Species: Ursus maritimus
Primarily in Canada and Alaska
Polar bears have a distinctive white appearance. The neck is longer and the head is narrower than other species of bear. The front paws are large and used like paddles for swimming. Paws are heavily furred. In contrast to their grizzly bear cousins, polar bears only have 4 functional mammae, which is related to their smaller litter size. In contrast to other northern bears, only pregnant females will enter dens for extended periods over winter: all other bears remain active.
The polar bear is the largest land carnivore alive in the world today. Males weigh from 880 to 1750 pounds (399 to 794 kg), and average about 1200 pounds (544 kg). Females weigh from 440 to 660 pounds (200 to 299), averaging 500 pounds (227 kg). At birth, cubs weigh about one pound (454 grams). Life expectancy in the wild can be as long as 30 years. In captivity, life expectancy can reach 40 years
Females reach maturity at 4 to 6 years. Polar bears mate from late March to late May, and cubs are born between late November and early January. Litter size ranges from 1 to 3 cubs; two is most common. Cubs remain with their mothers until they are two-and-a-half years of age. (In western Hudson Bay , some females used to wean cubs at 1.5 years).
Polar bears tend to be solitary animals except for females accompanied by cubs. Adult males can be social during the ice-free period and have been observed play-fighting. Their home ranges overlap; polar bears are not territorial.
Polar bears are the most carnivorous of all the bears, and live almost entirely on ringed seals and bearded seals. They rely on the thick blubber layer of the seals to provide an energy-rich diet. Their diet may include other seal species and young walruses, as well as grass, kelp, and berries. They will also scavenge on the carcasses of terrestrial or marine mammals.
The preferred habitat of polar bears is the annual ice over the continental shelves adjacent to the shorelines of the continents and archipelagoes throughout the circumpolar Arctic. During ice-free periods, males tend to remain along the coast, while family groups and sub-adults move further inland. The furthest south that polar bears live all year round is James Bay in Canada . During the winter, polar bears move as far south as Newfoundland and into the northern Bering Sea . Polar bears are found as far north as the North Pole, but here the densities are low. Dens usually occur on land but are commonly found on the sea ice in Alaska.
The population of polar bears is estimated between 20,000 and 25,000 in 19 subpopulations with varying status (some stable, some declining, and some unknown). Global warming is affecting some populations, although others are not showing any indication of impact. Pollution is a concern in some areas.
Polar bears are classified as marine mammals. Cubs will ride on their mother’s backs to cross open water, or when weak or cold. Polar bears are excellent swimmers. The soles of a polar bear’s foot have small papillae and vacuoles like suction cups to make the bear less likely to slip on the ice. While polar bears appear white, their skin is black.
photo credit: Tom Mangelsen