U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010-5125

Demography of the Pryor Mountain Wild Horses, 

1993–2007
Front and back cover photographs. Jason I. Ransom, U.S. Geological Survey.
Demography of the Pryor Mountain Wild Horses, 

1993–2007

By James E. Roelle, Francis J. Singer, Linda C. Zeigenfuss, Jason I. Ransom, 

Linda Coates-Markle, and Kathryn A. Schoenecker

Scientific Investigations Report 2010-5125

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

For more information on the USGS—the Federal source for science about the Earth,

its natural and living resources, natural hazards, and the environment:

World Wide Web: http://www.usgs.gov

Telephone: 1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.

Suggested citation:

Roelle, J.E., Singer, F.J., Zeigenfuss, L.C., Ransom, J.I., Coates-Markle, L., and Schoenecker, K.A., 2010, 

Demography of the Pryor Mountain wild horses, 1993–2007: U.S. Geological Survey Scientific Investigations 

Report 2010-5125, 31 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Study Area.......................................................................................................................................................2

Herd Behavior and Management.......................................................................................................4

Methods...........................................................................................................................................................5

Field Data Collection.............................................................................................................................5

Analysis ..................................................................................................................................................5

Results .............................................................................................................................................................6

Population Size and Growth................................................................................................................6

Removals................................................................................................................................................7

Sex and Age Structure.........................................................................................................................7

Survival...................................................................................................................................................8

Foaling Rates .......................................................................................................................................10

Harem Stallions and Band Composition .........................................................................................13

Bachelors.............................................................................................................................................16

Population Regulation........................................................................................................................16

Density Dependence.................................................................................................................16

Weather.......................................................................................................................................17

Discussion.....................................................................................................................................................17

Population Size and Growth..............................................................................................................17

Sex and Age Ratios.............................................................................................................................19

Survival.................................................................................................................................................20

Foaling Rates .......................................................................................................................................22

Harem Stallions and Band Composition .........................................................................................24

Bachelors.............................................................................................................................................25

Population Regulation........................................................................................................................25

Acknowledgments.......................................................................................................................................25

References Cited..........................................................................................................................................27
Figures

1. Map showing location of the Pryor Mountain Wild Horse Range and study area............3

2-14. Graphs showing:

2. Trend in growth rate of the Pryor Mountain wild horse (Equus caballus) population,

1993–2007 .......................................................................................................................................7

3. Sex ratio of wild horse (Equus caballus) foals born in the Pryor Mountain herd,

1993–2007 .......................................................................................................................................8

4. Variation in the sex ratio of wild horse (Equus caballus) foals by age of dam in the

Pryor Mountain herd, 1993–2007................................................................................................8

5. Annual sex ratio of adult (≥1 year old) wild horses (Equus caballus) in the Pryor

Mountain herd, 1993–2007...........................................................................................................8

6. Average age of (A) male and (B) female wild horses (Equus caballus) in the Pryor

Mountain herd, 1993–2007...........................................................................................................9

7. Trend in annual survival of Pryor Mountain wild horses (Equus caballus) for adults

and foals, 1993–2007...................................................................................................................11

8. Age of wild horse (Equus caballus) stallions holding harems in the Pryor Mountain

herd, 1996–2007...........................................................................................................................14

9. Age at which wild horse (Equus caballus) stallions first held a harem in the Pryor

Mountain herd, 1996–2007.........................................................................................................15

10. Frequency distribution of wild horse (Equus caballus) harem size in the Pryor

Mountain herd, 1996–2007.........................................................................................................15

11. Mean number of wild horse (Equus caballus) mares held relative to age of harem

stallions in the Pryor Mountain herd, 1996–2007...................................................................15

12. Number of bachelor wild horses (Equus caballus) in the Pryor Mountain herd,

1996–2007 .....................................................................................................................................16

13. Age distribution of bachelor wild horses (Equus caballus) in the Pryor Mountain

herd, 1996–2007...........................................................................................................................17

14. Relation of (A) survival and (B) population growth rate to the previous year’s

population for wild horses (Equus caballus) in the Pryor Mountain herd.........................18
v

Tables

1. Size and growth rate of the Pryor Mountain wild horse (Equus caballus) herd, 

1993–2007 .......................................................................................................................................7

2. Removals of wild horses (Equus caballus) from the Pryor Mountain herd, 1993–2007 ....8

3. Annual survival rates of Pryor Mountain wild horses (Equus caballus), 1993–2007.......10

4. Age-specific survival rates of Pryor Mountain wild horses (Equus caballus), 

1993–2007 .....................................................................................................................................11

5. Annual foaling rates of Pryor Mountain wild horse (Equus caballus) mares, 

1993–2007 .....................................................................................................................................12

6. Total foals born to Pryor Mountain wild horse (Equus caballus) mares, 1993–2007 ......12

7. Age-specific fecundity of Pryor Mountain wild horse (Equus caballus) mares,

1993–2007 .....................................................................................................................................13

8. Number of wild horse (Equus caballus) bands, mean age of harem stallions, mean

harem size (mares ≥2 years old), and mean band size (all members) in the Pryor

Mountain herd, 1996–2007.........................................................................................................14

9. Results of stepwise multiple regressions between wild horse (Equus caballus) herd

survival, foal survival, and foaling rate (mares ≥3 years of age) and various

weather parameters for the Pryor Mountain herd................................................................19

10. Survival rates reported for North American wild horse (Equus caballus) herds ............21

11. Foaling rates reported for North American wild horse (Equus caballus) herds..............23
vi

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

yard (yd) 0.9144 meter (m)

Area

acre 0.4047 hectare (ha)

square mile (mi2

) 259.0 hectare (ha)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

 °F=(1.8×°C)+32

Acronyms Used in Report

AML Appropriate Management Level

APHIS Animal and Plant Health Inspection Service

BLM Bureau of Land Management

FS Forest Service

HMA Herd Management Area

NCDC National Climatic Data Center

NPS National Park Service

PMWHR Pryor Mountain Wild Horse Range

RAWS Remote Automated Weather Station

USGS U.S. Geological Survey
Demography of the Pryor Mountain Wild Horses, 

1993–2007

By James E. Roelle,1 Francis J. Singer,1,2 Linda C. Zeigenfuss,1 Jason I. Ransom,1 Linda Coates-Markle,3 and 

Kathryn A. Schoenecker1

1

 U.S. Geological Survey, Fort Collins, Colorado.

2

 Deceased.

3

 Bureau of Land Management, Wheat Ridge, Colorado.

Abstract

Wild horses (Equus caballus) at Pryor Mountain were 

studied by direct observation from 1993 through 2007. All 

horses present were individually identifiable on the basis of 

coat coloration, head and leg markings, gender, and band 

associations. Of the 609 horses either present prior to foaling in 1993 or born since, ages were precisely known for 

491 (observed as a foal). Ages for 52 horses were estimated 

through tooth eruption and wear patterns, and for the remaining 66 horses through body size, morphology, and anecdotal 

evidence concerning when they were present on the range. At 

varying intensities, never less than 30 days per year, all horses 

were inventoried and their band associations noted. Foals were 

paired with dams based on observations of attachment during 

the early days and weeks of life. Year of death was determined by identification of the carcass where possible. In the 

absence of finding a carcass, an animal that was not observed 

for 2 years was considered to have died in the year that it 

went missing. Animals that were removed from the herd and 

mares that were part of a contraception study were excluded 

from calculations of survival and foaling rates, respectively, as 

appropriate.

The average prefoaling population over the 15 years 

of the study was 148.8 animals (range = 120–187), and the 

annual foal crop averaged 32.1 (range = 23–40). Large removals (19–60 animals) in four years helped maintain the herd at 

this level; apparent growth rate (calculated as though removals 

had not occurred) was 9.6 percent annually (λ = 1.096, range 

= 0.977–1.220). This annual growth rate is relatively low 

compared to that for many western horse herds, at least in part 

because of a decline in foal survival. Sex ratio of the foal crop 

varied widely among years, but pooled across years did not 

differ from 50:50. Sex ratio in the herd changed mostly as a 

result of removals. The average age of both males and females 

in the herd increased during the course of the study. Annual 

survival of males did not differ from that of females, nor did 

gender affect annual survival of foals. Pooled across years, 

ages, and sexes, the annual survival rate was 0.899. Annual 

foal survival rate was 0.697 and declined through time, with 

a tendency toward recovery in 2005–2007. Foal survival was 

higher in larger bands, but did not differ between foals born to 

primiparous and multiparous mares. A few 2-year-old mares 

produced foals; foaling rate (excluding contracepted mares 

and foals they produced) increased through age 10, remained 

high through age 15, and declined thereafter. Overall foaling 

rate for mares ≥3 years of age was 0.576 foals per mare, with 

no apparent trend during the period of our study. Foaling rate 

in years following gathers was somewhat lower than in other 

years. There was a positive relation between foaling rate and 

band size. Primiparous mares were somewhat less likely to 

foal in the following year than were multiparous mares. Most 

stallions that acquired a harem did so at age 5 or 6, and the 

average age of harem stallions increased during our study. 

Most harems had 1–3 mares ≥2 years of age, but harem size 

varied with age of the stallion, increasing through about age 

11 and declining thereafter. About 6 percent of bands had a 

satellite stallion (≥5 years of age), but the mean number of 

mares did not differ between single- and multistallion bands. 

Most stallions left their natal band at age 2 or 3, but 17 percent 

remained with their natal band until age 4 or 5.

Foal survival rate was positively related to precipitation, 

suggesting a possible link to forage production and availability 

mediated through mare fitness. There also was evidence for 

density-dependent population regulation, as both population 

growth rate and survival rate were negatively correlated with 

population size from the previous year. These and other factors were not sufficient to stabilize the population during our 

period of study, however, as evidenced by the necessity for 

large removals in several years.

Introduction

The Pryor Mountain Wild Horse Range was created 

in 1968 by order of the Secretary, U.S. Department of the 

Interior, who directed that wild horses (Equus caballus) be 

managed in “a balanced program which considers all public values and without impairment of the productivity of the 

land” (Federal Register, Document 68-11056, Sept. 11, 1969; 
Bureau of Land Management, 2006, p. 5). The Bureau of Land 

Management (BLM) was designated as the lead agency for 

management of wild horses, though the range also contains 

lands administered by the U.S. Forest Service (FS; Custer 

National Forest) and the National Park Service (NPS; Bighorn 

Canyon National Recreation Area). The Wild Free-Roaming 

Horses and Burros Act of 1971 (P.L. 92-195) further directed 

the FS and BLM to manage wild horses at Pryor Mountain and 

wild horses and burros (Equus asinus) wherever they occur “in 

a manner that is designed to achieve a thriving natural ecological balance on the public lands.”

While laudable as general management goals, the conditions implied by phrases such as “a balanced program which 

considers all public values without impairment of the productivity of the land” and “a thriving natural ecological balance” 

have proven difficult to achieve for many wild horse herds. 

With the protection afforded by the Act and a dearth of natural 

predators, many wild horse herds can increase at annual rates 

of 15–25 percent (Eberhardt and others, 1982; Garrott and 

Taylor, 1990; Garrott and others, 1991b; Singer and others, 

2000), thus threatening to exceed the carrying capacity of 

their ranges. BLM has addressed this problem largely through 

a gather and removal program in which excess horses are 

removed from the range and offered to the general public for 

adoption or sale to good homes. While hugely successful by 

some measures (BLM has adopted out more than 220,000 

horses since 1971; http://www.blm.gov/wo/st/en/prog/wild_

horse_and_burro/wh_b_information_center/Fact_Sheet.html), 

supply has exceeded demand, leaving nearly 32,000 horses in 

BLM corrals and long-term holding pastures. This approach 

is not embraced by all members of the public. Wild horse 

advocacy groups often favor a policy of minimum disturbance 

and intervention, arguing that there are too few, rather than too 

many, wild horses and that the gather and removal process is 

detrimental to horse health and well-being. On the other hand, 

ranchers and conservationists often see wild horses as competing with domestic cattle (Bos taurus) and native wildlife for 

forage, and would thus prefer that BLM reduce the size of 

many horse herds.

Given the contentious nature of the issues involved, 

BLM has a continuing need for the best scientific information available on the ecology and demography of wild horse 

herds. Since creation of the range in 1968, information on 

wild horses has been gathered nearly continuously by trained 

investigators and volunteers, making the Pryor Mountain 

horses perhaps the most studied herd in the Western United 

States. Furthermore, all of the horses at Pryor Mountain are 

individually identifiable (unique color markings), and many 

are of known (observed as a foal) or closely estimated (tooth 

eruption and wear patterns) age, making the resulting data 

dependable. The first study of horses at Pryor Mountain 

was initiated in 1970 by J.D. Feist, who focused largely on 

behavior (Feist, 1971), but also gathered some information on 

demography of the population (Feist and McCullough, 1975). 

Shortly thereafter, Hall (1972) performed an assessment of 

the herd and possible management alternatives. Perkins and 

others (1979) studied population dynamics, age distribution, 

and band composition in the Pryor Mountain herd from 1974 

through 1978. Garrott and Taylor (1990), using information 

gathered by BLM personnel, reported on many aspects of herd 

demography from 1972 through 1986. F.J. Singer and several colleagues began studies at Pryor Mountain in the early 

1990s, broadening their scope to include vegetation dynamics and conservation genetics as well as demography (Singer 

and Schoenecker, 2000). Prior to his untimely death in 2005, 

Singer also participated in work at Pryor Mountain on contraception and population estimation.

Here we report on the demographics of the Pryor Mountain wild horse herd from 1993 to 2007. The results contained 

herein represent a more detailed analysis of the data presented 

by Singer and others (2000) and an extension covering the 

next decade. While specific to the Pryor Mountain herd, and 

thus not necessarily representative of the other 180 Herd Management Areas (HMAs) overseen by BLM, we hope that our 

results will provide wild horse managers with useful insights 

as they continue to meet the challenges of managing wild 

horse herds in the Western United States.

Study Area

During the period of our study, horses actually used an 

area larger than the Pryor Mountain Wild Horse Range as originally designated. The northwestern and northern boundaries 

of our study area are not well defined, as there are no natural 

or manmade barriers to horse movement in that direction. As 

shown in figure 1, however, the study area encompasses about 

22,300 hectares (ha). The following description of the area 

largely follows Singer and others (2000) and Ricketts (2004).

The study area lies along the Montana-Wyoming border 

at approximately latitude 45°4’N and longitude 108°19’W, 

76 kilometers (km) south of Billings, Mont., and 14 km north 

of Lovell, Wyo. (fig. 1). The area is topographically diverse, 

dominated by narrow canyons and steep cliff faces. Elevations 

range approximately from 1,175 to 2,670 meters (m), with 

corresponding variation in climate and vegetation. Temperatures range from 40 oC in the summer to -34 oC in the winter. 

Mean annual precipitation ranges from 15 centimeters (cm) 

at lower elevations to nearly 70 cm at higher elevations, with 

much of the annual total falling in April–June. Midelevation 

ridges and plateaus often remain snow free or accumulate <10 

cm of snow as a result of winds and southern and western 

exposures. Higher elevations often have winter snow accumulations of 1 m or more. In water years 1993–1998 (water year 

1993, for example, runs from October 1992 through September 1993), mean annual precipitation at Lovell, Wyo. (http://

www.wrcc.dri.edu/summary/climsmwy.html), was 17.2 cm, 

essentially identical to the 30-year average (Bureau of Land 

Management and others, 2008). From 1999 to 2004, however, 

severe drought conditions prevailed (mean annual precipitation = 12.3 cm). Prolonged drought can decrease plant health 

2 Demography of the Pryor Mountain Wild Horses, 1993–2007
Figure 1. Location of the Pryor Mountain Wild Horse Range and study area.

Creek

Com

missary Creek

D e a d m a n C re e k

Crooked Creek

Bobcat Creek

N o rth F o r k Tra il C re e k

G y p s u m C re e k

Layout Creek

Porcupine Creek

Crooked Creek

South F o rk

Trail Creek

River

Crooked

Cree k

Cave Creek

Lost Water Creek

Big h o r n

Crow

Indian

Reservation

Bighorn

Canyon

National

Recreation

Area

Custer

National

Forest

Pryor Mountain

Wild Horse Range

Montana

Wyoming Big Coulee

Dry Head

Sykes

Ri d ge

R

o

ad

Burnt

T

mi b

er Rid

g

e

Roa

d

245605

245109

108°24'W 108°21'W 108°18'W 108°15'W

45°9'N 45°6'N 45°3'N 45°0'N

Bighorn

Lake

94

90

25

94

Study

area

M o n t a n a M o n t a n a

W y o m i n g W y o m i n g

EXPLANATION

Weather stations

Water sources

Roads

Intermittent streams

Pryor Mountain Wild Horse Range

Project study area

Private land

State land

Crow Indian Reservation

Custer National Forest

Bureau of Land

Management land

Bighorn Canyon National

Recreation Area

Wilderness Study Areas

Bighorn Tack-On

Burnt Timber Canyon

Pryor Mountain

0 21

MILES

0 21 KILOMETERS

245605

Study Area 3
4 Demography of the Pryor Mountain Wild Horses, 1993–2007

and vigor and forage production. Temporary relief from the 

drought occurred in 2005 (annual precipitation = 21.2 cm) 

and 2007 (annual precipitation = 14.3 cm for 11 months, data 

missing for June) At least in 2005, greater precipitation probably resulted in improved forage production (Bureau of Land 

Management, 2006).

Vegetation in the study area changes dramatically with 

elevation. At the highest elevations, subalpine meadows (Idaho 

fescue, Festuca idahoensis; alpine timothy, Phleum alpinum; 

alpine bluegrass, Poa alpina; Ross sedge, Carex rossii; silky 

lupine, Lupinus sericeus) are interspersed with stands of 

subalpine fir (Abies lasiocarpa) and Engelmann spruce (Picea 

engelmannii). At somewhat lower elevations, Douglas fir 

(Pseudotsuga menziesii) occurs with an understory of snowberry (Symphoricarpos spp.). Still lower is a shrub community 

dominated by big sagebrush (Artemisia tridentata), rubber 

rabbitbrush (Ericameria nauseosus), shrubby cinquefoil 

(Dasiphora fruticosa), Utah juniper (Juniperus osteosperma), 

and curl-leaf mountain mahogany (Cercocarpus ledifolius) 

and a variety of grasses and forbs such as bluebunch wheatgrass (Pseudoroegnaria spicata), threeawn (Aristida purpurea), needle-and-thread (Hesperostipa comata), and blue 

grama (Bouteloua gracilis). For a more complete description 

of vegetation communities on the Pryor Mountain Wild Horse 

Range, see Ricketts (2004).

Herd Behavior and Management

The social structure of the Pryor Mountain wild horse 

population consists of bands that include a harem stallion, 

occasionally along with a satellite stallion, plus adult mares 

and yearlings and foals of both sexes. In general, we use the 

term band to refer to all members of the group and harem to 

refer to mares ≥2 years of age. Males older than yearlings that 

have not acquired a band usually are found in bachelor groups. 

Distribution and movements of the bands largely are restricted 

by topography to the west (Crooked Creek drainage) and east 

(Bighorn Canyon), and fences of varying integrity to the north 

and south (fig. 1). The wild horses occur in three more or less 

discrete subpopulations (Burnt Timber Ridge, Sykes Ridge, 

and Dryhead). These areas are separated by Big Coulee (a 

deep canyon) and a long, nearly vertical cliff face. However, 

the horses are able to move between these areas at high elevations and below Big Coulee, as well as occasionally by way 

of trails on the cliff face. Varying numbers of horses also use 

U.S. Forest Service land to the north and west of the original 

horse range boundary. Interchange of horses between subpopulations occurs regularly, but not with high frequency. The 

horses move with the seasons, spending winters at low and 

middle elevations where snow is less deep and summering at 

higher elevations. Distribution is restricted by available water 

in summer, when the horses tend to congregate near water 

sources. Water has less effect on distribution in winter, when 

the horses use snow to meet their water needs. 

In the past, the herd has been managed consistent with 

national selective removal policy for the BLM Wild Horse and 

Burro Program, removing mostly young animals and conserving the core breeding component of the population. In general, 

BLM defines Appropriate Management Level (AML) as the 

number of adult wild horses (6 months and older) determined 

through the agency planning process to be consistent with 

the objective of achieving and maintaining a thriving natural ecological balance and multiple-use relation (Bureau of 

Land Management, 2006). Initially, the AML for the Pryor 

Mountain herd was set at 115–127, but was revised to 85–105 

horses in 1992. However, based on genetic research (Cothran 

and Singer, 2000; Gross, 2000a,b), from 1996 to 2006 BLM 

attempted to manage the herd at an average of 140 adult 

horses to help conserve genetic diversity (Bureau of Land 

Management, 2006). More recently, BLM has determined that 

management at an AML of 90–120 adult horses is necessary 

to protect and improve rangeland health (Bureau of Land Management, 2009).

The BLM has managed the size of the Pryor Mountain 

herd primarily through a gather, removal, and adoption program. In 1994, wranglers on horseback attempted to capture 

the entire herd by moving horses into corral traps. In 1997 

and 2001, a helicopter was used to move most of the family bands (individual bachelors typically were not gathered) 

into corral traps; approximately 125 animals were gathered in 

each of these years. Smaller gathers aimed at removing excess 

bachelor stallions and yearling females were conducted in 

2003 and 2006. The 2003 gather involved a helicopter to move 

groups containing target horses into a corral trap. The 2006 

gather was conducted using bait trapping to lure the horses 

into a trap, after which they were transported by vehicle to a 

holding corral.

Since 2001, some mares in the Pryor Mountain herd 

have received fertility control treatments using the contraceptive porcine zona pellucida [PZP; see Kirkpatrick and Frank 

(2005) for a recent review of contraception in equids and other 

free-ranging animals] as part of a cooperative research project 

between BLM, the U.S. Geological Survey (USGS), and the 

U.S. Department of Agriculture, Animal and Plant Health 

Inspection Service (APHIS). In 2001, six mares (age 1–2) that 

had not yet reached breeding status were treated with PZP by 

hand injection during the gather. A booster inoculation was 

administered to three of these females in August 2002; the 

remainder had either died or could not be located to deliver the 

booster. Fertility control was subsequently applied to younger 

mares (age 1–2, 2002–2004) and older mares (>14 years, 

2002–2004; >10 years after 2004) that had already contributed 

to the genetics of the population. After 2001, all inoculations (both primers and boosters) were delivered remotely by 

darting. Because of the timing of booster inoculations relative 

to the breeding season, the first contraceptive effects were 

expected in 2004. From 2004 through 2007, 20.0, 29.0, 10.8, 

and 30.8 percent of the mares (≥3 years old) in the herd were 

presumptively contracepted. 
Methods 5

BLM also has attempted to manage the herd to adjust 

the sex ratio in favor of males (Bureau of Land Management, 

2006) and occasionally has introduced a limited number of 

horses (both mares and stallions) from outside herds in order 

to supplement the herd genetically. Introduction of outside 

horses was discontinued in 1994.

Methods

Field Data Collection

All data reported in this study were gathered through 

direct observations of the herd by trained BLM and USGS 

employees and volunteers. From 1995 to 1997, efforts were 

focused on the peak foaling season, May 1 to June 30. From 

1998 to 2007, daily observations were made during the 

peak foaling and breeding period, May 1 to August 30. This 

effort totaled approximately 100 hours of observation per 

week during 1995–1997 and 100–150 hours per week during 

1998–2007. Semi-weekly to weekly observations were made 

March–April and September–November. Fewer observations 

were made during midwinter, and they were limited to accessible areas of the range, which varied from year to year. Observations were made from the ground during systematic surveys 

along roads and trails that traverse the study area. Ground 

observations were augmented with two helicopter flights each 

fall or winter, 1995–2001, and one flight in late 2006, as well 

as 20–25 days of horseback surveys throughout each year, 

1995–2005. 

Greater than 90 percent of the bands were located at least 

twice per week during the May–July period, and all bands 

usually were located at least once per week from April to 

mid-May and August to mid-October. Reduced effort in the 

remainder of the year resulted in less than one observation 

per week per band. Observers collaborated during the peak 

observation season to develop a weekly list of band composition and foal presence.

Wild horses were approached within 100–250 m and 

could be observed for periods of 2–6 hours with no apparent 

effect on their behaviors. All horses were identified individually based on color, markings, gender, and band associations 

since 1993. All stallions, mares, yearlings, and foals observed 

in bands, bachelor groups, or alone were recorded. Lead harem 

stallions were identified based on behavior as described in 

Feist and McCullough (1975), Oom and Santos-Reis (1986), 

and Berger (1986). Interchanges between bands by mares and 

stallions were recorded, as were returns to the original band. 

Each horse in the population was assigned a unique 

number during data compilation. Of the 609 animals in the 

population from 1993 to 2007, horses whose birth year was 

known from foal records included 480 of the 481 foals born 

during 1993–2007 and 11 horses born prior to 1993. Ages 

of an additional 52 animals were estimated by a veterinarian 

based on tooth eruption and wear patterns (American Association of Equine Practitioners, 2002). Ages of the remaining 

66 animals were based on body size and morphology and 

anecdotal evidence of years the animals were on the range. 

Death year was determined by identification of the carcass of 

the deceased individual or, if an individual was not observed 

for two consecutive years, the animal was considered to have 

died in the year it went missing. Number of foals produced 

was determined by observation as described above. It is possible that some foals were born and died without being seen; 

however, given the intensity of the observations, we believe 

that this was rare. Identification of an individual’s dam was 

based on observations of attachment of foal to mare during the 

early days and weeks of the foal’s life.

Analysis

We conducted all of our analyses on the basis of biological years, which we defined as just prior to foaling in one year 

to the same time the next year. Thus, the biological year 1993 

encompasses the period from spring 1993 to spring 1994. 

Because we defined the biological year in this way, because 

we made no attempt to pinpoint when during a year a horse 

was born or died, and because we have the benefit of hindsight 

(other reported data were often projections), our data will in 

some cases not match exactly those presented elsewhere (for 

example, various Environmental Assessments prepared by 

BLM).

We calculated annual population growth using the expression λt

 = Nt / (Nt – 1 – Rt – 1), where λt is the growth multiplier 

from year t – 1 to year t, N is the population size, and Rt – 1 is 

the number of animals removed in the period t – 1 to t. In this 

formulation, λ represents the apparent growth rate of the population had horses not been removed (Garrott and Taylor, 1990).

Survival rates were calculated as the number of animals 

present at the end of a year divided by the number present at 

the beginning of that year. Animals that were removed from 

the herd were excluded from survival calculations in the year 

they were removed. Foaling rates were calculated as the number of foals born (regardless of how long they lived) divided 

by the number of mares present. Mares that were presumptively contracepted and foals that they produced were excluded 

from calculations of foaling rate.

Band composition has been recorded on “horse lists” 

regularly (usually weekly) since 1996. These are lists of all 

the harem stallions present and all the members of their bands, 

plus lists of bachelor males and animals in transit between 

bands. To derive information on harem stallions and band 

composition, we used two of these lists for each year from 

1996 through 2007, one from the breeding season (usually 

early June) and one from fall (September–November, usually 

the last list available for the calendar year). For each harem 

stallion present, we calculated the number of mares ≥2 years 

of age in the band for each of the two time periods and then 
6 Demography of the Pryor Mountain Wild Horses, 1993–2007

took the maximum of those two values as the number of 

mares held in the harem in that year. We calculated total band 

size in an analogous manner, and we used these same lists to 

derive information on bachelors and satellite stallions, which 

we defined as ≥5 years of age. We defined a satellite stallion 

in this way in an effort to distinguish true multistallion bands 

from cases in which a male simply remained with his natal 

band longer than usual.

We looked for evidence of density dependence in foaling 

rate, herd survival, foal survival, and population growth rate 

in the Pryor Mountain wild horse population using stepwise multiple regression with the current year’s postfoaling 

population and the previous year’s postfoaling population as 

independent variables. For these analyses, we used data from 

Garrott and Taylor (1990) for 1976–1986 (foaling rate, herd 

survival, and population growth rate), as well as data from our 

study for 1993–2007. We excluded data from the winter of 

1977–1978, during which 51 percent of the population died 

as a result of heavy snow and ice accumulations (Garrott and 

Taylor, 1990), from the analyses of density dependence in herd 

survival and growth rate. We also excluded data from 2004, 

when 96 percent of the foal crop was lost, from the analysis of 

foal survival. Foal losses in 2004 are suspected to have been 

the result of predation by mountain lions (Puma concolor). We 

believe that these exclusions are justified because the biological processes operating in these years were clearly different 

from those operating in other years.

It would be desirable to have consistent, long-term data 

on annual forage production for comparison with demographic 

parameters, but we are unaware of such data for the study 

area. As a possible surrogate, we examined weather records 

from a number of stations in the area, reasoning that forage 

production in this semiarid environment likely is related to 

precipitation, even though large elevation differences across 

the study area make it unlikely that any single station will be 

representative of the entire area. In terms of location, the best 

available data are from a Remote Automated Weather Station 

(RAWS; Wild Horse Montana, NWS ID #245605, elevation 

2,675 m), which was operated in the study area from October 

1991 to July 1997 (http://raws.dri.edu/wraws/emtF.html) (fig. 

1). Unfortunately, overlap between this period of record and 

the period of our study is minimal. A National Climatic Data 

Center Station (NCDC #484770, elevation 1,170 m) at Lovell, 

Wyo., has a much longer period of record (1948–present; 

http://www.wrcc.dri.edu/summary/climsmwy.html), but its 

lower elevation caused us to question its relevance to higher 

elevations in the study area. For the 7 years of data in common, however, there was a strong correlation between the two 

stations for both total annual precipitation (R2

 = 0.921, P = 

0.0006) and the minimum of the monthly average minimum 

temperatures (R2 = 0.763, P = 0.01). We therefore used data 

from the Lovell station and stepwise multiple linear regression 

to analyze relations between weather and herd survival rate, 

foal survival rate, and foaling rate (mares ≥3 years of age). 

Data on annual herd survival and foaling rate were available from both our study and the work of Garrott and Taylor 

(1990), but data on annual foal survival were available only 

from our study. For the reasons cited above, we again excluded 

2004 from the analysis of foal survival.

Other investigators (Heitschmidt and others, 2005; Smart, 

2005) showed that spring precipitation (April–June) is related 

to annual forage production in the Great Plains. We therefore 

divided precipitation data into spring (April–June), summer 

(July–September), and winter (October–March). For each 

year, we also calculated the mean of the monthly average 

minimum temperatures (December–February) and the mean of 

the monthly average maximum temperatures (April–September) as indices of winter cold and summer heat.

For herd survival, we used the covariates winter precipitation in the previous year, spring precipitation in the current 

year, and summer precipitation in the current year (hypothesizing that all might have a positive relation with forage production in the current year), winter precipitation in the current 

year (potential negative effect of snow accumulation on 

forage availability in the current year), and average monthly 

minimum temperature in the current year (potential negative 

effect on ability to meet energy demands). For foal survival, 

we used the same set of covariates and added average monthly 

maximum temperature (possible effect of heat stress on more 

susceptible foals). For foaling rate, we used the covariates 

summer precipitation in the previous year (potential positive 

relation with forage production during early pregnancy), winter precipitation in the previous year (potential negative effect 

of snow accumulation on forage availability), average monthly 

minimum temperature (potential negative effect on ability to 

meet energy demands), and spring precipitation in the current 

year (possible positive relation with forage production during 

late pregnancy).

All data were entered into Microsoft Excel® spreadsheets 

(Microsoft Corporation, Redmond, Wash.). Analyses were 

conducted in SAS® v. 9.1 (SAS Institute, Cary, N.C.) using 

standard statistical techniques as noted in the Results section.

Results

Population Size and Growth

Prior to foaling in spring 1993, there were 128 horses 

≥1 year of age in the study area. From 1993 to 2007, 481 

horses were born; thus, our study encompasses 609 individual 

animals. During the period of study, the prefoaling population averaged 148.8 animals (range = 120–187), the foal crop 

(number born) averaged 32.1 (range = 23–40), and the postfoaling population averaged 180.9 animals (range = 149–227) 

(table 1). As we calculated it, the postfoaling population 

represents the maximum number of animals that could have 

been present at any time during the year, as it assumes no mortality of foals or adults during the foaling period. The number 

of deaths per year apparently increased during 1999–2006, 

while the foal crop remained relatively constant throughout 
Results 7

the period of study. The mean annual apparent growth rate, 

calculated as though removals had not occurred, was 9.6 

percent (λ = 1.096, range = 0.977–1.220), or 10.5 percent if 

the year (2004) of heavy foal loss is excluded. Although the 

growth rate rebounded somewhat in 2006 and 2007, there was 

a significant (R2 = 0.514, P = 0.004) overall decline during the 

period of study as determined by linear least squares regression (fig. 2).

Removals

Major removals of wild horses occurred in 1994, 1997, 

2001, and 2006, with smaller numbers of horses removed in 

four other years (table 2). For the entire period, adults (≥2 

years of age) were removed most often (62.6 percent; 

n = 182), followed by yearlings (28.6 percent) and foals (8.8 

percent). Across all age classes, slightly more males (56.6 

percent; n = 182) were removed than females.

Sex and Age Structure

During the period 1993–2007, 477 foals of known sex 

were born; four foals disappeared before sex could be ascertained. Slightly more males (243) were born than females 

(234), but this difference was not statistically significant 

(χ2 = 0.170, P = 0.68). On an annual basis, however, the 

proportion of males in the foal crop varied widely (range = 

34.8–71.9 percent) (fig. 3). The proportion of males produced 

also varied by age of mare, declining from age 3 through about 

age 10, and then increasing in older mares (fig. 4).

The sex ratio of animals ≥1 year of age varied only moderately during the period of study, ranging from a high of 55.2 

percent males in 1997 to a low of 44.2 percent males in 2000 

and 2007 (fig. 5). The largest interannual changes in sex ratio 

occurred as a result of removals in 1994 (27 males, 33 females 

Figure 2. Trend in growth rate of the Pryor Mountain wild horse 

(Equus caballus) population, 1993–2007. POPULATION GROWTH RATE

0.95

1.00

1.05

1.10

1.15

1.20

1.25

YEAR

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Y = 30.325 - 0.0146X

Table 1. Size and growth rate of the Pryor Mountain wild horse (Equus caballus) herd, 1993–2007.

[Growth rate (λ) is calculated based on the postfoaling population. —, not applicable; SE, standard error of the mean.]

Year

Prefoaling 

population Foals born

Postfoaling 

population Deaths Removals λ

1993 128 23 151 1 1 —

1994 149 34 183 3 60 1.220

1995 120 29 149 7 0 1.211

1996 142 32 174 7 2 1.168

1997 165 35 200 15 46 1.163

1998 139 23 162 7 1 1.052

1999 154 38 192 27 0 1.193

2000 165 37 202 15 0 1.052

2001 187 40 227 31 46 1.124

2002 150 32 182 28 0 1.006

2003 154 30 184 32 7 1.011

2004 145 28 173 38 0 0.977

2005 135 36 171 19 0 0.988

2006 152 30 182 16 19 1.064

2007 147 34 181 9 0 1.110

Total 2,232 481 2,713 255 182 —

Mean 148.8 32.1 180.9 17.0 12.1 1.096

SE 4.18 1.30 5.14 3.02 5.36 0.0228
8 Demography of the Pryor Mountain Wild Horses, 1993–2007

removed), 1997 (33 males, 13 females), 2001 (18 males, 28 

females), and 2006 (14 males, 5 females).

The age structure of the Pryor Mountain wild horse herd, 

on the other hand, changed markedly between 1993 and 2007 

(fig. 6A,B). In 1993, males in the herd averaged 4.4 years of 

age and females averaged 4.2 years. In 2007, the comparable 

values were 8.1 and 6.8 years, respectively. Both of these 

trends are statistically significant as judged by linear least 

squares regression (males, P < 0.0001, R2 = 0.911; females, 

P < 0.0001, R2 = 0.832).

Survival

Pooled across years, ages, and sexes (n = 2,531), the 

annual survival rate of horses on the study area was 0.899. 

Excluding four foals that disappeared before sex could be 

ascertained, survival of males (0.896, n = 1,246) did not 

differ from that of females (0.905, n = 1,281) (χ2 = 0.486, 

Table 2. Removals of wild horses (Equus caballus) from the Pryor Mountain herd, 1993–2007.

Foals Yearlings Adults

Year Males Females Males Females Males Females Total

1993 0 0 0 0 1 0 1

1994 8 6 7 7 12 20 60

1996 0 0 0 0 2 0 2

1997 2 0 11 1 20 12 46

1998 0 0 0 0 1 0 1

2001 0 0 10 6 8 22 46

2003 0 0 1 0 6 0 7

2006 0 0 4 5 10 0 19

Total 10 6 33 19 60 54 182

PERCENT MALES

0

10

20

30

40

50

60

70

80

YEAR

1993 1995 1997 1999 2001 2003 2005 2007

5 36

37 41 48

44 44 41

43

31

28

22

17

14 7

2

PERCENT MALES

0

50

100

AGE OF MARE, IN YEARS

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Figure 3. Sex ratio of wild horse (Equus caballus) foals born in the 

Pryor Mountain herd, 1993–2007.

Figure 4. Variation in the sex ratio of wild horse (Equus caballus) 

foals by age of dam in the Pryor Mountain herd, 1993–2007. Values 

above the bars are number of foals born.

% MALES

30

40

50

60

70

YEAR

1993 1995 1997 1999 2001 2003 2005 2007

Figure 5. Annual sex ratio of adult (≥1 year old) wild horses 

(Equus caballus) in the Pryor Mountain herd, 1993–2007.
Results 9

A MALES

B FEMALES

9

8

7

6

5

4

Y = -548.46 + 0.277x

AVERAGE AGE, IN YEARS

7

6

5

4

AVERAGE AGE, IN YEARS

Y = -399.56 + 0.202X

YEAR

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Figure 6. Average age of (A) male and (B) female wild horses (Equus caballus) in the Pryor Mountain herd, 1993–2007.
10 Demography of the Pryor Mountain Wild Horses, 1993–2007

P = 0.486), nor was the difference between the sexes significant for foals (χ2

 = 2.500, P = 0.114, n = 461). Data from both 

sexes were therefore pooled for calculations of survival by 

year.

Survival by year varied from a high of 0.993 in 1993 to 

a low of 0.780 in 2004 (table 3), when 27 of 28 foals were 

lost, with predation by mountain lions suspected as the cause 

(Bureau of Land Management, 2006). For the period of our 

study, there was a trend toward decreasing foal survival (fig. 7; 

P = 0.020, R2 = 0.353), but there was no trend in survival of 

animals ≥1 year old (P = 0.282, R2 = 0.088). Foal survival 

increased in the most recent three years of our study, however.

More than one half of the horses that died from 1993 to 

2007 were foals, and annual foal survival pooled across years 

was 0.697 (table 4). Survival increased gradually from age 0 to 

age 3, remained high through age 15, and then declined gradually. Horses ≥18 years of age were pooled for this analysis 

because of small sample sizes in these age classes. The oldest 

animal recorded during the period of study was a male that 

died at age 26 in 2005. Nine horses lived to be 20 years of age 

or older, seven of which were males. Four horses (3 males, 1 

female) age 20 and 21 were present on the range at the end of 

our study.

As judged by logistic regression, there was a positive 

relation between the probability of a foal surviving and the 

mean (average of summer and fall in a given year) number of 

members in the band (n = 393 foals in 1996–2007; Wald χ2

 = 

14.964, odds ratio = 1.244 with 95 percent CI = 1.114–1.390, 

P = 0.0001). Thus, for example, the regression yields a 

predicted foal survival rate of 0.457 (95 percent CI = 0.351–

0.567) for a band size of two, and a predicted survival rate of 

0.757 (95 percent CI = 0.688–0.815) for a band size of eight.

We also examined survival of foals in relation to parity 

of their dams—primiparous (those giving birth to their first 

foal) compared to multiparous (those that had given birth one 

or more times). For this analysis, we excluded all mares born 

before 1993 (since parity could not be determined with certainty) and any foals they produced. We also excluded mares 

that were presumptively contracepted or were present postcontraception (hereafter referred to as treated and posttreated) and 

foals that they produced. For the remaining subset of foals, 

there was no difference in survival based on parity of the dam 

(n = 189, χ2

 = 0.957, P = 0.328).

Foaling Rates

For ease of comparison with other studies, we calculated 

foaling rates of mares ≥2 years of age, ≥3 years of age, and 

≥4 years of age. Treated and posttreated mares were excluded 

from these calculations, as were foals born to unknown dams. 

Pooled across years, productivity was 0.501 foals/mare (range 

= 0.254–0.705) for mares ≥2 years of age, 0.576 foals/mare 

(range = 0.300–0.795) for mares ≥3 years of age, and 0.597 

foals/mare (range = 0.311–0.795) for mares ≥4 years of age 

(table 5). Regression analysis showed a slight increase in 

foaling rate of mares over time (for mares ≥3 years of age, P = 

0.088), but variability was high (R2 = 0.208).

From 1993 to 2007, 464 of the 481 foals produced in the 

study area were born to 118 known or highly probable dams 

(table 6), an average of 3.93 foals per mare. Many of the dams 

represented in table 6 are still producing foals; thus, the data 

represent only a single point in time. Twenty-seven dams that 

each produced seven or more foals accounted for a total of 237 

foals, or 51.1 percent of the production by known dams. Of the 

54 dams that have been monitored since birth (that is, those 

born in 1993 or later) and have not been contracepted, 6 (11.1 

percent) first produced a foal at age 2, 29 (53.7 percent) at age 

3, 14 (25.9 percent) at age 4, 3 (5.6 percent) at age 5, and 2 

(3.7 percent) at age 7. The average age at first foaling for these 

dams was 3.4 years. Age-specific fecundity was 0.048 foals 

per mare for 2-year-old mares, increased nearly monotonically 

through age 10, remained high through age 15, and dropped 

rapidly thereafter (table 7). The maximum age at which any 

mare gave birth during the study period was 17 years, although 

that mare was presumptively contracepted and thus is not 

represented in table 7. A single set of twin foals (both females) 

was born during the period of our study, to a 6-year-old mare 

in 1994.

We used logistic regression to examine the probability 

of foaling in years following a gather as compared to other 

years for three age groups of mares: 2 years old, 3 years old, 

and ≥4 years old. Foal production (yes or no) was the dependent variable and year was the independent variable, and we 

used a specific hypothesis test (or contrast) to compare the 

mean probabilities of foaling. For this analysis we considered 

Year n Deaths Survival

1993 150 1 0.993

1994 123 3 0.976

1995 149 7 0.953

1996 172 7 0.959

1997 154 15 0.903

1998 161 7 0.957

1999 192 27 0.859

2000 202 15 0.926

2001 181 31 0.829

2002 182 28 0.846

2003 177 32 0.819

2004 173 38 0.780

2005 171 19 0.889

2006 163 16 0.902

2007 181 9 0.950

Total 2,531 255 0.899

Table 3. Annual survival rates of Pryor Mountain wild horses 

(Equus caballus), 1993–2007.

[Horses that were removed were excluded from survival calculations in the 

year of their removal.]
Results 11

Age

Males Females Total

n Deaths Survival n Deaths Survival n Deaths Survival

0 233 77 0.670 228 60 0.737 1

465 141 0.697

1 115 9 0.922 137 12 0.912 252 21 0.917

2 91 5 0.945 108 9 0.917 199 14 0.930

3 77 3 0.961 89 2 0.978 166 5 0.970

4 72 1 0.986 84 5 0.941 156 6 0.962

5 70 1 0.986 87 4 0.954 157 5 0.968

6 67 4 0.940 76 3 0.961 143 7 0.951

7 60 2 0.967 73 0 1.000 133 2 0.985

8 60 1 0.983 69 0 1.000 129 1 0.992

9 56 1 0.982 65 1 0.985 121 2 0.984

10 53 0 1.000 57 4 0.930 110 4 0.964

11 49 3 0.939 48 3 0.938 97 6 0.938

12 38 2 0.947 40 5 0.875 78 7 0.910

13 34 1 0.971 32 4 0.875 66 5 0.924

14 33 1 0.970 27 1 0.963 60 2 0.967

15 30 1 0.967 22 2 0.909 52 3 0.942

16 30 1 0.967 18 4 0.778 48 5 0.896

17 26 4 0.846 9 2 0.778 35 6 0.829

≥18 52 12 0.769 12 1 0.917 64 13 0.797

 Total 1,246 129 0.896 1,281 122 0.905 1

2,531 255 0.899 1

 Includes four foals that disappeared before sex could be ascertained. 

Table 4. Age-specific survival rates of Pryor Mountain wild horses (Equus caballus), 1993–2007. 

[Horses that were removed were excluded from survival calculations in the year of their removal.]

YEAR

SURVIVAL RATE 

Figure 7. Trend in annual survival of Pryor Mountain wild horses (Equus caballus) for adults and foals, 1993–2007.
12 Demography of the Pryor Mountain Wild Horses, 1993–2007

only the gathers conducted in 1994, 1997, and 2001 in which 

most of the family bands were gathered; in 2003, only groups 

containing animals targeted for removal were gathered, and 

in 2006 a subset of the herd was captured by bait trapping. 

For mares 2 or 3 years old, the data were too sparse to allow 

calculation of the maximum likelihood estimate in the logistic 

regression (for example, in some years no mares in these age 

groups produced foals). For mares ≥4 years old the probability 

of foaling was lower in years following a gather than in other 

years (P = 0.03). The absolute value of this difference was 

small, however. The mean predicted foaling rate in years following a gather was 0.522 (n = 3 years, 134 mares) as opposed 

to 0.622 (n = 12 years, 531 mares) in other years.

We also used logistic regression to examine the probability of a mare foaling as a function of number of members 

in the band in summer and found a positive relation (n = 577 

mare-years, Wald χ2

 = 24.122, odds ratio = 1.254 with 95 

percent CI = 1.146–1.372, P < 0.0001). As an example, the 

regression yields a predicted probability of foaling of 0.427 

(95 percent CI = 0.347–0.511) with a band size of two, as 

compared to a probability of foaling of 0.743 (95 percent CI = 

0.682–0.797) with a band size of eight.

Year

Mares ≥ 2 years old Mares ≥ 3 years old Mares ≥ 4 years old

Foals Mares Foals/mare Foals Mares Foals/mare Foals Mares Foals/mare

1993 15 59 0.254 15 50 0.300 14 45 0.311

1994 32 64 0.500 32 59 0.542 30 50 0.600

1995 23 46 0.500 23 42 0.548 20 38 0.526

1996 32 51 0.627 30 44 0.682 27 40 0.675

1997 34 66 0.515 33 50 0.660 29 43 0.674

1998 23 55 0.418 23 48 0.479 22 43 0.512

1999 38 70 0.543 37 52 0.712 34 46 0.739

2000 37 76 0.487 36 61 0.590 27 44 0.614

2001 40 88 0.455 40 75 0.533 34 60 0.567

2002 32 63 0.508 32 57 0.561 28 53 0.528

2003 30 65 0.462 29 56 0.518 29 50 0.580

2004 28 52 0.538 28 48 0.583 28 48 0.583

2005 31 44 0.705 31 39 0.795 31 39 0.795

2006 23 42 0.548 23 42 0.548 23 37 0.622

2007 21 35 0.600 21 29 0.724 21 29 0.724

Total 439 876 0.501 433 752 0.576 397 665 0.597

Table 5. Annual foaling rates of Pryor Mountain wild horse (Equus caballus) mares, 1993–2007.

[Treated and posttreated mares in 2004–2007 and foals that they produced were excluded, as were foals born to unknown dams.]

Foals / Mare

Number

of mares Total foals

Percent of 

total foals

1 43 43 9.3

2 12 24 5.2

3 10 30 6.5

4 5 20 4.3

5 16 80 17.2

6 5 30 6.5

7 9 63 13.6

8 6 48 10.3

9 4 36 7.8

10 2 20 4.3

11 4 44 9.5

12 1 12 2.6

14 1 14 3.0

Total 118 464 100

Table 6. Total foals born to Pryor Mountain wild horse (Equus 

caballus) mares, 1993–2007. 

[Includes only foals born to known dams. Many of the mares represented are 

still producing foals.]
Results 13

In addition, we compared the probability of foaling in the 

next year for primiparous and multiparous mares. Primiparous 

females were less likely to foal (χ2 = 27.29, P < 0.0001) in the 

year following their first foal (proportion foaling = 0.26, n = 

43) than were multiparous females (proportion foaling = 0.72, 

n = 110). On the other hand, multiparous females were somewhat more likely to foal (χ2

 = 2.60, P = 0.107) if they foaled 

in the previous year (proportion foaling = 0.72, n = 110) than 

if they did not foal in the previous year (proportion foaling = 

0.61, n = 76).

Harem Stallions and Band Composition

During the 12 years (1996–2007) for which information 

was available, the number of bands varied from 29 to 38 (table 

8), with an increasing trend through time (least squares regression, P = 0.012, R2 = 0.48). There was no relation, however, 

between number of bands and band size (least squares regression, P = 0.88); that is, band size did not decline significantly 

as the number of bands increased. Bands were held by 64 

different stallions during this period, the youngest being age 

3 and the oldest being age 23 (fig. 8). The vast majority of 

bands, however, were held by stallions aged 5–17 (fig. 8; n = 

399). A single stallion gained his first harem at age 3, but 18 

of the 29 harem stallions (62.1 percent) for which we have 

records since birth (born in 1993 or later) acquired their first 

harem at age 5 or 6 (fig. 9). As might be expected from the 

increasing age of all males in the herd (fig. 6A), the average 

age of harem stallions also increased through time (table 8) 

(least squares regression, P < 0.0001, R2

 = 0.82).

Harems with as many as six breeding-age females (≥2 

years old) were recorded occasionally, but more than 86 percent of the harems contained 1–3 breeding-age females (fig. 

10; n = 399). Harem size varied by age of stallion, increasing 

from age 3 to about age 11 and declining thereafter (fig. 11; 

n = 399). There were four somewhat anomalous cases in 

which a male was listed as a harem stallion but the harem size 

was zero (fig. 10). In two of these cases there were no harem 

members at all when the observations were recorded. These 

represent harems in flux, with one or more mares sometimes 

with the stallion and sometimes not. In the other two cases 

the only band members were the harem stallion and a yearling female, and we counted only females ≥2 years old in our 

calculations of harem size.

Of 399 band-year combinations for which we have 

observations, only 25 (6.3 percent) involved satellite stallions, 

which we defined as a male ≥5 years old that was associated 

with a harem stallion. Three pairs of stallions that lived in 

the same band for 6, 6, and 5 years accounted for 17 of these 

25 associations. The remaining eight associations lasted only 

1 or 2 years. In only one case were three stallions involved, 

and that association was maintained for a year or less. Two 

of the long-term associations (≥5 years) involved stallions 

that were the same age (7 through 12 years of age) or 1 year 

apart (harem stallion 5 through 9 years of age; satellite stallion 

6 through 10 years of age). The third long-term association 

included a harem stallion 17 through 22 years of age and a 

satellite stallion 7 through 12 years of age. The mean number 

of mares (≥2 years old) did not differ between single- and multistallion bands (2.2 mares in both cases), although 5 of the 25 

multistallion bands had 4–6 mares.

Of the 180 male foals born from 1993 through 2003 

(those born after 2003 would have been only 3 years old or 

younger at the end of our study), 124 died or were removed 

before reaching age 4 and thus had virtually no chance of 

becoming a harem stallion. An additional 17 died or were 

removed at age 4 or 5 and thus had a much reduced chance 

of becoming a harem stallion. Three died at age 6, and two 

were removed at age 7 without having held a harem. Of the 

original 180 male foals born, 29 became harem stallions. Of 

the remaining five animals, two were still bachelors at the end 

of our study (ages 4 and 6 in 2007), two briefly held harems 

(at ages 4 and 8) but reverted to bachelor status, and one was a 

satellite stallion in his natal band (at age 5).

Age

Number of 

foals

Number of 

mares Foals/mare

0 0 234 0.000

1 0 156 0.000

2 6 124 0.048

3 36 87 0.414

4 34 81 0.420

5 37 72 0.514

6 43 69 0.623

7 41 68 0.603

8 44 70 0.629

9 42 69 0.609

10 43 59 0.729

11 31 48 0.646

12 28 40 0.700

13 22 30 0.733

14 14 23 0.609

15 14 20 0.700

16 4 9 0.444

17 0 4 0.000

≥18 0 3 0.000

Total 439 1,266 —

Table 7. Age-specific fecundity of Pryor Mountain wild horse 

(Equus caballus) mares, 1993–2007.

[Treated and posttreated mares in 2004–2007 and foals that they produced 

were excluded, as were foals born to unknown dams. —, not applicable.]
14 Demography of the Pryor Mountain Wild Horses, 1993–2007

Year Number of bands

Mean age of harem 

stallions Mean harem size Mean band size

1996 29 8.76 1.90 5.14

1997 31 9.68 2.06 5.23

1998 31 10.26 1.87 4.90

1999 31 10.71 2.68 5.74

2000 31 11.10 2.68 5.87

2001 38 10.71 2.39 5.24

2002 34 11.06 1.94 4.59

2003 36 10.89 2.06 4.53

2004 34 11.21 2.06 4.03

2005 35 12.11 2.11 4.20

2006 35 13.00 2.20 4.66

2007 34 11.97 2.38 5.18

Total 399 — — —

Table 8. Number of wild horse (Equus caballus) bands, mean age of harem stallions, mean harem size (mares ≥2 years old), 

and mean band size (all members) in the Pryor Mountain herd, 1996–2007.

[ —, not applicable.]

AGE OF STALLION, IN YEARS

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

PERCENT OF HAREMS HELD

0

1

2

4

5

6

7

8

9

10

11

12

41 40

23

25

31

39

23 22

17

9

4 4

2 1 1

39

31

24

18

4

1

3

Figure 8. Age of wild horse (Equus caballus) stallions holding harems in the Pryor Mountain herd, 1996–2007. Values above the bars are 

number of harems.
Results 15

1

4

18

24 31

39 41

40

39

31

25

23 23

22 17

9 4 4

2

NUMBER OF MARES

1 1

0

1

2

3

AGE OF STALLION, IN YEARS

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Figure 11. Mean number of wild horse (Equus caballus) mares held relative to age of harem stallions in the Pryor Mountain herd, 

1996–2007. Values above the bars are number of harems.

Figure 9. Age at which wild horse (Equus caballus) stallions first 

held a harem in the Pryor Mountain herd, 1996–2007. Values above 

the bars are percent of the total stallions represented on the 

chart.

Figure 10. Frequency distribution of wild horse (Equus caballus) 

harem size in the Pryor Mountain herd, 1996–2007. Values above 

the bars are number of harems.

3.45%

10.34%

37.93%

24.14%

6.90% 6.90% 6.90%

3.45%

NUMBER OF STALLIONS

0

1

2

3

4

5

6

7

8

9

10

11

12

AGE OF STALLION, IN YEARS

3 4 5 6 7 8 9 10

 HAREMS, IN PERCENTS

NUMBER OF MARES

0 1 2 3 4 5 6 0

10

30

20

40

4

125

133

86

31

17

3
16 Demography of the Pryor Mountain Wild Horses, 1993–2007

Bachelors

The number of bachelor males in a wild horse herd 

changes through the course of a year, as young males are 

expelled from or leave bands, middle-aged males (formerly 

bachelors) gain bands, and older males gain and lose bands. 

In the Pryor Mountain herd, the number of bachelor males 

in summer (early to mid-June) varied between 16 and 36 

during the period of study (fig. 12). The largest interannual 

change occurred between 1997 and 1998, in part because of 

the removal of 18 bachelors in fall 1997. In total, 40 bachelors 

were removed from the herd from 1996 through 2007. Pooled 

across years, 188 of 328 bachelors (57.3 percent) were 2 

through 5 years of age (fig. 13).

To examine the question of when young stallions leave 

their natal bands and become bachelors, we used a subset of 

our data that included all males born in 1996 or later (thus 

ensuring that we had complete information on their band associations since birth). Of the 59 males that were born in 1996 or 

later and reached bachelor status (many died or were removed 

before becoming bachelors, others were born too late in the 

study period to have an opportunity to become bachelors), 4 

(6.8 percent) became bachelors at age 1 (though one subsequently returned to his natal band), 32 (54.2 percent) at age 2, 

13 (22.0 percent) at age 3, 7 (11.9 percent) at age 4, and 3 (5.1 

percent) at age 5.

Population Regulation

Density Dependence

Regression analyses showed no relation between the foaling rate of mares ≥3 years of age and population size in either 

the current year or the previous year (P ≥ 0.143). Foaling rate 

of 2-year-old mares may have declined somewhat with larger 

population in the previous year (n = 13 years, P = 0.090), but 

variability was high (R2 = 0.239). Other young age classes 

(3–5 years) did not show similar relations, nor did combinations of these age classes.

Overall herd survival was related negatively to the 

previous year’s postfoaling population (fig. 14A) (n = 23 

years, P = 0.006), but there was a great deal of variability 

(R2

 = 0.305). Foal survival, however, was unrelated to population size (n = 13 years, P > 0.15). There also was a relation between population growth rate and the previous year’s 

postfoaling population (fig. 14B) (n = 23 years, P = 0.002, 

R2 = 0.382). This is perhaps not surprising, because survival 

rate is an important component of population growth rate.

NUMBER OF BACHELORS

0

10

20

30

40

YEAR

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Figure 12. Number of bachelor wild horses (Equus caballus) in the Pryor Mountain herd, 1996–2007.
Discussion 17

Weather

None of the covariates that we used entered the stepwise 

multiple regression model for herd survival (P > 0.15; table 9). 

For foal survival, both summer and winter precipitation in the 

current year met the criterion for entry in the model (P < 0.15), 

and both were related positively to foal survival. In the case 

of winter precipitation in the current year, the positive relation 

was contradictory to our hypothesis. In combination, these 

parameters accounted for about 81 percent of the variability 

in foal survival. In terms of mare foaling rate, only the mean 

of the monthly average minimum temperature in the previous 

winter entered the model, and the relation accounted for only 

about 15 percent of the variability.

Discussion

Population Size and Growth

From 1993 to 2007, the prefoaling (adult) population 

averaged about 149 animals, only slightly more than the target 

population of ≈140 adults established on the basis of genetic 

considerations (Bureau of Land Management, 2006). Removals and a declining survival rate for foals appear to have been 

the primary factors keeping the population at this level. Contraception also may have played a role if it prevented the birth 

of any foals. However, it seems likely that the effect of the 

contraceptive program on population growth was small, given 

that relatively few mares were treated and that those mares 

were not in the most productive age classes.

The subject of population growth in wild horse populations has been controversial since early reports (for example, 

Cook, 1975) of rates as high as 30 percent annually. Other 

investigators challenged the accuracy of such estimates, and 

several used simulation modeling in combination with the 

scant data then available for wild horses and extrapolation of 

data from other ungulates to show that growth rates of 20–30 

percent seemed highly unlikely given apparently reasonable 

assumptions about model parameters (Conley, 1979; Wolfe, 

1980). Frei and others (1979) suggested that the apparently 

inflated growth rates might be an artifact of improved aerial 

census techniques and greater experience of observers.

As additional data were accumulated, however, it 

became apparent that initial assumptions regarding population parameters, particularly survival rates and age of mares 

at first reproduction, were probably conservative for many 

wild horse herds (Wolfe, 1986). Eberhardt and others (1982) 

calculated rates of increase of 20 and 22 percent for two herds 

Figure 13. Age distribution of bachelor wild horses (Equus caballus) in the Pryor Mountain herd, 1996–2007. Values above the bars are 

number of bachelors.

4

40

53 54

41

23

16

13

6 5 6

3

6

9

4

6

9

11

8

3 4

1 1 1 1

PERCENTAGE OF TOTAL BACHELORS

0

5

10

15

20

AGE, IN YEARS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 24 25 26 
18 Demography of the Pryor Mountain Wild Horses, 1993–2007 SURVIVAL RATE

0.75

0.80

0.85

0.90

0.95

1.00

1.05

Y = 1.0987 - 0.0011X

POPULATION GROWTH RATE

0.95

1.00

1.05

1.10

1.15

1.20

1.25

1.30

POPULATION SIZE IN PREVIOUS YEAR

80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230

Y = 1.3933 - 0.0016X

A

B

Figure 14. Relation of (A) survival and (B) population growth rate to the previous year’s population for wild horses (Equus caballus) in 

the Pryor Mountain herd. Includes data from this study and from Garrott and Taylor (1990), excepting the abnormally severe winter of 

1977–1978.
Discussion 19

in Oregon and, by comparing pre- and postremoval populations with known removals, showed that the aerial counts used 

to derive the growth rates must have been reasonably accurate. 

Berger (1986, p. 76), based on a 5-year study of horses in the 

Granite Range of Nevada, reported an average annual growth 

rate of 31 percent. [Note, however, that Berger (1986, p. 77) 

also reported an exponential growth rate of 0.188, which is 

equivalent to a finite growth rate of only 20.7 percent annually.] Wolfe (1986), using data from an unspecified number of 

BLM herds (>40,000 animals), calculated an annual growth 

rate of 14 percent for the period 1981–1983 and 15 percent for 1983–1985. Garrott and others (1991b) reported an 

unweighted mean growth rate of 21 percent (range = 15–27 

percent) for 12 western herds. High rates of population growth 

are not limited to western horse populations. Franke Stevens 

(1990), for example, reported that the horse population at 

Rachel Carson National Estuary, made up of four small islands 

off the coast of North Carolina, increased from 44 animals 

in 1984 to 68 animals in 1986, from which we calculated an 

annual growth rate of 24.3 percent.

Reported growth rates of wild horse herds have not 

been universally high, however. Keiper and Houpt (1984), 

for example, stated that the horse population on Assateague 

Island National Seashore grew from 42 to 80 animals during 

the period 1975 to 1982, which represents an average annual 

growth rate of 9.6 percent. Goodloe and others (2000) found 

an average annual growth rate of 4.3 percent (range = 3.0–6.8 

percent) for horses on Cumberland Island, Georgia, from 1986 

to 1990 and suggested that the low rates likely were the result 

of density-dependent response to limited forage. Greger and 

Romney (1999), studying a horse population at the Nevada 

Test Site (now the Nevada Wild Horse Range), found an average growth rate of -7.0 percent, which they attributed to very 

low foal survival as a result of predation by mountain lions.

The first multiyear study of horses at Pryor Mountain was 

that of Perkins and others (1979). While they did not calculate 

population growth rates, they presented information sufficient 

to do so for two years. Based on information in their table 1, 

we calculated growth rates of 7.5 and 16.0 percent for 1976 

and 1977, respectively, for the postfoaling population. Garrott 

and Taylor (1990) reported growth rates at Pryor Mountain for 

1977–1986 with a mean of 11.9 percent, but 17.8 percent if 

a year with heavy winter mortality (51 percent of the population died) was excluded. Singer and others (2000) found a 

mean growth rate of 17.6 percent at Pryor Mountain from 

1992–1997.

Our mean annual growth rate (9.6 percent; 10.5 percent if 

the year of heavy foal loss is excluded) for the period 1993–

2007 is thus toward the lower end of the range reported for the 

Pryor Mountain and other herds. The declining trend in growth 

rate through time appears to be a result of decreasing survival 

of foals, though both foal survival (2005–2007) and growth 

rate (2006–2007) increased in recent years. Contraception also 

may have played a small role in reducing the average growth 

rate, though it is difficult to separate the effects of multiple 

factors.

Sex and Age Ratios

Studies at Pryor Mountain and elsewhere have found foal 

sex ratios favoring both males and females, with the pattern 

sometimes switching for the same herd in different time periods. At Pryor Mountain, for example, Feist and McCullough 

(1975) reported that 54.3 percent of 35 foals were males, 

Singer and others (2000) found that 58.3 percent of “surviving” foals were males (sample size not given), but Perkins 

and others (1979) found 48.5 percent males among 103 foals. 

Similarly, Keiper (1979) reported 52.6 percent males among 

190 foals on Assateague Island, but Keiper and Houpt (1984) 

found 46.6 percent males in 103 foals born at the same location. In many of these studies with relatively small sample 

sizes, of course, the results did not differ significantly from 

parity, which also is true of most studies with larger samples, 

including ours. Speelman and others (1944), for example, 

found 49.4 percent males in 568 foals born to domestic mares 

raised under range conditions in Montana. Garrott (1991), who 

Table 9. Results of stepwise multiple regressions between wild horse (Equus caballus) herd survival, foal survival, and foaling rate 

(mares ≥3 years of age) and various weather parameters for the Pryor Mountain herd.

[Includes data from Garrott and Taylor (1990) and this study for herd survival and mare foaling rate, but only from this study for foal survival. n, sample size; 

SE, standard error of the estimate; R2, coefficient of determination; P, probability; —, no variables entered the model; SUMP, summer precipitation in the current year; WINP, winter precipitation in the current year; LMNT, average monthly minimum temperature (December–February) in the previous year.]

Population

parameter

n

 (years) Covariate

Parameter

estimate SE Partial R 2 Model R 2 P

Herd survival 19 — — — — — —

Foal survival 10 SUMP 0.164 0.041 0.71 0.71 0.002

WINP 0.117 0.059 0.10 0.81 0.090

Foaling rate 19 LMNT 0.008 0.005 0.15 0.15 0.105
20 Demography of the Pryor Mountain Wild Horses, 1993–2007

studied the sex ratio of more than 10,000 foals removed from 

74 BLM herds, found no significant difference from parity in 

65 of those herds; the foal sex ratio favored females in seven 

herds and males in two herds. Interestingly, one exception to 

this pattern is found in the study by Garrott and Taylor (1990) 

at Pryor Mountain, who reported a sex ratio of 39.7 percent 

males (P < 0.001) in a sample of 330 foals from 1972 to 1986. 

Garrott and Taylor offered no explanation for this result, but 

they did comment that other data from their study might suggest low forage availability. Other investigators have hypothesized that females in poor condition (as through low forage 

availability) should produce fewer male offspring (Trivers and 

Willard, 1973).

The foal sex ratio did not differ from parity during the 

course of our study, and there was no overall difference in 

survival between the sexes. Thus, apart from minor variations resulting from each foal crop entering the population, 

consistent departures from a 50:50 sex ratio are the result 

of removals. This is evidenced by the fact that the largest 

interannual changes in sex ratio occurred following the larger 

removals. The drop in the proportion of males in the herd in 

2007 was because of the removal of 14 males and 5 females in 

2006, but also because natural mortality in 2006 consisted of 

13 males and only 3 females. The increasing age of the herd 

also is mostly explained by removals; for all animals removed 

during the period of study, the average age was 3.2 years 

for females and 2.7 years for males. This is in keeping with 

BLM’s management strategy of retaining harem stallions and 

core breeding-age mares (6–10 years) on the range (Bureau of 

Land Management, 2006). Low survival of foals in 2003 (36.7 

percent) and 2004 (3.6 percent) also reduced the proportion 

of young animals in the herd and thus increased the average 

age. To the extent that it prevented any mares from foaling, the 

contraceptive program also would have contributed to increasing age of the herd, but the magnitude of any such effect is 

unknown.

Survival

The most striking feature of the survival data from our 

study is the general decline in foal survival in 1993–2004, followed by an apparent increase in the next 3 years. It would be 

tempting to attribute at least the declining part of this pattern 

to increased observational effort and hence increased detection of neonatal loss. We do not believe this to be the case, 

however, because there was no trend in the number of foals 

produced through time (linear least squares regression, R2

 = 

0.06, P = 0.38), and because all animals present in the herd in 

1993 and all foals born since then have been accounted for. 

Predation by mountain lions is suspected as the cause of low 

foal survival in 2004 and perhaps in 2003, though only four 

foals were confirmed to have been taken by mountain lions 

(Bureau of Land Management, 2006). Three mountain lions 

were removed from the area by hunters in winter 2004–2005 

(Bureau of Land Management, 2006), which might in part 

explain the increase in foal survival observed in 2005–2007. 

It also seems possible that predation began on a smaller scale 

much earlier than 2003, but we know of no evidence to support this hypothesis. Predation by mountain lions has been 

shown to be an important factor in foal mortality in at least 

one herd (Turner and Morrison, 2001) and has been implicated 

in others (Salter, 1979; Greger and Romney, 1999). Wolves 

(Canis lupus), when present, are also suspected of preying 

on wild horses (Salter, 1979; Oom and Santos-Reis, 1986). 

Another possible contributor to the observed pattern in foal 

survival is drought. The drought began in 1999, the same 

year as the first real drop in foal survival, and was somewhat 

ameliorated in 2005 and 2007, at which time foal survival 

increased. The results of our regression analyses between 

precipitation and foal survival would seem to support this 

explanation. On the other hand, the initial response to reduced 

forage availability would likely be a decline in body condition 

of mares and foals, and such a decline was not observed during the early years of the drought (LC-M).

Overall, survival rates from our study for 1993–2007 are 

similar to those reported by Perkins and others (1979), but 

lower than those reported by Garrott and Taylor (1990) and 

Singer and others (2000) for the same study area (table 10). 

The foal survival rate reported by Garrott and Taylor, however, 

probably is an overestimate because it covers the period from 

midsummer to midsummer; neonatal mortality, which often 

constitutes the majority of foal mortality (Boyd, 1979; Keiper, 

1979; Berger, 1986; Monard and others, 1997; Goodloe and 

others, 2000), likely was missed. Age of the herd also may 

account for some of the difference between our survival rate 

and that calculated by Garrott and Taylor (1990). At the end 

of their study in 1986, about 22 percent of the herd was ≥11 

years of age, whereas the comparable number for 2007 was 

39 percent. Perkins and others (1979) and Singer and others 

(2000) did not provide sufficiently detailed age data to make 

similar comparisons. Survival rates from our study were also 

moderate compared to those reported previously for other 

horse herds in North America (table 10).

Relations between foal survival and band size have not 

often been reported in the literature on wild horses, and the 

few available results are equivocal. We found that foals from 

larger bands tended to survive better than those from smaller 

bands. Working on Camargue horses in France, Feh (1999) 

found that there was less foal mortality in bands with an 

alliance of two stallions (3.2 percent), than in single-stallion 

bands (22.0 percent). Multiple stallions often are associated 

with larger bands. Cameron and others (2003), on the other 

hand, found that among Kaimanawa horses in New Zealand 

more foals died in multistallion bands (survival = 80 percent, 

n = 30) than in single-stallion bands (survival = 94 percent, 

n = 50), but the difference was not quite significant (P = 

0.056). Similarly, evidence for a relation between mare age 

or parity and foal survival is equivocal. We found no relation between foal survival and parity of the mare. Keiper and 

Houpt (1984) reached the same conclusion, but only 10 foals 

died in the course of their study. In contrast, Cameron and 
Source and location

Survival rate (n)

Foals Yearlings Adults Herd Comments

Greger and Romney (1999), 

NV

0.12 NA NA NA Foal survival is a mean for 8 years and is a maxi

neonatal mortality probably went undetected.

Turner and Morrison 

(2001), CA and NV

0.32 0.88 0.92 NA All data represent means for 10 years.

Goodloe and others (2000), 

GA

0.600 (105) NA 0.923 (263) NA Adult rate is for mares only, probably ≥3 years o

This study, (PMWHR) 0.697 (465) 0.917 (252) 0.949 (1,814) 0.899 (2,531)

Perkins and others (1979), 

PMWHR

0.75 NA NA 0.903 (579) Foal survival is a mean for 4 years. Herd surviva

lated from Table 1, Perkins and others (1979)

Boyd (1979), WY 0.824 (51) NA NA NA Data for 1978 only.

Singer and others (2000), 

PMWHR

0.87 NA NA Males, 0.965

Females, 0.975

Not clear that data reported for the herd actually 

Siniff and others (1986), 

Pine Nut, NV

0.875 (40) NA 0.972 (72) NA Adult rate is a weighted mean for 2 years repres

≥2 years old

Siniff and others (1986), 

Pah Rah, NV

0.877 (73) NA 0.975 (81) NA Adult rate is a weighted mean for 2 years repres

≥2 years old

Keiper and Houpt (1984), 

MD and VA

0.883 (86) NA NA NA Foal rate is a weighted mean for 8 years.

Pellegrini (1971), NV 0.905 (21) NA NA NA Data are for 1969. However, in 1968 no foals we

only one foal was observed. 

Berger (1986), NV 0.917 (120) 0.969 (64) 0.951 (329) 0.945 (513) Herd rate calculated from data on the three age g

Garrott and Taylor (1990), 

PMWHR

0.968 (249) 0.977 (173) 0.962 (865) 0.965 (1,287) Excludes data from a severe winter. Adult and h

Table 2, Garrott and Taylor (1990). Foal rate mto midsummer and may be an overestimate.

Table 10. Survival rates reported for North American wild horse (Equus caballus) herds.[Data are arranged in order of increasing foal survival. Sample size (n), when available, is given in parentheses. NA, data not available; PMWHR, Pryor Mountain Wild Horse R
22 Demography of the Pryor Mountain Wild Horses, 1993–2007

others (2000) found significant differences in mortality of 

foals born to mares 3–5 years old (18 percent mortality), 6–8 

years old (3 percent), and ≥9 years old (9 percent).

Foaling Rates

Foaling rates of mares at Pryor Mountain most often 

have been low to moderate when compared to those reported 

for other North American herds (table 11). In fact, the rate of 

0.432 foals/mare for mares ≥4 years of age calculated from 

Feist and McCullough (1975) is among the lowest for any 

herd, although their study was primarily behavioral and lasted 

only 1 year. In addition, they attempted to classify mares aged 

1, 2–3, and ≥4 years without the benefit of examining tooth 

eruption and wear, and they admit that some classification 

errors probably occurred. If there was a tendency to misclassify 3-year-old mares as being ≥4 years old, the resulting 

foaling rate would be biased low. Garrott and Taylor (1990), 

however, reported similarly low rates, and mares in their study 

were aged by dental characteristics. As noted above, Garrott 

and Taylor (1990) reported on foals present in midsummer; 

some neonatal loss likely was missed, which would tend to 

bias foaling rates low. Perkins and others (1979) did not report 

age-specific foaling rates of mares, but for the three years of 

their study for which data are complete (1975–1977), the foal 

crop was 15.1 percent of the prefoaling population (weighted 

average, n = 411 animals ≥1 year of age). The comparable 

value during our study was 21.6 percent (table 1, n = 2,232 

animals). These values, of course, are affected by the sex and 

age structure of the herd. Similarly, Singer and others (2000) 

did not report age-specific foaling rates in sufficient detail 

to be included in table 11. They did, however, show rates of 

0.107 foals/mare for mares of age 2, 0.54 for age 3, 0.67 for 

ages 4–11, and 0.46 for ages ≥12 in the period 1992–1997. 

Comparable values from our study (1993–2007) were 0.048, 

0.414, 0.588, and 0.636, respectively. Thus, it appears that 

foaling rates at Pryor Mountain have varied considerably 

through time, with those from our study being intermediate.

Several investigators have suggested that forage availability can affect the condition of feral mares and thus foaling 

rates (Green and Green, 1977: Nelson, 1978; Berger, 1986; 

Siniff and others, 1986; Garrott and Taylor, 1990). Hall 

(1972) described range conditions at Pryor Mountain as being 

poor to extremely poor through much of the area. Feist and 

McCullough (1975) reported a low foaling rate in 1971, as did 

Garrott and Taylor (1990) in the first 3 years (1976–1978) of 

their study. Furthermore, Garrott and Taylor (1990) suggested 

that low foaling rates likely were a result of poor forage, 

also noting that the foaling rate increased markedly in 1979 

following loss of 51 percent of the herd in the severe winter 

of 1977–1978. Singer and others (2000) reported somewhat 

higher foaling rates than were observed in the earlier studies 

and also noted that former managers who visited Pryor Mountain in 1997 remarked about the overall improvement in “plant 

condition.” Again, our values for foaling rate appear to be 

intermediate. It also may be relevant that mean annual precipitation in the biological years covered by the study of Singer 

and others (17.55 cm; 1992–1997, all years included regardless of missing data) was greater than that for the remainder of 

our study (13.79 cm; 1998–2007).

In addition to its effects on foaling rates of adult animals, forage availability may affect age of first reproduction 

by mares. Feist and McCullough (1975), Perkins and others 

(1979), and Garrott and Taylor (1990) all worked at times 

when forage production may have been poor at Pryor Mountain and all found that mares first foaled at age 3. Singer 

and others (2000), on the other hand, at a time when forage 

production may have been somewhat better, found that 10.7 

percent of 2-year-old mares produced foals; the comparable 

value from our study was 4.8 percent. First foaling at age 2 

(Salter and Hudson, 1982; Berger, 1986; Lucas and others, 

1991) and age 3 (Boyd, 1979; Keiper and Houpt, 1984; Goodloe and others, 2000) has been reported for other herds, with 

the rate for 2-year-olds varying both among herds and within 

herds in different years (Garrott and others, 1991a). In at least 

two studies, low foaling rates in young mares have been attributed to high rates of fetal and neonatal loss (Lucas and others, 

1991; Linklater and others, 2004).

Other factors that have been shown or hypothesized to 

affect foaling rate include gathers, band stability, and parturition history of the mare. In the case of gathers, we found a 

small but statistically significant effect, with foaling rate being 

lower in years following gathers. It must be remembered, 

however, that we did not design an experiment specifically to 

test a hypothesis about the effect of gathers. In particular, we 

had to assume that the majority of mares were affected by the 

gather (either actually moved into a trap or significantly disturbed by helicopter activity) in each of the three years that we 

considered to be gather years. Given the nature of the gathers 

in those three years, we believe that such an assumption is reasonable. Other evidence for an effect of gathers on foaling rate 

is equivocal. Hansen and Mosley (2000) studied two herds, 

one in Idaho and one in Wyoming, and found no difference in 

foaling rate between mares ungathered, gathered and released, 

or gathered and adopted. The ungathered samples were small, 

however, consisting of only 14 mares in each herd. Ashley and 

Holcombe (2001), on the other hand, found that in one of the 

two years of their study, ungathered mares foaled at a higher 

rate than did those that were gathered and released or gathered 

and removed. They found no evidence for an effect of a gather 

in the other year.

Evidence for an effect of band stability on mare foaling 

rate is more consistent. Berger (1983) found that mares in 

bands with resident stallions produced foals at a higher rate 

than those in bands taken over by a new stallion. Kaseda and 

others (1995), working on Misaki horses in Japan, reported 

that mares in “stable consort relationships” had higher reproductive success than those that changed stallions. Similarly, 

Goodloe and others (2000) found that mares that changed 

bands had a lower probability of foaling than those that did 
Source and location

 Foals/mare (n)

Mares ≥2 

years old

Mares ≥3 

years old

Mares ≥4 

years old Comments

Siniff and others (1986), Pine Nut, NV 0.385 (104) NA NA Combined data for 2 years. Age of mares inferred from other in

Feist and McCullough (1975), PMWHR NA NA 0.432 (81) Some animals may have been aged incorrectly. Includes two fo

Greger and Romney (1999), NV NA 0.360 NA Unweighted mean for 5 years.

Garrott and Taylor (1990), PMWHR NA 0.488 (600) NA Calculated from Table 3, Garrott and Taylor (1990).

Wolfe and others (1989), NV 0.432 (206) NA NA Calculated from Table 5, Wolf and others (1989), for lactating m

Boyd (1979), WY, 1979 NA 0.526 0.545 Sample sizes not given.

Turner and others (1992), CA and NV NA 0.537 NA Unweighted mean for 6 years.

Keiper and Houpt (1984), MD NA 0.571 NA Unweighted mean for 8 years.

Nelson, (1978), NM NA NA 0.553 (38)

This study, PMWHR 0.501 (876) 0.576 (752) 0.597 (665)Keiper (1979), MD NA 0.614 NA Unweighted mean for 5 years.

Salter and Hudson (1982), Alberta 0.521 (71) 0.621 (58) NA Calculated from Table I, Salter and Hudson (1982).Seal and Plotka (1983), OR NA 0.626 (99) NA Foaling rate based on lactating mares from Table 4, Ashley and 

Ashley and Holcombe (2001), Granite 

Range, NV, 1994

NA NA 0.714 (28) Calculated only for ungathered horses.

Ashley and Holcombe (2001), Granite 

Range, NV, 1997

NA NA 0.741 (54) Calculated only for ungathered horses. Age of mares inferred fr

Wolfe and others (1989), WY 0.535 (172) NA NA Calculated from Table 5, Wolfe and others (1989), for lactating 

Garrott and others (1991a), NV 0.584 (901) NA NA Foaling rate based on lactating mares gathered in June. Calcula

and others (1991a).

Keiper (1979), VA NA 0.744 NA Unweighted mean for 4 years.

Ashley and Holcombe (2001), Garfield 

Flat, NV, 1997

NA NA 0.824 (17) Calculated only for ungathered horses. Age of mares inferred fr

Boyd (1979), WY, 1978 NA 0.781 0.863 Sample sizes not given.

Siniff and others (1986), Pah Rah, NV 0.670 (109) NA NA Combined data for 2 years. Age of mares inferred from other in

Table 11. Foaling rates reported for North American wild horse (Equus caballus) herds. [To the extent possible, data are arranged in order of increasing production by mares ≥3 years old. Sample size (n), when available, is given in parentheses. NA, data not availab

Wild Horse Range.]
24 Demography of the Pryor Mountain Wild Horses, 1993–2007

not. Data on mare interchanges are available from the Pryor 

Mountain herd, but have not yet been analyzed.

The question of whether parturition history of the mare 

affects foaling rate has been of interest since early investigators reported what they perceived to be a tendency to foal in 

alternate years (Green and Green, 1977; Nelson, 1978). This 

question has been addressed in a number of ways and results 

have varied. Seal and Plotka (1983) found that 68.4 percent 

(n = 76) of pregnant mares also had a foal at their side or were 

lactating, and Wolfe and others (1989) found no difference 

in pregnancy rates for lactating and nonlactating mares in 

six different populations. Neither of these studies, however, 

addressed the question of whether lactating and nonlactating mares are equally successful in bringing their pregnancies to term. On Assateague Island, 10.2 percent of 49 mares 

followed for 4 years foaled in alternate years, whereas 55.1 

percent foaled in either 3 or 4 years (Keiper, 1979). Similarly, 

Keiper and Houpt (1984) could not see any tendency to foal in 

alternate years among 14 mares followed for 8 years. At Pryor 

Mountain, in both our study and that of Garrott and Taylor 

(1990), primiparous mares were less likely to foal in the next 

year than were multiparous mares. Cameron and Linklater 

(2000), on the other hand, found that the probability of foaling 

in a given year was reduced significantly if the mare had a 

foal in the previous year. Thus, while not universal, it appears 

that foaling in one year can sometimes reduce the probability 

of foaling in the subsequent year, particularly in primiparous 

mares.

Harem Stallions and Band Composition

Comparisons among band or harem sizes reported by 

various investigators are hampered by inconsistencies in 

defining what constitutes a harem member, or failure to do 

so at all. We defined harem members as mares ≥2 years of 

age, and found that the mean harem size was 2.2 mares with a 

range of 1.9 to 2.7 over 12 years; the number of bands ranged 

from 29 to 38. In other studies at Pryor Mountain, Feist and 

McCullough (1975) reported only average band size (5.0 

including all members, not just mares), but they found 44 

bands and 106 females ≥2 years of age, so the average harem 

size was likely somewhere near 2.4 mares. Note that the total 

population during the single summer of their study was about 

225 animals, which probably explains the somewhat larger 

number of bands. Perkins and others (1979), working on a 

population that ranged from 95 to 168 animals (1974–1978), 

found 19–27 bands with a mean size of 4.9 (total animals), 

but did not report harem size in terms of number of mares. 

During a period (1992–1997) when the mean population was 

161 animals, Singer and others (2000) reported an increase in 

the number of bands (18 to 31) and a decline in the number 

of mares (age unspecified) per harem from four to two. They 

attributed these trends to an increasing proportion of males 

in the population, death or waning vigor of a number of older 

males, and removal of members from the bands of several stallions aged 10–15 years. Investigators working on other herds 

have found similar or perhaps slightly larger average harem 

sizes. Mean harem size on Cumberland Island was 1.8 mares 

(Goodloe and others, 2000); the authors did not specify the 

age of mares included, but only mares ≥3 years old produced 

foals. Greger and Romney (1999) reported an average harem 

size of 2.4, which included all females ≥1 year of age, excluding daughters. Berger (1986, p. 130) found an average harem 

size of 3.1 mares with a range of 2.73–3.67 across years; age 

of mares was not specified but, because 2-year-old females 

reproduced, mares ≥2 years old may have been included. In 

Misaki horses (Japan), the annual average harem size ranged 

from 1.8 to 5.3 mares ≥2 years old; one stallion in one year 

had a harem of nine mares, and 55.0 percent of 60 harems (14 

harem stallions) observed from 1979 to 1994 had four or more 

mares (Kaseda and Khalil, 1996).

In our study, 62.1 percent of harem stallions first acquired 

a harem at age 5 or 6. Other investigators have found comparable results: Berger (1986, p. 142), age 5–6; Kaseda and 

Khalil (1996), age 4–6; Asa (1999), age 5; Singer and others 

(2000), mean age = 6 years. In the last two years of our study, 

the mean age of harem stallions was 13 and 12 years, which is 

remarkably similar to the mean of 12.8 years reported at Pryor 

Mountain by Perkins and others (1979). Hall (1972), however, 

reported a mean age of 7.8 years, which is more similar to the 

data from the early years of our study. Age structure of removals is undoubtedly an important determinant of such results.

During our study, 6.3 percent of bands had multiple stallions. Feist and McCullough (1975) did not mention whether 

any of the 44 bands present during their study had multiple 

stallions. Perkins and others (1979) found only two multistallion bands in the 19–27 bands present during the 4 years of 

their study; if the mean number of bands present was, say, 23 

per year, multistallion bands would have made up about 2.2 

percent of the total. These percentages are lower than those 

reported for other herds: Goodloe and others (2000), 10.7 

percent, n = 177; Nelson (1978), 11.8 percent, n = 17; Berger 

(1986, p. 129), 12 percent, sample size not given; Salter and 

Hudson (1982), 17.4 percent, n = 23; Asa (1999), 28 percent and 33 percent in 2 years, n = 18 and 9; Miller (1979), 

23–45 percent for 3 years, n = 12–18; Bowling and Touchberry (1990), 39.1 percent, n = 69; Franke Stevens (1990), 50 

percent, n = 12; Green and Green (1977), 45 percent and 54 

percent in 2 years, n = 53 and 67, respectively. Kirkpatrick 

and Turner (1986) suggested that differences in proportions 

of bands with multiple stallions may be related to population 

density and sex ratio, with a greater ratio of males to females 

tending to lead to multistallion bands, and noted that some of 

the reported proportions may be high as a result of inaccurate 

aging of the animals based on body size [for example, Green 

and Green (1977)].
Acknowledgments 25

Bachelors

We have information on the total number of bachelors, 

but not on individual groups, which makes comparisons 

with other studies at Pryor Mountain difficult. Hall (1972) 

reported that the average size of bachelor groups was three, 

with individuals ranging in age from 1 to 5 years, and that 

older stallions usually lived alone after losing their harems. 

Feist and McCullough (1975) found 23 bachelor groups with 

a mean of 1.8 animals per group. Perkins and others (1979) 

found six bachelor groups, all with 2–3 individuals ranging in 

age from 3 to 8 years, as well as six older stallions that lived 

alone. With allowances for varying population size, our results 

do not appear to be greatly different from those found at other 

times at Pryor Mountain.

Most evidence suggests that males leave or are expelled 

from their natal band when they reach sexual maturity, which 

usually occurs at age 3 and occasionally at age 2 (Angle and 

others, 1979; Kirkpatrick and Turner, 1986). Our data show 

a somewhat different pattern. A few males left their natal 

band at age 1, most (54.2 percent) left at age 2, but 17 percent 

remained with their natal band until age 4 or 5. Of the three 

individuals that remained in their natal bands until age 5, two 

subsequently acquired harems and one was still a bachelor (6 

years old) at the end of our study. Interestingly, one of these 

stallions took over his natal band, at which point his dam left 

the band for another. It seems possible that instances of stallions remaining with their natal band until age 4 or 5 account 

for some of the reports in other studies of a high proportion of 

multistallion bands.

Population Regulation

Several investigators have suggested a relation between 

forage availability and foaling rate, but few studies have been 

long enough and had sufficient variation in population size to 

allow detection of density-dependent effects. Berger (1986, 

p. 97) found little evidence for density dependence in his data 

from the Granite Range and suggested that the confounding 

effects of weather were responsible. He did, however, find 

some evidence for a decline in foaling rate with increasing population size in data from Keiper and Houpt (1984). 

Singer and others (2000) found that population growth rate at 

Pryor Mountain declined with increasing population size, but 

this result was clearly dominated by data from a single year 

[Singer and others (2000), fig. 5]. They did not find evidence 

for a density-dependent effect on either foaling rate or survival 

rate. Jenkins (2000) examined data from the Granite Range 

and Pryor Mountain herds, both having individually identified animals, as well as five additional herds where population 

size was estimated using aerial surveys. In all seven cases, 

he found that population growth rate decreased with increasing population size, but only one was statistically significant 

(P ≤ 0.05). However, his meta-analysis of the entire data 

set indicated that the probability of the same result through 

chance alone was only 0.001.

Our results are similar to those of both Singer and 

others (2000) and Jenkins (2000) in that we also found that 

population growth rate declined with increasing population 

size, albeit population size from the preceding year, which 

suggests that lag effects may be important. Furthermore, we 

found similar evidence for a negative relation between overall 

herd survival and population size, and perhaps some weaker 

evidence that the foaling rate of 2-year-old mares is affected 

by population size. We did not, however, find evidence for 

a density-dependent effect on foal survival, perhaps in part 

because Garrott and Taylor (1990), whose study encompassed 

a range of population sizes, did not provide annual foal survival rates.

We used population size as a surrogate for population 

density in these analyses, as did all of the authors cited in this 

section. In doing so, we made the implicit assumption that the 

area used by horses did not change significantly through the 

course of the study. In the case of the Pryor Mountain herd, 

however, horses probably made increasing seasonal use of 

Forest Service lands north and west of the original wild horse 

range. It seems likely that any density-dependent effects would 

have been more pronounced had this increase in use of Forest 

Service land not occurred.

In all cases where we found evidence for density dependence, variability was high, suggesting that other factors also 

may be important in regulating the Pryor Mountain wild horse 

herd. Assuming that the cause of mortality was correctly identified, mountain lion predation may have had a dramatic effect 

on foal survival in at least one year. In the latter years of our 

study, contraception also may have had some effect on foaling 

rate, though it seems likely that this effect was small given 

that relatively few mares were treated and that they were not 

in the most productive age classes. One of the most important 

regulatory mechanisms appears to be precipitation, which 

we found to be strongly related to foal survival. Both density 

dependence and precipitation effects are consistent with an 

interpretation of forage availability per individual as an important parameter. All of these factors combined, however, were 

not sufficient to stabilize the population during our period 

of study, as evidenced by the necessity for large removals in 

several years. 

Acknowledgments

Thanks to the Bureau of Land Management’s Billings 

Field Office for logistical support and making historical data 

available, and to the National Park Service, Bighorn Canyon 

National Recreation Area, for providing access and logistical support for field work on NPS lands. The integrity of 
26 Demography of the Pryor Mountain Wild Horses, 1993–2007

long-term data for the Pryor Mountain herd can be partially 

attributed to the commitment and dedication of the late Reverend Floyd Schwieger, who graciously shared his work and 

answered many questions before his passing. Special thanks to 

the BLM and USGS field technicians who spent their summers 

following horses and accurately collecting the data: Hanem 

Abouelezz, Genie Amelung, Erin Anderson, Laura Caldwell, 

Steve Cordts, Kira Donnelly, Darla Dunaway, Jason Dungan, Melissa Esser, Jill Fanning-Rodgers, Don Galvin, Kayla 

Grams, Patti Grigsby, Susan Hahn, Christine Hardy, Heather 

Jensen, Martina Keil, Dan Kraemer, Megan Lallier, Samantha Lukas, Ian McCluskey, Sean Mead, Jessica Meredith, 

Jill Nannenga, Jane Nibler, Christopher Papouchis, Christina 

Petrandis, Justin Rodgers, Lisa Selner, Jennifer Severude, 

Aaron Swallow, Nani Teves, and Jamie Wages. Thanks also 

to Dan Baker, Jared Bybee, Bob Garrott, Al Kane, and Alan 

Shepherd for providing critical reviews of the manuscript, to 

Tammy Fancher for preparing the map of the study area, and 

to Jennifer Shoemaker for assistance with graphics.
References Cited 27

References Cited

American Association of Equine Practitioners, 2002, Guide 

for determining the age of the horse (6th ed.): Lexington, 

Ky., American Association of Equine Practitioners, 39 p.

Angle, M., Swain, S., Kirkpatrick, J.F., Turner, J.W., Jr., Kenney, R.M., and Ganjam, V.K., 1979, Androgens in feral stallions, in Denniston, R.H., conference coordinator, Symposium on the ecology and behavior of wild and feral equids: 

Laramie, University of Wyoming, p. 31–38.

Asa, C.S., 1999, Male reproductive success in free-ranging 

feral horses: Behavioral Ecology and Sociobiology, v. 47, 

p. 89–93.

Ashley, M.C., and Holcombe, D.W., 2001, Effect of stress 

induced by gathers and removals on reproductive success of 

feral horses: Wildlife Society Bulletin, v. 29, p. 248–254.

Berger, J., 1983, Induced abortion and social factors in wild 

horses: Nature, v. 303, p. 59–61.

Berger, J., 1986, Wild horses of the Great Basin: University of 

Chicago Press, Ill., 326 p.

Bowling, A.T., and Touchberry, R.W., 1990, Parentage of 

Great Basin feral horses: Journal of Wildlife Management, 

v. 54, p. 424–429.

Boyd, L., 1979, The mare-foal demography of feral horses 

in Wyoming’s Red Desert, in Denniston, R.H., conference 

coordinator, Symposium on the ecology and behavior of 

wild and feral equids: Laramie, University of Wyoming, 

p. 185–204.

Bureau of Land Management, 2006, Environmental Assessment for the Pryor Mountain Wild Horse Range, Population Control 2006, EA # BLM-MT-010-FY06-19: Billings, 

Mont., BLM Field Office.

Bureau of Land Management, 2009, Pryor Mountain Wild 

Horse Range/Territory Environmental Assessment, MT-101-

08-24, and Herd Management Area Plan: Billings, Mont., 

BLM Field Office.

Bureau of Land Management, National Park Service, and 

Forest Service, 2008, Pryor Mountain Wild Horse Range 

evaluation: Billings, Mont., BLM Field Office.

Cameron, E.Z., and Linklater, W.L., 2000, Individual mares 

bias investment in sons and daughters in relation to their 

condition: Animal Behaviour, v. 60, p. 359–367.

Cameron, E.Z., Linklater, W.L., Stafford, K.J., and Minot, 

E.O., 2000, Aging and improving reproductive success in 

horses—declining residual reproductive value or just older 

and wiser?: Behavioral Ecology and Sociobiology, v. 47, 

p. 243–249.

Cameron, E.Z., Linklater, W.L., Stafford, K.J., and Minot, 

E.O., 2003, Social grouping and maternal behaviour in feral 

horses (Equus caballus)—the influence of males on maternal protectiveness: Behavioral Ecology and Sociobiology, 

v. 53, p. 92–101.

Conley, W., 1979, The potential for increase in horse and ass 

populations—a theoretical analysis, in Denniston, R.H., 

conference coordinator, Symposium on the ecology and 

behavior of wild and feral equids: Laramie, University of 

Wyoming, p. 221–234.

Cook, C.W., 1975, Wild horses and burros—a new management problem: Rangeman’s Journal, v. 2, p. 19–21.

Cothran, E.G., and Singer, F., 2000, Analysis of genetic variation in the Pryor Mountain wild horse herd, in Singer, F.J., 

and Schoenecker, K.A., compilers, Managers’ summary—

ecological studies of the Pryor Mountain Wild Horse Range, 

1992–1997: Fort Collins, Colo., U.S. Geological Survey, 

Midcontinent Ecological Science Center, p. 91–104.

Eberhardt, L.L., Majorowicz, A.K., and Wilcox, J.A., 1982, 

Apparent rates of increase for two feral horse herds: Journal 

of Wildlife Management, v. 46, p. 367–374.

Feh, C., 1999, Alliances and reproductive success in Camargue stallions: Animal Behaviour, v. 57, p. 705–713.

Feist, J.D., 1971, Behavior of feral horses in the Pryor Mountain Wild Horse Range: Ann Arbor, University of Michigan, 

Master’s thesis, 129 p.

Feist, J.D., and McCullough, D.R., 1975, Reproduction in 

feral horses: Journal of Reproduction and Fertility, Supplement 23, p. 13–18.

Franke Stevens, E., 1990, Instability of harems of feral horses 

in relation to season and presence of subordinate stallions: 

Behaviour, v. 112, p. 149–161.

Frei, M.N., Peterson, J.S., and Hall, J.R., 1979, Aerial census 

of wild horses in western Utah: Journal of Range Management, v. 32, p. 8–11.

Garrott, R.A., 1991, Sex ratios and differential survival of feral 

horses: Journal of Animal Ecology, v. 60, p. 929–937.

Garrott, R.A., Eagle, T.C., and Plotka, E.D., 1991a, Agespecific reproduction in feral horses: Canadian Journal of 

Zoology, v. 69, p. 738–743.

Garrott, R.A., Siniff, D.B., and Eberhardt, L.L., 1991b, 

Growth rates of feral horse populations: Journal of Wildlife 

Management, v. 55, p. 641–648.

Garrott, R.A., and Taylor, L., 1990, Dynamics of a feral horse 

population in Montana: Journal of Wildlife Management, 

v. 54, p. 603–612.
28 Demography of the Pryor Mountain Wild Horses, 1993–2007

Goodloe, R.B., Warren, R.J., Osborn, D.A., and Hall, C., 2000, 

Population characteristics of feral horses on Cumberland 

Island, Georgia and their management implications: Journal 

of Wildlife Management, v. 64, p. 114–121.

Green, N.F., and Green, H.D., 1977, The wild horse population of Stone Cabin Valley—a preliminary report, in Proceedings, National Wild Horse Forum: Reno, University of 

Nevada, Cooperative Extension Service, p. 59–65.

Greger, P.D., and Romney, E.M., 1999, High foal mortality 

limits growth of a desert feral horse population in Nevada: 

Great Basin Naturalist, v. 59, p. 374–379.

Gross, J.E., 2000a, Genetic and demographic consequences 

of removals and contraception on wild horses in the Pryor 

Mountain Wild Horse Range, in Singer, F.J., and Schoenecker, K.A., compilers, Managers’ summary—ecological 

studies of the Pryor Mountain Wild Horse Range, 1992–

1997: Fort Collins, Colo., U.S. Geological Survey, Midcontinent Ecological Science Center, p. 105–120.

Gross, J.E., 2000b, A dynamic simulation model for evaluating effects of removal and contraception on genetic 

variation and demography of Pryor Mountain wild horses: 

Biological Conservation, v. 96, p. 319–330.

Hall, R., 1972, Wild horse biology and alternatives for management—Pryor Mountain Wild Horse Range: Billings, 

Mont., Bureau of Land Management, no pagination.

Hansen, K.V., and Mosley, J.C., 2000, Effects of roundups on 

behavior and reproduction of feral horses: Journal of Range 

Management, v. 53, p. 479–482.

Heitschmidt, R.K., Klement, K.D., and Haferkamp, M.R., 

2005, Interactive effects of drought and grazing on Northern 

Great Plains rangelands: Rangeland Ecology and Management, v. 58, p. 11–19.

Jenkins, S.H., 2000, Density dependence in population dynamics of feral horses: Denver, Colo., Bureau of Land Management, National Science and Technology Center, Resource 

Notes No. 26, 2 p.

Kaseda, Y., and Khalil, A.M., 1996, Harem size and reproductive success of stallions in Misaki feral horses: Applied 

Animal Behaviour Science, v. 47, p. 163–173.

Kaseda, Y., Khalil, A.M., and Ogawa, H., 1995, Harem stability and reproductive success of Misaki feral mares: Equine 

Veterinary Journal, v. 27, p. 368–372.

Keiper, R.R., 1979, Population dynamics of feral ponies, in

Denniston, R.H., conference coordinator, Symposium on 

the ecology and behavior of wild and feral equids: Laramie, 

University of Wyoming, p. 175–183.

Keiper, R., and Houpt, K., 1984, Reproduction in feral 

horses—an eight-year study: American Journal of Veterinary Research, v. 45, p. 991–995.

Kirkpatrick, J.F., and Frank, K.M., 2005, Contraception in 

free-ranging wildlife, in Asa, C.S., and Porton, I.J., eds., 

Wildlife contraception—issues, methods, and applications: 

Baltimore, Md., The Johns Hopkins University Press, 

p. 195–221.

Kirkpatrick, J.F., and Turner, J.W., Jr., 1986, Comparative 

reproductive biology of North American feral horses: 

Equine Veterinary Science, v. 6, p. 224–230.

Linklater, W.L., Cameron, E.Z., Minot, E.O., and Stafford, K.J., 2004, Feral horse demography and population 

growth in the Kaimanawa Ranges, New Zealand: Wildlife 

Research, v. 31, p. 119–128.

Lucas, Z., Raeside, J.I., and Betteridge, K.J., 1991, Noninvasive assessment of the incidences of pregnancy and 

pregnancy loss in the feral horses of Sable Island: Journal of 

Reproduction and Fertility, Supplement 44, p. 479–488.

Miller, R., 1979, Band organization and stability in Red Desert 

feral horses, in Denniston, R.H., conference coordinator, 

Symposium on the ecology and behavior of wild and feral 

equids: Laramie, University of Wyoming, p. 113–128.

Monard, A-M., Duncan, P., Fritz, H., and Feh, C., 1997, 

Variations in the birth sex ratio and neonatal mortality in a 

natural herd of horses: Behavioral Ecology and Sociobiology, v. 41, p. 243–249.

Nelson, K.J., 1978, On the question of male-limited population growth in feral horses (Equus caballus): Las Cruces, 

New Mexico State University, Master’s thesis, 68 p.

Oom, M.M., and Santos-Reis, M., 1986, Ecology, social organization and behaviour of the feral horses in the PenedaGerês National Park: Arquivos de Museu Bocage (sêrie A), 

v. 3, p.169–196.

Pellegrini, S.W., 1971, Home range, territoriality and movement patterns of wild horses in the Wassuk Range of western Nevada: Reno, University of Nevada, Master’s thesis, 

39 p.

Perkins, A., Gevers, E., Turner, J.W., Jr., and Kirkpatrick, J.F., 

1979, Age characteristics of feral horses in Montana, in

Denniston, R.H., conference coordinator, Symposium on 

the ecology and behavior of wild and feral equids: Laramie, 

University of Wyoming, p. 51–58.

Ricketts, M.J., 2004, Pryor Mountain Wild Horse Range 

survey and assessment: Bozeman, Mont., U.S. Department 

of Agriculture, Natural Resources Conservation Service, 

146 p.
References Cited 29

Salter, R.E., 1979, Biogeography and habitat-use behavior of 

feral horses in western and northern Canada, in Denniston, 

R.H., conference coordinator, Symposium on the ecology 

and behavior of wild and feral equids: Laramie, University 

of Wyoming, p. 129–141.

Salter, R.E., and Hudson, R.J., 1982, Social organization of 

feral horses in western Canada: Applied Animal Ethology, 

v. 8, p. 207–223.

Seal, U.S., and Plotka, E.D., 1983, Age-specific pregnancy 

rates in feral horses: Journal of Wildlife Management, v. 47, 

p. 422–429.

Singer, F.J., and Schoenecker, K.A., compilers, 2000, Managers’ summary—ecological studies of the Pryor Mountain 

Wild Horse Range, 1992–1997: Fort Collins, Colo., U.S. 

Geological Survey, Midcontinent Ecological Science 

Center, 131 p.

Singer, F.J., Zeigenfuss, L., Coates-Markle, L., and Schwieger, 

Rev. F., 2000, A demographic analysis, group dynamics, 

and genetic effective number in the Pryor Mountain wild 

horse population, 1992–1997, in Singer, F.J., and Schoenecker, K.A., compilers, Managers’ summary—ecological 

studies of the Pryor Mountain Wild Horse Range, 1992–

1997: Fort Collins, Colo., U.S. Geological Survey, Midcontinent Ecological Science Center, p. 73–89.

Siniff, D.B., Tester, J.R., and McMahon, G.L., 1986, Foaling 

rate and survival of feral horses in western Nevada: Journal 

of Range Management, v. 39, p. 296–297.

Smart, A.J., 2005, Forecasting forage production, in Proceedings, The Range Beef Cow Symposium XIX: Rapid City, S. 

Dak., p. 101–106.

Speelman, S.R., Dawson, W.M., and Phillips, R.W., 1944, 

Some aspects of fertility in horses raised under western 

range conditions: Journal of Animal Science, v. 3, 

p. 233–241.

Trivers, R.L., and Willard, D.E., 1973, Natural selection of 

parental ability to vary the sex ratio of offspring: Science, 

v. 179, p. 90–92.

Turner, J.W., Jr., and Morrison, M.L., 2001, Influence of predation by mountain lions on numbers and survivorship of a 

feral horse population: The Southwestern Naturalist, v. 46, 

p. 183–190.

Turner, J.W., Jr., Wolfe, M.L., and Kirkpatrick, J.F., 1992, Seasonal mountain lion predation on a feral horse population: 

Canadian Journal of Zoology, v. 70, p. 929–934.

Wolfe, M.L., Jr., 1980, Feral horse demography—a preliminary report: Journal of Range Management, v. 33, p. 

354–360.

Wolfe, M.L., 1986, Population dynamics of feral horses in 

western North America: Equine Veterinary Science, v. 6, 

p. 231-235.

Wolfe, M.L., Ellis, L.C., and MacMullen, R., 1989, Reproductive rates of feral horses and burros: Journal of Wildlife 

Management, v. 53, p. 916–924.
Publishing support provided by: 

Denver Publishing Service Center

For more information concerning this publication, contact:

Center Director, USGS Fort Collins Science Center

2150 Centre Ave., Bldg. C

Fort Collins, CO 80526-8118

(970)226-9398

Or visit the Fort Collins Science Center Web site at:

http://www.fort.usgs.gov/

30 Demography of the Pryor Mountain Wild Horses, 1993–2007
 31
Roelle and others— Demography of the Pryor Mountain Wild Horses, 1993–2007—Scientific Investigations Report 2010–5125