THE POLAR BEAR (order Carnivora, family Ursi-dae) is the largest bear species and is thought to have evolved from brown bears, Ursus arctos, approximately one million years ago. There are 19 recognized populations, distributed in Canada, the United States (Alaska), Norway (Svalbard Islands), Denmark (Greenland), and Russia. The current estimated worldwide population is 20,000-25,000.
Polar bear territories can cover tens of thousands of sq. km. They live solitarily, but often congregate around food sources. Their diet consists primarily of ringed seals, but includes other seals, walruses, and beluga whales. They get the majority of their nutritional intake in the spring and summer from seal pups, which can be as much as 50 percent fat. Polar bears can swim for 37 mi. (60 km.) without resting, at speeds up to 6.2 mi. per hour (10 km. per hour). They possess several adaptations for a semi-aquatic existence, including partially webbed front paws, eyes adapted to see underwater, and a thick fat layer (eight to 12 cm.) that provides buoyancy and insulation in water.
Polar bears are also highly adapted to the Arctic climate. Arctic temperatures can drop to minus 49 degrees F (minus 45 degrees C) for days or weeks, but polar bears can withstand this due to their thick fat layer, a dense undercoat of fur with longer guard hairs, and black skin (which absorbs heat from sunlight). Other Arctic adaptations include white-seeming fur for camouflage (the hairs are actually colourless and hollow), small ears and tail (which reduce heat loss), large furry feet (which act like snowshoes), and a digestive system very effective at absorbing and storing fat.
Females become sexually mature at four to five years of age. Males may not mate successfully until they are 8 to 10 years old. Mating occurs from April to June and each male may mate with more than one female. The females have induced ovulation, mating multiple times causes the release of an egg. The implantation of the blastula (the fertilized egg after several cell divisions) is delayed until September/ October, and in November/December the female excavates a den in the snow. She eventually gives birth in December or January. One to three cubs are born (two-thirds of cubs are twins) and they are nursed in the den until March/April, when they emerge. Cubs are weaned at the age of two or three years. During
this period, they learn important hunting skills from their mother. Occasionally, cubs are attacked and killed by males, so mothers are fiercely protective. Females breed after they wean the current cub(s), but will not reproduce at all if conditions are unfavorable. Cub mortality rates are high.
The most common cause of death for subadult bears is starvation, as they do not yet have a territory and must compete with larger bears, pushing many into marginal habitats. In adulthood, mortality drops sharply (to less than 5 percent annually); the most common natural cause of death in adults is attacks by other bears. Human activities and impacts that pose the greatest risk to polar bears include hunting, pollution, industrial development, and climate change.
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