Beringia Natural History Notebook - Ursus Maritimus

Polar Bear Ursus maritimus
Description

Only recently in evolutionary time have bears adapted to life on arctic seas, but these great creatures have mastered the water and ice environment superbly. Over time they evolved a luxuriant white coat and layer of blubber for camouflage and warmth. Oversize feet serve as paddles for extensive swimming and spread their weight, helping this largest of modern carnivores (excepting Orcas) to traverse ice too thin to support a person.


Evolution

During the Ice Age, seals adapted to life in icy northern seas. Their need to breathe and reproduce at the surface put a rich year-around food resource within reach of a population of brown bears that began to live more and more out on the ice. Natural selection favored those bears best able to catch seals, and they became more thoroughly carnivorous than other bears. By 100,000 years ago they had evolved into something like the polar bear of today. Although polar and brown bears now look and act rather differently, their genetic closeness is demonstrated by matings in zoos that produce fertile offspring.


Distribution and Abundance

Polar bears' range is circumpolar. A few have been spotted close to the pole, but heavy perennial ice there provides poor seal hunting, so most are found further south where the ice is thinner and less continuous.

Formerly it was believed that polar bears migrated freely all across the Arctic, but modern research suggests that there are actually a number of more or less distinct populations. Russian and American are investigating the possibility that Beringian bears comprise a single group which during winter is distributed from Wrangel Island south along the Asian coast and in the central Bering Sea as far as St. Mathew Island. In summer, those wintering in the Bering Sea return to the north with the retreat of pack ice. Beringian bears seldom mingle with another population found in the Beaufort Sea east of Pt. Barrow, Alaska.

In 1981 the IUCN Polar Bear Specialist Group agreed that the world population was between 20,000 and 40,000. As of 1988 the most accepted estimate for the Alaska populations was 3,000-5,000.


Life History

The "Arctic Ring of Life" is the name given by Soviet biologist Savva Uspenski to the system of polynyas, or lakes of open water in pack ice, that persist through the winter. Wind, upwellings and currents along some shores or downwind from islands bring nutrients to the surface here, and keep the sea ice from coalescing. The coastal shear zone between shore- fast ice and the moving pack is also rich for similar reasons. These areas are vital to marine mammals and migrating birds, and to polar bears, which often hunt in such places.

Ringed seals are the bears' principal prey. They also hunt bearded seals and occasionally the more dangerous walrus. Normally solitary hunters, they have an impressive range of strategies, learn quickly, and show immense patience, power and speed. It has been calculated that their caloric needs require one ringed seal every six and a half days. Arctic foxes live on the sea ice in winter by scavenging polar bear kills.

Since their prey is available year-round, polar bears do not hibernate like brown bears, except pregnant females, who spend about five months in dens to give birth to their cubs. The female must greatly increase her weight, mostly in fat, to carry off a successful pregnancy and denning. The cubs, usually two, are born in December or January, weighing only 0.5 to 0.9 kilograms (one to one and a half pounds). By the time the family breaks out of the den in March or April the cubs weigh 10-15 kilograms (25-30 pounds). Cubs generally remain with their mother for two and a half years. Females are therefore able to bear young only every three years. This low rate of reproduction is balanced by a long life and low rates of natural mortality.

Moving in autumn from drifting ice to suitable denning sites requires a remarkable and little understood navigational ability. An important denning area for the Beringia population is on Wrangel Island. Denning also occurs on the northeastern coast of Alaska, although a majority of the Beaufort population dens on sea ice.


Relationships with People

About 4,000 years ago the ancestors of present day Eskimos moved into an ecological niche not yet occupied by people: hunting marine mammals of the northern seas. Once they learned this life-style they spread quickly along Arctic coasts. They had discovered much the same niche as the polar bear and may even have learned from bears, for their seal hunting methods are strikingly similar.

Polar bears have a preeminent place in Eskimo cultural and spiritual life. The spiritual guardians of shamans were usually polar bears, and it was believed that the spirits of people and bears sometimes interchanged. Killing a bear was a major event, requiring ceremonial propitiation of its spirit. Sometimes it was the bear who killed the person, for the predator-prey relationship went both ways.


From:

Beringia Natural History Notebook Series - September, 1992
National Audubon Society
Alaska-Hawaii Regional Office
308 G. Street, Suite 217 Anchorage, AK 99501

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