Polar Bear - biology.

Polar Bear - biology. Polar Bear, common name for a white bear found on the sea ice of the Arctic Circle throughout the North Polar basin. The only marine bear, it is longer than other bears and streamlined for aquatic life. It has the plantigrade feet (heel and sole touching the ground) typical of bears, with five sharp, curved claws on each foot for grasping the ice and holding its prey. Long hair between the pads protects the bear's feet from the cold and provides traction on the ice. Stiff hairs on the forelegs, and very broad front feet, help the bear swim. All polar bears are white, although their fur may yellow in the summer. The largest wild bears ever weighed--more than 800 kg (1,760 lb)--have been polar bears. However, most male polar bears weigh an average of about 350 kg (about 880 lb), and most females weigh about 250 kg (550 lb). Polar bears have a strong navigational sense and an extremely good sense of smell, and they are unusually clever at solving problems in order to obtain food. They eat primarily ringed seals, and occasionally bearded seals, walruses, or white whales. They also feed on berries, sedges, mussels, and kelp. As with other true bears, polar-bear mothers and young form strong bonds. The young are very small when born: about 1 kg (about 2 lb). Their eyes remain closed for about 40 days and they must nurse every few hours. The mother holds them close to keep them warm. Except during the breeding season, male polar bears are solitary and roam over vast expanses of sea ice while hunting. During the breeding season (May to June), the males fight furiously over females. Both the male and female may mate with other individuals as well. Polar bears have little interest in other Arctic species, and they often ignore people entirely. Polar-bear hunting is regulated or prohibited in many areas through which the bears roam, including parts of Canada, Greenland, Russia, Alaska, and Norway. Polar-bear populations have declined in some areas, such as western Hudson Bay in Canada, and a variety of factors threaten their status, including global warming and toxic pollutants in the food chain. Global warming appears to be reducing the amount and thickness of the Arctic sea ice that forms an important part of the bears' habitat and that helps these predators hunt seals and other prey. In 2005 wildlife biologists in Alaska documented that some polar bears have drowned as they tried to swim long distances between ice floes in the search for food. In June 2005 wildlife biologists representing the five nations with polar-bear populations warned that polar bears are susceptible to a variety of airborne toxic chemicals that originate in consumer products and industrial processes and are carried by prevailing winds to the Arctic. In a study published in December 2005 in Environmental Science and Technology, the scientists reported high levels of chemicals used as flame retardants in the fatty tissues of polar bears. These chemicals, known as polybrominated diphenyl ethers (PBDEs), have unknown effects on polar bears but in laboratory animals, they are associated with brain and motor skill damage, changes to thyroid and sex hormones, and a weakening of the immune system. Animals high in the food chain receive concentrated amounts of the chemicals. The study found that 139 bears captured in ten different locations had concentrations of some PBDEs 71 times higher than in ringed seals, their principal prey. Polar bears are particularly susceptible to pollutants that end up in fatty tissues because their diet is almost entirely fatty meat. However, scientists have not found a direct cause for the decline in polar-bear populations in some areas, such as western Hudson Bay, where the population has declined from 1,100 in 1995 to fewer than 950 in 2004. Scientific classification: The polar bear belongs to the family Ursidae in the order Carnivora. It is classified as Ursus maritimus. Contributed By: Charles Joseph Jonkel Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.