Polar bears are the top predator in the arctic marine ecosystem and
evolved from brown bears during the Pleistocene, the time period that
spanned from 1.8 million to 11,000 years ago. They have adapted to life
in the north where temperatures do not exceed 10°C (50°F) in
summer and typically fall to -30°C (-22°F) during winter. Their
thick coat is made up of water repellent hairs that conserve heat. Under
their dense fur is black skin, good for absorbing the rays of the arctic
sun. A layer of fat up to 11 cm (4.3 in) thick keeps the bears warm,
especially while swimming. Polar bears are so well insulated that they
have to move slowly to avoid overheating.
Adult males typically measure 200 to 250 cm (6.5 to 8 feet) from the tip
of the nose to the tip of the tail and weigh 400-600 kg (880-1300 lbs).
Females are about half this size. These bears have enormous paws that
function like snowshoes, distributing their weight to keep them from
breaking through ice and snow. The pads of their feet are covered with
soft, tiny growths called papillae, which increase friction between paw
and ice and reduce the chance of slipping.
Polar bear claws are shorter, and more solid than their grizzly bear
cousins, better suited for walking on ice or climbing steep banks. They
have sharp, jagged back teeth, and canines that are larger and sharper
than grizzly teeth, but they swallow their food in large chunks rather
than chewing.
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Their fur reflects light, giving a polar bear
its white coloration. Oxidation from the sun, and oil from a diet heavy
on seal blubber, can make their coat look yellow or brown.
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