Friday, August 3, 2007

Polar Bears

Description and Natural History

Physical Description
Polar bear information. The polar bear is the largest member of the bear family and is easily recognized by its white fur. It has a relatively longer neck and smaller head than the other bear species. The body is stocky and lacks the shoulder hump so obvious in the grizzly bear. The nose, lips and skin are black, and the front paws, which are used for paddling, are very large. When swimming, their back legs hang behind.Polar bears are well adapted to life on the sea ice. They have a translucent coat with waterproof protective guard hairs and a dense undercoat, small ears (to minimize heat loss), and pads on the bottom of their feet that help provide traction on the ice.Adult males (boars) measure 2.5 - 3.0 meters (8 - 10 feet) and weigh between 250 - 600 kilograms (550 - 1,320 pounds), although they can weigh as much as 800 kilograms (1,700 pounds). Adult females (sows) are smaller, measuring 1.8 - 2.5 meters (6 - 8 feet) and weighing 100 - 300 kilograms (200 - 660 pounds). Cubs are born weighing about 600 grams (1.3 pounds) and are about 30 centimeters (12 inches) long. Polar bears vary in size depending on geographic location, the largest inhabiting the Chukchi Sea region of Russia.
Natural History
Polar bear information. Polar bears have a discontinuous circumpolar distribution on the annual shoreline pack ice (extensive, free floating areas of annually formed ice) of Greenland, Norway, Russia, Canada and north/northwest Alaska. There are six apparently distinct populations in the main polar basin: Wrangel Island and western Alaska (the Chukchi Sea population); northern Alaska and northwestern Canada (the Beaufort Sea population); the Canadian Arctic archipelago; Greenland; Spitsbergen-Franz Josef Land; and central Siberia.Sub-populations exist within the Canadian Arctic archipelago, and James and Hudson Bays. James Bay, Canada, is the furthest south that polar bears can be found year round, although they may ride ice packs as far south as Newfoundland and into the Bering Sea. The bears then move northward with the receding pack ice in the summer.Polar bears rarely enter areas of heavy ice because there is little food in these regions. In places such as Hudson Bay – where the ice melts completely in late summer – polar bears spend time on land until the return of seals at freeze up. Males tend to remain along the coast while females with young, and sub-adults, go further inland. Polar bears are nomadic and, although they occasionally travel great distances, tagging studies show that they tend to stay in the same general area. While they have large home ranges that overlap with other polar bears, they do not defend territories. Polar bears are solitary except for females with cubs, or adults when mating. A number of animals may gather at a food site or on land during the open-water season in some parts of Canada.Females reach sexual maturity between four and six years of age and give birth every three to four years. Males reach sexual maturity at about three years of age, but probably do not successfully mate until after six years. Mating takes place on the ice between late March and early May and, with delayed implantation of the embryo (the embryo floats freely in the womb for a certain amount of time before it implants in the wall of the uterus) until September or early October, cubs are born between late November and early January in maternity dens made under drifted snow in denning areas.Two cubs are most common born; triplets occur occasionally. Like other bear species, polar bear cubs are born blind and are dependent on their mothers for the first month of life. The cubs emerge from their den in late March or early April weighing upwards of 45 kilograms (99 pounds). Cubs remain dependent on their mothers for 2.5 years. In the wild, they probably live to about 25 years of age.Polar bears are the most carnivorous of the bears and have specialized teeth. They hunt mostly on the ice but willingly enter the water where they can swim for great distances. They eat mostly ringed seals (Pusa hispida) which they catch by waiting at breathing holes or cracks (known as leads) in the ice, or by pouncing on basking animals.Polar bears also prey upon ringed seal pups in their birthing lairs under the snow, and on other seal species including bearded seals (Erignathus barbatus) and spotted seals (Phoca largha). They may also eat hooded seals (Cystophora cristata), young walruses (Odobenus rosmarus) and beluga whales, and will feed on carrion such as whale carcasses. They may also supplement their diet with small mammals, birds, eggs, berries, grass, kelp or other vegetation, but these are not a major components of their diet.The world population of polar bears is estimated at 22,000 - 27,000, although numbers in some areas for polar bear information remain unknown.