Polar bears (Ursus maritimus) are circumpolar in distribution, inhabiting the majority of Arctic seas and coastlines. They range across territory owned by Canada, the United States, Russia, Sweden, Finland, Norway and Denmark.
An estimated 20,000 to 40,000 polar bears remain in the wild. The United States Alaskan population is thought to number around 2,000, while Canada’s polar bear population is estimated at between 13,000 and 15,000 individuals.
Polar bears are thought to have evolved from brown bears between 70 - 100,000 years ago. During the Pleistocene period, brown bears were isolated by the advance of glaciers. This resulted in a series of rapid evolutionary changes to adapt to arctic environments, including the development of a number of unique physiological attributes. In evolutionary terms, polar bears should be viewed as bears first, and as arctic inhabitants second.
Polar bears are the largest living land carnivore, with males reaching a weight of up to 650 kg (1,433 lbs.) and a length up to nearly 3 m (9.9 ft). Females are smaller, weighing up to 250 kg (551 lbs.) and reaching a length of 2.5 m (8.2 ft). The largest polar bear ever recorded weighed 1,002 kg (2,209 lbs.) and measured 3.7 m (12 ft) in length.
Polar bears can achieve an age of more than twenty-five years in the wild, and more than thirty-five years in captivity.
Adaptations To Cold Weather
The body shape of polar bears differs from other bears in that they have elongated bodies and relatively long, slender necks, a streamlining adaptation conducive to swimming. Their heads are small when compared to their overall body size, and their snout is arched. Their ears are small and rounded, and are laid flat when swimming underwater.
Polar bears have thick stocky legs, with the hind limbs being longer than the forelimbs. The paws are very large, sometimes reaching more than 12 inches in diameter, and serve as a snowshoe, spreading out the bear’s weight as it moves across ice and snow. Each paw has one non-retractable claw, used for grasping prey and for traction on slippery surfaces.
The sole of each foot has a thick black pad covered with tiny bumps, and long hairs grow between the pads and the toes. Both of these characteristics help create friction between the foot and the ground preventing slippage. Their physical characteristics and swinging method of locomotion cause polar bears to use more energy to move at a given speed than other mammals. Their average walking speed is about 5.5 kph (3.4 mph), but they can reach speeds of 40 kph (25 mph) at times.
Polar bears have black skin, a broad black nose, and a small tail. The black skin facilitates the retention of heat from sunlight that reaches the body surface.
Fur covers the entire body of polar bears, except for the nose and footpads. Dense underhair serves to insulate the body, and is covered by a thinner layer of stiff, clear, hollow guard hairs. The guard hairs reflect sunlight down the shaft of each hair to the body surface. The fur is oily and water repellent, and shakes dry.
Polar bear hair reflects light, giving the bears their white coloration. Depending upon the angle of the sun and the season, polar bears may appear yellowish or light brown in colour.
In addition to their thick fur and tough skin, polar bears are equipped with a layer of blubber, up to 11 cm (4.3 inches) thick, to help them maintain a body temperature of 98.6 degrees F. Polar bears are so well insulated, they easily become overheated. To prevent this from happening, polar bears move slowly and rest often, and will swim to cool down on warm days. Excess heat is released from the body through areas where fur is absent or minimal such as the snout, footpads, ears, and inner thighs, and by panting.
Sight, Hearing & Smell
Polar bears have hearing and eyesight comparable to humans. Their sense of smell is acute, and is extremely important in detecting food sources. Polar bears are able to smell a seal from a distance of more than 32 km (20 miles). Little research has been conducted regarding the polar bear’s sense of touch, but bears in captivity and in the wild have been observed manipulating small objects with great dexterity.
Each polar bear has its own home territory which varies in size depending on food availability and weather conditions. Individual bear territories may overlap with the territories of other bears.
Polar bears mate and give birth every two to three years, with pregnancies lasting approximately eight months. Females seek out maternity dens as early as August, but most enter sometime in October. The majority of dens are situated on land, within 16 km (10 miles) of the coast, but dens have been found as far inland as 100 km (62 miles). Polar bears of both sexes will occasionally occupy dens and shelters to overcome severe weather, food scarcity or to escape from summer heat and insects.
Cubs are born in the den from November to January, and emerge with their mother sometime in March or April weighing approximately 10 to 15 kg (22-33 lbs.). The most common litter size is two.
The major area of social interaction for polar bears is between females and cubs. Polar bear cubs learn many of their behaviours, including hunting, by following along and observing their mother. At about 30 months of age, when their mother is ready to breed again, she chases the cubs away.
Polar bears feed mainly on seals, but also scavenge for carcasses of whales and walruses. They will also consume reindeer, small mammals, birds, fish, eggs, vegetation and human refuse. They are capable of eating as much as 20% of their body weight at one time.
Polar bears utilize several methods for hunting. The most common is remaining motionless by a breathing hole or ice edge, and grabbing a seal when it surfaces to breathe. Other methods of hunting include stalking on land, stalking in the water, and searching for seal birth dens.
Polar bears are most active during the morning hours, with activity levels decreasing as the day progresses. Adult female bears with cubs spend about 19% of their time hunting during the spring, and about 38% of their time during the summer. For adult males, it’s about 25% and 40% respectively.