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NATURAL HISTORY

POLAR BEAR } Ursus maritimus
FAMILY: Ursidae


DESCRIPTION: The largest of the bear species, the polar bear sports luxurious white fur with water-repellent guard hairs and dense underfur. It also has a layer of blubber up to five inches thick, black skin, a short-furred snout, small ears, and a streamlined body with large, oar-like feet. Males measure from eight to 11 feet from nose to tail and generally weigh about 1,300 pounds but can reach more than 1,700. Females measure about six to eight feet and are usually about half the weight of males.

HABITAT: Polar bears live throughout the ice-covered waters of the circumpolar Arctic, with distribution dependent on food availability and sea-ice conditions; they are most often found at the convergence of sea ice and open water, and where seals congregate. These bears are totally reliant on the sea ice as their primary habitat, using it for a number of essential activities including hunting and feeding on seals, seeking mates and breeding, making long-distance movements, accessing terrestrial maternity denning areas, and sometimes even maternity denning itself. Polynyas — areas of open water surrounded by ice and caused by fluctuations in wind, tide or current — are sites of increased marine mammal and bird concentrations and are extremely important to polar bears.

RANGE: This circumpolar species is found in and around the Arctic Ocean, with its southern range limited by pack-ice availability and its southernmost occurrence at James Bay in Canada. The world’s currently recognized 20 polar bear populations occur within the jurisdictions of the United States ( Alaska), Canada, Denmark (Greenland), Norway, and Russia.

MIGRATION: Some polar bears make extensive north-south migrations in response to ice packs receding northward in the spring and advancing southward in the fall. In addition, individuals may travel vast distances to find mates or food and have been seen 100 miles from the nearest land- or icefall. In October and November, males head out onto the pack ice where they spend the winter, while pregnant females seek sites on land or nearshore sea ice to dig dens in the snow, where they spend the winter and give birth.

BREEDING: Like other members of the bear family, female polar bears have small litters, reach breeding age late in life, and produce few young in their lifetime. They mate on the sea ice in either April or May, after which a female must accumulate sufficient fat reserves to live and to support her cubs from the time she enters the maternity den between late October and mid-November until the time the family emerges in the spring and she again begins to feed. Cubs are born in snow dens between late November and early January, with timing varying by region and population. Because of their vulnerability at birth, cubs must remain in the maternity den, where the temperature warms to near freezing. They nurse inside the den until sometime between late February and the middle of April, depending on the latitude. The age at which mothers wean their cubs also varies by region, though in most areas cubs are weaned at approximately 2.5 years of age, resulting in a three-year reproductive cycle. After a period of several weeks’ acclimatization, the mother and cubs begin their trek to the sea ice to feed on seals.

LIFE CYCLE: Polar bears can live up to 25 or 30 years in the wild.

FEEDING: The top Arctic predators, polar bears primarily eat ringed seals but also hunt bearded seals, walrus, and beluga whales, and will scavenge on beached carrion such as whale, walrus, and seal carcasses found along the coast. These bears often eat only seals’ skin and blubber, leaving the carcass for other animals to scavenge and thus playing a critical role in the Arctic food chain.

THREATS: The greatest threat to polar bears is global warming, which is affecting the Arctic far more intensely than the rest of the world and is rapidly causing the bears’ sea-ice habitat to melt away. Other grave threats include oil and gas development, environmental contaminants such as PCBs, industrial noise and harassment from increased Arctic shipping and other activities, and overhunting in some areas. Global warming will likely interact with several of these additional threats to further increase the polar bear’s peril.

POPULATION TREND: Polar bear numbers increased following the establishment of hunting regulations in the 1970s and today stand at 20,000 to 25,000. The rapid decline of Arctic sea-ice due to global warming has reversed this trend, and currently at least five of the 19 polar bear populations including those in Western Hudson Bay are declining. Scientists estimate that if the Arctic continues its melting trend, the worldwide polar bear population will decline by two-thirds by 2050 and will be near extinction by the end of the century. As actual sea-ice melting has proceeded much faster than predicted by scientific models, population declines may occur much faster as well.

Photo © Thomas D. Mangelsen, ImagesOfNatureStock.com