Polar Bear, the Largest Living Carnivorous Mammal

The ice killer

By on 27 Nov 2007, 18:07 GMT
1.All living bears are the descendants of a fox-sized 20 million-year-old bear called Ursavus. The genus Ursus including modern brown bear (grizzly is part of this species), black bear and polar bear appeared 5-10 million years ago. Ursus etruscus that lived in Europe 1.5-2 million years ago is the ancestor of the cave bear (Ursus spelaeus), extinct 12,000 years ago. In Asia, from Ursus prearctos 600,000 years ago the brown bear (Ursus arctos) evolved. Brown bear colonized the whole Eurasia and 100,000 years ago crossed the Behring into North America, giving birth to the grizzly forms.

Some individuals remained prisoners on the ice packs, they adapted to this environment and through adaptations and color change gave birth to the polar bear (Ursus maritimus). Thus, the polar bear could be considered the youngest bear species.

2.Polar bears are solitary, gathering only around the carcass of a stranded whale (for example). A polar bears travels up to 70 km (42 mi) daily, some 3,200 km (2,000 mi) per year. They live on or around the ice shelf, tundra and rarely polar bears enter 150 km (92 mi) the taiga. Some polar bears can wander along a year over an area of 20,000 square mi (50,000 square km). Usually, only young males move on very large distances, of hundreds of km. Females move less than the males. It is considered there are 4-7 polar bear populations isolated in the Arctic.

On the run, polar bears can reach 30-40 km (19-25 mi) per hour, much faster than a human.

3.Like in the case of the brown bear, the female "hibernates" (it's just a profound sleep) in the den 4-6 months annually (meanwhile they do not urinate or defecate). Males and sterile females spend the winter on the ice pack, in places rich in seals. Young polar bears stay in dens for about 2 months annually, just like the females.

The dens are dug in snow or ice, are 2.5 m (8.3 ft) long, 1.5 m (5 ft) tall and located 2-3 m (7-10 ft) under the snow, on solid ground. Inside the temperature varies between 10 to 20o C, due to the heat expelled by the female's body.

During summer, polar bears follow the shores of the islands and mainland.

4.Seals make 90 % of the polar bear diet (this is the most carnivorous bear): ringed seals (Pusa hispida), harp seal (Pagophilus groenlandica), bearded seal (Erignathus barbatus) and hooded seal (Cystophora cristata). Male walruses are rather avoided because of the size and tusks.

Seals are taken by surprise on ice pack or bears wait for them at the breathing holes made in the ice (in many cases for hours) and kill them with a paw blow (the paw has 30 cm or one foot in diameter!) or through a bite on the nape. This hunter must be skilled and patient, as only 18 % of the attacks succeed and the bear lays on its belly with its nose close to the seal's orifice for hours. Preferred preys are pup seals, taken from shelters dug under the soft snow, with only one small orifice left, necessary for the breathing of the small seals. Bears detect them using its smell. A 6 weeks pup weighs 20 kg and is filled with fat.

During summer, with the retreat of the ice pack, they eat roots, grasses, algae, fish, water birds and their eggs, lemmings, arctic hares, cadavers, reindeer, muskox, food wastes.

During winter, polar bears consume only the skin and the bubbler of the seals (the rest of the carcass is eaten by other animals, like polar foxes and gulls). The bear does not eat meat rich in proteins because eliminating the nitrogen resulting wastes requires a lot of water and ingesting the cold ice consumes energy.

Polar bears must eat one seal at each 5-6 days and their stomach has a capacity of 70 kg (155 pounds).

Due to the fine olfaction, a polar bear smells a seal from 30 km (19 mi) away.

5.Adult polar bears play in a way which looks like wrestling, which can turn real during the mating season. The mating takes place in April-June, but the egg's implantation and its development start in autumn. They give birth in November-January, in the den, and after the 7-9 months gestation two cubs are born. At birth the offspring have just 500-700 grams (1.1-1.8 pounds) (the smallest birth size, compared to the mother's, amongst placental mammals). The female could not feed larger cubs while remaining with them in the den, as she does not eat all this time, the milk being formed from her fat reserves. When they are 3 months old, the cubs weigh 5-12 kg (11-27 pounds). Female suckles the young for 8 months, a period when she lasts 45 % of her weight. Toxins from milk increase by 12-34 % after the females start hunting seals in April. A 5 years old female is 1.7 m long and weighs 166 kg. A 12 years old male is 2.2 m long and weighs 360 kg.

The female takes care of the offspring for 18-30 months. Sexual maturity is reached after 3-4 years in the case of females, and 6-10 years for males. The female reproduces once at every 2-4 years. Polar bears can live up to 34 years.

Pregnant females go to solid ground and polar bear breeding places are the northern shore of Alaska, southwestern Hudson Bay, the territory King Christian X (Greenland), Franz Joseph archipelago and Wrangel Island (northern Siberia) (where there is a population of 180-200 breeding females). The largest concentration of breeding polar bear females is found on Cape Drem (Wrangel), where 57 breeding dens were found on a surface of 0.37 square km.

Currently, there are 15-40,000 individuals, but the current global warming could doom this species. In Greenland, Eskimoes (Inuit) can hunt 100-150 polar bears annually. Pesticides (like organochlorines and dioxins) have been found in polar bear blood.

6.Polar bears are considered rather sea mammals. They have been found many times swimming up to 100 mi (160 km) off the shore, and sometimes they can swim for days. Their fingers are webbed, an adaptation for swimming. The fore limbs are used for propulsion and the rear limbs as rudders. Their ears are small, so that they do not lose heat through them. The skull is 37-41 cm long (1 ft 3 in-1 ft 5 in) in length, being prolonged and flattened compared to other bears, an adaptation for swimming. Limbs too are longer than in the case of other bears, and the soles are hairy for adherence on the ice.

The underskin isolating fat layer is 5-10 cm (2-4 in) thick and they have two types of hairs, short and long. This bear is so well isolated that it loses heat only through the nose area, and sometimes, during winter, at tens of -35o, it is seen refreshing in the snow!

Male polar bears reach length up to 2.5 m (8.3 ft), females just 2.1 m (7 ft). The tail is 7.5-13 cm (3-5 in) long. The average weight is 410 kg (911 pounds) for males and 320 kg (711 pounds) for females. The largest polar bear ever was 3.65 m (12.2 ft) long and weighed one ton (2.200 pounds). The head is 40 cm (1.3 ft) long, with a circumference of 30 cm (1 ft). The weight can experience enormous fluctuations, of even 400 kg (900 pounds)! After 14 days without food, polar bears enter into "hibernation".

However, brown bear races from Alaska and Kamchatka are larger than the polar bear, and also have a stronger neck and head. Here, the polar bear loses the race because its head and neck are more adapted to swimming, which caused them to become thinner and lighter, thus more maneouvrable. Still, a polar bear has smaller and sharper teeth.

7.One thing. Polar bears are not white! Their hairs are transparent and present an empty nucleus which disperses and reflects the sunlight, which makes them appear white, just like in the case of the snow. The hair act like optic fibers, conducting the light radiation towards the black skin where it is absorbed. This can be so effective, that it explains why polar bears can overheat. And they appear white only soon after the seasonal molt as in time sun oxidation and grease stains slur it to yellow.

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