Polar Bear

Appendix A—Background
APPENDIX A—BACKGROUND
Brief Overview/Species Status
Polar bears (Ursus maritimus) occur in 19 relatively
discrete subpopulations (Plan Fig. 1) throughout
the seasonally and permanently ice-covered marine
waters of the northern hemisphere (Arctic and
Subarctic), in Canada, Denmark (Greenland),
Norway, Russia and the United States (U.S.).
The status of each of these subpopulations varies
(Polar Bear Specialists Group Status Table; http://
pbsg.npolar.no/en/status/status-table.html). The
U.S. contains portions of two subpopulations: the
Chukchi Sea (CS) (also called the Alaska-Chukotka
subpopulation in the U.S.–Russia Bilateral Agreement)
and the Southern Beaufort Sea (SB) subpopulation.
The polar bear was listed as a threatened
species under the U.S. Endangered Species Act of
1973, as amended (16 USC 1531 et seq.)(ESA) on
May 15, 2008 (73 FR 28212). The total circumpolar
population is estimated to be 26,000 (95% CI =
22,000 – 31,000) polar bears (Wiig et al. 2015).
Species Biology and Life History
Physical characteristics. Polar bears are the largest
living bear species (Demaster and Stirling 1981), and
are characterized by large body size, a stocky form,
and have a longer neck and proportionally smaller
head than other ursids. Their hair is non-pigmented.
Fur color varies between white, yellow, grey, or
almost brown, and is affected by oxidation, i.e.
exposure to the air, light conditions, and soiling or
staining due to contact with fats obtained from prey
items (Amstrup 2003). They are sexually dimorphic;
females weigh 181 to 317 kilograms (kg) (400 to 700
pounds (lbs) and males up to 654 kg (1,440 lbs).
Adaptations. Polar bears evolved in Arctic sea ice
habitats and are evolutionarily well adapted to this
habitat. Their unique physical adaptations include:
(1) non-pigmented pelage with water-repellent
guard hairs and dense underfur; (2) a short, furred
snout; (3) small ears with reduced surface area;
(4) teeth specialized for a carnivorous rather than
an omnivorous diet; and (5) feet with tiny papillae
on the underside, which increase traction on ice
(Stirling 1988). In addition, they have large, paddlelike
feet (Stirling 1988), and claws that are shorter
and more strongly curved than brown bear (Ursus
arctos) claws, and larger and heavier than those of
black bears (Ursus americanus) (Amstrup 2003)
used mainly for clutching prey.
Breeding and reproduction. Polar bears are a
K-selected species, characterized by late sexual
maturity, small litter sizes, and extended parental
investment in raising young. All of these factors
contribute to the species’ low reproductive rate
(Amstrup 2003). Females generally mature and
breed for the first time at 4 or 5 years and give
birth at 5 or 6 years of age. Litters of two cubs are
most common, but 3-cub litters are seen on occasion
across the Arctic (Amstrup 2003). The minimum
reproductive interval for adult females is three
years.
Females enter a prolonged estrus between March
and June, when breeding occurs. Though bears
ovulate in the spring, implantation is delayed until
autumn. The timing of implantation, and therefore
the timing of birth, likely depends on body condition
of the female, which is determined by many
environmental factors. When foraging conditions
are difficult, polar bears may “defer” reproduction
in favor of survival (Derocher and Stirling 1992,
Eberhardt 2002). Pregnant females that spend the
late summer on land prior to denning may not feed
for eight months (Watts and Hansen 1987) which
coincides with the time when the female gives birth
and nourishes new cubs.
Altricial, newborn polar bears have fur, but are
blind, and weigh only 0.6 kg (1.3 lb) (Blix and
Lentfer 1979). Cubs grow rapidly, and may weigh
10 to 12 kg (22 to 26 lbs) by the time they emerge
from the den in the spring. Young bears will stay
with their mothers until weaning, which occurs most
commonly in early spring when the cubs are 2 1/2
years old. Female bears are available to breed again
after their cubs are weaned.
Survival. Polar bears are long-lived and are not
generally susceptible to disease or parasites. Due to
extended maternal care of young and low reproductive
rates, polar bears require high adult survival
rates, particularly females, to maintain population
levels (Eberhardt 1985; Amstrup and Durner 1995).
Survival rates are generally age dependent, with
cubs-of-the-year having the lowest rates and prime
age adults (prime reproductive years are between
approximately 5 and 20 years of age) having survival
rates that can exceed 90 percent (Regehr et al.
2007b). Survival rates exceeding 90 percent for adult
females are essential to sustain polar bear populations
(Amstrup and Durner 1995).
New studies (Rode et al. 2010a, 2014b) conducted on
the SB subpopulation are consistent with previous
findings (Regehr et al. 2006) which concluded
that declines in body size, body condition, and
recruitment in recent decades were associated with
declining sea ice availability. Additionally, Regehr et
al. (2010) suggested several years of reduced sea ice
in the mid-2000s were associated with low breeding
Polar Bear Conservation Management Plan 61