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Appendix A—Background APPENDIX A—BACKGROUND Brief Overview/Species Status <strong>Polar</strong> 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 (<strong>Polar</strong> <strong>Bear</strong> 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. <strong>Polar</strong> 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. <strong>Polar</strong> 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. <strong>Polar</strong> 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. <strong>Polar</strong> 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 <strong>Polar</strong> <strong>Bear</strong> Conservation Management Plan 61
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