Polar Bear
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The polar bear (Ursus maritimus), also known as the white bear, northern bear, or sea bear, is a large bear native to the Arctic. It is one of the two largest land carnivore species and the apex predator within its range. It is well-adapted to its habitat: its thick blubber and fur insulate it against the cold and its translucent fur (which appears white or cream-coloured) camouflages it from its prey. The polar bear hunts well on land and on the sea ice, as well as in the water.

Natural range
The polar bear is a circumpolar species sometimes regarded by authorities as a marine mammal found in and around the Arctic Ocean whose southern range limits are determined by pack ice (their southernmost point is James Bay in Canada). While their numbers thin north of 88 degrees, there is evidence of polar bears all the way across the Arctic. Population estimates are generally just over 20,000.

Their main population centers are:

Wrangel Island and western Alaska
Northern Alaska
Canadian Arctic archipelago
Greenland
Svalbard-Franz Josef Land
North-Central Siberia
Their range is limited by the availability of sea ice that they use as a platform to hunt seals, the mainstay of their diet. The destruction of its habitat on the Arctic ice, which has been attributed to global warming, threatens the bear's survival as a species; it may become extinct within the century. Signs of this have already been observed at the southern edges of its range.

Size and weight
The largest extant species of land carnivore, a male Polar Bear can be twice the weight of a Siberian tiger. Most adult males weigh from 300 to 600 kg (660 to 1300 lb) and measure 2.4 to 2.6 m (7.9 to 8.5 ft) in length. The largest polar bear ever on record was a bear shot in Kotzebue Sound, Alaska in 1960. According to Guinness World Records 2006, this colossus weighed an estimated 880 kg, or 1960 lbs, and, mounted, it was 3.38 m (11 ft 11 in) tall. Adult females are generally about half the size of males and normally weigh 190 to 300 kg (420 to 650 lb). They typically measure 1.9 to 2.1 m (6.25 to 7 ft). At birth, cubs weigh 600 to 700 g.

A 2004 National Geographic study showed that polar bears that year weighed, on average, fifteen per cent less than they had in the 1970s.

Subspecies
It is generally believed that there are no living polar bear subspecies. In fact, because polar bears bred with brown bears have produced fertile hybrids, it can be argued that polar bears are a subspecies of Brown Bear.

The number of distinct populations depends on who is counting. The IUCN/SSC Polar Bear Specialist Group ("PBSG"), the preeminent international scientific body for research and management relating to polar bears, currently recognizes twenty populations, or stocks, worldwide. Other scientists recognize six distinct populations, but no (living) subspecies:

Chukchi Sea population on Wrangel Island and western Alaska
Northern and northwestern Alaska and northwestern Canada (the Beaufort Sea population)
Canadian Arctic archipelago
Greenland
Spitzbergen-Franz Josef Land
Central Siberia
Other sources list these subspecies:

Ursus maritimus maritimus
Ursus maritimus marinus

Fur and skin
A polar bear's nose and skin are black and the fur is translucent despite its apparent white hue. The fur is good camouflage as well as insulation. Stiff hairs grow on the soles of its paws; these insulate and provide traction on ice.

Unlike other arctic mammals, polar bears never shed their coat for a darker shade in the summer. It was originally hypothesized that the hollow hairs of a polar bear coat acted as fiber optic tubes to conduct light to the black polar bear skin, where it could be absorbed. However, a number of recent studies have demonstrated that this is not true. This thick undercoat does, however, insulate the bears to the point where they overheat at temperatures above 10 °C (50 °F) and are nearly invisible under infrared photography; only their breath and muzzles can be seen. Growing through the undercoat is a relatively sparse covering of hollow guard hairs about 6 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, several polar bears in the Brookfield Zoo turned green as a result of algae growing in their hollow guard hair tubes. (Chicago experienced an extremely hot and humid summer that year.) However, the zoo took no action since it was shown the algae does not negatively affect the bears in any way. The staff believed that exposing the bears to chlorine or bleach would be more harmful than letting the algae run its course. Previously, in February 2004, two polar bears in the Singapore Zoo turned green due to algae growth. A zoo spokesman said that the algae had formed as a result of Singapore's hot and humid conditions. The bears were washed in a peroxide blonde solution to restore their expected colour. A similar algae grew in the hair of three polar bears at San Diego Zoo in the summer of 1980. They were cured by washing the algae away in a salt solution.

Hunting, diet and feeding
The polar bear is the most carnivorous member of the bear family, and the one that is most likely to prey on humans as food. It feeds mainly on seals, especially ringed seals that poke holes in the ice to breathe, but will eat anything it can kill: birds, rodents, shellfish, crabs, beluga whales, and young walruses, occasionally musk oxen, and very occasionally other polar bears. They are enormously powerful predators, not uncommonly dispatching beluga whales and young walruses, but they rarely kill adult walruses, which are twice the polar bear's own weight. Orcas, humans, and larger bears of their own species are the only predators of polar bears, although walruses may occasionally kill a polar bear during a struggle to defend themselves.

Polar bears are excellent swimmers and can often be seen in open waters miles from land. Recently, polar bears in the arctic have undertaken longer than usual swims to find prey, resulting in four recorded drownings in the unusually large ice pack regression of 2005. They also hunt very efficiently on land due to their prodigious speed; they are more than capable of outrunning a human. Still, caribou and musk oxen can easily outrun a polar bear, and polar bears overheat quickly: thus the polar bear subsists almost entirely on seals and on walrus calves or adult carcasses.

As a carnivore which feeds largely upon fish-eating carnivores, the polar bear ingests large amounts of Vitamin A, which is stored in its liver; in the past, humans have been poisoned by eating the livers of polar bears. Polar bears also feed on carrion (e.g. beached whales), and although mostly carnivorous, they may also eat some vegetable matter: mainly berries and roots in the late summer, as well as kelp.

Polar bears are enormous, aggressive, curious, and extremely dangerous to humans. It is best to remember that wild polar bears, unlike most other bears, are often barely habituated to people and will quickly size up any animal they encounter as potential prey. A polar bear should never be approached and if one is spotted, it is best to retreat slowly on foot, preferably to an indoor location, or move away in a vehicle.

Regrettably, like other bear species, they have developed a liking for garbage; the dump in Churchill, Manitoba is frequently scavenged by polar bears.

Breeding
Polar bears mate in the spring; pairing is temporary and only lasts for the actual mating; males and females form no permanent bond. The gestation period is 240 days (8 months), and the cubs are born in early winter in a cave dug by the mother in deep snow in October. Usually, two cubs are born, less often one or three; litters of four cubs have been recorded. Like other Ursus bears, the cubs are very small at birth, typically 30 cm long and weighing 700 g. The cubs are born helpless and blind, and open their eyes at about one month old, are able to walk at 1.5 months, and start eating solid food at 4-5 months. They remain with their mother, learning how to hunt and protect themselves against adult males (which have been known to cannibalize cubs), until 10 months old. Females nurse their young for up to two and a half years on milk that contains approximately 33% fat, higher than that of any other species of bear and comparable to that of marine mammals. Sexual maturity is reached at 3-4 years. Adult polar bears are known to live over 30 years. Polar bears do not hibernate, though lactating females will not emerge from their cave while the cubs are very young; they often go without eating for a period of nine months and rely on stored body fat (also known as blubber) to keep themselves and their cubs alive.

The 2004 National Geographic study found no cases of cubs being born as triplets, a common event in the 1970s, and that only one in twenty cubs were weaned at eighteen months, as opposed to half of cubs three decades earlier.

Evolutionary relationships
The raccoon and bear families are believed to have diverged about 30 million years ago and around 13 million years ago the spectacled bear split from the other bears. The 6 distinct ursine species originated some 6 million years ago. According to both fossil and DNA evidence, the polar bear diverged from the brown bear around 200 thousand years ago, and crosses between the two species have produced fertile grizzly-polar bear hybrids. Fossils show that it was only between 10 and 20 thousand years ago that their molar teeth changed significantly from those of brown bears.

In a widely-cited paper published in 1996 by Sheilds and Talbot of the University of Alaska Fairbanks, a comparison of the DNA of various brown bear populations showed that the brown bears of Alaska's ABC islands actually shared a more recent common ancestor with polar bears than with any other brown bear population in the world.

http://en.wikipedia.org/wiki/Polar_bear

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