Maturation and Reproduction of

Northern Flying Squirrels in

Pacific Northwest Forests

Lisa J. Villa, Andrew B. Carey, Todd M. Wilson, and

Karma E. Glos

United States

Department of

Agriculture

Forest Service

Pacific Northwest

Research Station

General Technical

Report

PNW-GTR-444

July 1999
LISA J. VILLA is a wildlife biologist, ANDREW B. CAREY is a principal research

biologist, TODD M. WILSON is a wildlife biologist, and KARMA E. GLOS is a

biological technician, Pacific Northwest Research Station, 3625 93d Ave. SW.,

Olympia, WA 98512-9193.

Authors
Villa, Lisa J.; Carey, Andrew B.; Wilson, Todd M.; Glos, Karma E. 1999.

Maturation and reproduction of northern flying squirrels in Pacific Northwest

forests. Gen. Tech. Rep. PNW-GTR-444. Portland, OR: U.S. Department of

Agriculture, Forest Service, Pacific Northwest Research Station. 59 p.

Northern flying squirrels are the primary prey of northern spotted owls and are

important dispersers of fungal spores in Pacific Northwest forests. Despite the

importance of these squirrels in forest ecosystems, information is lacking on life

history and methods for determining age and reproductive condition. In the

laboratory, we measured epiphyseal notch, femur length, rostrum length, least

interorbital breadth, rump pelage length and color, and tail width of dead squirrels. We analyzed weight, pelage color, and reproduction for field-collected data

from live squirrels. We also described age-class differences in facial features

and tail size and shape in the Puget Trough. A birth-year-based age-class

system had several advantages over the traditional juvenile-subadult-adult class

system. Three age classes can be distinguished in the field, each with varying

degrees of accuracy. Reproduction of northern flying squirrels can be reliably

determined in live-trapping studies. Enlargement, turgidity, and redness of

genitalia are detectable signs of reproductive activity, whereas reduction, flaccidity, and paleness signal inactivity for both males and females. Nipple length is the

best indicator for distinguishing sexual maturity of inactive females. The most

effective way to avoid mistakes and ambiguity about age and reproductive status

in the field is to take descriptive notes. Maturation and reproductive patterns were

similar among the Puget Trough, Olympic Peninsula, and northern Cascade

studies in Washington. Squirrels in the Coast Range of Oregon, however,

displayed different patterns. Regional populations differed in proportion of

females reproductively active, the reproductive maturity of yearling females, and

survivorship across age classes. Regional variations suggest accurate assessment of age and reproduction is a prerequisite for understanding flying squirrel

ecology.

Keywords: Northern flying squirrel, maturation, reproductive biology, Oregon,

Washington, live-trapping, necropsy.

Abstract
There has been substantial research on old-growth Douglas-fir (Pseudotsuga menziesii

[Mirb.] Franco) forests and wildlife that depend on them (Ruggiero and others 1991).

Northern flying squirrels (Glaucomys sabrinus [Shaw 1801]) have been of particular

interest because they seem to be keystone species in late-seral forests—a main prey of

strigids and mustelids on one trophic level, and a consumer and disseminator of fungal

spores on another. Through its mission to develop forest management practices that

steward biodiversity, the Ecological Foundations of Biodiversity Research Team has

focused research on the relation between northern flying squirrels and forest health.

Despite the importance of northern flying squirrels in Pacific Northwest forest ecosystems, information is lacking on life history and methods for determining age and reproductive condition. We began studying flying squirrel populations in 1985 in Douglas-fir

forests of the Coast Ranges physiographic province in southwestern Oregon (Franklin

and Dyrness 1973). At that time, there was limited guidance for determining age and

reproduction of flying squirrels in the field (Davis 1963, Wells-Gosling 1985). Since

then, we have studied flying squirrels in the interior valleys and Klamath Mountains

of Oregon, and the Olympic Peninsula, northern Cascades, and Puget Trough of

Washington. We combined the skills of field biologists and laboratory technicians to

assemble and refine laboratory and field methods for determining age and reproduction.

This report is one in a series of method and biology papers that provide biologists with

tools for designing and implementing research and monitoring efforts for Pacific

Northwest forests (appendix 1).

1 Introduction

1 World Status of Flying Squirrels

1 Ecology in the Pacific Northwest

1 Importance of Age and Reproduction

2 Goals of This Study

3 Study Areas

3 Sampling Method

4 Sample Sizes

4 Statistics

4 Technique Development—Determining Age

5 Age Classification

5 Epiphyseal Notch

9 Femur Length

Preface

Contents
9 Skull Characteristics

11 Body Mass

13 Pelage and Molt

18 Reproductive Status

19 Recommendations for Assessing Age

20 Technique Development—Reproductive Condition and Litter Size

21 Reproductive Status

22 Necropsy Results

26 Field Comparisons to Necropsy Results

27 External Characteristics in the Field

30 Litter Size

31 Recommendations for Assessing Reproduction

32 Reproductive Biology

33 The Reproductive Cycle in the Pacific Northwest

35 Breeding Season

35 Ratios of Reproductive Activity

37 Population Characteristics

38 Sex Ratio

38 Age Class Ratio and Survivorship

39 Implications

39 Environment and Maturation

41 Density

41 Predation

42 Body Size and Energy Requirements

43 Selection Factors—r and K
43 Acknowledgments

43 Metric Conversions

44 Literature Cited

50 Appendix 1: Sampling Methods

52 Appendix 2: Arboreal Rodent Trapping Data Processing

54 Appendix 3: Female Reproductive Cycle

56 Appendix 4: Male Reproductive Cycle

57 Appendix 5: Necropsy Data Form

58 Appendix 6: Trapping Data Code Sheet

59 Appendix 7: Arboreal Rodent Live-Trapping Data Sheet
1

Throughout the world, flying squirrels (Sciuridae: Rodentia) and their habitat are

threatened by human activities. Flying squirrels, in various genera, seem to be

declining; 11 species and subspecies of flying squirrels are currently considered

threatened due to forest depletion (Nowak 1991; table 1). These squirrels are

becoming rare, and some species may become extinct before they are even

partly understood. Thorough studies of flying squirrels and the forests they

inhabit are essential to our understanding of forest ecosystems. Rapid habitat

alteration worldwide makes this work urgent.

The northern flying squirrel (Glaucomys sabrinus [Shaw 1801])1

 is an important

member of the arboreal rodent community of Pacific Northwest Douglas-fir

(Pseudotsuga menziesii Mirb. Franco), western hemlock (Tsuga heterophylla

[Raf.] Sarg.), and mixed-coniferous/deciduous forests. This nocturnal squirrel

shares habitats with the diurnal Douglas squirrel (Tamiasciurus douglasii

[Bachman 1839]) and semiarboreal diurnal Townsends chipmunk (Tamias

townsendii [Bachman 1839]) in the Pacific Northwest (Carey 1991). Northern

flying squirrels are the primary prey of northern spotted owls (Strix occidentalis

caurina [Xántus de Vesey 1860]) west of the Cascade crest (Carey and others

1992, Forsman 1984) and are preyed on by weasels (Mustela spp.), marten

(Martes americana [Turton 1806]), and other carnivores (Wells-Gosling and

Heaney 1984, Wilson and Carey 1996). Flying squirrels consume sporocarps of

ectomycorrhizal fungi that live symbiotically with forest plants, in particular with

conifers in the family Pineaceae (Carey 1995; Maser and others 1985, 1986).

Soil is inoculated with fungal spores from squirrel feces. The fungi enhance root

systems of Douglas-fir and other conifers, improving water and nutrient uptake

(Maser and Maser 1988). Studying flying squirrels will enhance our knowledge of

aboveground and belowground processes that can be used to improve forest

health and enhance biological diversity (Carey and others 1996).

Age structure indicates whether populations are increasing, decreasing, or stable

(Hutchinson 1978). Fertility and mortality differ by age for many species, and

population models can be developed with age-dependent birth and death rates

(Johnson 1994). Age structure combined with reproductive development can help

determine specific factors that regulate populations. For example, data on reproductive rates, sex ratios, and age structures can be used to calculate gross natality (Harder and Kirkpatrick 1994). Age structure and reproduction are also good

indicators of habitat quality (Maguire 1973), whereas density could be misleading as a measure of habitat quality by failing to distinguish between ecological

sources and sinks (Van Horn 1983). In addition, reproductive potential and

carrying capacity of prey determine the amount of predation a population can

sustain and, thus, carrying capacity for predators (Maguire 1973).

Techniques that measure continuous morphological development are commonly

used to determine age and reproductive characteristics. These techniques are

based on skeletal features, pelage, and genitalia that progressively change from

birth to death and provide a permanent record of attained age and reproduction

(Larson and Taber 1980). Criteria for ideal age-determination techniques include

Introduction

World Status of Flying

Squirrels

Ecology in the Pacific

Northwest

Importance of Age and

Reproduction

1 Scientific names for animals are from Banks and others

1987.
2

(1) independence from irregular nutritional and physiological variations, (2) clear

separations into age classes or year classes without subjective judgment, (3)

suitability for living animals of all ages, and (4) ease of application by semiskilled

technicians (Larson and Taber 1980).

Research on flying squirrel age and reproduction is limited. Wells-Gosling and

Heaney (1984) provide an overview of flying squirrel habits and biology. Carey

(1991, 1995) summarizes information on arboreal rodents specific to the Pacific

Northwest. Carey and others (1991) describe methods for assessing flying

squirrel populations. Davis (1963) describes the reproductive biology of northern

flying squirrels in Saskatchewan and methods for aging. Muul (1969b) and

Sollberger (1943) describe the reproductive biology of southern flying squirrels

(Glaucomys volans [Linnaeus 1758]). McCravy and Rose (1992) analyze

external features that may be used as predictors of reproductive status of small

mammals in the field. Morris (1972) reviews aging methods used to assess both

relative and absolute age in mammals.

Our goal is to provide information on the life history of northern flying squirrels in

Pacific Northwest forests, emphasizing maturation and reproductive development.

Our objectives are to (1) describe methods and data used to determine age and

reproductive class in the laboratory and field; (2) evaluate field techniques based

on laboratory findings and suggest criteria for determining age and reproductive

status of live flying squirrels; (3) summarize the reproductive biology of flying

squirrels with an emphasis on reproductive cycle, breeding season, and ratios of

reproductively active animals; and (4) summarize population characteristics of

flying squirrels in the Pacific Northwest.

Table 1List of threatened and endangered flying squirrels worldwide

Area Species Common name

China (Beijing) Aeretes melanopterus Grove-toothed flying squirrel

China (Yunnan) Trogopterus xanthipes Complex-toothed flying squirrel

Finland Pteromys volans European flying squirrel

Himalayas Belomys pearsoni Hairy-footed flying squirrel

Nepal Hylopetes alboniger Arrow-tailed flying squirrel

Pakistan Eupetaurus cinereus Woolly flying squirrel (Zahler 1996)

Sumatra Petinomys spp. Dwarf flying squirrels

Thailand, Malaysia Pteromyscus pulverulentus Smoky flying squirrel

U.S. (Southeast) Glaucomys sabrinus fuscus Northern flying squirrel (Federal

 Register 50:126)

U.S. (Southeast) Glaucomys sabrinus coloratus Carolina Northern flying squirrel (USDI

 FWS 1992)

U.S. (West) Glaucomys sabrinus californicus Northern flying squirrel (USDI FWS 1994)

Source: Nowak (1991) except where otherwise noted.

Goals of This Study
3

We studied flying squirrels in nineteen 16-ha stands (eight old growth, three

mixed aged, and eight young) in the transition from the Western Hemlock Zone

to the Mixed-Conifer/Mixed Evergreen Zone (Franklin and Dyrness 1973) in the

Coast Range of southwestern Oregon (1985-90; table 2). We examined fourteen

16-ha stands (six old growth, two mixed age, and six young) on the Olympic

Peninsula during 1987-89. In the Puget Trough, we studied sixteen 13-ha stands

in two second-growth Douglas-fir forests in Thurston County during 1991-97. In

the northern Cascades, we studied three 16-ha old-growth stands in Whatcom

County in 1991, and two 16-ha young Douglas-fir stands in Skagit County in

1992 and 1996. Most of our data come from the Coast Range and Puget Trough.

Most squirrels were captured, marked, and released during spring and fall trapping sessions (April-June, September-November). Trapping efforts are most

successful in these seasons presumably due to rainfall, which stimulates the

fruiting of hypogeous fungi and a concomitant increase in ground foraging by

squirrels (Carey and others 1991). Summer trapping efforts in the Coast Range

and Olympic Peninsula yielded low captures; we subsequently stopped trapping

during this period. Rosenberg and Anthony (1992) trapped for flying squirrels in

the western Cascades of Oregon in late summer and also had low captures.

Live-trapping grids usually were trapped for six to eight nights within a 2-week

period. When many grids were trapped, trapping sessions lasted up to 6 weeks.

We found 2-week trapping periods adequate for sampling flying squirrel populations in most years. Rosenberg and others (1995) had low capture rates and

recommended trapping for 3 weeks. We do not recommend this as we believe it

violates assumptions of closure (the size of the population is constant over the

Table 2Pacific Northwest forests studied by the Ecological Foundations of Biodiversity Team, Oregon

and Washington, 1985-97

No. of

Provincea Location Years grids Grid size Elevation Forest age Forest type

Ha m

Coast Rangesb Roseburg, OR 1985-90 19 16 79-610 Old-growth, young Douglas-fir, western

stands w/ hemlock, mixed

old-growth conifer

legacies, young

Olympic Olympic 1987-89 14 16 85-610 Old-growth, Western hemlock,

Peninsula Peninsula, WA young Douglas-fir,

Pacific silver fir

Northern Baker Lake, WA 1991 3 16 475-610 Old-growth Western hemlock,

Cascades Darrington, WA 1992, 1996 2 16 Young Pacific silver fir,

Douglas-fir

Puget Trough Fort Lewis, WA 1991-97 16 13 116-146 Young Douglas-fir

a From Franklin and Dyrness (1973).

b

 And adjunct interior valleys and Klamath Mountains.

Sampling Method

Study Areas
4

period of investigationno recruitment or losses occur; White and others 1982).

Instead we recommend following procedures that result in high probabilities of

capture (Carey and others 1991):

 Carefully set traps so that they are rigid, stable, and immovable with moderate

hand pressure.

 Trap during spring or fall rains.

 Carefully adjust traps to release crisply and completely with light pressure.

 Check adjustment and add bait (peanut butter, oats, and molasses) daily.

 Use large 10 by 10 grids with 30- to 40-m spacing and two traps per station

(200 per grid).

Squirrels were live-trapped in the Puget Trough from 1991-95. Additional data

were collected from squirrels found in nest boxes (part of an experimental study)

in the Puget Trough from 1993-97. Nest boxes were examined during August to

collect information on reproductive success. All squirrels (including young) were

eartagged and released.

Our studies occurred over a 13-year span (and continue still). Sampling efforts

differed for each technique we examined depending on time of analysis and

specimen condition and availability; sample sizes are listed for each technique.

Overall, we captured 1,386 individual northern flying squirrels 4,971 times.

Squirrels were eartagged, weighed, and carefully examined for age and reproductive condition; 208 squirrels died and were necropsied. These animals were

intentionally removed for necropsy during 1987-88 in the Coast Range or died in

live traps (usually from hypothermia or predation). All reproductive tissues were

preserved. Most Coast Range specimens were prepared as study skins and

skeletons by the Burke Museum at the University of Washington.

Descriptive statistics and probability plots were calculated for data sets and

assessed for normal distribution. Parametric techniques were used for all

variables that were normally distributed. For comparisons of two groups, we

used Students-t test. For comparisons of more than two groups, we used oneway ANOVA with Duncans multiple comparison test. All comparisons were evaluated at α = 0.05.

Here we will discuss the development of techniques for determining age and

reproduction in northern flying squirrels. For each technique, we provide a brief

background on its use, the specific process we used, and the results of our

application.

When mammals are caught in traps, it is often difficult to assign an accurate age

to individuals unless a long capture history or date of birth is available. Relative

age, however, can be assigned based on comparison with other individuals in the

sample, and age classes can be approximated (DeBlase and Martin 1981, Morris

1972). As many features as possible should be assessed to enhance accuracy

(Dimmick and Pelton 1994, Morris 1972). Here we review use of epiphyseal

notch, femur length, cranial measurements, body mass, pelage, and reproductive

status as measures of age.

Technique

Development

Determining Age

Statistics

Sample Sizes
5

Field determination of age for small mammals is usually limited to two classes:

juvenile and adult. Squirrels (Sciuridae), however, often can be classified into

three age classes: adult, subadult, and juvenile (Dimmick and Pelton 1994).

Adults are the large, breeding members of a population. Subadults are adult size

or slightly smaller, may or may not be in breeding condition, but do not breed.

Often subadults are yearlings. Juveniles are smaller than subadults or adults,

often have a distinct pelage (DeBlase and Martin 1981), and most are young of

the year.

We used three categories to age flying squirrels in the Coast Range and Olympic

Peninsula based on relative size, appearance, and evidence of reproductive

maturityjuveniles were defined as small with uniform grey pelage and with no

reproductive development, subadults were animals at or near adult size with no

sign of reproductive activity, and adults were defined as full-grown animals in

breeding condition or with evidence of past reproductive activity. Weight classes

for each category were developed from early data sets and incorporated into

aging guidelines.

Over time, however, we noticed subtle external differences among individuals

and suspected that additional external characteristics might be used to determine

relative age. We also sought to separate age classification from reproduction

and weight. In the Puget Trough, therefore, we developed a birth-year-based

age class system. This classification is commonly used with animals that live for

several years and have no more than one reproductive cycle per year, such as

flying squirrels. Birth-year class is similar to year-class, which is the actual

chronological age in years (Larson and Taber 1980). Each age class represents

a cohort born in a given calendar year (January 1-December 31). We labeled

classes numerically: class I individuals were born in the current calendar year

in which they were examined (trapped), class II individuals were born the previous year, class III the year before class II, and so forth. We attempted to age

individuals as far back as possible by using external characteristics.

The most reliable reported measure of age for northern flying squirrels is closure

of the epiphyseal notch (Davis 1963). Bone growth and development can be

measured by the thickness and ossification of the cartilage plate adjoining the

epiphysis and diaphysis of leg bones. Thickness of the epiphyseal cartilage is

reduced as an animal grows older and gradually disappears until complete ossification of the cartilage has occurred (Carson 1961). Unfused epiphyses are

therefore an indication of continuing growth and may serve as an index of age

(Morris 1972). This growth can be determined by x-ray. In addition, if cartilage is

removed by boiling or with chemicals, the epiphyseal notch is exposed (Davis

1963). The presence or absence of the epiphyseal notch can then be determined

without x-rays (fig. 1).

Examining the condition of epiphyseal cartilage was originally proposed by

Thomsen and Mortensen (1946) for classifying the age of cottontail rabbits

(Sylvilagus floridanus [J.A. Allen 1890]; Hale 1949). Petrides (1951) used x-rays

of gray squirrels (Sciurus carolinensis [Gmelin 1788]) to examine the degree of

Age Classification

Epiphyseal Notch
6

fusion of the epiphyses of the radius and ulna at the wrist to distinguish young

from adults. Carson (1961) extended use of x-rays in gray and fox squirrels (S.

niger [Linnaeus 1758]) by developing a reference set of x-rays from known-age,

captive squirrels. Expanding Petridess (1951) designation of young and adult,

Carson (1961) found that, within certain limits, squirrels could be placed in three

age classes (juvenile, subadult, and adult) based on condition of the epiphyseal

cartilage of the radius and ulna. The epiphyseal cartilage had three distinct

thicknesses during the longitudinal growth of these bones. In both fox squirrels

and gray squirrels, however, multiple litters and extended breeding seasons made

placing individuals into three distinct groups difficult.

Northern flying squirrels are believed to produce only one litter per year (WellsGosling and Heaney 1984), and thus epiphyseal cartilage development likely

would exhibit less overlap among cohorts than observed in other sciurids. Davis

(1963) was the first to use epiphyseal notch to assess age in flying squirrels.

Following Kirkpatrick and Barnett (1957), Davis (1963) examined the thickness

of the epiphyseal cartilage not only by radiograph but also by direct observation

of cleaned and dried epiphyseal notches of flying squirrels. Daviss age classification of juvenile, subadult, and adult was based on presence and degree of

Figure 1Epiphyseal notch condition of the distal end of the northern flying

squirrel femur by age class. The notch is wide in an age class I animal (A),

becomes a line at age class II (B), and nearly disappears by age class III

(C).
7

ossification and closure of the epiphyseal notch and was similar to that used by

Carson (1961). More recently, epiphyseal notch and femur length were used by

Forsman and others (1994) to determine the age of flying squirrels from remains

found in owl pellets on the Olympic Peninsula.

MethodWe examined the epiphyseal notches from the proximal end of femurs

from 140 Coast Range and 35 Puget Trough flying squirrels. Bones were first

boiled, then examined under a dissecting microscope at 10X magnification.

Femurs were sorted into three groups: little or no ossification, moderate to mostly

ossified, and completely ossified (fig. 1). We assumed complete ossification

occurred around 16 to17 months, during late fall (Davis 1963).

ResultsEpiphyseal notches of animals that died during spring and fall were

easily sorted into three groups. During spring, two notch groups could be

foundanimals born the previous year (8 to 12 months old) were placed in

notch group 2 (moderately ossified), and notch group 3 consisted of animals

> 24 months (completely ossified). All three notch groups could be found in fall.

Assuming that young of the year are born from May through August, the first

notch group (little or no ossification) consisted of juveniles 2 to 5 months old, the

second group (mostly ossified) were squirrels 14 to 17 months old, and the third

notch group (completely ossified) were animals > 26 months.

Comparison of epiphyseal notch aging with field agingIn the Coast Range,

aging was heavily dependent on reproductive condition and less emphasis was

placed on chronological age. As a result, direct comparisons between epiphyseal

notch classes and field age classes (using external characteristics of live animals) were difficult. Notch group 1 individuals (n = 6), with the least amount of

ossification, were identified as subadults (n = 6 captures) and juveniles (n = 3

captures) in the field. Notch group 2 individuals (n = 48), with moderate ossification, were called juveniles, subadults, and adults in the field, depending on weight

and reproduction. In spring, notch group 2 females were identified as subadults

(34 of 57 observations) more often than adults (22 of 57 observations) or juveniles (1 observation). This was likely due to no signs of past reproduction, and

they could thus be more easily distinguished from older adults. Notch group 2

males, however, were already scrotal by spring trapping and thus were called

adults (41 of 50 observations) rather than young. In fall, when three age classes

were present (with the addition of young of the year) males in notch group 2

were identified as adults (12 of 19 observations), young (7 of 19 observations),

and juveniles (2 of 19 observations). Notch group 2 females were called adults

(four of five observations) and young (one observation). In both seasons, animals

in notch group 3, with complete ossification, usually were called adults in the field

(females = 29 of 29 observations in spring and 3 of 4 observations in fall; males

= 18 of 24 observations in spring and 16 of 16 observations in fall).

The birth-year-based age system used in the Puget Trough more closely followed

that of epiphyseal notch groups. Notch group 1 individuals (n = 15), with little or

no ossification, were aged by trappers as class I in the field 28 of 36 times during October and November. Fourteen of these fifteen individuals (93 percent)
8

were aged as class I after routine laboratory review of data, which took into consideration technician experience, multiple records, and detailed notes (appendix

2). The one misidentified squirrel was captured in early October and was unusually large (148 g); it is likely that weight biased technician decisions. Notch group

3 individuals were aged as class III 100 percent of the time (26 captures) in

spring and 83 percent (18 captures) in fall (October to mid-November). All of

these individuals (n = 13) were identified as age class III after data review.

We had a sample of only three individuals aged as class II by epiphyseal notch.

This was insufficient to evaluate the ability of technicians to identify class II

squirrels in the field. We therefore examined 79 live-capture records from 32

squirrels; 3 aged by epiphyseal notch and 29 aged as class I the previous fall

(determined by multiple external characteristics or found in nest boxes as weanlings). During January and February, when class II squirrels were 7 to 9 months

old, age class was accurately assessed in the field 92 percent of the time (11 of

12 captures). By May, technicians correctly aged 77 percent (n = 52 captures)

of the squirrels; 58 percent of the incorrect observations in spring were made by

inexperienced technicians. In fall, 47 percent (n = 15 captures) of the squirrels

were correctly aged. All incorrect observations in fall were of technicians misidentifying class II squirrels as class III; none of the squirrels were misidentified

as class I squirrels.

Epiphyseal notch age corresponds with birth-year age during most of the year.

However, the change from little ossification to moderate ossification and the

change from moderate ossification to full ossification both occur during late fall

(and not on January 1 as calendar aging does). Four specimens we collected in

late November, and December could not be grouped by age class without knowing the months in which they were collected. This does not present problems for

trapping studies in spring and fall, but it could affect winter trapping studies or

studies where mortality dates are unknown.

Epiphyseal notch seems to be the most reliable and objective technique for assigning age to flying squirrels (Davis 1963); however, methods based on annuli of

tooth cementum have not been explored. For field studies of live flying squirrels,

epiphyseal notch is most useful for providing known-age references from specimens. These specimens then can be used to develop external characteristic

standards for each age class to account for regional differences. Also, use of a

known-age reference set is helpful in providing external criteria for separating

class II from class III squirrels as both can be similar in maturation and size.

For clarity and ease of comparison, subsequent sections in this report use only

data from epiphyseal notch-aged flying squirrels for the Coast Range and

Olympic Peninsula, except where otherwise noted. This allowed us to convert the

age of these squirrels to a birth-year class system for more direct comparisons

with the Puget Trough.
9

MethodFemurs from 87 Coast Range and 19 Puget Trough squirrels were

cleaned and examined according to Davis (1963). Femur length was measured

from head to medial condyle on the anterior surface.

ResultsIn the Coast Range, there were significant differences among all three

age classes (F = 32.25, df = 2, p < 0.01; table 3). Femurs from Puget Trough

class II squirrels were not represented. Femurs from class I Puget Trough

squirrels, however, were not significantly larger (t = 1.59, df = 12, p = 0.14) than

Coast Range class I squirrels. Femurs were longer in class III Puget Trough

squirrels than in class III Coast Range squirrels (t = 2.56, df = 42, p = 0.01).

Femur length can be used to separate age classes; regions with large-bodied

squirrels, such as the Puget Trough, may show even larger separation of age

classes. Femur length also provides a quantifiable measure that supports epiphyseal notch classes. The entire bone (including epiphyseal notch), however, must

be present to measure femur length. Femur length would not be a substitute

measurement when the epiphyseal notch was absent or for broken bones (e.g.,

as may occur in owl pellets).

Cranial characteristics of many mammals may be used to identify immature animals (DeBlase and Martin 1981). Accurate and consistent skull measurements

can be taken, which are not normally influenced by large seasonal variations or

nutritional status (Morris 1972). Measurements can include subjective evaluation

of cranial suture closure, number and condition of teeth, cementum annuli in

teeth, and measure of skull width and length. We explored the latter.

In the field, we noticed that class I flying squirrels had small, pointed heads

with close-set eyes, whereas class II and older animals tended to exhibit a

thickened snout, broader, more angular or blocky forehead, and widely spaced

eyes (fig. 2). Class II and III physiognomy appeared robust; class I individuals

appeared delicate. Field notes also suggested that adults might be separated

further, with older adults having even blockier and broader faces.

MethodWe used two cranial measurements to determine if facial features

observed in the field could be quantified in the laboratory. We measured least

interorbital breadth and rostrum length of intact skulls from 62 Coast Range

squirrels aged by epiphyseal notch (study skulls were not prepared from Puget

Trough specimens).

ResultsBoth interorbital breadth and rostrum length increased as squirrels

aged. There was a significant increase (F = 5.44, df = 2, p < 0.01) in interorbital

breadth between class I and older animals; overall, breadth increased 11 percent

from class I to class III. There was no significant difference between class II

(spring and fall) and class III individuals. Rostrum length differed significantly

(F = 3.17, df = 2, p = 0.049) between class I and fall class II (and older) individuals. Length, however, increased by only 5 percent between class I and class III.

Skull Characteristics

Femur Length
10

These cranial measurements suggest that, in live animals, as the breadth of the

face increases, the snout does not increase proportionately. This creates a wide,

blocky-appearing head in older squirrels and supported our field observations.

This difference is most noticeable between class I and older individuals. With

experienced personnel, class II squirrels can be separated from class III squirrels in spring; it appears that this separation between class II and III cannot be

reliably made by fall however. We also noticed a difference in facial hair between

age groups that may account for perception of age; a mixture of golden and dark

pelage provides more contrast and definition in adult facial appearance than dark

gray pelage gives to juvenile facial appearance.

Table 3External and internal characteristics for determining age in northern flying squirrels

(Coast Range and Puget Trough, 1985-94)

Characteristic Age class I Age class II Age class III

External

Pelage color and condition Generally grey and short, tail More gold-tipped hair, tail Gold-tipped hair, tail golden

narrow and pointed, face golden and thicker, face and square, face golden

cream and grey, fur thin golden orange, fur orange, fur lustrous and

and drab lustrous and thick very thick

Munsell soil color Dorsal 5/6 (4-6/4-6) Dorsal 6/7 (4-8/4-8) Dorsal 6/7 (4-7/3-8)

Chart 10YR Ventral 8/3 (7-8/2-4) Ventral 7/6 (5-8/3-8) Ventral 7/6 (7-8/4-8)

Value/chroma Tail tip 3/1 (3-4/1) Tail tip 3/1 (2-4/1) Tail tip 3/1 (2-4/1)

Tail width (mm)

Mean ± S.E. CR a

 37.30 ± 1.84 n = 6 CR 38.85 ± 0.68 n = 43 CR 45.24 ± 0.94 n = 24

Pelage length (mm)

Mean ± S.E. CR 10.78 ± 0.36 n = 6 CR 13.21 ± 0.17 n = 44 CR 13.88 ± 0.18 n = 27

PT 10.50 ± 0.34 n = 8  PT 13.70 ± 0.29 n = 11

Facial features Small, delicate features, face Less delicate features, Mature, blunt features, face

pointed and narrow face slightly pointed blocky and wide

Body weight (g)

Mean ± S.E. (range) CR b

 84.2 ± 3.6 n = 17 (51-111) CR 115.8 ± 1.6 n = 46 (83-149) CR 132.4 ± 1.8 n = 22 (120-149)

PT c 100.3 ± 1.8 n = 58 (62-148) PT 129.0 ± 2.5 n = 28 (108-155) PT 141.8 ± 1.7 n = 97 (110-185)

Internal

Interorbital breadth (mm)

Mean ± S.E. CR 7.04 ± 0.23 n = 5 CR 7.65 ± 0.08 n = 38 CR 7.81 ± 0.10 n = 19

Rostrum length

Mean ± S.E. CR 12.14 ± 0.23 n = 5 CR 12.56 ± 0.08 n = 38 CR 12.77 ± 0.13 n = 19

Femur length (mm)

Mean ± S.E. CR 34.45 ± 0.82 n = 6 CR 38.24 ± 0.19 n = 48 CR 39.02 ± 0.19 n = 33

PT 35.99 ± 0.57 n = 8  PT 39.99 ± 0.33 n = 11

Epiphyseal notch condition Notch present and wide, Notch reduced to a crack or Notch absent, bone

epiphyseal cartilage thick, line, epiphyseal cartilage smooth, epiphyseal

sandwiched, head of femur reduced to thin disk, head of cartilage completely

poorly developed femur developed ossified, head of femur

well developed

 = no data available

a

 CR = Coast Range, PT = Puget Trough.

bCoast Range field body weights from epiphyseal notch-aged animals only.

cPuget Trough field body weights from epiphyseal notch-aged animals, animals captured originally as weanlings, animals with >2 years

of trapping records, or animals described as reproductively active the previous year.
11

Mass can be used with other factors to assess age (Uhlig 1955). Mass is a

measure of size and, if increase in size is directly correlated with increasing

age, then mass may be used as an age index (Morris 1972). This is likely during initial development, when growth in mammals is continuous, providing a

means for separating the youngest individuals from adult members of a population (DeBlase and Martin 1981). Once animals reach adult size, mass is not a

reliable indicator of age because weight will differ with seasonal and year-to-year

physical condition.

In Saskatchewan, flying squirrel young weighed 3.5 g at 2 days old. They rapidly

increased in weight to 6 weeks old, then grew slowly until reaching adult weight

(130 to 212 g) (Davis 1963). Davis (1963) reported that class I squirrels could be

separated from older animals by body weight alone only until mid-September.

MethodsWe analyzed body weight for each age class. In the Coast Range,

only squirrels aged by epiphyseal notch were included in the sample. For Puget

Trough squirrels, we included all field-determined or epiphyseal-notched class I

individuals, class II individuals aged by juvenile records or epiphyseal notch, and

class III individuals aged by epiphyseal notch, > 2 years of trapping records, or

trapping records showing reproductive activity during the previous year. For

Olympic Peninsula and northern Cascade squirrels, we summarized weights for

juveniles and older animals (combining subadults and adults) because birth-year

age class was not used, and we had few mortalities to obtain epiphyseal notches.

Weights from first-captures in each trapping session were used in analyses.

Trappers used 300-g Pesola2 scales and took weights to the nearest gram.

Scales were tested for accuracy before each trapping session.

ResultsEighty-five Coast Range, 183 Puget Trough, 102 Olympic Peninsula,

and 29 northern Cascade squirrels were examined from captures in spring

(April-June) and fall (October-November). There were significant differences

between sexes, among seasons, and among age classes (tables 3 and 4).

2

 The use of trade or firm names in this publication is for

reader information and does not imply endorsement by the

U.S. Department of Agriculture of any product or service.

Body Mass

Figure 2Northern flying squirrel facial features in profile by age class. Note the narrow, pointed

features of an age class I animal (A), the broadening of an age class II head (B), and the wide, blocky

head of age class III (C).
12

Coast RangeDuring spring, age class III females were significantly heavier

(t = -4.77, df = 26, p < 0.01) than class II females. Class III males were also

significantly heavier (t = -4.53, df = 26, p < 0.01) than age class II males. There

were no significant differences in weight between males and females for class II

(t = -0.38, df = 36, p = 0.71) or class III (t = 0.41, df = 16, p = 0.69).

In fall, class I males were significantly lighter (f = 20.51, df = 2, p < 0.01) than

class II and III males; there was no significant difference between class II and III

males (t = -0.82, df = 8, p = 0.43). There were also no significant differences

between males and females for class I (t = -0.45, df = 15, p = 0.66). There were

no differences in weights between spring and fall for class II males (t = -1.52, df

= 24, p = 0.14) or class III males (t = 1.10, df = 10, p = 0.30). Sample sizes for

class II and III females in fall were insufficient to evaluate statistically.

Puget TroughIn spring, class II females were significantly smaller than class

III females (t = -4.39, df = 34, p < 0.01); there was no significant difference

between class II and III males (t = -2.44, df = 36, p = 0.16). During fall, class I

females were lighter than class II or III females (F = 62.51, df = 2, p < 0.01);

there was no difference between class II and III individuals.

In most seasons, weights between sexes were similar. Class III females, however,

were significantly heavier than class III males in spring (t = -5.27, df = 50, p <

0.01), but not in fall (t = -1.43, df = 43, p = 0.16). Across seasons, there were

Table 4Northern flying squirrel weights (grams), Oregon and Washington, 1985-95

Age class I Age class II Age class III

Study area Sex Fall Spring Fall Spring Fall

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Mean ± SE (range) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Coast Rangea % 82.3 ± 5.8 (51-101) 113.7 ± 2.5 (83-130) 122.2 ± 5.8 (110-149) 134.3 ± 3.3 (121-149) 128.5 ± 3.2 (123-137)

n = 7 n = 20 n = 6 n = 8 n = 4

& 85.6 ± 4.7 (71-111) 114.9 ± 2.3 (104-147) 126.0 ± 1.0 (125-127) 132.5 ± 2.8 (120-145) NA

n = 10 n = 18 n = 2 n = 10

Puget Troughb % 98.9 ± 2.4 (73-123) 122.9 ± 3.9 (108-147) 131.0 ± 1.0 (130-132) 132.2 ± 2.2 (110-172) 140.8 ± 2.5 (118-161)

n = 26 n = 10 n = 2 n = 28 n = 26

& 101.4 ± 2.7 (62-148) 129.1 ± 3.3 (109-147) 143.3 ± 8.0 (120-155) 152.8 ± 3.4 (120-185) 146.5 ± 3.1 (114-170)

n = 32 n = 12 n = 4 n = 24 n = 19

Olympic % 98.0 ± 4.7 (52-126) Fall only 135.9 ±3.1c (100-173)

 Mountains n = 16 n = 32

& 97.9 ± 4.9 (63-127) Fall only 140.8 ± 3.4c

 (94-194)

n = 14 n = 40

North Cascades % 88.0 ± 4.5 (74-98) Fall only 128.1 ±5.1c

 (110-150)

n = 5 n = 9

& 72.0 ± 0.0 (72) Fall only 133.2 ± 5.1c (105-174)

n = 2 n = 13

NA = not applicable

a First-capture (live) body weights from epiphyseal notch-aged animals only.

bPuget Trough first-capture (live) body weights from epiphyseal notch aged animals, animals captured originally as weanlings, animals with

>2 years of trapping records, or animals described as reproductively active the previous year.

c

 Age class II and III are combined.
13

two major differences in weight; class III males were lighter in spring than in fall

(t = -2.91, df = 52, p < 0.01), and class II females were smaller in spring than in

fall (t = -1.97, df = 14, p = 0.07).

Olympic Peninsula and northern CascadesThere were no objective methods

to retrospectively separate age classes in the Olympic Peninsula and northern

Cascades, so we did not make statistical comparisons. General weight ranges,

however, were similar to those of Puget Trough squirrels, and likely there are

differences among age classes in these provinces as observed in the Coast

Range and Puget Trough.

In the field, body weight is useful in distinguishing class I from class II and III

squirrels in early fall. We found that class I squirrels in the Puget Trough generally could be distinguished from adults through October. Weight can be used

as a general guide to separate age classes in spring, although there is some

weight overlap between age classes, particularly for males. Weight, by itself,

should not be used to distinguish ages. In addition to weight overlap between

age classes, we found evidence of gorging in both Coast Range and Puget

Trough flying squirrels. On necropsy, three heavy squirrels (152 to 165 g) had

large, food-engorged stomachs weighing 24 to 71 g. We also have observed

squirrels consuming over 15 g of food at one feeding on release from traps.

This gorging behavior may result in dramatic weight changes throughout the

year. Using multiple weights to determine age also can be misleading for trap

happy squirrels. Squirrels that are repeatedly captured in traps also may have

irregular foraging opportunities and thus exhibit weight fluctuations over the

trapping period (Witt 1991).

Pelage characteristics are commonly used to age squirrels. In 1955, Uhlig used

pelage length and color, condition of the eyes and ears, eruption of the teeth,

hind foot measurements, and length-weight correlations to age live nestling and

subadult gray squirrels in weeks. Sharp (1958) tested and refined an aging criteria for gray squirrels based on tail-pelage characteristics which, unlike other

aging methods such as bone development, could be used in the field. Sharp

found subadults and juveniles could be distinguished by tail pelage even in fall

when it is typically difficult to distinguish these ages in gray squirrels. In addition,

Barrier and Barkalow (1967) were able to assign 80 to 95 percent of gray squirrels to one of three age categories (nestling, juvenile, and adult) based on pelage

color.

Color and appearance of pelage have been used to determine age classes for

flying squirrels (Davis 1963). Like many mammals, fur of nestling flying squirrels

is replaced by juvenile pelage that is distinct in color and texture from subadult

and adult pelage (Davis 1963, Ingles 1965). Davis was able to distinguish juvenile

pelage until 5 to 6 months of age and found a Munsell soil color chart 10YR

Pelage and Molt
14

(Inter-Society Color Council 1955) was useful in identifying colors found in flying squirrel pelage. There are currently five recognized subspecies of northern

flying squirrels in Washington (Hall 1981; fig. 3), each demonstrating a wide

range of pelage coloration from pale wood brown to rufous brown and pecan

(Booth 1947).

The molting patterns for southern flying squirrels have been described well

(Jackson 1961, Wells-Gosling 1985). Most records indicate these squirrels have

only one primary molt annually, beginning in late summer or fall. This molt begins

at the nose and proceeds up and over the head and continues down the back

where the molt pattern then becomes irregular. Lactating females do not begin

this primary molt until after weaning, and juveniles begin molting by 12 weeks old,

after they are weaned in late summer. Pelage of all squirrels begins to show

considerable wear on the rump, head, and nape by spring and summer. Adults

experience a partial molt, of the head and neck only, in April and May. This partial molt begins at the nose and proceeds up the face and along the cheeks. This

molt is complete at 4 to 8 weeks when it reaches the back of the neck.

Golden blond

No 

flying 

squirrels

recorded

Golden yellow

Golden orange

Pale golden brown

Brownish orange

G. s. bangsi

G. s. latipes

G. s. 

columbiensis

No 

flying 

squirrels

recorded

G. s. 

klamathensis

G. s. 

fuliginosus

G. s. 

oregonensis

G. s. olympicus

 (historic)

Figure 3Regional pelage color variation of northern flying squirrels (Glaucomys sabrinus) in

Washington and Oregon as determined from museum specimens (subspecies distribution from

Booth 1947 and Hall 1981).
15

The molting pattern of northern flying squirrels has been described less fully than

that of the southern flying squirrel. Captive squirrels appear to have only one molt

per year, which begins on the face in late May or June (Wells-Gosling 1985).

Davis (1963) found a distinct molt line on the shoulders of adults collected in

summer but not on juveniles until September.

Hair and pelage color is affected by the kind, amount, and distribution of pigment granules, each of which produce different effects (DeBlase and Martin

1981). Cuticle texture, thickness of the hair, and amount of air space in the

medulla alter how light is reflected by the hair, and therefore change the appearance of pigments. Guard hairs of flying squirrels derive color from two pigment

types. Eumelanin produces blacks and browns, and pheomelanin produces reds

and yellows.

The hair of most mammals is shed and replaced periodically by molting. During

annual molts, all hairs are replaced in a relatively short period. The molt pattern

differs with species and sometimes with the age of an individual animal (DeBlase

and Martin 1981). When many mammals prepare to molt, black pigment accumulates in the skin. When new hairs start, they are deep in the skin and highly pigmented. As hairs develop, they move to the outer edge of the skin and become

more erect, and pigment leaves the root, migrating out the medulla of the hair and

toward its tip (Ingles 1965). Skin under fully molted pelage is clear, whereas skin

under molting pelage is dark or bluish due to undeveloped hairs. Appearance of

pigmentation is helpful in detecting molt.

MethodsWe used Munsell soil color chart 10YR3

 to identify colors found in

flying squirrel pelage on the body and tail. Color codes for tail tip, base of tail,

and dorsal and ventral pelage were recorded for 77 Coast Range and 19 Puget

Trough specimens. Pelage and hairs were examined under magnification for

pigmentation patterns, hair thickness, and condition. Pelage length was measured (millimeters) from base to tips on rump guard hairs, and tail width was

measured at the greatest width. To improve our understanding of the progression of molt and how molt related to age, we examined 60 prepared skins from

the Coast Range. Skin and hair were examined under 10X magnification from

the rostrum to the tail, dorsally and ventrally. Molt patterns, hair loss, guard hair

abnormalities, and the length of emerging hairs also were noted.

ResultsDorsal pelage of class I squirrels generally appeared gray with a pale

wash of yellowbrown; ventral pelage was generally grey with a wash of pale to

bright yellow (fig. 4a). Dorsal underfur was uniform gray and highly crimped

(wavy). As squirrels aged to class II and III, dorsal and ventral pelage changed

to a more golden-orange/brown color (fig. 4b, c). Class II animals had mildly

crimped underfur, whereas no crimping was seen in older squirrels.

3

 Munsell Color Company, Inc., 2441 N. Calvert St.,

Baltimore, MD 21218.
Figure 4—Development of northern flying squirrel pelage color, length,

and thickness by age class (A; top = age class I, middle = age class

II, bottom = age class III), ventral pelage of an age class III adult (B),

and dorsal pelage of an age class III adult (C).

A

C

B

16
17

Age class I dorsal guard hairs were a mix of solid black and agouti (black tipped

followed by successive bands of pheomelaninlight bands, and eumelanin

dark bands) as found by Davis (1963). Class II animals had some hair similar to

class I animals. Additionally, they also had agouti-tipped guard hairs followed by

a golden band, giving a dark golden appearance to the pelage. Class III adults

appeared more golden because the guard hair shaft at the golden band was

often thicker and the golden band longer than that of class II squirrels.

In the Coast Range, average pelage length increased significantly (F = 22.07,

df = 2, p < 0.01) with each age class and was distinctly shorter in class I individuals (table 3). Tail width was significantly narrower (F = 17.42, df = 2, p <

0.01) in class I and II squirrels than in class III squirrels; there was no difference

between class I and II squirrels. As flying squirrels aged, tails became wider and

squared at the end. There were no apparent differences in pelage length between Coast Range and Puget Trough squirrels.

In fall, we usually were able to distinguish class I squirrels by pelage color and

length; juvenile pelage was most apparent in class I squirrels < 6 months old. In

spring, class II squirrels were generally distinct from class III animals by their

grayish golden pelage and narrower tail. In addition, the underside of class II

tails often was edged with grey, whereas older animals lacked this edging. By fall,

class II squirrels were nearly class III size and had begun to molt into class III

pelage, becoming more golden and developing thicker pelage and a thicker tail.

In the field, these two classes were indistinguishable by pelage color and length.

We examined 90 flying squirrel specimens from 22 counties (Oregon and

Washington) in two museum collections (fig. 3). It was readily apparent that

regional pelage variation could easily lead to misidentification of age. Adult

squirrels of different subspecies displayed pelage variation from pale blond to

brown. Even squirrels within the same subspecies, G. s. oregonensis, appeared

to have varying pelage color from golden blond (King County, WA) to brownish

orange (Coos County, OR). Class III flying squirrels from our study sites in

Thurston County, Washington, had a grayer brown-orange pelage that closely

resembled class II squirrels from our Coast Range study sites.

Thirty-four of thirty-six spring-collected (April-May) squirrels lacked signs of molt

but did exhibit wear and patchiness on the head and rump. Two animals collected

in June showed molt on the rostrum and forehead. This molt was indicated by

dark pigmentation in the skin and the eruption of new hairs 1.5 to 2.5 mm long.

Eighteen of twenty-four fall-collected (October-November) squirrels were in various stages of molt. All age classes showed similar patterns. The earliest collected

animal (12 October) was the only individual to still be molting laterally. All others

showed various patterns covering the neck, shoulders, back, rump, and tail. Tail

molt was not evident until late November. Molt probably ends in November with the

tail molting last.

Field recognition of molt patterns may be difficult. In early summer, facial molts

appear to be quite distinct. Many animals, however, displayed lines similar to molt

lines, which were merely irregularities in the fur. Even heavy fall molts proved
18

visually difficult to discern without close examination. Wells-Gosling (1985) also

found molt difficult to discern because of pelage thickness.

Pelage color can be a useful aid in distinguishing age classes. Only after a wide

range of known-age individuals has been examined within a region, however,

should pelage characteristics be used to determine age class. We suggest carefully examining several specimens from each age class from each population to

develop a feel for how coloration will appear in the field. Museum collections are

a useful reference, keeping in mind that chemical alteration and foxing or reddening of the pelage can occur while in storage (Hodges 1989). In addition to the

Munsell Soil Color Chart 10YR, alternate color charts, such as the 10R, may be

needed. Color chart plates should be masked to protect them from damage and

fading. Descriptions and comparisons should be made in natural light when possible as artificial light may be inadequate to accurately determine colors (Hodges

1989). Low-light field conditions and wet squirrels also can make it difficult to

distinguish subtle shades in pelage color.

Molt does not seem to play a critical role in determining age. However, awareness of the timing of molt, particularly the fall molt from juvenile to adult pelage, is

important. Because dorsal pelage molts in October and November, young animals

may begin to display older pelage characteristics, thus making it more difficult to

determine age. Fortunately, the tail is the last to molt and may still be used to

determine age classes due to differing widths; class I squirrels molting into the

golden pelage of class II squirrels most likely will retain the thin, gray, wispy tail

into November. Additionally, in the Puget Trough, we observed that class I squirrels retained a thin, creamy white line along the ventral side of the patagium edge

well into winter. This may be a key characteristic for separating class 1 squirrels

from adults, once class 1 squirrels begin their first molt into adult pelage. Molt

patterns may differ with altitude, climate, or habitat; each population should be

individually assessed.

Sexual maturation occurs with age, and condition of reproductive organs can be

used to differentiate between juveniles and adults, at least during part of the year

(Morris 1972). Development of primary and secondary sexual organs is, therefore, an indicator of age class (Larson and Taber 1980). Degree of scrotum

development and amount of scrotal hair and condition of mammary glands are

characteristics used for aging fox squirrelsjuvenile male squirrels have abdominal testes and minimally pigmented scrotums; juvenile females have minute,

barely discernible nipples (Larson and Taber 1980).

ResultsReproductive characteristics of flying squirrels are described in detail

in the next section. We found reproductive status to be of limited use for aging

squirrels. In early fall, we could distinguish between class I and older males by

external reproductive development. Testes and scrotum of class I males showed

no development or were small and just developing. Testes and scrotum of class

II and III males enlarged during the breeding season in almost all cases, then

regressed to a moderate size but were usually distinguishable from undeveloped class I males. In winter, however, reproductive status became less reliable

Reproductive Status
19

because class I males began sexual development and scrotum and testes looked

and felt similar to those of older squirrels. Age classes II and III males could not

be distinguished by reproductive criteria in either season.

Age of females could only be distinguished by external evidence of previous

reproduction. External reproductive development did not begin before first estrus

in spring, 8 to 11 months after birth. Sign of previous sexual development, therefore, indicated age class III in spring, and age class II or III in fall. Lack of reproductive development, however, does not necessarily indicate age because some

females do not reproduce the first year after birth, and some may remain infertile

and show no development.

We recommend using a birth-year-based age class system for northern flying

squirrels. Birth-year systems follow a cohort of squirrels through time and can be

directly correlated with epiphyseal notch closure. Unlike a system based primarily on reproductive competency, the birth-year-based system allows reproductive

potential, reproductive success, and survival to be evaluated separately for each

cohort. Distinct differences in reproductive maturation among regions make the

use of the category subadult confusing because in one region, many yearlings

did not breed and thus remained subadults, yet in another, yearlings readily bred

and thus were reproductively adult (discussed under Reproductive Biology).

We also discovered that comparisons among regions with different reproductive

strategies can be confusing if there is not common terminology. Figure 5 provides an illustration of the overlap we observed in our studies.

Using a birth-year age-class system to describe flying squirrel age has several

other advantages: (1) classes are based on a group of individuals coming into

the population during a seasonal, finite period (2- to 4-month period during

summer); (2) each class represents the survivorship for a given year; (3) age

class is independent from trapping season; and (4) there is less variation among

individuals within an age class than a reproductive class. A birth-year-based age

class system avoids ambiguity, provides useful, relative ages, and can be used

successfully to determine population characteristics for northern flying squirrels.

External characteristics can be used to determine age in flying squirrels provided

that multiple and appropriate characteristics are examined. In the Puget Trough,

we identified class I and III individuals by using a combination of facial features,

weight, pelage, and reproduction. Class II individuals can be classified reliably in

spring with experience. By fall, however, class II squirrels are difficult to distinguish from class III because facial features and weights are similar between

class II and III, class II individuals are or have molted into the class III pelage

and, in some regions, most animals have reproduced.

We believe that two extrinsic factors can improve the accuracy of age determination:

Recommendations for

Assessing Age
20

• Detailed notes describing external characteristics each time an animal is

captured helps biologists and data analysts make retrospective decisions on

age. Notes also help technicians focus on appropriate characteristics needed

to adequately distinguish age classes.

• Multiyear studies or continuous trapping over a several-year period allows

class I individuals to be followed through time. This provided live examples of

“known-age” squirrels for training new technicians.

Part of the high success we experienced in aging Puget Trough squirrels may

be attributed to technician experience—field crew turnover rates were low; thus

technicians, on average, were highly experienced. A summary of external

characteristics for each age class is given in table 3.

Knowledge of reproduction and natality is a fundamental requirement for understanding population dynamics. Estimates of reproductive potential and attainment

in squirrel populations can be obtained by examining external and internal reproductive organs from both sexes and by determining litter size. Color, swelling,

texture, and size and development of reproductive organs, tracked over time,

make it possible to determine breeding seasons and determine what proportion

of the population is reproductively active at a given time (Kirkpatrick 1980). Uteri

(for embryos and placental scars) and nests (for nestlings) can be examined to

determine the number of young produced by reproductive females.

Observations and histological evaluations of internal organs provide the most

accurate determination of reproductive status in mammals. Testis size, weight,

and volume; androgen levels; spermatozoa counts; and testicular biopsies are

commonly used for males, whereas ovarian activity, follicular development, and

embryo development are used for females (Harder and Kirkpatrick 1994).

Figure 5—Age class systems used in the Coast Range (1985-90) and Puget Trough (1991-95) for

northern flying squirrels. Note the “gray areas” of age determination in the traditional aging system.

Technique

Development—

Reproductive

Condition and Litter

Size
21

Various external features have been used to obtain estimates of the reproductive

status in live animals. To accurately assess squirrel populations, external reproductive indicators must be recognizable and accurate. McCravy and Rose (1992)

used necropsy to evaluate breeding condition in small rodents. They found nipple

size was a better predictor of breeding status than condition of the pubic symphysis or vagina. In males, testis position was the best indicator of reproductive

activity.

Although several authors have assessed reproductive status of flying squirrels in

the field, few have described methods for doing so (Raymond and Layne 1988,

Weigl and others 1992, Wells-Gosling 1985). Reproductive characteristics of

southern flying squirrels have been well described by Sollberger (1943). Asdell

(1964) discussed the development of the vulva for southern flying squirrels, and

Wells-Gosling (1985) described the swelling of the testes. For northern flying

squirrels, Muul (1969b) reported that females become sexually active in late May,

when genitalia are swollen. Davis (1963) gave internal histological descriptions

for stages of the breeding cycle but did not describe external or gross internal

features.

Our trapping efforts in spring and fall coincided with the onset of breeding activity

and influx of young of the year, respectively. Field determination of reproduction

was usually based on appearance of external reproductive organs; in some

circumstances, body weight and age also were used. We created reproductive

categories to describe development during both breeding and nonbreeding

periods (appendices 3 and 4). Males in breeding condition were called scrotal.

Reproductively active females were categorized as estrus, pregnant, lactating, or

postlactating. Nonbreeding squirrels were categorized by reproductive maturity

inactive males were either never active or regressed; females were nulliparous or

regressed.

External and internal reproductive organs were closely examined on 226 squirrels

(123 males and 103 females; appendix 5). Males were described externally by

testicular and scrotal development. For females, the vulva was described by

color, turgor, and whether open (opening was prominent) or closed; color, turgor,

and shape of nipples were noted; and nipple length was measured. When mammary disks were distinct, diameter was measured. After external examinations,

entire reproductive tracts of females and testes of males were removed and

photographed. For males, length, width, and depth of both testes were measured,

and shape and position (within scrotal sacs or abdomen) were described. Testis

volume (length by width by depth) was calculated for one testis on each Coast

Range male; testis area (length by width) was calculated for Puget Trough and

Olympic Peninsula males. For females, ovaries and uterine horns were measured, described, and inspected for corpora lutea, embryos, and placental scars.

Fetus number and length were recorded.

Field descriptions of reproductive condition from 107 captures of 62 Coast

Range males, 72 captures of 49 Coast Range females, 54 captures of 17 Puget

Trough males, 31 captures of 19 Puget Trough females, and 9 Olympic Peninsula

squirrels (two males and seven females) were compared to necropsy results to

Reproductive Status
22

evaluate technician accuracy. In addition, we evaluated external reproduction

characteristics described in the field for 2,136 male and 1,798 female captures

from all three physiographic provinces.

MalesNecropsy results from 102 Coast Range males, 19 Puget Trough, and 2

Olympic Peninsula males were grouped by reproductive characteristics and

placed into never-active, scrotal, and regressed categories (table 5). Coast

Range squirrels accounted for 79 percent of all necropsy specimens. Males with

undeveloped testicles (testes not fully developed and descended into scrota) and

very small, rounded, turgid testes were classed as never active (n = 9 Coast

Range, 13 Puget Trough, and 2 Olympic Peninsula). Mean testis volume was 79

mm3 (SE 7.4 mm3, n = 7) for age class I Coast Range males (fig. 6). Testes were

positioned abdominally in three class I Coast Range males. Three Coast Range

males had testicles partly descended into scrotal sacs, and one had testes

positioned completely within the scrotum. In two class I Coast Range males, small

testes could be felt externally. Mean testis length and width of Puget Trough and

Olympic Peninsula never-active males were not significantly different from Coast

Range males.

Scrotal males had well-developed testicles with enlarged, turgid testes usually

within the scrotal sacs. We examined 76 Coast Range and 2 Puget Trough

scrotal males. Testes volume for 70 of 76 Coast Range males caught during the

breeding season exceeded 1000 mm3 and averaged 3000 mm3 (SE 98.2 mm3).

Age was determined by epiphyseal notch for six of eight Coast Range males that

had testis volumes less than 585 mm3; all were age class II squirrels. Testis

volume averaged seven times larger for class III scrotal males than males in

nonbreeding condition (fig. 6). One age class II squirrel with a testis volume of

2166 mm3 had testes positioned entirely within the abdominal cavity, whereas

four other squirrels in age classes II and III had one or both testes located partly

in the abdomen or inguinal canal, with testis volume ranging from 282 mm3 to

4355 mm3. This suggests that there was no absolute relation among testis

position, testis size, and age class. We did not explore any relation between

testes position and fertility.

Males were considered regressed if testes size indicated previous development,

but testes were not presently enlarged (n = 17 Coast Range and 3 Puget Trough

males). Regressed testes appeared wrinkled compared to the smooth, round

texture of testes found in never-active or scrotal males. During the nonbreeding

season, developed scrota were easily discernible but reduced in size. Testes

width and depth measured < 50 percent of average scrotal testes size, thereby

resulting in a flattened and flaccid condition. Testes length was usually not greatly

reduced in regressed testes. In seven cases where testis length was almost 50

percent of average scrotal testes length, however, testes were more rounded and

semiturgid but were larger than undeveloped testes. Testes were sometimes

difficult to palpate from the exterior of the animal. Four Coast Range regressed

males had testes located partially or entirely within the abdomen.

Necropsy Results
23

Figure 6—Testes volume (length by width by depth) of male

northern flying squirrels, Coast Range, Oregon, 1985-90.

Females—Necropsies were performed on 77 Coast Range, 19 Puget Trough,

and 7 Olympic Peninsula females. Reproductive condition was determined by

size and appearance of uterus and ovaries, along with presence of corpora

lutea, embryos, or placental scars (tables 6 and 7). Nulliparous females had

short, thin, and uniform uterine horns, and uterine walls often looked translucent;

Table 5—Descriptions and measurements of reproductive organs and genitalia of male northern flying

squirrels from the Pacific Northwest, 1985-94

Scrotal

Characteristic Never active Age class II Age class III Regressed

External

Scrotal sac Not discernible or very small Large, evident Very large, evident Small to large

Testes Very small, rounded; firm, Large, rounded, firm, Large, rounded, firm, Small, rounded or flat,

 impalpable or palpable palpable palpable flaccid; impalpable

or palpable

Internal

Coast Range:

Testes lengtha b 6.25 ± 0.18, n = 16 16.47 ± 0.35, n = 50 20.08 ± 0.18, n = 63 10.25 ± 0.46, n = 32

Testes widtha b 4.07 ± 0.08, n = 16 11.34 ± 0.39, n = 50 14.22 ± 0.12, n = 63 5.60 ± 0.17, n = 32

Testes deptha b 3.09 ± 0.06, n = 16 9.10 ± 0.37, n = 50 11.80 ± 0.10, n = 63 3.63 ± 0.17, n = 30

Puget Trough:c

Testes lengtha b 6.75 ± 0.73, n = 8 18.68 ± 0.12, n = 2 10.57 ± 0.20, n = 4

Testes widtha b 3.88 ± 0.20, n = 8 11.28 ± 0.47, n = 2 6.51 ± 0.51, n = 4

Olympic Peninsula:

Testes lengtha b 5.80 ± 0.16, n = 4

Testes widtha b 3.56 ± 0.09, n = 4

Testes deptha b 2.84 ± 0.05, n = 3d

a

 Mean (mm) ± standard error.

b

 Includes both left and right testicles from each individual.

c

 Testes depth not measured.
24

Table 6Descriptions of reproductive organs for female northern flying squirrels Coast Range,

Oregon, 1985-88

Characteristic Nulliparous Estrus Pregnant Postlactating Regressed

Vulva Small, white, closed Swollen, pink, open Unswollen, light Small, pale to pink, Small, pale,

to dark pink, open or closed closed

open or closed

Nipples Tiny dots or small Bumps or < 5 mm, Bumps or < 3-6 mm, conical, 2-3 mm, flaccid

bumps (<1 mm) flaccid to semi- 5 mm, flaccid turgid to semiturgid

turgid to turgid

Uterus Thin, uniform, clear to Enlarged, turgid, Thick, extended, Coiled, turgid, Flaccid, opaque,

pale pink pink to red red placental scars placental scars

Table 7Internal measurements of female northern flying squirrel reproductive organs in the

Pacific Northwest, 1985-95

Estrus

Characteristic Nulliparous Class II Class III Pregnant Postlactating Regressed

Coast Range:

Uterus lengtha b 15.45 ± 0.41 27.21 ± 1.85 36.70 ± 1.31 41.54 ± 2.86 25.98 ± 0.99 25.80 ± 0.86

n = 68 n = 12 n = 12 n = 20 n = 8 n = 22

Uterus widtha b 1.35 ± 0.05 3.37 ± 0.31 3.93 ± 0.28 3.12 ± 0.16 2.14 ± 0.20 2.24 ± 0.08

n = 68 n = 12 n = 12 n = 20 n = 8 n = 22

Ovary lengtha c 4.28 ± 0.07 4.33 ± 0.11 5.72 ± 0.26 5.32 ± 0.18 5.25 ± 0.30 4.45 ± 0.15

n = 70 n = 11d n = 12 n = 20 n = 8 n= 22

Ovary widtha c 1.94 ± 0.04 2.03 ± 0.12 2.80 ± 0.08 2.53 ± 0.11 2.81 ± 0.15 2.25 ± 0.08

n = 70 n = 11d n = 12 n = 20 n = 8 n = 22

Puget Trough:

Uterus lengtha b 18.60 ± 1.83 22.91 ± 1.38 49.90 ± 0.75 24.48 ± 2.55 24.01 ± 2.11

n = 8 n = 2 n = 2 n = 4 n = 6

Uterus widtha b 1.32 ± 0.12 2.16 ± 0.45 4.88 ± 0.13 3.19 ± 0.13 2.29 ± 0.19

n = 8 n = 2 n = 2 n = 4 n = 6

Ovary lengtha c 3.85 ± 0.15 4.05 ± 0.25 5.95 ± 0.40 4.69 ± 0.50 3.36 ± 0.28

n = 8 n = 2 n = 2 n = 4 n = 6

Ovary widtha c 1.85 ± 0.07 1.93 ± 0.07 4.32 ± 0.17 2.62 ± 0.09 1.99 ± 0.13

n = 8 n = 2 n = 2 n = 4 n = 6

Olympic Peninsula:

Uterus lengtha b 16.52 ± 0.94 29.20 ± 0.60e 19.93 ± 0.87

n = 8 n = 2 n = 4

Uterus widtha b 1.10 ± 0.05 5.45 ± 0.05e 1.90 ± 0.14

n = 8 n = 2 n = 4

Ovary lengtha c 4.36 ± 0.33 7.30 ± 0.20e 4.67 ± 0.38

n = 8 n = 2 n = 4

Ovary widtha c 1.93 ± 0.14 4.95 ± 0.45e 1.72 ± 0.13

n = 8 n = 2 n = 4

a Mean (mm) ± standard error.

b Includes both left and right uterine horns from each individual.

c

 Includes both ovaries from each individual.

d

 One ovary damaged.

e

 Olympic Peninsula females were all lactating.
25

ovaries were small and smooth (Coast Range: n = 35; Puget Trough: n = 10; and

Olympic Peninsula: n = 4). Nulliparous reproductive organs were sometimes

coiled behind the colon and difficult to see. Externally, the vulva was closed and

showed no sign of swelling or reddening, and mammary glands were imperceptible. Flying squirrels have four pair of nipples and mammae: one pectoral, two

abdominal, one inguinal. Nipples were commonly described as tiny dots on the

skin, whereas those that projected from the skin did not exceed 1 mm in length

(fig. 7a). Twenty-three nulliparous squirrels were caught in spring; all were from

the Coast Range. Twenty-one of these were aged by epiphyseal notch, and all

but one were age class II.

Uterine horns of estrous females (Coast Range: n = 15; Puget Trough: n = 2)

were elongated and pinkish, with thickened, turgid walls. Ovaries were larger

than those in nulliparous squirrels and usually showed formation of follicles at

the surface; distinct, round lumps, which varied from light to dark in color, were

evident in some squirrels; indicating current ovulation. Open, swollen, and

reddened vulvae were outward signs of estrus, but the amount of swelling and

color differed. Nipple development at this stage ranged from no development to

5 mm in length; developed nipples were flaccid to semiturgid (fig. 7b).

In pregnant females (Coast Range: n = 10; Puget Trough: n = 1), uterine horns

were reddened due to increased vascularization and were thick and extended.

Either one or both horns contained embryos. Corpora lutea were generally

evident on the adjoining ovary. Vulvae were found open or closed on pregnant

females; there was little to no swelling, but color was usually light to dark pink.

Development of nipples remained variable at this stage, the smallest measuring

1 mm in length (fig. 7b).

Only one lactating female, captured in the Olympic Peninsula, was necropsied.

The uterine horns were thick and extended, and opaque pink. Corpora lutea

were present in both ovaries. Mammae appeared as white, sparsely haired

disks extending from the body (fig. 7c). Large fat deposits were present under

the mammary disks. Nipples were pink and turgid; 5 mm long.

Figure 7Nipple and mammary development of female northern

flying squirrels: nulliparous (A); regressed, estrous, or pregnant (B);

lactating (C); and postlactating (D).
26

Seven postlactating (Coast Range: n = 5; Puget Trough: n = 2) and 18 regressed

(Coast Range: n = 12; Puget Trough: n = 4; Olympic Peninsula: n = 2) females

were examined. At the end of the breeding cycle, uterine horns decreased in

size and became coiled and flaccid but usually retained a thickened, opaque

appearance. Placental scars were recognizable in both spring and fall. Postlactating females were distinguished externally by conspicuous mammae (10 to

24 mm in diameter) and regrowth of hair around the nipples; nipple length ranged

from 3 to 6 mm. Regressed females had 2 to 3 mm, flaccid nipples and mammary glands were unnoticeable (fig. 7d). For both categories, most vulvae

regressed to a small, closed, and pink to pale state.

MalesIn the Coast Range, technicians correctly identified reproductive status

90 percent of the time, and distinguished between breeding and nonbreeding

condition 94 percent of the time. All specimens collected in spring (n = 39) were

reproductively active or developing. Two class II males were described as never

active with testis volumes of 181 mm3

 and 282 mm3, respectively (mean testis

volume of age class II scrotal males = 1878 ± 182 mm3 SE, n = 25). Because

these testis were larger than for never-active age class I males, these squirrels

were likely just beginning reproductive activity.

In fall, three regressed males were described as scrotal on four field occasions.

Two of these squirrels had the second and third largest testis volume (323 mm3

and 409 mm3) of all regressed males (mean testis volume of regressed males in

fall = 206 ± 29 mm3 SE, n = 15) (testis volume for the third squirrel was not

available because testis depth was not measured). Technicians had some difficulty distinguishing between inactive categories. Five of fifteen regressed males

were misidentified as never active. Age class was determined by epiphyseal

notch for four cases; all were age class II males. All never-active males (n = 8)

were accurately assessed in the field.

In the Puget Trough, two males collected in spring were reproductively active,

and were correctly identified by technicians in all captures (n = 10). In fall,

technicians accurately described each of three regressed males at every capture (n = 7). Never-active males (n = 12) were correctly described 54 percent of

the time (37 occasions). In 17 captures, technicians described never-active

males as regressed. One never-active male was misidentified eight times, twice

in the last week of November, and the rest in early December. Another neveractive male with two captures in December was called regressed both times.

FemalesIn the Coast Range, 36 of 49 females were in nonbreeding condition,

a high proportion. Forty-two of fifty captures (84 percent) of inactive females

were described correctly; 36 were nulliparous. Nulliparous females were called

regressed four times (three during May), and one regressed female was recorded as nulliparous. Technicians misidentified three inactive females as active

during spring. Two were recorded as in estrus, and one regressed female was

described as postlactating.

Field Comparisons to

Necropsy Results
27

Thirteen necropsied females were in breeding condition; 10 of 34 specimens

collected in spring and 3 of 15 specimens from fall. Technicians identified the

correct reproductive code in only 5 of 22 captures. Twelve of the seventeen

incorrect field descriptions involved captures of six pregnant females. All six of

these females were described correctly as reproductively active but in a wrong

category (estrus or lactating); pregnancy was identified only once. Each pregnant female also was called regressed once. Crown-to-rump length of embryos

removed from these females ranged from 4 to 13 mm, yet none showed mammary development or abnormally heavy weights. Estrus was accurately reported

in the field twice in six observations; four estrous females were recorded in the

field as inactive, but necropsy notes described the vulva of one squirrel as open

with no swelling, and the vulva of another as closed with only slight swelling. The

three reproductively active females collected in fall were postlactating; two of

these three were correctly identified by technicians.

In the Puget Trough, technicians reported the correct reproductive status for 14

inactive females 96 percent of the time (n = 25). Ten nulliparous females were

accurately identified in all captures (n = 20). Age class was determined by epiphyseal notch for eight of the nulliparous females; all were age class I. Each of

the four regressed females was correctly described once, and one was described as postlactating during a fall capture. Three females caught in fall and

two in spring were in breeding condition. Technicians recognized reproductive

activity in all cases. Two postlactating squirrels were identified in three fall captures. Another fall collected female was categorized in estrus during necropsy

and was described as pregnant in the field. In spring, one estrous and one

pregnant female were collected. Technicians described both as in estrus.

MalesWhen observed ventrally, the testicles (testes and scrota) of young,

never-active males were undeveloped; the skin around the groin generally

appeared taut, and testes were usually not detectable. Variation in scrotal

development, however, occurred among class I males, perhaps due to minor

differences in age.

Some class I males had small, but detectable, scrotal sacs. Truly scrotal males

had large, heavily furred scrotal sacs that protruded from the body, and testes

were fully developed and enlarged, filling the scrotum. The testes of regressed

males were much reduced in size and were not always palpable. Regressed

testicles differed in size and shape. The scrotal sacs of some regressed males

remained large and rounded, similar to scrotal males, but were empty when felt.

Other regressed scrota were shrunk and flattened so that sacs were not readily

seen but were distinguishable by touch.

Reproductive status of males can be accurately identified by scrotum and testes

size and feel in most cases. The easiest to distinguish are age class I before late

fall and class III males. Early born age class I males may be increasingly misidentified as regressed from late November-December, especially by less experienced technicians, because of beginning scrotal sac development. The moderate size of age class II males also can be a source for error. In the breeding

External Characteristics

in the Field
28

season, some age class II males with relatively small (perhaps still developing),

active testes may appear inactive in comparison to the much larger size of age

class III males. Similarly, scrota of age class II males with moderate development during their first breeding season may appear more like never active than

regressed scrota during the nonbreeding season. Testes position presents an

additional, yet uncommon problem for externally determining reproduction. Enlarged, active testes are not likely to be detected when located partly or entirely

within the abdomen, and these males would thus be described as either never

active or regressed depending on scrotum size. Some inactive males with regressed testes not positioned within the scrotal sacs showed reduced scrotum

development, and their status as never active or regressed may be unclear and

inconsistently identified by observers.

FemalesThe vulvae of nulliparous females were small (2 to 3 mm), white, and

completely closed. Nipples usually appeared as small, white or pink bumps raised

just above the skin, or as tiny red dots on the surface of the skin (fig. 7). The

vulvae of females in estrus were noticeably swollen, and the color usually darkened from pink to red (a few swollen vulvae appeared whitish). When vulvae were

swollen large, the opening inverted so that folds could be seen along inside walls.

The nipples of some squirrels, previously bred or not, lengthened or became

erect at this stage, but more often nipples remained inactive until pregnancy.

Vulval swelling subsided at the end of estrus, but dark coloration usually was

retained. At the onset of pregnancy, vulvae had an unswollen, closed appearance, yet opened easily when touched, and color began to fade. Nipple development was apparent during this stage, but timing of development seemed to differ

widely from early to late pregnancy. Outward sign of mammary development

occurred close to parturition. Mammae were most obvious during lactation and

appeared as white, sparsely haired disks. Nipples were pink or red (often with a

bright red tip), turgid, and > 3 mm in length. Milk could be expressed from

nipples. During lactation and postlactation, vulvae returned to an inactive state.

Postlactation was differentiated by diminished mammary development and

regrowth of hair around nipples. Nipples of postlactating females were long (3 to

6 mm), conical, semiturgid, and sometimes dark tipped or ringed due to scarring.

Fully regressed females had imperceptible mammae but usually retained long

nipples. Regressed nipples were pale and flaccid, but length and shape differed

from short and slender to long and flat. Completely regressed vulvae appeared

small, closed, and pale or light pink but were larger than vulvae of nulliparous

females.

During the female reproductive cycle, external changes can be subtle, but accuracy in determining reproductive status is possible through careful examination. Early stages of breeding activity are detectable from the change in vulva

size and color. The degree of swelling and coloring differs, however, during the

progression from the onset of estrus through early pregnancy and differs among

individuals. Individual condition (cold, wet vs. warm, dry) might influence vulval

appearance. Comparisons of vulvae among different females with different

breeding histories and in different stages of estrus and pregnancy may cause

misidentification of some estrous females because sign of active vulvae with less
29

obvious swelling and color may be overlooked. Pregnancy is probably the most

difficult stage to determine in the field. Swelling of the vulva recedes during early

pregnancy and color begins to fade, yet changes in nipples, mammary glands,

and abdomen do not usually occur until near parturition. In most cases, however,

there is a noticeable difference between unswollen vulvae and inactive vulvae,

and recognizing this difference is critical in identifying pregnancy. Vulvar tissue

after swelling is deflated and flaccid, and the opening is conspicuous or becomes

so by touch, as compared with the taut, firm, and closed inactive vulva. Also, the

underlying, darker coloring of many unswollen vulvae provides an additional clue

to sexual activity. For six Coast Range females collected during the second week

of spring trapping and determined pregnant by necropsy, estrus was described

four times, and pregnancy once, during first-week captures. All six females were

identified as regressed on their last capture date. Two technicians noted reduced

swelling but were likely unaware of the appearance of vulvae following estrus.

Although only one lactating specimen was necropsied, lactating females were

observed in the field many times. Lactation is externally obvious by protrusion of

some or all of the four pairs of white, and sparsely haired or hairless mammary

disks, positioned along the ventral side of the abdomen. Milk usually can be

expressed from nipples.

Postlactation is used to distinguish those females that are no longer nursing but

show evidence of having reproduced during the year. Pairs of mammae, and

long, semiturgid nipples, accentuated by the shorter hair surrounding them as

hair regrows, are usually noticeable signs of postlactation. Cessation of milk

production and reduction of mammary disks, however, are not clearly distinguishable at weaning, and external signs of lactating and postlactating females

may be interpreted differently by technicians. Later in the breeding season, as

individuals continue to regress, careful examination can still yield signs of recent

reproductive activity.

Nipples are generally good indicators of breeding history for inactive females.

Accuracy is highest during late summer and fall, when young of the year usually

show no sign of nipple development (in many cases, nipples are difficult to find)

and nipples of mature squirrels that reproduced during the year are still regressing. Nulliparous squirrels were correctly described most of the time during fall

(Coast Range = 13 of 14; Puget Trough = 19 of 19), whereas accuracy dropped

in spring (Coast Range = 23 of 27). Some confusion in spring may be attributed

to the onset of hormonal activity causing nipples to form into small cones projecting from the skin surface before vulvar swelling indicative of estrus. Size, shape,

and turgidity of regressed nipples differ widely among individuals and are affected by the amount of time passed since a litter was produced. Long (>2 mm) or

flattened nipples are adequate determinants of regressed females. Reproductive

history, however, cannot be determined externally for inactive females with relatively short (<2 mm in length) and slender (<1 mm width) nipples. Nipples of

some females known to be regressed have resembled those of females that

have never reproduced. A few age class I females have shown early development in fall with tiny conical nipples protruding from the skin, and class II or older
30

females that have not successfully reproduced, but have been in estrus, also

develop such nipples. It is useful to include nipple length criteria in descriptions

of inactive categories (<1 mm nulliparous, >1 mm regressed) to achieve consistency among technicians. Reproductive status can be refined during data review

(appendix 2).

In studies of northern flying squirrels, direct observation of young has been the

standard method to determine litter size (Davis 1963, Muul 1969a, Rust 1946,

Wells-Gosling 1985). Embryo and uterine scar counts also have been used to

estimate litter size. At parturition, a portion of the uterine wall is torn away by the

placenta. The resulting scar on the uterine wall appears as a darkened raised

band or disc in the uterine lumen (DeBlase and Martin 1981, Kirkpatrick 1980).

Placental scars, however, are only an approximation of the number of young

produced. Conaway (1955) found that the main source of error arises from

resorption of fetuses that leaves scars indistinguishable from those left by fully

developed fetuses. Counts of fetuses, or uterine swellings when fetuses are not

yet developed from embryos, are considered the best index to number of young

produced (Kirkpatrick 1980). Davis (1963) used embryo counts to determine

litter size but encountered partial litter resorption, thereby indicating that embryo

and placental scar counts could overestimate litter size.

During Coast Range studies, embryos and uterine scars were counted to determine litter size. Uterine scars appeared as red bands or stretched areas on the

uterine horn. Embryos, although often under 3-mm crown-to-rump length, were

easily discernable as lumps in the uterus. These lumps were cut open, and

embryos were extracted and measured under magnification.

Pregnant Coast Range females (n = 8) contained a mean of 2.5 ± 0.19 embryos.

Counts of uterine scars (0 = 3.1 ± 0.04) were higher than embryo counts. No

pregnant females were found among Olympic Peninsula and Puget Trough

mortalities.

In the Puget Trough, 5 years of direct observations were made during checks of

nest boxes and artificial cavities (Carey and others 1996; table 8). Mean litter

size over this period was 2.8 young per adult female (range 1-4), but year-toyear variability was high in these second-growth forests.

Litter size is difficult to determine in wild populations of flying squirrels. Counts of

embryos and placental scars can be done only on dead animals, but embryo

resorption may bias estimates. Direct litter observations, such as those during

nest box examinations, reveal the minimum number of healthy young that survive

to parturition but not necessarily the number born. Number of young weaned

may be the ecologically most significant indicator of reproductive success.

Litter Size
31

We recommend that technicians (1) examine all external sexual features and

consider the combination of signs to best determine a reproductive category, (2)

describe the condition of genitalia with detailed notes, and (3) report on reproduction at every capture to determine the reproductive status of flying squirrels

during live-trapping studies.

Table 8Northern flying squirrel litter dates, sizes, and mean weights, during nest box checks

in the Puget Trough, Washington, 1993-97

Age of Weight of No. of No. of No. of Mean weight Estimated date

Date of capture mother mother young males females of young of parturitiona

Grams Grams

16 August 1993 3 176 3 1 2 44 19 July 93

17 August 1993 3 166 4 1 3 55 14 July 93

19 August 1993 2 146 1 0 1 49 24 July 93

19 August 1993 2  1 0 1 82 26 June 93

23 August 1993 3 180 3 2 1 53 25 July 93

17 August 1994   2 2 0 68 10 July 94

24 August 1994 3 160 1 1 0 74 12 July 94

24 August 1994   2 1 1 97 18 June 94

24 August 1994   1 1 0 90 25 June 94

24 July 1995 3 180 4 3 1 42 1 July 95

31 July 1995 3 192 3 0 3 30 13 July 95

31 July 1995 3 158 4 2 2 50 5 July 95

8 August 1995 3 196 4 3 1 37 18 July 95

21 August 1996 2 160 2 1 1 54 23 July 96

22 August 1996 2 154 2 1 1 49 27 July 96

22 August 1996   2  1 60 20 July 96

22 August 1996   2 1 1 68 15 July 96

26 August 1996 2 146 1 0 1 76 13 July 96

26 August 1996 2 156 1 1 0 54 27 July 96

27 August 1996 3 157 2 1 1 76 14 July 96

27 August 1996   2 2 0 61 25 July 96

28 August 1996 3 174 3 1 2 64 23 July 96

13 August 1997 2 134 1 1 0 67 6 July 97

13 August 1997 2 118 2 1 1 39 22 July 97

13 August 1997 3 180 3 1 2 53 15 July 97

18 August 1997 3 159 1 1 0 38 28 July 97

18 August 1997 3 184 1 1 0 54 19 July 97

20 August 1997 3 179 4 2 2 43 27 July 97

21 August 1997   2 0 2 63 16 July 97

21 August 1997 2 158 3 2  32 2 August 97

25 August 1997 3 168 3 0 3 67 18 July 97

 = data not obtained or not available.

a Determined by estimating age in days by mass (Davis 1963) and back dating from date of capture.

Recommendations for

Assessing Reproduction
32

Before each trapping session, technicians should review flying squirrel reproduction, especially stages of reproductive cycles and corresponding changes in

sexual organs. Sampling protocols should include descriptions of external indicators of reproductive categories, including stages within each category. The

protocol establishes a standardized language and a consistent set of descriptives, and facilitates verbal and written communication among technicians and

between technicians and data analysts (appendices 3 and 4). Group discussions

of protocols and project objectives can reinforce learning. A summary of the

protocol that includes categories, codes, and key indicators should be carried as

a field reference and checklist for field examinations. For field training, new technicians should be accompanied by an experienced technician during trapping

until they have seen examples of each reproductive category expected during a

trapping session. As technicians gain experience, the increase in number of

squirrels examined improves understanding of the reproductive cycles, recognition of reproductive stage by external signs, and confidence and accuracy in

reporting reproductive status.

Most studies of reproductive biology in Glaucomys have been in populations of

southern flying squirrels in the Eastern United States. Southern flying squirrels

have two distinct breeding periods, typically peaking in March and July (Hibbard

1935, Linzey and Linzey 1979, Muul 1969b, Sollberger 1943, Stapp and Mautz

1984). Testes of males begin to descend in January and may regress in late

summer or early fall (Muul 1969b, Sollberger 1943, Wells-Gosling 1985). Females typically enter estrus slightly later, in mid-February, and are active for a

shorter period of time than males (Muul 1969b, Wells-Gosling 1985). Vulvas

regress 5 to 8 days after mating. Vulvas remain perforate, however, until 4 to 5

weeks after parturition (Sollberger 1943). After a 40-day gestation period, most

southern flying squirrel litters are born in early April. Additional litters can be

produced in fall (Muul 1969b, Uhlig 1956, Wells-Gosling 1985). Average litter

size is three young (range 1-6), and fall litters are commonly larger than spring

litters (Hibbard 1935, Linzey and Linzey 1979, Stapp and Mautz 1984, WellsGosling 1985).

There have been no definitive studies of reproductive biology of flying squirrels

in the Pacific Northwest. Northern flying squirrels are reported to have a single

breeding season and mate from late March through May (Wells-Gosling and

Heaney 1984). Flying squirrels breed as early as March in Saskatchewan

(Davis 1963) and produce offspring as late as November in California (Raphael

1984). In Saskatchewan, males become scrotal in late January and testes are

regressed by June (Davis 1963). Females enter estrus as early as March and

have a 16-day estrous cycle (Davis 1963). After gestation of 37 to 42 days (Muul

1969a), a single litter of three young (range 1-4) is produced (Booth 1947, Davis

1963, Rust 1946) between May 1 and June 30, with northern litters being born

slightly earlier than those in southern regions (Davis 1963, Forsman and others

1994, Jackson 1961). Few litters, most anecdotal observations, have been

observed in the wild. Rust (1946) found one to three young per litter (sample size

unknown) in northern Idaho, with two being the most common in grass nests and

three in cavity nests. Wells-Gosling (1985) captured one female in Michigan that

Reproductive

Biology
33

gave birth to a litter of six, but found the average to be three. Captive females

have been found to produce litters of three to four, whereas their wild counterparts were found to have 2.5 (range 2-3) live embryos (Davis 1963, Muul 1969a).

MalesIn the Puget Trough, age class II and III males matured and became

active (scrotal) by the end of January. The proportion of scrotal males peaked in

May; >95 percent of all males became scrotal each year. We observed scrotal

males as late as September, although testes in some males had regressed as

early as late May. In the Coast Range, timing of scrotal development was similar,

with most males scrotal in late May, but some squirrels retained scrotal testes

until early November.

FemalesPuget Trough females matured as yearlings, and most were sexually

active their first spring. Both age class II and III females exhibited estrus during

late April (when trapping began)late May with a peak in early May (fig. 8).

Pregnant squirrels first appeared in early May, and numbers increased until early

June. Pregnancy was observed as late as July 27. Lactation began in late May

and ended by October when most young were weaned. Regression of reproductive organs began as early as August, and most females were regressed by

November.

Coast Range females came into estrus beginning in late April. Peak of estrus was

late May to early June (fig. 9). Estrous females were captured as late as June

27. Pregnancy peaked in early June. Lactating females were captured from late

May through early November with no clear general cessation of lactation. Regression of breeding females began in late August and continued through

December (fig. 10). Nulliparous and nonbreeding females comprised the largest

proportion of females captured in most months.

Conception and parturitionDate of parturition was estimated for litters found

in nest boxes in the Puget Trough (table 8). We combined samples from 1993 to

1997 and determined mean mass for young in each litter and used mass to

determine relative litter age in days based on Saskatchewan squirrels (Davis

1963). Although Booth (1946) studied the development of one litter of flying

squirrels closer to our geographic region, these squirrels tended to be smaller

than Puget Trough squirrels; at 365 days of age, the captive squirrels Booth

examined were just beginning to approach mass and sizes of 4- to 5-month-old

Puget Trough squirrels.

Age of litters ranged from 18 to 67 days. Estimates of parturition were from

June 18 to August 2. Assuming a 40-day gestation (Sollberger 1943), conception was between May 10 and June 23. Peak estrus was therefore mid-May to

early June. A peak of parturition in July corresponded to a consistent den shift

by many female squirrels to low structures in June and July (Carey and others

1997). Female flying squirrels are known to seek out maternal dens 1 to 2 weeks

before parturition (Wells-Gosling 1985). Parturition estimates from nest box data

were 1 to 2 weeks later than indicated by trapping data. Puget Trough squirrels

had slightly less mass than those studied by Davis (1963) in Saskatchewan.

The Reproductive Cycle

in the Pacific Northwest
34

Percent of females captured

16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46

0

20

40

60

80

100

Regressed Nulliparous

Apr May Jun Jul Aug Sep Oct Nov

Figure 8—Percentage of female flying squirrels captured in active states of reproduction by week (Julian calendar), Puget Trough, Washington, 1991-95. Weeks with no

bars indicate no trapping or sample sizes of active females <5.

Figure 9—Percentage of female flying squirrels captured in active states of reproduction by week (Julian calendar), Coast Range, Oregon, 1985-90. Weeks with no bars

indicate no trapping or sample sizes of active females <5.

Figure 10—Percentage of female flying squirrels captured in inactive states of

reproduction by week (Julian calendar), Coast Range, Oregon, 1985-90. Weeks with

no bars indicate no trapping or sample sizes of inactive females <5.
35

Consequently, Puget Trough squirrels may appear younger than their true age.

This would explain the difference in parturition dates between trapping (and

necropsy) data and nest-box data. Additionally, litter size likely influences mass

of young; larger litters would have more competition for milk and thus mass

development would be slower than for small litters of the same age.

There were significantly more young (t = -3.11, df = 22, p < 0.01) with age class

III females (0 = 2.87 ± 0.29) than with class II females (0 = 1.56 ± 0.24).

Additionally, class III females were significantly heavier (t = -4.83, df = 21, p <

0.01) than class II females (0 = 174 ± 3 for class III; 0 = 147 ± 5 for class II).

Coast Range and Puget Trough squirrels seem to have similar reproductive

cycles, but the breeding season for Coast Range squirrels is slightly later and

more extended than for Puget Trough squirrels (figs. 11 and 12). Coast Range

males tended to remain scrotal 3 months longer than Puget Trough males, and

the females showed signs of estrus well into June and July. In the Puget Trough,

pregnancy peaked in June with lactation peaking in late July, whereas peak of

pregnancy in the Coast Range occurred in July and lactation in late August. In

the Coast Range, lactation continued well into late November. In both studies,

young appeared in the population (postweaning) beginning in August. Both the

Puget Trough and Coast Range breeding seasons began considerably later than

that found by Davis (1963) who found females in estrus in late March and a peak

of pregnancy in late April to early May in Saskatchewan.

Although sample sizes in the Olympic Peninsula and North Cascades studies

were small and conducted primarily in fall, patterns appeared similar to the Puget

Trough. Male squirrels began regressing as early as July in the North Cascades

and September in the Olympics, and all were fully regressed by October. Some

lactating and postlactating females were found as late as October, but most adult

females were regressed by November. The first young of the year were trapped

in early October.

In the Puget Trough field study, most (90 percent) age class II and III females

reproduced by summer (fig. 13). In contrast, a small proportion (39 percent) of

age class II and III females in the Coast Range field studies showed signs of

active reproduction (estrus, pregnancy, lactation, or postlactation) in the field.

These data suggest onset of reproductive activity in age class II females is

density dependant, delayed in high-density populations. Necropsy data also

suggest that a large proportion of Coast Range females did not breed in any

given year. Of the epiphyseal notch-aged Coast Range females (collected in all

seasons), 86 percent of the age class II and 63 percent of the age class III

females were nulliparous or reproductively inactive. In addition, 73 percent of

age class III females captured during the breeding season (April-July) were

reproductively inactive, and 89 percent of the age class II females were nulliparous. In fall, nearly twice as many necropsied females showed signs of current

reproductive activity (lactation or postlactation) than did field-observed females

during spring, thereby suggesting that some breeding occurred after our spring

trapping sessions.

Breeding Season

Ratios of Reproductive

Activity
36

74

90

14

23

39

24

Percent of females captured

Spring Summer Fall

0

20

40

60

80

100

Puget Trough Coast Range

Figure 13Percentage of female flying squirrels found reproductively active

by season, Coast Range, Oregon (1985-90), and Puget Trough, Washington

(1991-95).

Figure 12Projected reproductive patterns of female flying squirrels by week

(Julian calendar), Puget Trough, Washington, 1991-95. Lines based on number

of females in each reproductive state per trap night.

Figure 11Projected reproductive patterns of female flying squirrels by week

(Julian calendar), Coast Range, Oregon, 1985-90. Lines based on proportion of

females in each reproductive class captured during live-trapping studies.
37

There was no apparent difference in the percentage of inactive females between

young and old stands in the Coast Range, although sample sizes for young

stands were small. Overall, 68 percent of the adult females captured were inactive. During summer (June-August), when most females should be showing

signs of reproductive activity, only 39 percent of female Coast Range squirrels

were reproductively active.

Several factors may account for this low percentage of reproductively active

females in the Coast Range field studies. Females recorded as regressed after

early May each year actually may have been in the undetectable early stages of

pregnancy or may have come into estrus as the season progressed but were not

subsequently recaptured. However, because 76 percent of adult females in the

field in fall were recorded as inactive, with no signs of recent lactation (and many

were recorded as still nulliparous), a high percentage of females did not reproduce in any given year. In contrast, regressed females in the Puget Trough

typically showed signs of recent lactation.

Population densities in the Coast Range young stands were, on average, 1.5 to

three times higher than in the Puget Trough; densities in old-growth stands were

four to seven times those in the Puget Trough (Carey 1995; Carey and others

1992, 1996). Breeding may be density dependent. Rates of reproduction of

Puget Trough female squirrels were similar to those studied by Davis (1963) in

Saskatchewan. Female squirrels in Saskatchewan bred at 1 year of age. All older

adult females caught during spring and summer showed signs of reproductive

activity. In the Puget Trough, over 70 percent of the field-assessed age class II

females bred at 1 year of age, and 97 percent of age class III females showed

signs of reproductive activity.

In the Olympic and North Cascades field studies, conducted primarily in late

summer and fall, densities (Carey 1995) and ratios of reproductively active females fell between the Coast Range and Puget Trough. In late summer (August),

57 percent of adult females were active (lactating or postlactating), and in fall, the

proportion dropped to 21 percent. Proportions were not directly comparable to

Coast Range and Puget Trough because the summer trapping was conducted so

late. Most adult (age class II and III) females, however, probably did breed

because few nulliparous adults were trapped.

Several population characteristics may be determined from age, sex, and reproductive data. Age and sex ratios combined with information on maturation and

reproductive attainment reveal many aspects of flying squirrel life history. In

Saskatchewan, from a sample collected in July, 56 percent were juveniles, 28

percent young adults, and 15 percent older adults (Davis 1963). These data

suggested a 50-percent mortality rate and almost a complete population turnover

by the third year. Davis (1963) also found a ratio of 27 females to 26 males for

juvenile squirrels and, for adults, 69 females to 81 males. This ratio was not

significantly different from a 1:1 ratio, and the ratio of males to females differed

little during different months sampled.

Population

Characteristics
38

Few studies have examined northern flying squirrel populations in Pacific Northwest forests. Forsman and others (1994) explored seasonal relative proportions of juvenile and adult flying squirrels in spotted owl diets on the Olympic

Peninsula, Washington. By inspecting owl pellets and assigning age categories

based on size of skeletal parts, tooth development, and degree of fusion of the

epiphyseal cartilage, age ratios, and timing of the initial recruitment of juveniles

were estimated. Conclusions could not be drawn about the entire population

because juvenile flying squirrels probably were more susceptible to predation

than adults.

Sex ratios of populations of northern flying squirrels studied in all regions followed the general trend for mammal populations; all populations showed an

approximate 1:1 sex ratio in fall. In both the Coast Range and Puget Trough,

however, males were captured more often than females in spring and, therefore,

inflated the male proportion in that season. Movements of flying squirrels during

the breeding season may account for higher male captures; male flying squirrels

move greater distances than females, thus covering more territory.4

 In addition,

as females begin to give birth and lactate, they may be less active and less likely

to be captured. Sex ratio of juveniles captured in nest boxes was 1:1 (34 males

and 34 females). Sex ratio did not differ among age classes and appeared to be

maintained as cohorts aged.

In Coast Range and Puget Trough populations, age ratio was similar from season to season for the youngest cohort (fig. 14). In early fall, when most Puget

Trough class I individuals entered the population, they accounted for about 36

percent of the population. In spring, class II squirrels accounted for about 37

percent of the population, thereby suggesting overwinter mortality was similar to

older animals. Coast Range and Olympic Peninsula squirrels followed a similar

pattern with class I accounting for 29 to 34 percent of the population in fall.

Age classes were equally represented for Coast Range squirrels aged by epiphyseal notch. In fall, 12 individuals from each age class were captured, in

spring, 26 class II individuals and 20 class III individuals were captured, and in

summer, 35 and 33 individuals, respectively, were captured. It was unclear

whether necropsied individuals were an unbiased sample of the population when

examining age ratio. Snap-trapped individuals were probably equally affected,

but young animals may die more often in live traps than do adults. Low body

weight, foraging habits, and other factors may cause young squirrels to perish in

live traps more often than adults, thus skewing the age ratio of dead squirrels

toward younger individuals.

We used Coast Range and Puget Trough animals initially aged as class I to evaluate survivorship between age classes. If individuals were not captured within two

seasons (1 year) after last capture, we assumed they were dead (not a strictly

Sex Ratio

Age Class Ratio and

Survivorship

4

 Carey, A.B. 1998. Unpublished data. On file with: Pacific

Northwest Research Station, Forestry Sciences Laboratory,

3625 93d Ave., SW, Olympia, WA 98512.
39

true assumption). Because trapping effort differed by stand, individuals in the

sample were not included once they did not have the opportunity to be trapped

for at least two seasons after last capture.

From age class I, 190 Coast Range and 163 Puget Trough squirrels were followed through time (table 9). Because we knew the birth year of individuals, we

could follow them past age class III (unlike using epiphyseal notch or external

characteristics). About 50 percent of individuals survived from class I to II in

Coast Range old-growth and mixed-age stands. Only 7 percent of individuals

captured in young Coast Range and 30 percent in Puget Trough stands survived

to age class II. One-third of class II squirrels survived to class III in all three

stand types in the Coast Range, but only 22 percent survived in the Puget

Trough. Although no individuals from the sample survived beyond class III in

mixed-age or young stands, almost 60 percent of class III squirrels survived to

class IV in old-growth stands in the Oregon Coast Range.

In the Puget Trough, the proportion of individuals that survived to subsequent

age classes diminished over time. Overall, Puget Trough squirrels were less likely

to survive to the next age class than in Coast Range old-growth or mixed-age

stands. During our study, we recorded squirrels living longer than data in table 9

suggests; three individuals (one Coast Range, two Puget Trough), initially captured as adults, were known to be at least 7 years old.

The greatest difference in flying squirrel reproductive biology between the Coast

Range and Puget Trough was the proportion of females that were reproductively

active. In the Coast Range, class II squirrels tended not to reproduce, and few

class III individuals reproduced in any one year. In contrast, in the Puget Trough,

nearly all class II and III females bred. Diet and climatic conditions are environmental factors known to influence reproductive development in mammals and thus

Figure 14—Percentage of flying squirrels captured from each age class, Coast Range,

Oregon (1985-90), and Puget Trough, Washington (1991-95). Age classes were

assigned to Coast Range animals retrospectively based on trapping notes and history.

Implications

Environment and

Maturation
40

the age at which they become fertile (Bronson 1989). Environment could influence onset of reproduction and proportion of adults breeding through seasonal

differences in harshness of climate and food availability. Coast Range winters

are colder and wetter than Puget Trough winters (Franklin and Dyrness 1973,

USDC National Oceanic and Atmospheric Association 1982, USDA Soil Conservation Service 1990). Harsh winters in the mountainous Coast Range could limit

production of the primary food of squirrelstruffles (Carey 1995). Temperature

and precipitation govern fungal productivity (Fogel 1981); temperatures <0 to

6 °C cause production to cease (Hunt and Trappe 1987). In spring, with increases in temperature and precipitation, fungi produce a burst of truffles (Hunt

and Trappe 1987).

Restriction of food affects the estrous cycle, puberty, and onset of seasonal

breeding (Bronson 1989). Intraspecific competition for limited food also would be

more likely in the dense Coast Range populations. Small-bodied age class II

females in the Coast Range may not receive enough food during winter and

spring to reach a minimum body weight needed for estrus; thus, onset of reproduction could be delayed to the next year. Even age class III females also may

need a burst of food availability for estrus. Variation in fungal sporocarp abundance (temporal and absolute) is marked (Fogel 1981, Hunt and Trappe 1985,

Luoma 1991). Thus, variation in the nutritional status of females in spring could

result in variation in the onset of estrus, breeding, and parturition. Once primary

demands are met, excess energy can be used to support reproduction. When

nutrients are scarce, partitioning of energy must be decided between reproductive and nonreproductive needs (Bronson 1989).

Fungal production drops in summer with drought, which also can be marked in

the Coast Range (Franklin and Dyrness 1973, USDC National Oceanic and

Atmospheric Association 1981). Summer drought and late (well into fall) lactation

could place females in energetic deficits going into winter, further delaying or

even prohibiting reproduction the next year. If young are born late, they would

still be nursing during the fall burst of truffles. Young would then face the postweaning period, with its need for high energy for growth and maturation, in winter

when food is scarce. Thus, late-born young would likely not breed the following

year.

Table 9Proportion of northern flying squirrels in each age class that

survived to subsequent age classes, 1985-95

Flying squirrel age class

Region Class I to II Class II to III Class III to IV Class IV to V

Oregon Coast Range:

Old growth 0.54 (107) 0.33 (58) 0.58 (19) 0.18 (11)

Mixed age .43 (42) .33 (18) 0 (6) NA

Young .07 (41) .33 (3) 0 (1) NA

Puget Trough:

Young .30 (163) .22 (45) .10 (10) 0 (0)

NA = not applicable.

Note: Sample sizes are shown in parentheses.
41

Puget Trough winters are mild and wet (Franklin and Dyrness 1973, USDC

National Oceanic and Atmospheric Association 1982, USDA Soil Conservation

Service 1990), and population densities were low. Both age class II and III

squirrels gain weight rapidly in fall and early winter, and Puget Trough squirrels

may enter spring with greater nutritional balance than in the Coast Range. Early

spring blooms of truffles could then enable both age classes to gain adequate

weight and come into estrus, with a greater synchronicity than in the Coast

Range.

Greater seasonal differences in temperature, precipitation, and food availability

may simply be met by delaying or prolonging maturation and reproduction in the

Coast Range. Lower predictability of food abundance due to greater climatic

extremes may be best approached by hedging bets and extending periods of

reproductive activity. This would provide continued reproductive success by

ensuring that the timing of maturation for some young each year would occur

during periods of adequate forage production.

Flying squirrel densities in the Coast Range (Carey 1995, Carey and others

1992) were much higher than in the Puget Trough (Carey and others 1996)

and reduced proportions of flying squirrels breeding could reflect density

dependencethe greater the density, the fewer squirrels breeding. Like other

rodents, the reproductive potential of flying squirrels can be relatively high. In

high-density populations such as the Coast Range, mechanisms seem to limit

excessive production of offspring by large numbers of adults. Mechanisms of

density dependences differ; for example, food restriction, effects on social

behavior, or pheromone-mediated reproduction. In contrast, low-density populations like the Puget Trough do not need to limit the number of reproductively

active adults; most individuals breed when they reach their first breeding season.

In the Coast Range, breeding season was long and various stages of reproduction (estrus, pregnancy, and lactation) overlapped (fig. 11). In contrast, flying

squirrels in the Puget Trough demonstrated synchronicity in breeding condition,

with distinct peaks of estrus and parturition (fig. 12). Timing of recruitment of

class I individuals into the population may be an adaptation to reduce susceptibility of young to predation. Coast Range young were recruited gradually, whereas

Puget Trough young were recruited all at once.

In the Coast Range, spotted owls were important predators of flying squirrels and

could lower flying squirrel populations (Carey and others 1992). Owls manifested

strategies that seem to reflect avoidance of depleted prey populations. They also

avoided depleting prey populations by rotating among foraging patches (Carey

and Peeler 1995). The extended breeding season in the Coast Range may be an

adaptation to reduce high predation of weanlings by owls. Spotted owls probably

track prey resources within their home range (Carey and Peeler 1996); a sudden influx of weanlings into a local population could attract or maintain intensive,

continuous foraging by owls. A drawn-out breeding season and recruitment of

class I flying squirrels could maintain intermittent foraging by spotted owls; fewer

Density

Predation
42

young would be available for any given period of foraging. Gradual recruitment

also would reduce competition for food among young squirrels and between

young, naive squirrels and experienced adult squirrels until fungal blooms in fall.

Spotted owls were not present in Puget Trough stands. Weasels (Mustela spp.),

which are less mobile than owls, appeared to be major predators of flying

squirrels in these second-growth stands (Wilson and Carey 1996). Many of the

maternal dens used by females in the Puget Trough were low (<2 m aboveground), and safe maternal dens may have been lacking (Carey and others

1997). Highly synchronous reproduction could swamp weasels with young

during the critical period of nursing, when females and young occupy nests

close to the ground, thus allowing some young to survive long enough to learn

predator avoidance. After weaning, squirrels move to arboreal dens and are less

susceptible to predation by terrestrial predators. Thus, it could be to the advantage of Coast Range populations to space weaning for predator avoidance, but

for Puget Trough populations to concentrate reproduction.

On average, Puget Trough squirrels were larger than Coast Range squirrels. In

addition to density-dependent factors, differences in weight among and between

populations could reflect differences in breeding season behavior and thermoregulation. In general, home range size was larger in the Puget Trough than in

the Coast Range. Stored energy (reflected in mass) was used differently by

males and females during the breeding season.

Male squirrels in the Puget Trough covered large distances (>5 km) in search of

females during the breeding season and thus had tremendous energy needs

(see footnote 4). Weights were heavier for age class III males than for age class

II males in the Coast Range during spring where large movements by males were

not observed. Thus, similar weights of age class II and III males in the Puget

Trough in late spring may suggest that older males come into early spring with

larger weights, use more energy, and perhaps travel farther distances than class

II males in search of females. In both provinces, age class III females were

heavier than age class II females in spring, and mass parallels the ability of older

individuals to reproduce (Coast Range) or have more offspring (Puget Trough).

By fall, weights were heavier but similar between sexes and between class II and

III squirrels, thereby suggesting reduced activity levels allowed accumulation of

stored energy reserves (fat) for winter.

Large body size in the Puget Trough also may reflect an increased need for

thermoregulation, even though the climate is relatively mild. In the Puget Trough,

up to 48 percent of the dens used during the year were external nests comprised

of sticks, leaves, and moss; few external nests were observed in the Coast Range

(Carey and others 1997, Gillesberg and Carey 1991). External dens generally

provide less thermal protection than cavities, and thus body size may be larger in

the Puget Trough to compensate for increased thermoregulation needs. Additionally, with lower densities than the Coast Range, Puget Trough squirrels may have

less group denning opportunities, or den in smaller groups, and therefore have

fewer opportunities to reduce energy expenditures through sharing body heat.

Body Size and Energy

Requirements
43

Differences in population structure and survivorship between the Coast Range

and Puget Trough suggest differences in survival strategy. Coast Range squirrels

exhibit the K-selection characteristics of dense populations, low reproduction,

high adult survivorship, and high juvenile survival (Wilson and Bossert 1971).

Puget Trough squirrels exhibit r-selection traits of low-density populations, high

reproduction, low survivorship of older age classes, and high juvenile mortality.

The K-strategists often are found in more stable habitats; the Coast Range may

provide more relatively established and secure habitat (food diversity and dens)

even though climatic conditions are harsher than that of the Puget Trough. Puget

Trough second-growth stands represent young, disturbed forests with low habitat

diversity, and the r-selection strategies seen in the Puget Trough may represent a

more unstable environment for flying squirrels.

It is always difficult to determine cause and effect in biological systems. Most

situations are multifactorial, with various causes for each observed effect. As

we proceed with our experiments on the influence of forest structure on flying

squirrel biology (Carey and others 1996), we will gain further insight into the

causes and effects. Nevertheless, being able to accurately assess age and

reproduction and applying it to life history characteristics is a prerequisite for

understanding the ecology of flying squirrels.

We thank technicians Greg Braden, Bruce Haveri, Charley Knox, and all alumni

trappers for their collection of field data and observations. We also thank

Shannon Hirst for laboratory assistance, Angela Lovato for many illustrations,

and Judy Putera for extensive laboratory work. Helpful review comments were

provided by Ellen Kritzman, Douglas Ransome, Peter Weigl, and Suzanne

Wilson. Funding for this work was provided by the U.S. Department of

Agriculture, Forest Service, Pacific Northwest Research Station; the U.S. Army,

Ft. Lewis, WA; the U.S. Department of the Interior, Bureau of Land Management,

Oregon State Office; and the U.S. Department of Agriculture, National Research

Initiative.

1 hectare (ha) = 2.471 acres

1 kilometer (km) = 0.6214 mile

1 meter (m) = 3.28084 feet

1 centimeter (cm) = 0.3937 inch

1 millimeter (mm) = 0.03937 inch

1 gram (g) = 0.035 ounce

Selection Factors

r and K

Acknowledgments

Metric Conversions
44

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49

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Zahler, P. 1996. Rediscovery of the wooly flying squirrel (Eupetaurus cinereus).

Journal of Mammalogy. 77(1): 54-57.
50

The following is a series of publications on sampling methods and biology for use

in designing and implementing research and monitoring efforts in Pacific Northwest forests. This list of publications does not duplicate those listed in Literature

Cited.

Bury, R.B.; Corn, P.S. 1991. Sampling methods for amphibians in streams in

the Pacific Northwest. Gen. Tech. Rep. PNW-GTR-275. Portland, OR: U.S.

Department of Agriculture, Forest Service, Pacific Northwest Research

Station. 29 p.

Carey, A.B.; Biswell, B.L.; Witt, J.W. 1991. Methods for measuring

populations of arboreal rodents. Gen. Tech. Rep. PNW-GTR-273. Portland,

OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest

Research Station. 24 p.

Carey, A.B.; Castellano, V.E.; Chappell, C. [and others], tech. comps. 1990.

Training guide for bird identification in Pacific Northwest Douglas-fir forests.

Gen. Tech. Rep. PNW-GTR-260. Portland, OR: U.S. Department of

Agriculture, Forest Service, Pacific Northwest Research Station. 28 p.

Carey, A.B.; Horton, S.P.; Reid, J.A. 1989. Optimal sampling for radiotelemetry

studies of spotted owl habitat and home range. Res. Pap. PNW-RP-416.

Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific

Northwest Research Station. 17 p.

Carey, A.B.; Witt, J.W. 1991. Track counts as indices to abundances of

arboreal rodents. Journal of Mammalogy. 72(1): 192-194

Corn, P.S.; Bury, R.B. 1990. Sampling methods for terrestrial amphibians and

reptiles. Gen. Tech. Rep. PNW-GTR-256. Portland, OR: U.S. Department of

Agriculture, Forest Service, Pacific Northwest Research Station. 34 p.

Forsman, E.D. 1983. Methods and materials for locating and studying spotted

owls. Gen. Tech. Rep. PNW-GTR-162. Portland, OR: U.S. Department of

Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment

Station. 8 p.

Guetterman, J.H.; Burns, J.A.; Reid, J.A. [and others]. 1991. Radio telemetry

methods for studying spotted owls in the Pacific Northwest. Gen. Tech. Rep.

PNW-GTR-272. Portland, OR: U.S. Department of Agriculture, Forest Service,

Pacific Northwest Research Station. 43 p.

Manuwal, D.A.; Carey, A.B. 1991. Methods for measuring populations of small,

diurnal forest birds. Gen. Tech. Rep. PNW-GTR-278. Portland, OR: U.S.

Department of Agriculture, Forest Service, Pacific Northwest Research

Station. 23 p. (Ruggiero, L.F.; Carey, A.B., tech. eds.; Wildlife-habitat

relationships: sampling procedures for Pacific Northwest vertebrates).

Appendix 1:

Sampling Methods
51

Thomas, D.W.; West, S.D. 1989. Sampling methods for bats. Gen. Tech. Rep.

PNW-GTR-243. Portland, OR: U.S. Department of Agriculture, Forest Service,

Pacific Northwest Research Station. 20 p. (Ruggiero, L.F.; Carey, A.B., tech.

eds.; Wildlife-habitat relationships: sampling procedures for Pacific Northwest

vertebrates).
52

Guidelines follow for selecting a single age and reproductive code for individual

flying squirrels. For data summary and analysis, each captured individual should

have a single code describing age and reproductive status for the trapping

session. Animals with more than one capture, however, are sometimes described

with more than one age or reproductive code. This document explains how a

single code is determined for these individuals and provides guidelines for

following a consistent procedure.

Age-class systemFor flying squirrels, there are three age-class codes: class I

squirrels were born in the same year as the capture date (young-of-the-year),

class II squirrels were born the year before the capture date, and class III

squirrels were born in years earlier than one year before the capture date.

Reproductive statusThere are two inactive codes used for both males and

females: N and R. Animals that show no sign of ever having been reproductively

active are N for never active (males) or nulliparous (females), whereas regressed

individuals (R) show signs of past (but not present) reproductive activity. Males

have only one code for current reproductive activity: scrotal (S); females have

four: estrus (V), pregnant (P), lactating (L), and postlactating (G). The codes for

reproductive status are viewed hierarchically: never active and nulliparous are

considered the lowest level of reproduction, whereas scrotal and lactatingpostlactating are the highest levels. Regressed is viewed as a higher reproductive

level than nulliparous, even though we cannot be certain that regressed individuals were active in the current year. All active codes rank above inactive codes,

and active codes for females are higher as reproductive condition progresses.

If an individual is caught several times and is assigned two or more age or

reproductive codes, a combination of factors is used to select one code. The list

below is in order of greatest to least importance in steps used to determine a

single code. When the age of a squirrel has been described differently during

the trapping session, guidelines 1-7 described below are used to determine the

most accurate assessment of age. For reproductive status, all guidelines apply,

and the objective is to assign the highest level of reproduction there is evidence

to support.

 Capture historyWhen available, all information about an individual (such as

captures in previous trapping sessions) is reviewed along with current trapping

data for thorough age and reproductive assessment.

 Complete descriptionsComplete includes a description in the notes of all

the characteristics that determine the age or reproductive code (appendices 3

and 4; tables 3, 5, and 6).

 Number of trappers using the same codeMore individual trappers using one

code adds credence to that code, but there must be notes that correspond

with code choice for this to be a determining factor.

 Trapper experienceThe more animals and seasons a trapper has observed

increases reliability. Also, accuracy generally improves for all trappers as the

trapping session progresses.

Appendix 2:

Arboreal Rodent

Trapping Data

Processing

Guidelines for Selecting

Age and Reproductive

Code
53

 Age and reproductive statusIn fall, females with previous reproductive

activity cannot be age class I, and age class I males and females are reproductive code N.

 WeightWeight is of limited use in determining age or reproductive status. In

fall, a weight <100 g suggests age class I, whereas a weight >140 g is likely

an older squirrel (for the Puget Trough). A steady increase in the weight of a

heavy female combined with evidence of postestrus suggests pregnancy. A

sudden drop in weight (approximating 17 percent) within 3 to 4 days or less

may indicate parturition. Flying squirrels usually give birth from late May

through June in the Puget Trough.

 Population breeding stageParticipation in a trapping session and reviewing

all the captures over a session provides an overall picture of the breeding

stage of each population. During spring trapping, most male flying squirrels

are scrotal, whereas females exhibit various reproductive stages. During fall

trapping, most flying squirrels are sexually inactive. It is not uncommon,

however, to capture a few females that are postlactating, and it is feasible for

animals to be in active stages of reproduction. Individuals described as

reproductively active in fall should be carefully reviewed for conclusive evidence regardless of the number of captures or consistency of codes.

 If a decision on the final reproduction code cannot be determined with the

above guidelines, the reproductive field is blanked and the change is documented in the notes (as for all code modifications).
54

The following are external characteristics of the female flying squirrel

reproductive cycle:

Genitalia are reduced and pale, no sign of breeding activity. Vulva not swollen,

light pink to no color, closed. Mammary disks undeveloped (N) or unapparent (R).

Nipple size describes past breeding history:

Nulliparous (N)Never reproduced.

 Nipples are usually tiny pink dots or small nubs that do not project distinctly

above the skin surface; or

 Nipples may show some development: small, slender, conical shape <1 mm in

length, standing out from skin, pale to pink.

Regressed (R)Previously reproduced.

 Nipples developed. Conical or flattened, flaccid, >1 mm long, white or light

pink.

Begins with changes in vulva appearance, then a progression in the development

of nipples, abdomen, and mammary glands.

Estrus (V)

 Vulva enlarging-swollen and color darkening pink to red. Vagina opening

becomes more apparent through estrus until noticeably inverted and open.

Nipples undeveloped, developing, or fully developed yet flaccid.

 Mammary is showing no obvious development.

It should be noted when you suspect an animal to be in post-late estrus but it

could not be determined pregnant. Write R = V/P in your comments to indicate

this condition.

Pregnant (P)

 Vulva unswollen, usually retains dark coloration, labia flaccid yet easily opened

 Nipples just developing OR becoming more upright and turgid on previously

bred animals. Color is darkening pink.

 Abdomen enlarging-disproportionately large during later stages of pregnancy.

 Mammary shows no development or may start to protrude.

Lactating (L)

 Vulva reverting to small, pale state.

 Nipples developed, turgid, pink to red.

 Abdomen not distended.

 Mammary disk developed, milk production, hair loss.

Appendix 3: Female

Reproductive Cycle

Inactive

Active
55

Postlactating (G)

 Nipples developed but losing rigidity and color.

 Mammary diminished, hair growing back.
56

The following are external characteristics of the male flying squirrel reproductive

cycle:

Genitalia are reduced, underdeveloped, or absent (externally).

Never active (N)Never reproduced.

 Scrotum absent or developing (but small). Seen primarily in males born in the

current year.

 Testes tiny and undeveloped, often within abdominal cavity, descended into

scrotum on some males.

 Penis positioned nearer to anus on immature animals

Regressed (R)External signs of previous reproductive activity but not

currently in breeding condition.

 Scrotum evident to very evident, usually of a size large enough to contain fully

developed testes.

 Testes are small (having shrunk down to a nonbreeding condition), however,

and may or may not be detectable.

Genitalia are either enlarging, shrinking, or fully developed, depending on the

season.

Scrotal (S)Developed into breeding condition

 Scrotum is large, full, and obvious.

 Testes are large, palpable, and give the scrotum its full shape.

Flying squirrel males become sexually active well before females go into estrus

(around May). In the Puget Trough, we have observed scrotal males as early as

January. After female estrus and pregnancy, however, male testes rapidly diminish in size. The scrotal sac still remains relatively large and can be used as a

factor in distinguishing adults from young of the year when trapping in fall. It is

important to record notes on relative testes size so that we get an indication of

when sexual regression is occurring in our populations. Also remember that

some individuals will retract testes against the abdominal wall due to cold temperatures or on being touched, thereby making detection difficult.

Appendix 4: Male

Reproductive Cycle

Inactive

Active
57

Appendix 5: Necropsy Data Form

DISSECTOR 

CATNUM STD NAME STD# LOCATION 

DATE / / TAG1 SPP AGE SEX REPRO 

TOTAL LENGTH TAIL BODY LENGTH FOOT EAR WT. 

FEMALE

VULVA NIPPLES SCARS EMBRYOS UTERINE HORNS

OPEN/

CLOSED

AVERAGE

LENGTH NUMBER NUMBER

1

C-R

2

C-R

3

C-R

4

C-R LENGTH WIDTH

L

R

ADDITIONAL REPRO NOTES

CORPORA

LUTEA

OVARIES

 # AVE. DIA. LENGTH WIDTH

L

R

 MALE 

WELL-DEVELOPED SCROTAL SAC: Y OR N TESTES: FLACCID OR TURGID

POSITION LENGTH WIDTH COLLECTIONS

L PELLETS STOMACH REPRO SKIN OTHER

R

NOTES

PHOTOS TAKEN: Num Roll# Film Subject 
58

Appendix 6: Trapping Data Code Sheet

FIELD NAMES

DATE= D-MON-Y; use 1st 3 letters of month

SEA= 1-Spring (15 Mar)

2-Summer (15 Jun)

3-Fall (15 Sep)

4-Winter (15 Dec)

WK= week (1,2)

N= night (1,2,3,4)

OBS= first,middle,last initials

STA= Station (A1,H8, etc.)

T= trap: G-ground,T-tree, L-large, S-small

C-cavity, N-nestbox

SPP= species-(4 letter code1)

C= capture code: 1-New Capture

2-Recapture

4-Recapture & Retag

TAG1/TAG2- 5 character eartag label

A= Age: 1-young of year

2-born in previous year (for GLSA only)

3-adult (for GLSA, 3= born before previous year)

S= sex: M-male, F-female

R= reproductive stage2

GRWT= wt. of cone and animal

TRWT= wt. of cone

WT= weight of animal (GRWT-TRWT)

PELN= pellet vial3

 code number

D= put '*' if dead, whether or not collected

CATNUM= Catalog number for collected dead

animals. Write in notes.

RC= put '*' if animal carries radio collar

 _____________________________________________________________________________________

1SPECIES CODES

SPRU- Sprung trap NECI- Bushy tailed woodrat APRU- Mt. beaver 

GLSA- Flying squirrel NEFU- Dusky footed woodrat DIVI- Opossum

TATO- Townsend's chipmunk SPPU- Spotted skunk MUER- Ermine

TADO- Douglas' squirrel SCGR- W. gray squirrel MUFR- Long-tailed weasel

SYFL- Eastern cottontail LEAM- Snowshoe hare

Some common abbreviations in the 'notes' field for SPRU traps include:

NAR - no apparent reason NUT - nestbox under treadle

____________________________________________________________________________________

2REPRODUCTIVE CODES

Males Females

N- never been active N- nulliparous

R- regressed R- regressed

S- scrotal V- estrus

P- pregnant

L- lactating

G- post lactating

____________________________________________________________________________________

3PELLET VIAL LABEL

Stand Name 

Species

Station:Trap

Date (dy/mo/yr)

Stand #

Tag #s

Sex

Observer
59

Appendix 7: Arboreal Rodent Live-trapping Data Sheet 

STAND NAME/# ______________________ DATE __________________ SEA _____ WK _____ N _____ 

STA T SPP C TAG 1 TAG 2 A S R GRWT TRWT WT PELN D RC 
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Document continues on next page.
Villa, Lisa J.; Carey, Andrew B.; Wilson, Todd M.; Glos, Karma E. 1999. Maturation

and reproduction of northern flying squirrels in Pacific Northwest forests. Gen. Tech.

Rep. PNW-GTR-444. Portland, OR: U.S. Department of Agriculture, Forest Service,

Pacific Northwest Research Station. 59 p.

Northern flying squirrels are the primary prey of northern spotted owls and are important

dispersers of fungal spores in Pacific Northwest forests. Despite the importance of these

squirrels in forest ecosystems, information is lacking on life history and methods for

determining age and reproductive condition. In the laboratory, we measured epiphyseal

notch, femur length, rostrum length, least interorbital breadth, rump pelage length and

color, and tail width of dead squirrels. We analyzed weight, pelage color, and reproduction

for field-collected data from live squirrels. Reproduction of northern flying squirrels can be

reliably determined in live-trapping studies. Regional variations suggest accurate assessment of age and reproduction is a prerequisite for understanding flying squirrel ecology.

Keywords: Northern flying squirrel, maturation, reproductive biology, Oregon, Washington,

live-trapping, necropsy.

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increasingly greater service to a growing Nation.

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