Polar Bear

The polar bear (Ursus maritimus) is a carnivorous bear whose native range lies largely within the Arctic Circle, encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is a large bear, approximately the same size as the omnivorous Kodiak bear (Ursus arctos middendorffi). A boar (adult male) weighs around 350–700 kg (772–1,543 lb), while a sow (adult female) is about half that size. Although it is the sister species of the brown bear,[5] it has evolved to occupy a narrower ecological niche, with many body characteristics adapted for cold temperatures, for moving across snow, ice and open water, and for hunting seals, which make up most of its diet.[6] Although most polar bears are born on land, they spend most of their time on the sea ice. Their scientific name means "maritime bear" and derives from this fact. Polar bears hunt their preferred food of seals from the edge of sea ice, often living off fat reserves when no sea ice is present. Because of their dependence on the sea ice, polar bears are classified as marine mammals.
Because of expected habitat loss caused by climate change, the polar bear is classified as a vulnerable species, and at least three of the nineteen polar bear subpopulations are currently in decline. However, at least two of the nineteen subpopulations are currently increasing, while another six are considered stable. For decades, large-scale hunting raised international concern for the future of the species, but populations rebounded after controls and quotas began to take effect. For thousands of years, the polar bear has been a key figure in the material, spiritual, and cultural life of circumpolar peoples, and polar bears remain important in their cultures. Historically, the polar bear has also been known as the white bear.
== Description ==
The only other bear similar in size to the polar bear is the Kodiak bear, which is a subspecies of brown bear. Adult male polar bears weigh 350–700 kg (772–1,543 lb) and measure 2.4–3 metres (7 ft 10 in–9 ft 10 in) in total length. The Guinness Book of World Records listed the average male as having a body mass of 385 to 410 kg (849 to 904 lb) and a shoulder height of 133 cm (4 ft 4 in), slightly smaller than the average male Kodiak bears. Around the Beaufort Sea, however, mature males reportedly average 450 kg (992 lb). Adult females are roughly half the size of males and normally weigh 150–250 kg (331–551 lb), measuring 1.8–2.4 metres (5 ft 11 in–7 ft 10 in) in length. Elsewhere, a slightly larger estimated average weight of 260 kg (573 lb) was claimed for adult females. When pregnant, however, females can weigh as much as 500 kg (1,102 lb). The polar bear is among the most sexually dimorphic of mammals, surpassed only by the pinnipeds such as elephant seals. The largest polar bear on record, reportedly weighing 1,002 kg (2,209 lb), was a male shot at Kotzebue Sound in northwestern Alaska in 1960. This specimen, when mounted, stood 3.39 m (11 ft 1 in) tall on its hindlegs. The shoulder height of an adult polar bear is 122 to 160 cm (4 ft 0 in to 5 ft 3 in). While all bears are short-tailed, the polar bear's tail is relatively the shortest amongst living bears, ranging from 7 to 13 cm (2.8 to 5.1 in) in length.
Compared with its closest relative, the brown bear, the polar bear has a more elongated body build and a longer skull and nose. As predicted by Allen's rule for a northerly animal, the legs are stocky and the ears and tail are small. However, the feet are very large to distribute load when walking on snow or thin ice and to provide propulsion when swimming; they may measure 30 cm (12 in) across in an adult. The pads of the paws are covered with small, soft papillae (dermal bumps), which provide traction on the ice. The polar bear's claws are short and stocky compared to those of the brown bear, perhaps to serve the former's need to grip heavy prey and ice. The claws are deeply scooped on the underside to assist in digging in the ice of the natural habitat. Research of injury patterns in polar bear forelimbs found injuries to the right forelimb to be more frequent than those to the left, suggesting, perhaps, right-handedness. Unlike the brown bear, polar bears in captivity are rarely overweight or particularly large, possibly as a reaction to the warm conditions of most zoos.
The 42 teeth of a polar bear reflect its highly carnivorous diet. The cheek teeth are smaller and more jagged than in the brown bear, and the canines are larger and sharper. The dental formula is 3.1.4.23.1.4.3
Polar bears are superbly insulated by up to 10 cm (4 in) of adipose tissue, their hide and their fur; they overheat at temperatures above 10 °C (50 °F), and are nearly invisible under infrared photography. Polar bear fur consists of a layer of dense underfur and an outer layer of guard hairs, which appear white to tan but are actually transparent. Two genes that are known to influence melanin production, LYST and AIM1, are both mutated in polar bears, possibly leading to the absence on this pigment in their fur. The guard hair is 5–15 cm (2–6 in) over most of the body. Polar bears gradually moult from May to August, but, unlike other Arctic mammals, they do not shed their coat for a darker shade to provide camouflage in summer conditions. The hollow guard hairs of a polar bear coat were once thought to act as fiber-optic tubes to conduct light to its black skin, where it could be absorbed; however, this hypothesis was disproved by a study in 1998.
The white coat usually yellows with age. When kept in captivity in warm, humid conditions, the fur may turn a pale shade of green due to algae growing inside the guard hairs. Males have significantly longer hairs on their forelegs, which increase in length until the bear reaches 14 years of age. The male's ornamental foreleg hair is thought to attract females, serving a similar function to the lion's mane.
The polar bear has an extremely well developed sense of smell, being able to detect seals nearly 1.6 km (1 mi) away and buried under 1 m (3 ft) of snow. Its hearing is about as acute as that of a human, and its vision is also good at long distances.
The polar bear is an excellent swimmer and often will swim for days. One bear swam continuously for 9 days in the frigid Bering Sea for 700 km (400 mi) to reach ice far from land. She then travelled another 1,800 km (1,100 mi). During the swim, the bear lost 22% of her body mass and her yearling cub died. With its body fat providing buoyancy, the bear swims in a dog paddle fashion using its large forepaws for propulsion. Polar bears can swim 10 km/h (6 mph). When walking, the polar bear tends to have a lumbering gait and maintains an average speed of around 5.6 km/h (3.5 mph). When sprinting, they can reach up to 40 km/h (25 mph).
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