The polar bear (Ursus maritimus) is the largest terrestrial carnivore. Adult males can measure more than nine feet in length and weigh between 770 to 1,430 pounds. When standing erect, a male polar bear can look an elephant straight in the eye. The bear's body and neck are elongated, and the head is narrow and long with small, rounded ears.
The polar bear's coat, covering it completely except for the nose and foot pads, is superbly adapted to Arctic environments. Along with a thick layer of body fat, the water-repellent coat insulates the bear from cold air and water. It also serves as camouflage; in fact, polar bears can sometimes pass as snow drifts. The fur is 95 percent efficient in converting ultra-violet sun rays into usable heat. Its transparent hairs have a hollow inner core which scatters ultra-violet light by some unknown mechanism, converting it into heat when it reaches the bear's black skin. Surprisingly, the fur has no white pigment; it is the reflection of the sun which causes the fur to appear white.
Habitat and Distribution: The five million square-mile range of the polar bear circles the Arctic. They live mainly in Canada and Greenland and on Norwegian and Russian islands, but they have been seen within 150 miles of the North Pole.
Polar bears generally live by stretches of open water where their prey, seals, are easily caught. In winter they migrate south to the edge of the drifting ice floes which form a ring around the Arctic and, as the ring recedes in summer, the polar bears return north with them.
Diet: Polar bears prey primarily upon ringed seals and bearded seals. In late April and May, they hunt these seals by breaking into seal dens found in the snow covering the sea ice. The dens are not visible from above, but polar bears can detect seals beneath a layer of snow more than three feet thick. During the rest of the year, they wait patiently at a seal's breathing hole or at the water's edge. Sometimes, a large number of polar bears from a wide area follow a scent to the carcass of a walrus or whale. During the summer, polar bears may become omnivorous, eating rodents, salmon, seaweed and blueberries.
Reproduction: Polar bears breed in late March, April and May. The males actively seek out females by following their tracks on sea ice. They remain with the female for a short time, then leave in search of another female.
During November and December, the female excavates a maternity den in a drift of snow, maintaining and enlarging the chamber as the drifts cover her, snowing her in. Soon she gives birth to twins which cuddle in their mother's thick fur. She ceases to feed throughout the winter and instead lives off her stored fat. Her milk, high in fat content, enables the cubs to keep warm and grow rapidly before leaving the dark den in March or April.
Short trips are made to and from the den for several days as the cubs acclimate to the outside temperatures. Then the family leaves and makes its way to the sea ice where the mother feeds and protects her cubs. The family returns to the den the next winter and remains together during the following spring and summer. After two years together, the family disperses.
Status: With 20,000 to 40,000 polar bears living in the wild, the species is not currently endangered. In 1973, Canada, the U.S., Denmark, Norway and the former U.S.S.R. signed the International Agreement on the Conservation of Polar Bears and their Habitat. This agreement restricts the hunting of polar bears and directs each nation to protect their habitats.
The exploration and development of fossil fuels also threatens the species. For example, oil developments sometimes intrude upon principal denning areas. Furthermore, because of the noise and the early warming caused by the industry, females sometimes break out of their maternity dens too soon. This reduces the chance of survival for their cubs. Oil spills also foul up their fur and reduce their insulating abilities, causing them to freeze to death.