Polar Bear Basics

Let's learn about the sea bear

This is picture of a polar bear on a ledge.

Polar bears are possibly the most famous of the bears, and ironically the most likely to disappear. Polar bears are classified by the IUCN as "vulnerable," which is only one classification above "endangered." You may remember polar bears from those heart-warming Coca-Cola® Christmas commercials or maybe you have a white stuffed bear in your room. Polar bears are much more complex than that, and unfortunately (and it may come as a surprise), they don't drink Coke®.

My son really loves wildlife. And everytime he draws a polar bear I want to tell him there probably won't any by the time... he's my age. That's kinda hard to deal with. Thom Yorke

Scientific Classification

The polar bears' scientific name is the Ursus maritimus. Ursus maritimus is Latin for "sea bear". The polar bear is the only bear considered to be a marine mammal. It is only partially aquatic though, and the least aquatic of all marine mammals.

Habitat

Polar bears are found throughout the circumpolar Arctic in the Northern hemisphere. Polar bears, or their tracks, have been reported almost as far north as the pole; however, scientists believe few bears frequent areas north of 88° north latitude on the ice over the continental shelf. The northern Arctic Ocean has little food for them. The majority of polar bears are found near land masses around the edge of the polar basin, where the continental shelf makes conditions ideal for hunting.

The polar bears' southern range is limited by the amount of sea ice that forms in the winter. Polar bears prefer to travel on sea ice and must have ice from which to hunt seals. In the south, polar bears are annual visitors to southern Labrador, Newfoundland, and Norway. They can reach the Gulf of St. Lawrence and Bering Sea during years with heavy pack ice. The most southerly dwelling polar bears live year-round in James Bay, Canada.

Polar bears inhabit arctic sea ice, water, islands, and continental coastlines. However, they prefer sea ice. Some polar bears follow the southern edge of the ice pack year-round, making extensive migrations as the ice recedes and advances.

Some polar bears spend part of the year on land. They have been found as far inland as 402 km (250 mi.). Polar bears in warmer climates may become stranded on land. In summer, sea ice melts along the coastlines, and pack ice (floating sea ice, or floes, not connected to land) moves so far north, that polar bears can't reach it, even though they are excellent swimmers. Most pregnant females spend the autumn and winter on land in maternity dens.

Migration

Polar bears travel throughout the year within loose, individual home ranges. Polar bears undergo seasonal migrations, following the movements of the ice pack. Some bears prefer to remain at the edge of the ice pack year-round, making extensive migrations as the ice advances and recedes. On the southern shores of Hudson Bay, some bears move onto land for summer and disperse over ice for the winter.

Population

This is a polar bear comic poking fun at how the polar bears are losing pack ice to live around due to global warming.

The world polar bear population is estimated at 20,000 to 25,000 individuals.

The ratio of males to females is approximately one to one.

Most recently, some polar bear populations are declining due to loss of sea ice habitat.

Physical Characteristics

Size

The polar bear is the largest land carnivore. Male polar bears (boars) grow two to three times the size of females (sows). Boars weigh about 350 to more than 650 kg (772-1,433 lb.) and are about 2.5 to 3 m (8.2-9.8 ft.) long. Sows weigh about 150 to 250 kg (331-551 lb.) and are about 1.8 to 2.5 m (6.0-8.2 ft.) long. Pregnant females can weigh as much as 500 kg (1,102 lb.).

Shape and Build

This is a Cola Cola ad with two polar bears sharing a Coke.

Compared to other bears, polar bears have more slender bodies and longer necks and heads. A polar bear's head is oblong and relatively small compared to body size. The muzzle is elongated with a "Roman-nosed" (slightly arched) snout.

A polar bear's nose is broad and black. They have 42 specialized teeth used for catching food and aggressive behavior. The ears are small compared to those of other bears - an adaptation that enables them to conserve body heat. A polar bear's eyes are dark brown, set relatively close together, and face forward.

The hind limbs are longer than the forelimbs. This makes the large, muscular hind end stand higher than the shoulders.

Feet are five-toed paws. Polar bears have large paws compared to body size, reaching 30 cm (12 in.) in diameter. The large paws of a polar bear act like snowshoes, spreading out the bear's weight as it moves over ice and snow. The forepaws are round and partially webbed. The hind paws are elongated.

Each toe has a thick, curved, nonretractile claw. The claws are used for grasping prey and for traction when running or climbing on ice. The sole of a polar bear's foot has thick, black pads covered with small, soft papillae (dermal bumps). The papillae create friction between the foot and ice to prevent slipping. Long hairs growing between pads and toes also help prevent slipping.

Hair and Skin

A polar bear's skin, visible only on the nose and footpads, is black. The black color enables the bear to absorb sunlight energy to warm its body.

Polar bears are completely furred except for the nose and footpads. A polar bear's coat is about 2.5 to 5 cm (1-2 in.) thick. A dense, woolly, insulating layer of underhair is covered by a relatively thin layer of stiff, shiny, hollow guard hairs. Guard hairs may be as long as 15 cm (6 in.).

The coat can vary from pure white to yellow to light brown depending upon season and angle of light. Even though our eyes tell us differently, a polar bear's fur is not white. Each hair is completely color free and transparent with a hollow core. Polar bears look white because the hollow core scatters and reflects visible light, just like ice and snow does. When photographed with film sensitive to ultraviolet light, polar bears appear black.

Polar bear fur is oily and water repellent. The hairs don't mat when wet, allowing the polar bears to easily shake free of water and any ice that may form after swimming. Ice forms when the wet fur is exposed to air temperatures at or below freezing.

Polar bears completely molt (shed and replace their fur) annually, in May or June. The molt can last several weeks.

Adaptations

Swimming and Diving

This is a picture of a polar bear mother and her two cubs.Polar bears are strong swimmers; they swim across bays or wide leads without hesitation. They can swim for several hours at a time over long distances.

A polar bear's front paws propel them through the water dog-paddle style. The hind feet and legs are held flat and are used as rudders.

A thick layer of blubber (fat), up to 11 cm (4.3 in.) thick, keeps the polar bear warm while swimming in cold water. Polar bears can obtain a swimming speed of 10 kph (6.2 mph). A polar bear's nostrils close when under water.

Polar bears make shallow dives when stalking prey, navigating ice floes, or searching for kelp. Polar bears usually swim under water at depths of only about 3-4.5 m (9.8-14.8 ft.). They can remain submerged for as long as two minutes. No one knows how deep a polar bear can dive. One researcher estimates that polar bears dive no deeper than 6 m (20 ft.).

Thermoregulation

Body temperature, which is normally 37°C (98.6°F), is maintained through a thick layer of fur, a tough hide, and an insulating fat layer (up to 11 cm or 4.5 in. thick). This excellent insulation keeps a polar bear warm even when air temperatures drop to -37°C (-34°F).

Polar bears are so well insulated they tend to overheat. Polar bears move slowly and rest often to avoid overheating. Excess heat is released from the body through areas where fur is absent or blood vessels are close to the skin. These areas include the muzzle, nose, ears, footpads, inner thighs, and shoulders. Polar bears also swim to cool down on warm days or after physical activity.

Diet and Eating Habits

This is a picture of two alert polar bears.

Polar bears feed mainly on ringed and bearded seals. Depending upon their location, they also eat harp and hooded seals and scavenge on carcasses of beluga whales, walruses, narwhals, and bowhead whales. On occasion, polar bears kill beluga whales and young walruses.

When other food is unavailable, polar bears will eat just about any animal they can get, including reindeer, small rodents, seabirds, waterfowl, fish, eggs, vegetation (including kelp), berries, and human garbage.

A polar bear's stomach can hold an estimated 15% to 20% of its body weight. A polar bear generally eats this much only when its energy demands are high.

A bear can assimilate 84% of the protein and 97% of the fat it eats. Polar bears need an average of 2 kg (4.4 lb.) of fat per day to obtain enough energy to survive. A ringed seal weighing 55 kg (121 lb.) could provide up to eight days of energy for a polar bear. Hibernating polar bears do not eat.

Behaviors

Daily Activity Cycle

Polar bears are most active the first third of the day and least active the final third of the day.

When not hunting, polar bears are often sleeping or resting. From July to December in Canada's James Bay region, when lack of ice prevents seal hunting, a polar bear may spend up to 87% of its time resting.

Social Structure and Behavior

This is a picture of a polar bear laying on its back.Polar bears are basically independent.

Only two social units exist, adult breeding pairs and mothers/cubs.

The most constant social interaction occurs between mother and cubs. Polar bear mothers are attentive, frequently touching and grooming their cubs. Polar bear breeding pairs remain together for one week or more, mating several times.

Aggression occurs between males during the breeding season and when males attempt to steal food caught by other polar bears. Play fighting has been observed between aggregating subadult and adult male polar bears.

Hibernation

Polar bears aren't deep hibernators, but enter a state of carnivore lethargy. Their body temperatures do not drop substantially, and other body functions continue. Scientists, however, use the term "hibernation" in a general sense when referring to carnivore lethargy and the term is used in this booklet as well.

Only females, especially pregnant females, enter into a state of carnivore lethargy, or "hibernation". They do so from about October or November through March or April.

Reproduction

Mating

This is a picture of two polar bear cubs.Female polar bears reach sexual maturity at about four to five years. Male polar bears reach sexual maturity at about six years. Most male polar bears don't successfully mate until 8 to 10 years and older.

Breeding takes place from March to June on the sea ice, but most occurs during April and May. During the breeding season, males and females find each other by congregating in the best seal-hunting habitats.

Competition for females is intense. Females breed about once every three years; therefore, there are about three adult males to every breeding female.

Polar bears have many mates over their lifetime.

Birth

This is a picture of stranded polar bears on an ice cap.The total gestation period is about 8 months. Once mated, females begin depositing fat in preparation for cubbing. Females need to gain at least 200 kg (441 lb.) for a successful pregnancy. Some females may seek out maternity dens as early as late August, but most enter dens in mid to late October. Dens protect newborn cubs from winter's temperature extremes.

Polar bear cubs are born November through January in a den. Mother and cubs emerge from their den in late March or April. Most adult females give birth once every three years. In populations with access to abundant food, birth occurs once every two years. The most frequent litter size is two, followed by litters of one. Litters of three are less common than twins or singles, and litters of four are rare.

Cubs open their eyes within the first month. The cubs begin walking while in the den at about two months. By this time, they also have thick, whitish fur and their teeth have erupted. By the time the mother and cubs emerge from the den in late March or April, the cubs weigh 10 to 15 kg (22-33 lb.).

Longevity

Polar bears can live 20 to 30 years, but only a small percentage of polar bears live past 15 to 18 years. The oldest known polar bear in the Arctic lived 32 years. The oldest known polar bear in a zoological park lived 45 years.

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