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The polar bear (Ursus maritimus), also known as the white bear, northern bear, sea bear, or nanuq in some Inuit languages, is a species of bear that is native to the Arctic and the apex predator within its range. Its thick blubber and fur insulate it against the cold, and its translucent fur, which appears white or cream-colored, camouflages it from its prey. The bear has a short tail and small ears that help reduce heat loss, as well as a relatively small head and long, tapered body to streamline it for swimming. The polar bear is a semi-aquatic marine mammal that depends mainly upon the pack ice and the marine food web for survival. It has uniquely adapted for life on a combination of land, sea, and ice and is now dependent on this combination.[2]

Scientists now believe that the projected decreases in the polar sea ice due to global warming will have a significant negative impact or even lead to extinction of this species within this century.[3] [1] Population reductions of up to 20% have been recorded in recent years, the average weight of the bears has been declining significantly, and cub survival rates have plunged.

Size and weight
Polar bears rank with the Kodiak bear as among the largest living carnivores, and male polar bears may weigh twice as much as a Siberian tiger. There is great sexual dimorphism, with some males reaching more than twice the size of the females. Most adult males weigh 300-600 kg (660-1320 lb) and measure 2.4-2.6 m (7.9-8.5 ft) in length. Adult females are roughly half the size of males and normally weigh 150-300 kg (330-660 lb), measuring 1.9-2.1 m (6.25-7 ft).[4][5] At birth, cubs weigh only 600-700 g or about a pound and a half.

The largest polar bear on record was shot in Kotzebue Sound, Alaska, in 1960. According to Guinness World Records 2006, this bear weighed an estimated 580 kg (1960 lb) and was 3.38 m (11 ft 11 in) tall when mounted.[6]

Recent data, however, suggest that polar bear weights are declining, an indication of the current pressure on the bears. A 2004 National Geographic Society study showed that polar bears that year weighed, on average, fifteen percent less than they did in the 1970s.[7] In 2007 the females in Hudson Bay averaged only 230 kg, down from near 300 kg in 1980.[8]

Fur and skin

A Polar Bear resting.A polar bear's fur is translucent despite its apparent white hue, providing good camouflage and insulation. It may appear yellowish brown as they age. Its black skin has evolved to radiate heat out to its layer of thick fur, which helps keep the animal warm in the coldest weather. The bear has a black nose. Stiff hairs on the soles of its paws provide insulation and traction on ice.

Unlike other Arctic mammals, polar bears do not shed their coat for a darker shade in the summer. It was once conjectured that the hollow hairs of a polar bear coat acted as fiber-optic tubes to conduct light to its black skin, where it could be absorbed - a theory disproved by recent studies.[9] The thick undercoat does, however, insulate the bears: they overheat at temperatures above 10 °C (50 °F), and are nearly invisible under infrared photography; only their breath and muzzles can be easily seen.[10] These bears often sprawl upon the ice to cool off; on land, they may dig for the cooler permafrost layer beneath. Growing through the undercoat is a relatively sparse covering of hollow guard hairs about six inches long. These guard hairs are stiff, shiny and erect, and stop the undercoat from matting when wet. Water is easily shaken off before it can freeze. The bear also rolls in snow to shed moisture from the coat.

In July 2005, during an extremely hot and humid summer in Chicago, several polar bears in the Brookfield Zoo turned green as a result of algae growing in their hollow guard hair tubes. The staff let the harmless algae run its course, and did not subject the bears to chlorine or bleach treatment.
Speciation

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