and longevity of nitenpyram 

against adult cat fleas (Siphonaptera: Pulicidae): Journal of 

Medical Entomology, v. 40, p. 678–681.

Ryckman, R.E., 1971, Plague vector studies 3. The rate 

deparasitized ground squirrels are reinfested with fleas 

under field conditions: Journal of Medical Entomology, v. 8, 

p. 668–670.
280 Recovery of the Black-footed Ferret

Seery, D.B., Biggins, D.E., Montenieri, J.A., Enscore, R.E., 

Tanda, D.T., and Gage, K.L., 2003, Treatment of blacktailed prairie dog burrows with deltamethrin to control fleas 

(Insecta: Siphonaptera) and plague: Journal of Medical 

Entomology, v. 40, p. 718–722.

Tomlin, C.D.S., 2000, The pesticide manual: Farnham, U.K., 

British Crop Protection Council, 1,344 p.
Fleas and Small Mammal Hosts Within and Adjacent 

to the Coyote Basin White-tailed Prairie Dog Colony in 

Northeastern Utah

By Lianna K. Etchberger,1 William E. Stroh,2 Brent D. Bibles,3 Matthew R. Dzialak,1 and Richard C. Etchberger1

1

Utah State University–Uintah Basin Campus, 1680 W. Hwy. 40, Vernal, 

UT 84078.

2

U.S. Forest Service, Ashley National Forest, 355 N. Vernal Ave., Vernal, 

UT 84078.

3

Colorado Division of Wildlife, 317 W. Prospect Rd., Fort Collins, CO 

80526.

Introduction

The endangered black-footed ferret (Mustela nigripes) 

and its prairie dog (Cynomys spp.) prey are susceptible to 

sylvatic plague, an infectious disease caused by the bacterium 

Yersinia pestis. Plague is a contagious disease transmitted by 

bites of vector fleas (Insecta: Siphonaptera), consumption of 

infected tissue, or contact with infected animals. Epizootics of 

plague are a threat to prairie dog and ferret populations at most 

reintroduction sites due to high mortality rates of both ferrets 

and prairie dogs. While much effort is currently focused 

on protecting these species from plague by flea control and 

vaccine development, little is understood about the reservoirs 

of this disease in the wild. Contributions to this understanding will lend insight for designing plague monitoring and 

transmission prevention protocols and for any effort to expand 

habitat for black-footed ferrets and prairie dogs.

Factors influencing the occurrence of plague epizootics 

are not fully understood (reviewed in Gage and Kosoy, 2005). 

In the arid Southwest, for example, favorable climate conditions correlate with plague epizootics (Parmenter and others, 

1999; Enscore, 2002). While other models exist, there is 

speculation that climate may correlate with epizootics through 

increased rodent host populations (Parmenter and others, 

1999; Enscore and others, 2002; Stapp and others, 2004). High 

population density increases the likelihood of transmission of 

a contagious disease once the pathogen is introduced to the 

susceptible population (Madigan and others, 2000). Population 

growth can also lead to expansion into adjacent areas atypical of short grassland prairie dog habitat. Indeed, active and 

inactive burrows have been found in dense sagebrush and on 

hillsides in a white-tailed prairie dog (C. leucurus) colony in 

Coyote Basin in northeastern Utah (L. Etchberger, personal 

observation, 1999). White-tailed prairie dog expansion into 

new habitat may therefore result in increased exposure to 

one or more flea or small mammal host species that are more 

likely to transmit plague to the population. This increased 

exposure may be caused by either overlapping ranges between 

the prairie dogs and flea vector or reservoir host or by expansion into an area with increased frequency of either the vector 

or the reservoir. While different flea species have different 

biological transmission rates in laboratory settings, mechanical 

transmission has also been observed (reviewed in Gage and 

Kosoy, 2005), suggesting that most flea species may be potential candidates for transmission in the wild. In this study, we 

present preliminary analyses of data comparing small mammal 

hosts and their flea species collected from white-tailed prairie 

dog habitat to those collected from adjacent habitat with the 

potential for prairie dog expansion. Patterns of flea species 

associations with host or habitat may help identify potential 

plague reservoirs. 

Methods

We used field data on prairie dog distribution in Coyote 

Basin, Utah, and a geographic information system (ArcView®; 

Environmental Systems Research Institute, Inc., Redlands, 

Calif.) to delineate habitat occupied by white-tailed prairie 

dogs (PD) and adjacent habitat that was not occupied by 

prairie dogs (NOPD). Occupied habitat included a 50-m buffer 

adjacent to the colony perimeter; all habitat delineated as 

NOPD was at least 50 m from the colony boundary. We established small mammal trapping grids within PD and NOPD 

areas. To select specific grid locations, we used ArcView to 

generate 20 random points in each area, and we surveyed these 

locations systematically. Upon identifying a location in each 

area that we considered appropriate logistically, we selected 

that location as the southwest corner of a trapping grid. Grids 

consisted of 100 Sherman live traps placed 10 m apart in a 

10 × 10 array. Grids were revisited for two to four trapping 
282 Recovery of the Black-footed Ferret

sessions each summer. Small mammals were trapped for two 

consecutive nights on the four grids (800 trap nights) during 

each session. Animals trapped on the second night were 

processed by determining weight and sex and combing for 

fleas after anesthesia with isoflurane. We restricted processing 

to animals trapped on the second night to facilitate objectives 

of additional research not presented here. Fleas were collected 

in a tube with saline/Tween detergent solution, pooled for each 

individual host animal, and sent to the Bacterial Zoonoses 

Branch, Centers for Disease Control and Prevention (CDC) in 

Fort Collins, Colo., for species identification. We used Fisher’s 

exact test to evaluate differences in the occurrence of small 

mammal species, flea species, or flea host species distributed 

between PD and NOPD habitats. Significance was set at P ≤

0.05. 

Results

A preliminary analysis of pooled data collected between 

the years 2000 and 2004 is reported here. Spatial and temporal 

analyses will be reported elsewhere.

Small mammal species and their distribution between the 

PD and NOPD locations are shown in table 1. The majority (96 percent) of small mammals trapped were deer mice 

(Peromyscus maniculatus). We also trapped Ord’s kangaroo 

rats (Dipodomys ordii) and piñon mice (P. truei) along with 

one northern grasshopper mouse (Onychomys leucogaster) and 

one thirteen-lined ground squirrel (Spermophilus tridecemlineatus). Of the 822 small mammals trapped, 405 were trapped 

in PD grids and 417 were trapped in NOPD grids. Differences 

in the occurrence of small mammal species between PD and 

NOPD habitats were significant (P = 0.04), largely reflecting 

the differences in abundance of Ord’s kangaroo rats between 

sites.

Flea species and their distribution between the PD and 

NOPD locations are shown in table 1. The majority (86 

percent) of fleas collected were Aetheca wagneri; Meringus

sp., Epitidea wemmani, Amaradix euphorbi, and Orchopeas 

sexdentatus were also collected. There was some concern 

at the CDC as to whether the O. sexdentatus identification 

was correct because this species is not normally associated 

with the deer mouse host on which it was found. Of the 299 

fleas collected, 145 were collected from small mammal hosts 

trapped in PD grids, and 154 were from NOPD grids. The 

difference in the occurrence of flea species between PD and 

NOPD habitats was significant (P < 0.01), reflecting the 

increased occurrence of Meringus sp. and E. wemmani in 

NOPD locations.

All fleas collected were from deer mice and Ord’s 

kangaroo rats. The flea/host relationships and distributions are 

shown in table 2. Numbers represent host species infested with 

each flea species. The majority of infested hosts were deer 

mice with A. wagneri fleas. There was no statistical difference in the flea/host relationships between the PD and NOPD 

habitats; the occurrence of deer mice, Ord’s kangaroo rats, and 

total number of hosts infested with various flea species did 

not vary between the two habitats (P = 0.27, 0.29, and 0.44, 

respectively).

Discussion

Small mammals and their fleas were collected in the 

white-tailed prairie dog colony of Coyote Basin, Utah, for the 

 PD NOPD

Species 

total

Small mammals

Deer mouse

(Peromyscus maniculatus)

Ord’s kangaroo rat

(Dipodomys ordii)

Piñon mouse

(P. truei)

Northern grasshopper mouse

(Onychomys leucogaster)

Thirteen-lined ground squirrel

(Spermophilus tridecemlineatus)

396

5

3

1

0

396

17

3

0

1

792

22

6

1

1

Total 405 417 822

Fleas

Aetheca wagneri

Meringus sp.

Epitidea wemmani

Amaradix euphorbi

Orchopeas sexdentatus

136

2

2

5

0

122

17

10

3

2

258

19

12

8

2

Total 145 154 299

Table 2. Numbers of small mammals infested with fleas from 

habitats occupied (PD) and not occupied (NOPD) by white-tailed 

prairie dogs (Cynomys leucurus).

Table 1. Distribution of small mammals and fleas from habitats 

occupied (PD) and not occupied (NOPD) by white-tailed prairie 

dogs (Cynomys leucurus).

Deer mouse

(Peromyscus maniculatus)

Ord’s kangaroo rat

(Dipodomys ordii)

PD NOPD Total PD NOPD Total

Aetheca wagneri 70 63 133 1 0 1

Meringus sp. 1 1 2 1 5 6

Epitidea 

wemmani

2 7 9 0 0 0

Amaradix 

euphorbi

2 1 3 0 0 0

Orchopeas 

sexdentatus

0 1 1 0 0 0

Total infested 

hosts

75 73 148 2 5 7
Fleas and Small Mammal Hosts 283

past five seasons. By returning to the same grid locations every 

year in this ongoing study, we have boosted our small sample 

sizes to better detect patterns in species composition and 

abundance. By comparison to prairie dog-occupied habitat, we 

observed that Ord’s kangaroo rats and E. wemmani and Meringus sp. fleas were more abundant outside the colony boundary. 

Whereas deer mice were equally distributed, those infested 

with E. wemmani occurred more often in habitat outside of 

the colony. Also, Meringus sp. was found on both deer mice 

and Ord’s kangaroo rats, an interesting observation since 

many flea species associate with only one host species. The 

preliminary observation that these flea and host species occur 

more frequently outside but near the Coyote Basin white-tailed 

prairie dog colony may have implications in plague transmission to prairie dogs and black-footed ferrets from an unidentified reservoir.

References Cited 

Enscore, R.E., Biggerstaff, B.J., Brown, T.L., Fulgham, R.F., 

Reynolds, P.J., Engelthaler, D.M., Levy, C.E., Parmenter, 

R.R., Montenieri, J.A., Cheek, J.E., Grinnell, R.K., Ettestad, 

P.J., and Gage, K.L., 2002, Modeling relationships between 

climate and the frequency of human plague cases in the 

southwestern United States, 1960–1997: American Journal 

of Tropical Medicine and Hygiene, v. 66, p. 186–196.

Gage, K.L., and Kosoy, M.Y., 2005, Natural history of 

plague—perspectives from more than a century of research: 

Annual Review of Entomology, v. 50, p. 505–528.

Madigan, M.T., Martinko, J.M., and Parker, J., 2000, Brock 

biology of microorganisms (9th ed.): Upper Saddle River, 

N.J., Prentice-Hall, Inc., 991 p.

Parmenter, R.R., Yadav, E.P., Parmenter, C.A., Ettestad, P., 

and Gage, K.L., 1999, Incidence of plague associated with 

increased winter-spring precipitation in New Mexico: 

American Journal of Tropical Medicine and Hygiene, v. 61, 

p. 814–821.

Stapp, P., Antolin, M.F., and Ball, M., 2004, Patterns of extinction in prairie dog metapopulations—plague outbreaks follow El Niño events: Frontiers in Ecology and the Environment, v. 2, p. 235–240.
Conservation biology and animal behavior are two fields 

of science that can complement one another. Animal behavior 

research is important for understanding the complex needs 

of a species to be managed or restored to its native range and 

can be a critical part of the foundation for preservation of a 

species. Black-footed ferrets (Mustela nigripes) are endangered, nocturnal carnivores native to the North American 

prairie ecosystem. Siberian polecats (M. eversmannii) are 

their closest living relative and inhabit the steppe grasslands 

of Asia and Eastern Europe. Polecats were used as a surrogate 

species for behavioral studies in the initial stages of restoration 

attempts for the black-footed ferret. In previous studies, ferrets 

and polecats were highly susceptible to predation (Biggins, 

2000). In this study, we hypothesized that ferrets and polecats 

would react to moonlight similarly to small nocturnal mammalian prey species that decrease activity and increase use of 

cover with increasing moonlight to avoid predation (Kavanau, 

1969; Clarke, 1983; Falkenberg and Clarke, 1998; Zollner 

and Lima, 1999). We investigated the effects of moonlight 

on nocturnal cover usage and spatial learning abilities of 

black-footed ferrets. Multivariate general linear models with a 

repeated measures design were used to analyze data with P = 

0.05 chosen as the significance level.

We tested cover usage by black-footed ferrets (n = 8) in 

an indoor chamber (7 m2) under simulated new (0.05 lux), half 

(0.35 lux), and full (2.2 lux) moonlight levels. We measured 

use of cover (edge, burrows) and open areas. We detected no 

effect of moonlight level on use of cover versus open space 

for black-footed ferrets. Free-ranging ferrets and polecats 

studied previously increased their aboveground activity and 

movements with increasing moonlight levels, and black-footed 

ferret activity was low during primary activity periods of their 

principal predators, regardless of moonlight levels (Biggins, 

2000). Energetic demands of ferrets may not allow moonlight 

to be a principal determinant of activity even if they prefer 

certain light levels. Also, light may be beneficial for spatial 

learning of home ranges, finding burrows, and locating prey 

or mates.

Spatial learning refers to the ability to remember the 

location of key features in one’s environment (Gaulin and 

Fitzgerald, 1989; Lavenex and Schenk, 1998). Ferrets may use 

moonlight to examine their surroundings. We tested blackfooted ferret spatial learning abilities (as indexed by distance 

traveled before the subject found a goal in a faux burrow) in a 

hexagonal indoor chamber (9 m2) in new, half, and full moonlight levels. The ferrets typically stayed close to the walls 

of the arena during trials, a behavior known as thigmotaxy. 

Black-footed ferrets seemed to learn, but moonlight levels 

appeared to have no effect on that process. Polecats tested in 

another study that used similar methods (Sheffer, 2001) exhibited spatial learning abilities that appeared to be enhanced in 

half moonlight. Black-footed ferrets may be more nocturnally 

adapted than polecats (Biggins, 2000; Sheffer, 2001). Both 

species traveled less with successive spatial learning trials, 

suggesting that they either learned the goal location or the 

ritual for the test (fig. 1). Black-footed ferrets did not decrease 

the distance traveled to locate the goal in full moonlight; there 

was no evidence for a positive correlation between spatial 

learning and light level. Overall, black-footed ferrets traveled 

shorter distances than did polecats (fig. 1). Learning abilities of both species should be examined further to determine 

how cage rearing might affect spatial learning skills (e.g., 

Biggins and others, 1998). If these skills can be lost or fail 

1

School of Biological Sciences, University of Northern Colorado, Ross 

Hall, Greeley, CO 80639.

2

Current address: U.S. Fish and Wildlife Service, 6010 Hidden Valley Rd., 

Carlsbad, CA 92011.

3

U.S. Geological Survey, Fort Collins Science Center, 2150 Centre Ave., 

Bldg. C, Fort Collins, CO 80526.

Effects of Moonlight on Cover Usage and Spatial Learning 

of Black-footed Ferrets

By Samantha N. Marcum,1,2 Dean E. Biggins,3 and Jennifer A. Clarke1

Figure 1. Mean distance traveled (m) by black-footed ferrets 

(Mustela nigripes) and Siberian polecats (M. eversmannii) during 

15 consecutive trials.
Effects of Moonlight on Cover Usage and Spatial Learning 285

to develop without appropriate stimuli, then modifications in 

captive breeding facilities may be necessary. Understanding 

these aspects of ferret behavior may be critical to conservation efforts for the species, particularly the success of captive 

breeding programs and species restoration. For example, 

better understanding of ferret behaviors under varying light 

levels may lead to increased efficiency in searching for 

ferrets (Biggins, Godbey, Matchett, and others, this volume) 

and improved interpretations of both energetic relationships 

(Harrington and others, this volume) and interactions with 

other predators (Breck and others, this volume).

References Cited

Biggins, D.E., 2000, Predation on black-footed ferrets (Mustela nigripes) and Siberian polecats (M. eversmannii)—conservation and evolutionary implications: Fort Collins, 

Colorado State University, Ph.D. dissertation, 185 p.

Biggins, D.E., Godbey, J.L., Hanebury, L.R., Luce, B., Marinari, P., Matchett, M.R., and Vargas, A., 1998, The effect of 

rearing methods on survival of reintroduced black-footed 

ferrets: Journal of Wildlife Management, v. 62, p. 643–653.

Clarke, J.A., 1983, Moonlight’s influence of predator/prey 

interactions between short-eared owls (Asio flammeus) and 

deermice (Peromyscus maniculatus): Behavioral Ecology 

and Sociobiology, v. 13, p. 205–209.

Falkenberg, J.C., and Clarke, J.A., 1998, Microhabitat use of 

deer mice: effects of inter-specific interaction risks: Journal 

of Mammalogy, v. 79, p. 558–565.

Gaulin, S., and Fitzgerald, R., 1989, Sexual selection for 

spatial-learning ability: Animal Behaviour, v. 37, p. 322–

331.

Kavanau, J.L., 1969, Influences of light on activity of small 

mammals: Ecology, v. 50, p. 548–557.

Lavenex, P., and Schenk, F., 1998, Olfactory traces and spatial 

learning in rats: Animal Behaviour, v. 56, p. 1129–1136.

Sheffer, K.H., 2001, The effect of moonlight on spatial learning in steppe ferrets: Greeley, University of Northern Colorado, M.S. thesis, 48 p.

Zollner, P.A., and Lima, S.L., 1999, Illumination and the 

perception of remote habitat patches by white-footed mice: 

Animal Behaviour, v. 58, p. 489–500.
Introduction

Several models have been developed to estimate prey 

requirements and to assess habitat suitability of release sites 

for the black-footed ferret (Mustela nigripes) (e.g., Stromberg and others, 1983; Powell and others, 1985; Biggins and 

others, 1993). None of these models, however, addressed 

possible differences in energetic requirements between sites 

due to climatic differences within the ferret’s historical range. 

We used a simplified energetics model to examine the effect 

of variation in environmental conditions on ferret energetic 

requirements. The aim of the study was to determine whether 

the ferret might be more successful in one area than another.

The Model

The total daily energy expenditure (EDEE) of any mammal 

can be conceptualized as the sum of all mutually exclusive 

sources of energy expenditure (Ex) (Wunder, 1975; Powell and 

others, 1985). For a nonreproductive, fully grown adult, EDEE

can be modeled in the general form:

EDEE = Es + Ea + Et 

where Es is the energy cost of resting; Ea is the energy cost of 

activity, including, in this case, running (Er), digging (Ed), and 

standing (Est) (Powell and others, 1985); and Et is the energy 

cost of thermoregulation. We included thermoregulatory 

costs below the animal’s lower critical temperature (TLC) only 

and divided this into the cost above ground (Eta) and below 

ground (Etu). The inclusion of thermoregulation in the model 

was conditional upon Ta input. We estimated the total energy 

requirements of the animal for one day (in kJ) as: 

EDEE = Es + Er + Ed + Est + [if Taa < TLC] Eta + [if Tau < TLC] Etu

where Ei is estimated as Mi x ti (Mi is the energetic cost of 

activity i in kJ per hour; ti is the time spent in activity i in 

hours per day), Taa is the ambient temperature above ground, 

and Tau is the temperature within the burrow (details in 

Harrington, 2001). 

Model parameter estimates were from the literature, 

with empirical data on black-footed ferret metabolism from 

Harrington (2001) and Harrington and others (2003) and site 

temperature data (Taa) from meteorological records.

Model Simulations

For three hypothetical sites in the extreme north, south, 

and middle of the ferret’s historical range, the model was run 

for 11 different activity scenarios ranging from complete rest 

within burrows to 5 hours active above ground (activity data 

from Powell and others, 1985). For each model run, Taa was 

chosen at random from a hypothesized normal distribution 

approximating nighttime temperature for each site in summer 

and winter. Tau was chosen at random from a range of values 

from the literature for summer and winter (same for all sites). 

Means and variances were based on 100 runs of the model for 

each of the 11 activity scenarios, for each site, in winter and 

summer.

Results

Assuming all activity scenarios are carried out at all 

sites in winter and summer, the model predicted higher 

energy requirements in the north than in the south in winter. 

In summer, energy requirements were predicted to be lower 

in the south than in the middle of the ferret’s range. All other 

comparisons were nonsignificant. In all cases, variability 

within a site and season was high due to the inclusion of all 

possible activity scenarios in the simulations. Separating the 

analysis into low, medium, and high activity levels revealed 

that although trends tended to be similar (higher in the north 

than in the south), differences between sites were greatest at 

high activity levels and during winter. For resting ferrets, no 

differences between sites were detected; this was, however, an 

artifact of the model resulting from the use of a constant value 

Modeling Black-footed Ferret Energetics: Are Southern 

Release Sites Better?

By Lauren A. Harrington,1,2 Dean E. Biggins,3 and A. William Alldredge1

1

Department of Fishery and Wildlife Biology, Colorado State University, 

Fort Collins, CO 80523.

2

Current address: Wildlife Conservation Research Unit, Department of 

Zoology, University of Oxford, Tubney House, Abingdon Road, Tubney, 

Abingdon OX13 5QL, UK.

3

U.S. Geological Survey, Fort Collins Science Center, 2150 Centre Ave., 

Bldg. C, Fort Collins, CO 80526.
Modeling Black-footed Ferret Energetics 287

for Tau. If burrow temperatures do vary between sites, overall 

intersite differences are likely to be greater. 

Discussion

Although our model predicted statistically significant 

differences in energy requirements between northern and 

southern sites, these differences were small (<100 kJ per day 

between sites or about 11 percent of total mean expenditure 

during winter) and would require only small increases in prey 

consumption (one black-tailed prairie dog [Cynomys ludovicianus], the ferret’s main prey, provides between 4,000 and 

5,000 metabolizable kJ of energy; Powell and others, 1985). 

More biologically meaningful differences were found in 

consideration of energetic limits.

If maximum sustained metabolic rates for ferrets are 

limited at five times the basal metabolic rate (as they are for 

most other animals; Hammond and Diamond, 1997), maximum daily energy expenditure may be limited to approximately 1200 kJ per day, or less. Plotting predicted energy 

required per day in relation to above ground temperature 

demonstrated that, on this basis, high activity levels may be 

prohibitive at temperatures below -35oC (fig. 1). Although 

ferrets have been observed above ground at temperatures as 

low as -40oC (Richardson and others, 1987), it is not known 

how long they can stay above ground at such extremes. Ferret 

movements are shorter in colder temperatures; on the coldest 

days, ferrets simply may not be able to remain above ground. 

Ferret movements in late winter are principally for mating 

(Richardson and others, 1987); thus, restricted activity at this 

time could adversely affect reproductive potential.

Management Implications and 

Questions Remaining

This study does not provide definitive answers regarding 

the effect of climatic variability on ferret energy requirements. 

It does suggest, however, that ferret energetics and climate 

may be important factors to consider in evaluating potential 

release sites. If ferrets are to be successfully reintroduced into 

the wild, management plans should seek to minimize sources 

of stress to the extent possible. Winter energy requirements 

may be reduced by selecting more southerly reintroduction 

sites. As with all models, our predictions will require field 

validation. Questions remaining include (but are not limited 

to) the following. Is water stress greater in the south? How 

much do burrow temperatures vary between sites (and can 

ferrets manipulate their own burrow temperature by selecting 

depth)? How does ferret activity vary throughout their range 

(and in response to climate)?

References Cited

Biggins, D.E., Miller, B.J., Hanebury, L.R., Oakleaf, B., 

Farmer, A.H., Crete, R., and Dood, A., 1993, A technique 

for evaluating black-footed ferret habitat, in Oldemeyer, 

J.L., Biggins, D.E., Miller, B.J., and Crete, R., eds., Management of prairie dog complexes for the reintroduction of 

the black-footed ferret: Washington, D.C., U.S. Fish and 

Wildlife Service, Biological Report 13, p. 73–88.

Hammond, K.A., and Diamond, J., 1997, Maximal sustained 

energy budgets in humans and animals: Nature, v. 386, p. 

457–462.

Harrington, L.A., 2001, Application of an energetics model 

to evaluate release-site potential for the black-footed ferret 

(Mustela nigripes): Fort Collins, Colorado State University, 

M.S. thesis, 126 p.

Harrington, L.A., Biggins, D.E., and Alldredge, A.W., 2003, 

Basal metabolism of the black-footed ferret (Mustela 

nigripes) and the Siberian polecat (M. eversmannii): Journal 

of Mammalogy, v. 84, p. 497–504.

Powell, R.A., Clark, T.W., Richardson, L., and Forrest, S.C., 

1985, Black-footed ferret Mustela nigripes energy expenditure and prey requirements: Biological Conservation, v. 34, 

p. 1–15.

Richardson, L., Clark, T.W., Forrest, S.C., and Campbell, 

T.M., III, 1987, Winter ecology of black-footed ferrets 

Figure 1. Predicted daily energy requirements for a black-footed 

ferret (Mustela nigripes) in winter in relation to aboveground temperature and amount of time spent above ground. Bracketing lines 

depict the range of winter temperatures for example sites. Activity 

was modeled to include mostly running with some time spent 

standing; Tau was set at 7.5oC (midpoint of the winter range).
288 Recovery of the Black-footed Ferret

(Mustela nigripes) at Meeteetse, Wyoming: American Midland Naturalist, v. 117, p. 225–239.

Stromberg, M.R., Rayburn, R.L., and Clark, T.W., 1983, 

Black-footed ferret prey requirements—an energy balance 

estimate: Journal of Wildlife Management, v. 47, p. 67–73.

Wunder, B., 1975, A model for estimating metabolic rate of 

active or resting mammals: Journal of Theoretical Biology, 

v. 49, p. 345–354.
Roelle and others, eds.—Recovery of the Black-footed Ferret: Progress and Continuing Challenges—U.S. Geological Survey Scientific Investigations Report 2005-5293

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