Polar Bear

SUBFAMILY URSINAE
Ursus maritimus

APPEARANCE The polar bear is immediately recognizable from the distinctive white color of its fur. The neck of the polar bear is longer than in other species of bears. The head is elongated but the ears are relatively small. The front paws are large and are used like paddles for swimming while the hind legs trail behind. The nose, and the skin underneath the white fur, are black. The soles of the feet have small papillae and vacuoles like suction cups to make them less likely to slip on the ice.

SIZE The polar bear is the largest land carnivore alive in the world today. Adult males weigh from 400 to 600 kilograms (880 to 1,320 pounds) and occasionally exceed 800 kilograms (1,760 pounds). Females are about half the size of male s and normally weigh 200 to 300 kilograms (440 to 660 pounds). Immediately before entering the maternity den in the fall, the weight of a pregnant adult female can exceed 500 kilograms (1,100 pounds) because of the enormous amount of stored fat. Adult m ales measure 240 to 260 centimeters (95 to 105 inches) and females 190 to 210 centimeters (75 to 85 inches). At birth, cubs weigh 600 to 700 grams (1 pound 3 ounces to 1 pound 6 ounces).

HABITAT The preferred habitat of polar bears is the annual ice adjacent to the shorelines of the continents and archipelagos throughout the circumpolar Arctic. Wind and currents create cracks in the ice that concentrate the seals they hunt. Although polar bears have been recorded as far north as 88", they rarely enter the zone of heavy multiyear ice of the central polar basin because it is unproductive biologically and there is little to eat. In areas such as Hudson Bay, where the ice melts completely for a few months in the late summer and fall, bears spend the summer on land, resting to conserve energy and waiting for freeze-up. Males tend to remain along the coast, while family groups and sub adults go further inland.

DISTRIBUTION Polar bears are found throughout the circumpolar Arctic. The farthest south that polar bears live all year round is James Bay in Canada, which is about the same latitude as London, England. During winter, when the polar ice pack extends further south, polar bears move as far south as Newfoundland and into the northern Bering Sea. They then move back north as the southern edge of the pack ice recedes throughout the summer.

REPRODUCTION Polar bears mate from late March to late May. Implantation of the fertilized egg is delayed until late September to early October and the cubs are born between late November and early January. A little under 70 percent of t he litters consist of two cubs, 25 to 30 percent are singletons, and there are a small number of triplet litters. Litters of four cubs have been reported, but are extremely rare and it would be unlikely for all the cubs to survive. Cubs remain with their mothers until they are two-and-a-half years of age, so the most often that females normally breed is once every three years.

SOCIAL SYSTEM Throughout most of the year, polar bears are distributed as solitary individuals, except for females accompanied by their cubs. They have large overlapping home ranges but do not defend territories. The adult sex ratio is 1:1 but since most females reproduce only once every three years, only a third of them are available in each breeding season. This results in intense competition between males for mates, which is probably one of the reasons why males are twice the size of females.

DIET Polar bears are the most carnivorous of all the bears and live almost entirely on ringed seals, and to a lesser degree, on bearded seals. They are also known to prey on young walruses and occasionally even capture narwhals and belugas. In summer, if they are along the coast, they may eat some grass, kelp, or berries, and scavenge on the carcasses of terrestrial or marine mammals.

Image by David Kirshner
From Ian Stirling, ed. Bears, Majestic Creatures of the Wild.
Emmaus, Pennsylvania: Rodale Press, 1993. 240 pages.