Polar Bear

(Jrt)

Polar Bear
The polar bear or polar bear (Ursus maritimus, or Ursus arctos maritimus) is a large mammal originating in the Arctic regions. It is the largest land carnivore and is at the top of its food pyramid.

Perfectly adapted to its habitat, it has a thick layer of fat and a fur isolated from the cold, his white coat it provides ideal camouflage on the ice and his black skin enables him to better conserve warmth body. Provided with a short tail and small ears, the bear has a relatively small head and a body fuselée long adapted to swimming.

The polar bear is a marine mammal semi-aquatic, whose survival depends mainly on the ice and marine productivity. It hunting both on land and in water.

Size and Weight
The polar bear is - with kodiak bear - the largest living terrestrial carnivore. It is twice as large a Siberian tiger and weighs twice as heavy a lion. Adult males weigh between 400 and 600 kg and can sometimes reach 800 kg to a height of 2.4 to 3 meters long. The bear has a significant sexual dimorphism: females are generally two times smaller than the male, weighing 200 to 300 kg and measure from 1.9 to 2.1 metres. At birth, the cubs weigh not only between 600 and 700 grams. The record weight for a polar bear is now 1102 kg.

Polar bears have taken weight quite spectacular. For example, in Canada, a female polar bear took more than 400 pounds in nine months. In November, she weighed 92 kg, but in August, she was weighing 505 kg. This can be explained by the fat seals that are eaten in the spring.

Recent data suggest that the weight of polar bears is declining. These data can be taken as an indication of the pressures on them. A 2004 study by the National Geographic Society found that the weight of polar bears, on average, was 50% lower than their weight in the 1970. For example, in 2007, females Hudson's Bay had an average weight of only 230 kg, compared with a weight of 300 kg in 1980.

Their weight do not prevent them from being very fast on land. They can smoothly pass a man in the race.

Polar bears are excellent swimmers through their layer of fat. They can be seen on the open sea to hundreds of meters from any land. They swim using their front legs to propel itself and their hind legs as a rudder.

Skin and Fur
The polar bear is immediately recognizable with its white fur. And unlike many other arctic mammals (such as the Arctic fox), it never changes this coat for a darker color in the summer. The hairs are not really pigmented blank; they are non-pigmented and hollow, like white hair in humans.

An interesting feature of its fur is that it appears black if it is photographed under ultraviolet rays. Some people have hypothesized that the hair dig furrows capturing light to the black skin of the bear to help them keep warm during the cold winters and without sun, but this is contradicted by more recent studies. Measurements show that the hairs absorb strongly violet rays and ultraviolet rays. That is why the skin of the polar bear seems often yellow.

Polar bears renewal their furs from May to August. The fur is typically 5-15 cm in most parts of the body. However, on the front legs, males have hair longer and growing in length up to age 14 years. It is assumed that this is a form of attraction for females, in the manner of the lion's mane.

These bears are extremely well insulated to the point where they catch hot at temperatures above 10 ° C. As a result, they sometimes prefer the ice to cool off, and on land, they can dig in search of the permafrost layer of cooler under the ground.

Speciation
The raccoons and bears diverged there are about 30 Ma The bear was separated from other bears there are about 13 Ma The six distinct species of bears have emerged there are about 6 million years ago. The fossil evidence and analysis of their DNA have shown that the polar bear and brown bear differed there are about 200000 years.

Polar bears, however, have the possibility of producing fertile offspring in mating with brown bears, suggèrant they have a common ancètre close. This, according to the classic definition of a species (the ability to have a normally fertile offspring, should classify polar bears and brown bears within the same species.

In a widely cited article from 1996, a comparison of DNA from different brown bear Islands Admiralty, Baranof, and Chichagof of Alaska shows that these groups bear share a common ancestor with the most recent polar bear as with other populations of brown bears in the world. From the viewpoint of descent, describe the black bear as a group genetics (a taxon monophylétique) separate the polar bears did not think it was relevant.

Other index proximity between polar bears and black bears, we can see that polar bears still possess the substance HIT ( "Hibernation Induction Trigger", which helps hibernation) in their blood, but not use it as the l 'brown bear. They may, however, and occasionally enter in a state of drowsiness (for females in gestation in particular), even though their body temperature does not decrease during this period as might be the case for mammals hibernants characteristics.

Although the traditional definition of the polar bear as a separate species of black bears appear to be questionable according to the traditional taxonomic criteria, neither of the two species can not survive in the ecological niche of the other. Therefore, in addition to a morphology, a social behaviour, food and phenotypic quite different, both species are still classified as different.


Sub-species and sub-populations
It is generally believed that there is no sub-species in the polar bear. In fact, since crossing between brown bears and polar bears provide fertile hybrids, the polar bear is sometimes seen as representing a subset of the brown bear. It therefore considers that there is no sub-species of bear, but sub-populations.

The number of sub-populations depends very much on the body count. The IUCN / SSC PBSG (Polar Bear Specialist Group), a large body of international research and management on the Polar Bears, currently recognizes some twenty sub-populations in the world. Among the best known are:

* Those in the Chukchi Sea (Wrangel Island and west of Alaska)
* Those in the Beaufort Sea (Alaska in the north and north-western Canada and the north-west)
* Those in the Canadian Arctic Archipelago
* Those of Greenland
* Those Spitzbergen - Land of Franz Joseph
* Those in the north-central Siberia

Of these sub-populations, there are 13 in Canada, with about 15000 people in total.

However, some sources differ two sub-species: Ursus maritimus maritimus and Ursus maritimus marinus.

The number of populations of polar bears in decline has increased, from one in 2001 to five in 2006. There are only 19 populations of polar bears in the world, and this decline represents more than a quarter of the populations of the species.

The decline in populations of polar bears indicates that the entire Arctic is subject to a huge stress due to climate change. With the warming of the Arctic, which is more than twice as high as the rest of the world, and the fact that the ice must disappear in summer by the end of the century, polar bears are facing serious problems, especially because they depend on the ice to live, hunt and breed.

According to a report recently published by the specialist group of polar bears of the International Union for the Conservation of Nature (IUCN), two sub-populations are best studied polar bears in the world, the people of West Hudson's Bay in Canada and the people of the south of the Beaufort Sea (USA / Canada), have experienced a decline of 22% and 17% during the last two decades. The other three declining populations are those in the Baffin Bay and the Kane Basin - shared between Greenland and Canada - and the Bay of Norway to Canada.

The population of polar bears is estimated in the years 2000 between 16000 and 35000 people, 60% live in Canada and 25% in Alaska (USA).

Reproduction
Females small (one or two usually) every three years. They come into the world while the mother was entrenched in its lair to winter: they do not wake up, content to feed the rich milk in tétant for several weeks. The mother does take away from the den until they are aged three to four months: it is at this time only that they learn about the world around them. Young people remain long with their mother. She is the one who makes all their education: hunting, choice of a den, and so on. They will stay together forever from the mother at the age of three.

During this period, small take up a lot of weight thanks to the milk produced by the female and which contains 50% fat.