Sexual Dimorphism of Polar Bears

Abstract

Sexual dimorphism in body mass, body length, head width, head length, and foreleg guard hair length of polar bears (Ursus maritimus) was examined from live-captured polar bears in Svalbard, Norway. Limited evidence of sexual dimorphism was apparent in cubs shortly after den emergence but was marked after the 1st year of life. Sexual dimorphism in adults resulted from both a higher growth rate and prolonged growth period in males. In mature animals, sexual dimorphism was greatest in mass, followed by foreleg guard hair length, head width, body length, and head length. Foreleg guard hair length was age related and hypothesized to be a form of ornamentation. Geographic variation in sexual dimorphism was evident for mass and body length for seven different populations but there was no evidence of a hyperallometric relationship in sexual dimorphism.

Full text

SEXUAL DIMORPHISM OF POLAR BEARS
ANDREW E. DEROCHER,* MAGNUS ANDERSEN,AND ØYSTEIN WIIG
Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada (AED)
Norwegian Polar Institute, N-9296 Tromsø, Norway (MA)
Zoological Museum, University of Oslo, P.O. Box 1172 Blindern, N-0318, Oslo, Norway (ØW)
Sexual dimorphism in body mass, body length, head width, head length, and foreleg guard hair length of polar
bears (Ursus maritimus) was examined from live-captured polar bears in Svalbard, Norway. Limited evidence of
sexual dimorphism was apparent in cubs shortly after den emergence but was marked after the 1st year of life.
Sexual dimorphism in adults resulted from both a higher growth rate and prolonged growth period in males. In
mature animals, sexual dimorphism was greatest in mass, followed by foreleg guard hair length, head width,
body length, and head length. Foreleg guard hair length was age related and hypothesized to be a form of
ornamentation. Geographic variation in sexual dimorphism was evident for mass and body length for seven
different populations but there was no evidence of a hyperallometric relationship in sexual dimorphism.
Key words: polar bear, sexual dimorphism, Ursus maritimus
Sexual dimorphism, the morphological differentiation be-
tween males and females, is common among vertebrates, with
males usually larger than females (Andersson 1994; Eisenberg
1981; Ralls 1976; Schoener 1967; Selander 1966). The study of
sexual dimorphism can give insights into the ecology and life
history of a species although 3 main areas have been proposed
for the occurrence of dimorphism in a species. Sexual
dimorphism can arise, and be maintained, 1st, through sexual
selection; 2nd, through separation of parental roles for males
and females (e.g., greater maternal care of young); and 3rd,
through intersexual competition for food (e.g., differences in
prey species—Alexander et al. 1979; Darwin 1871; Ralls 1977;
Schoener 1967). In many species, sexual selection is thought to
be the ultimate cause for sexual dimorphism (Charnov 1992;
Clutton-Brock et al. 1977; Cox and LeBoeuf 1977; Fairbairn
1997; Hoogland 2003; Selander 1972). Often, larger body size
of males is correlated with higher reproductive success because
of intermale competition for access to females (e.g., Andersson
1994; Clutton-Brock et al. 1982; LeBoeuf and Reiter 1988;
Ralls 1976). Therefore, sexual selection is related to the mating
system and in polygamous or promiscuous species tends to
result in selection for larger males (Emlen and Oring 1977;
Ralls 1977; Selander 1966).
Most studies of sexual dimorphism focus on sexually mature
males and females. Even in the most dimorphic mammals,
sexual dimorphism at birth is low, although often present.
Maternal factors can affect sexual dimorphism of offspring
through differential investment after parturition (Boyd and
McCann 1989; Lee and Moss 1986; Trivers 1972). Sexual
dimorphism develops in most species through divergent growth
patterns, that is, sex-specific differences in growth rate and
growth duration (Badyaev 2002; Cheverud et al. 1992).
Another issue relevant to the examination of sexual
dimorphism pertains to secondary sex differences. Exaggerated
secondary sex differences are often referred to as ornamenta-
tion. Conspicuous examples in mammals are horns, antlers,
tusks, and hair patterns (Andersson 1994). Ornamentation of
males is presumed, and often demonstrated in empirical
studies, to be preferred by females and can evolve even if
female choice is costly (see review Mead and Arnold 2004).
Showier males may be preferred by females if fertility is linked
to phenotype (Blount et al. 2001; Sheldon 1994). Empirical and
theoretical studies of ornamentation are a current issue in avian
fauna (e.g., Evans 2004; Kraaijeveld et al. 2004; van Doorn
and Weissing 2004) but recent studies of ornamentation in
mammals are less common.
Sexual dimorphism tends to increase with size over a variety
of taxa (Abouheif and Fairbairn 1997; Rensch 1950; Smith and
Cheverud 2002). In species where males are larger than females,
studies often find that sexual dimorphism increases with
increasing body size (hyperallometry—Fairbairn and Preziosi
1994; Rensch 1959). Mammals commonly demonstrate geo-
graphic variation or trends in body size with latitude and
longitude (Derocher and Stirling 1998a; Geist 1987; Langvatn
and Albon 1986; McNab 1971) and, therefore, geographic
variation in sexual dimorphism is likely to result. Conditions
favorable for growth could result in increased sexual dimor-
phism, particularly if males and females are under different
* Correspondent: derocher@ualberta.ca
Ó2005 American Society of Mammalogists
www.mammalogy.org
Journal of Mammalogy, 86(5):895–901, 2005
895

selective pressures. If sexual selection is a key factor explaining
adult body size, then we would expect that an increase in adult
size (i.e., favorable conditions during development) would be
associated with an increase in sexual dimorphism (Fairbairn and
Preziosi 1994; Mahoney et al. 2001).
Among mammals, the most commonly identified dimorphic
taxa include primates, elephants, pinnipeds, ungulates, mus-
telids, and macropods (Ralls 1977; Weckerly 1998). Most
carnivores have a polygamous or promiscuous mating system,
and generally show distinct sexual dimorphism, with males
being larger (Ewer 1973; Ralls 1977). In Ursidae, sexual
dimorphism is poorly described but appears to be widespread
(Stirling and Derocher 1990). Polar bears (Ursus maritimus)
have been described as a sexually dimorphic species based on
the comparison of body-mass growth curves, with adult males
about twice the mass of adult females (Atkinson et al. 1996;
Derocher and Wiig 2002; Kingsley 1979; Ramsay and Stirling
1986). The mating system of polar bears is poorly described
and both a polygynous system (Berta and Sumich 1999) and
a polyandrous system (Ramsay and Stirling 1986) have been
suggested. Prolonged mother–offspring associations were
postulated to result in a skewed operational sex ratio with 2
or 3 males available for every estrous female (Bunnell and Tait
1981; Ramsay and Stirling 1986) and these authors postulated
that larger males may have preferential access to females.
Evidence suggests that male polar bears compete intensely for
access to estrous females and can suffer severe injuries while
fighting (Ramsay and Stirling 1986).
In this paper, we examine sexual dimorphism and the
ontogeny of sexual dimorphism in body length, body mass,
head length, and head width in live-captured polar bears.
Foreleg guard hairs, which are conspicuously long in adult
males, were examined as a possible example of male ornamen-
tation. We hypothesize that foreleg guard hair length is an age-
related trait in male polar bears. We also examine geographic
variation of sexual dimorphism to determine if size of males
increases hyperallometrically with size of females.
MATERIALS AND METHODS
Polar bears were captured as a part of a research program on the
ecology of the Svalbard–Barents Sea population in Norway. Sampling
occurred on the sea ice in the central Barents Sea (74–778N, 37–438E)
and on the islands and the surrounding sea ice at Spitsbergen,
Nordaustlandet, Edgeøya, Barentsøya, and Hopen Island, Svalbard,
Norway (74–818N, 15–458E) from 4 March to 9 May 1987–2002.
Bears in this area are part of the Barents Sea population that moves
between Norway and Russia (Mauritzen et al. 2002). Yearlings and
older bears were caught by remote injection of a dart (Cap-Chur
Equipment, Douglasville, Georgia) containing the drug Zoletil
(Virbac, Carros, France) fired from a helicopter (Stirling et al.
1989). Cubs were caught by hand injection of drug shortly after den
emergence at about 4 months of age. Yearlings were approximately 16
months of age at capture. Offspring are normally independent of their
mothers at about 2.5 years of age. Animal handling methods were
approved by the National Animal Research Authority (Norwegian
Animal Health Authority, Oslo, Norway) and were in accordance with
guidelines of the American Society of Mammalogists (Animal Care
and Use Committee 1998).
We attempted to sample all bears sighted and believe the sample is
representative of the population. All bears were permanently marked
for future identification by a tattoo (Ketchum Manufacturing Supply
Inc., Brockville, Ontario) applied to the inner surface of the upper lip
on each side, plastic tags placed in each ear (Edcan Industries,
Edmonton, Alberta, Canada), and a transponder chip (Tiris, Texas
Instruments, Dallas, Texas) placed subdermally behind the ear. A
rudimentary premolar tooth was extracted from all bears more than
1 year old for age determination (Calvert and Ramsay 1998). The
sex, reproductive status, and a series of standardized morphometric
measure were collected from each bear. Body length (cm) was
measured as the dorsal straight-line distance from the tip of the nose to
the caudal end of the last tail vertebra. All bears were measured while
lying sternally recumbent with the back legs straight behind and the
forelegs flexed forward at the elbows parallel to the body. In the same
position, axillary girth (cm) was measured as the circumference around
the chest at the axilla with a rope (0.4-cm diameter) tightened with
a tension of about 0.5 kg. Mass for bears .1 year old was estimated
from a regression model developed specifically for the study
population that used axillary girth and body length (Derocher and
Wiig 2002). A spring scale (Chatillon, Largo, Florida) was used to
determine mass (to nearest 250 g) of cubs ,1 year old. Head
measurements (mm) of captured bears were made with calipers. Head
breadth was the maximum head width between the zygomatic arches.
Head length was the straight-line length from between the upper middle
incisors at the gum line to the most posterior dorsal skull process of the
sagittal crest. Guard hair length (cm) on the back of the forelegs was
measured at 5 evenly spaced locations on each leg from the top of the
wrist to the elbow while the bear was lying on its side. We used the
maximum guard hair length for analyses of age and sex variation.
To examine geographic variation in sexual dimorphism, we used
published values of asymptotic body size from growth curves of polar
bears. We examined the hypothesis that the relationship between the
mass (log
10
) of male and female polar bears would be hyperallometric
with a regression slope .1.0 (Leutenegger 1978; Lindenfors et al.
2002; Mahoney et al. 2001).
We used parametric statistics for all analyses by using SAS
statistical software (SAS Institute Inc. 1989). Values are presented as
means 61SE. Some information was not available for all animals,
resulting in varying samples sizes between analyses. Ages were log
10
transformed for statistical analyses. All estimates of sexual di-
morphism were calculated as the ratio of males to females. Statistical
significance was set to P0.05.
RESULTS
Ontogengy of sexual dimorphism.— When combining litter
sizes of 1, 2, and 3 cubs, mass of female (
X¼11.2 kg 60.4 SE,
n¼86) and male cubs (
X¼11.2 60.3 kg, n¼88) in spring
(about 4 months old) did not differ significantly (t-test, P¼
0.97). Similarly, no difference (t-test, P¼0.99) was found in
body length (females, 
X¼75 61 cm, n¼86; males, 
X¼75 6
1 cm, n¼80). Differences were evident in head length (t-test,
P¼0.012), with females (
X¼163 61 mm, n¼85) slightly
smaller than males (
X¼167 61 mm, n¼92) resulting in
a sexual dimorphism ratio of 1.02. Similar differences were
found in head width (t-test, P¼0.003), with females (
X¼100
61 mm, n¼85) smaller than males (
X¼104 61 mm, n¼
93) resulting in a sexual dimorphism ratio of 1.04. To con-
trol for differences between mothers, we examined sexual
896 JOURNAL OF MAMMALOGY Vol. 86, No. 5

dimorphism within litters of mixed-sex twins but the patterns
were similar to the pooled sample, with males larger than
females in head length and width but not in mass or length
(paired t-test; Table 1). Within twin litters of same-sex cubs, no
differences were found in any of the 4 morphometric measure-
ments (paired t-test, all P.0.11, all n.24).
Males were larger than females in all body measures after the
1st year of life. Examination of yearlings dependent upon their
mother revealed differences in all 4 morphometric parameters
(t-test; Table 2). When using mean values, the ratio for sexual
dimorphism in mass was 1.30, in body length was 1.07, in head
length was 1.07, and in head width was 1.08 for yearlings.
When using mean values for each age and sex, sexual
dimorphism in mass increased with age, peaking at 18.7 years
of age, and was described by a quadratic relationship (mass
sexual dimorphism ¼1.0574 þ0.1098age 0.00294age
2
,
r
2
¼0.83; n¼409 females and 364 males; Fig. 1). Similarly,
head length (head length sexual dimorphism ¼1.025 þ
0.0233age 0.000574age
2
,r
2
¼0.89; n¼492 females and
448 males), head width (head width sexual dimorphism ¼
1.043 þ0.0139age 0.000433age
2
,r
2
¼0.76; n¼493
females and 448 males), and body length (body length sexual
dimorphism ¼1.031 þ0.0152age 0.000409age
2
,r
2
¼0.84;
n¼484 females and 434 males) increased in a quadratic
manner peaking at roughly 16–20 years of age (Fig. 2).
Foreleg guard hairs.—Measurements were collected from
40 females and 74 males (1 year old). The maximum guard
hair length ranged from 19 to 42 cm in males and from 17 to 28
cm in females. Mean maximum length of guard hairs in males
(32 60.6 cm) was longer (t-test, P,0.0001) than in females
(22 60.4 cm), resulting in a sexual dimorphism ratio of 1.45.
Guard hair length increased and then decreased with age in
males (quadratic regression, guard hair ¼20.2 þ2.068age 
0.0724age
2
,r
2
¼0.57) reaching a peak at 14.3 years (Fig. 3).
No significant age-related pattern was found for females (linear
regression, P¼0.48, quadratic regression P¼0.22; Fig. 3).
Geographic variation and allometry.— Comparison of 7
populations where growth curves were available revealed varia-
tion in sexual dimorphism between populations, with values
between 1.93 and 2.31 in mass and between 1.16 and 1.20 in
body length (Table 3). The most marked sexual dimorphism
was noted in mass followed by that of head width, body
TABLE 1.—Comparison of sibling female and male polar bear cubs
from litters of 2 for body mass, body length, head length, and head
width. Bears were captured in the Svalbard–Barents Sea area, Norway.
Sex
a
n
X6SE
P, paired
t-test
Mass (kg) f 36 10.8 60.3 0.071
m 36 11.2 60.4
Body length (cm) f 35 75 61 0.82
m35 7561
Head length (mm) f 34 162 61 0.0001
m 34 167 62
Head width (mm) f 34 101 61 0.012
m 34 103 61
a
f¼female, m ¼male.
TABLE 2.—Body mass, body length, head length, and head width
for yearling female and male polar bears captured in the Svalbard–
Barents Sea area, Norway (pooled for litters of 1 and 2 yearlings). The
t-test results indicate the intersex comparison.
Sex
a
n
X6SE P,t-test
Mass (kg) f 27 66 63 0.0022
m248666
Body length (cm) f 27 141 62 0.005
m 24 151 63
Head length (mm) f 28 269 63 0.0009
m 24 288 65
Head width (mm) f 29 145 62 0.0005
m 24 156 62
a
f¼female, m ¼male.
FIG.1.—Sexual dimorphism in body mass of polar bears
(males : females) captured in the Svalbard–Barents Sea area, Norway,
based on age-specific mean size. Curve represents the quadratic
regression (see text).
FIG.2.—Sexual dimorphism in body length, head length, and head
width of polar bears (males : females) captured in the Svalbard–
Barents Sea area, Norway, based on age-specific mean size. Curves
represent quadratic regressions (see text).
October 2005 897DEROCHER ET AL.—POLAR BEAR SEXUAL DIMORPHISM

length, and head length. Sexual dimorphism in head measure-
ments appeared less variable than sexual dimorphism in mass
or length but data were only available from 3 populations.
The allometric relationship between masses of males and
females (log
10
) for polar bears in 7 populations was not
significantly different from 1 with a slope of 1.13 (60.30) but
was significantly .0 (linear regression, P¼0.013, r
2
¼0.74).
The allometric relationship for body length (log
10
) for males
and females was not significant (linear regression, P¼0.071).
DISCUSSION
Measurement and description of sexual dimorphism is an
area of active discussion, with issues pertaining to the
appropriate morphometric parameters (e.g., body mass versus
body length) and methods of analysis as central concerns
(Lovich and Whitfield Gibbons 1992; Ranta et al. 1994;
Schulte-Hostedde and Millar 2000; Weckerly 1998). In our
study, we elected to examine several morphometrics to
examine sexual dimorphism.
Some evidence of sexual dimorphism was present in head
length and width in polar bear cubs shortly after they emerged
from dens but prominent intersex differences were not apparent
until the following year when they were yearlings. Similar to
our findings, an earlier study examining body length and mass
did not find sexual dimorphism in polar bear cubs at den
emergence (Derocher and Stirling 1998b). The similar mass and
length of female and male twins suggest that mothers invest
equally in young of both sexes up to den emergence. The larger
head size of male cubs reflected greater allocation of energy to
head growth given the similar mass of both sexes. With
increasing age, sexual dimorphism became more apparent in
dependent young. One year after den emergence, as yearlings,
males were larger than females in all 4 measured parameters. It
is unknown how body size while dependent upon their mother
affects adult size but given that size at den emergence is
correlated with size at 1.5 years of age (Derocher and Stirling
1998b), it is possible that adult size is affected. However,
countering this argument, the size of polar bears shortly after
weaning was correlated with their adult size in females but not
in males (Atkinson et al. 1996). These authors suggested that
sex-based difference in body size during maternal care were
unlikely to persist to adulthood. Female polar bears do not
appear to differentially invest in male and female offspring upon
den emergence, and the higher growth rates of dependent males
may result from greater intake rates from prey.
Adult polar bears are highly dimorphic, with males 2.1 times
the mass of females. This level of sexual dimorphism would
place them between the 2 most dimorphic taxa of mammals;
the Phocidae with a mean 1.81 for 17 species and Otariidae
with a mean of 2.98 for 10 species (Weckerly 1998). In mature
polar bears, sexual dimorphism was greatest in mass, followed
by foreleg guard hair length, head width, body length, and head
length, with the latter 2 similar.
Sexual dimorphism can result from sex-specific differences
in growth rate and growth duration (Badyaev 2002). In polar
bears, sexual dimorphism was produced by a combination of
higher growth rates, evident in larger 1-year-old males, and
from a prolonged growth period in males compared to females
(Derocher and Stirling 1998a; Derocher and Wiig 2002;
Kingsley 1979). The extended growth period of males was
noted in Svalbard polar bears, where females reach 97% of
their asymptotic body length by sexual maturity at 5 years
of age whereas males took an additional 2.2 years to reach 97%
of their asymptotic length (Derocher and Wiig 2002). Simi-
larly, the same study revealed that females reached 97% of
their asymptotic body mass at 7.3 years of age whereas males
took 13.5 years. The prolonged growth period of males re-
sult in sexual dimorphism not reaching maximal levels until
relatively late in life, roughly 16–20 years of age.
In general, pelage in carnivores is not sexually dichromatic
(Ortolani and Caro 1996), so gender identification or in-
tersexual display pelage is uncommon. Guard hair over most of
the body in polar bears is 5–15 cm long depending on season
(Obbard 1987; Uspenskii 1977), but the foreleg guard hairs of
both females and males are noticeably longer than hairs on the
rest of the body. Foreleg guard hair was sexually dimorphic in
polar bears. In males, length of foreleg guard hair tended to
FIG.3.—Maximum length (6SE) of foreleg guard hairs for female
and male polar bears in the Svalbard–Barents Sea area, Norway.
Curve represents the quadratic regression for data on males (see text).
TABLE 3.—Sexual dimorphism (males : females) in body mass,
body length, head length, and head width based on asymptotic size
from growth curves (Derocher 1991; Derocher and Stirling 1998a;
Derocher and Wiig 2002).
Population
Sexual dimorphism
Body
mass
Body
length
Head
length
Head
width
Beaufort Sea 2.12 1.17 1.16 1.33
Central Arctic 2.07 1.20 — —
High Arctic 1.94 1.18 — —
Western Hudson Bay 1.93 1.18 1.17 1.30
Foxe Basin 2.27 1.20 — —
Davis Strait 2.31 1.17 — —
Svalbard 2.10 1.16 1.14 1.30

X2.11 1.18 1.16 1.31
898 JOURNAL OF MAMMALOGY Vol. 86, No. 5

increase until 14 years of age, followed by a gradual decline.
We hypothesize that foreleg guard hairs may act as a form of
ornamentation in male polar bears and may be used by females
as an indicator of male quality. Males with longer guard hairs
may be more attractive to females. Ornamentation depends on
both the phenotypic condition and overall genotype of animal
(Andersson 1986). In African lions (Panthera leo), the mane is
highly variable and reflects male condition (West and Packer
2002). This study also found that mane length was associated
with fighting success and was associated with female choice.
Although associated data on mating success in male polar bears
is unavailable, foreleg guard hairs may indicate such qualities
in polar bears and warrant further study.
Hair growth in other large mammals was suggested to
increase apparent size of the animal (Andersson 1994). An
alternative function of foreleg guard hair is that males with
longer guard hairs may appear larger in intrasexual compet-
itions. Polar bear males have been postulated to have a loose
dominance hierarchy based on size (Derocher and Stirling
1990), which is assisted by social play outside of the breeding
season (Latour 1981). Evidence from wounds, scars, and canine
breakage in polar bear males suggest the risk of injury in
intrasex competitions is high (Ramsay and Stirling 1986).
Therefore, accurately assessing potential opponents for mates
would be advantageous (Clutton-Brock et al. 1979). However,
additional data are required to assess foreleg guard hair length
as an indicator of male quality. We were unable to determine the
length of time that the guard hairs take to grow, so it is difficult
to correlate length with the condition of males at capture.
However, polar bears undergo a single annual molt, with
gradual hair replacement from May to August (Kolenosky
1987). It is possible that the long guard hairs are maintained
over more than 1 year so conclusions about the utility of it as
a visual signal of condition at the time of capture are difficult to
determine. Length of foreleg guard hairs was used as an indictor
of age in adult females (Ovsyanikov 1998) but our results do not
support its use as an age-related indicator in females.
Geographic variation in body size of mammals is common
and often follows environmental gradients or varies with popu-
lation density (e.g., Derocher and Wiig 2002; Fowler 1990;
Kingsley et al. 1988; McNab 1971). Geographic variation in
sexual dimorphism has been documented in some mammals
(Levenson 1990; Ralls and Harvey 1985) and differences
between the sexes in sensitivity to environmental conditions
are thought to be a major cause of variation in sexual dimor-
phism between populations (Badyaev 2002). Mahoney et al.
(2001) predicted hyperallometry in sexual dimorphism in black
bears (Ursus americanus) between populations but found no
such trend. Further, a recent study of pinnipeds found no
evidence of hyperallometry and suggested that sexual di-
morphism was not a consequence of an allometric relationship
between sizes of males and females (Lindenfors et al. 2002).
We found no evidence of hyperallometry of sexual dimorphism
in polar bears but this could be revisited with data from
additional populations.
We recognize that numerous factors may influence sexual
dimorphism in polar bears. Differential niche use could be a
factor affecting sexual dimorphism in polar bears and males
have been suggested to feed more often on the larger (about
400-kg) bearded seal (Erignathus barbatus), whereas females
prefer the smaller (about 60-kg) ringed seals (Phoca hispida—
Stirling and Derocher 1990). However, as Ralls (1976) noted,
differences in the size of food eaten does not clarify the roll of
diet in sexual dimorphism because it cannot be separated from
other selective pressures including sexual selection. Addi-
tional factors also can affect sexual dimorphism, and in bighorn
sheep (Ovis canadensis) sexual dimorphism in mass was
influenced by population density (LeBlanc et al. 2001). With the
exception of the Svalbard population, all polar bear popula-
tions investigated here undergo a substantial harvest, and pop-
ulation densities may vary between populations and thus affect
sexual dimorphism.
The ultimate cause of sexual dimorphism in polar bears and
its implications for the ecology of the species are unknown.
Study of a population over a longer period may yield insight
into the dynamics of sexual dimorphism, which would allow
some hypotheses to be examined. In addition, information on
the mating success of individual males and the prey preferences
of females and males could provide quantitative tests of sexual
dimorphism hypotheses.
ACKNOWLEDGMENTS
Assistance in logistics was kindly provided by the Governor of
Svalbard. We are grateful for the help of the Hopen Radio staff, who
provided an excellent base for operations, and to M. Mauritzen, who
provided assistance in the field. The Norwegian Polar Institute funded
this study with assistance from the Norwegian Research Council and
the World Wildlife Fund (Arctic Programme). Ages of bears were
determined by D. Andriashek and C. Spencer of the Canadian Wildlife
Service, Environment Canada, Edmonton, Alberta.
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Submitted 7 August 2004. Accepted 3 February 2005.
Associate Editor was Enrique P. Lessa.
October 2005 901DEROCHER ET AL.—POLAR BEAR SEXUAL DIMORPHISM