Identification

Figure 1. Polar bear, Ursus maritimus

The polar bear (Fig. 1) is the largest member of the family Ursidae. Males are approximately twice the size of females. On average, adult males weigh 500 to 900 pounds (250 to 400 kg), depending on the time of year. An exceptionally large individual might reach 1,320 to 1,760 pounds (600 to 800 kg). Adult females weigh 330 to 550 pounds (150 to 250 kg) on average, although a pregnant female just prior to going into a maternity den could be double that weight.

Polar bears have a heavy build overall, large feet, and a longer neck relative to their body size than other species of bears. The fur is white, but the shade may vary among white, yellow, grey, or almost brown, depending on the time of year or light conditions. The pelage consists of a thick under fur about 2 inches (5 cm) in length and guard hairs about 6 inches (15 cm) long. Polar bears have a plantigrade gait and five toes on each paw with short, sharp, nonretractable claws. Females normally have four functional mammae. The vitamin A content of the liver ranges between 15,000 and 30,000 units per gram and is toxic to humans if consumed in any quantity.

General Biology, Reproduction, and Behavior

Polar bears mate on the sea ice in April and May. Implantation of the embryo is delayed until the following September. The adult sex ratio is even, but because females normally keep their young for about 2 1/2 years, they usually mate only once every 3 years. This creates a functional sex ratio of three or more males per female that results in intensive competition among males for access to estrus females.

Maternity dens are usually dug in deep snow banks on steep slopes or stream banks near the sea by late October or early November, depending on the availability of snow. In the Beaufort Sea, a large proportion of the females den on the multiyear pack ice several hundred miles (km) offshore. On the Ontario and Manitoba coasts of Hudson Bay, female polar bears may have their maternity dens 30 to 60 miles (50 to 100 km) or more inland, though this is quite unusual elsewhere in polar bear range.

Pregnant females normally have 2 young between about late November and early January. At birth, cubs weigh about 1.3 pounds (0.6 kg), have a covering of fine hair, and are blind. They are nursed inside the den until sometime between the end of February and the middle of April, depending on latitude. When the female opens her den, the cubs weigh 22 to 26 pounds (10 to 12 kg). The family remains near the den, sleeping in it at night or during inclement weather for up to another 2 weeks while the cubs exercise and acclimatize to the cold, after which they move to the sea ice to hunt seals.

The mean age of adults in a population is 9 to 10 years and average life expectancy is about 15 to 18 years. Maximum recorded age of a male in the wild is 29 years. Few male polar bears live past 20 years because of the intense competition and aggression among them. The oldest age recorded for a wild female polar bear is 32 years.

Depending on the age and sex class, polar bears spend 19% to 25% of their total time hunting in the spring and 30% to 50% of their time hunting in the summer. Polar bears capture seals mainly by stalking them, by waiting for them to surface at a breathing hole or, in the spring, by digging out seal pups and sometimes adults from birth lairs beneath the snow. When a polar bear kills a seal it immediately eats as much as it can and then leaves. Polar bears do not cache food and normally only remain with a kill for a short time. In the case of a large food supply such as a dead whale or a garbage dump, individual bears may remain in an area for several days or even weeks.

Polar bears sleep about 7 to 8 hours a day. They tend to be more active at “night” during the 24-hour daylight that prevails in the summer months, and to sleep during the day. Within 1 or 2 hours after feeding, they will usually sleep, regardless of the time of day. Before sleeping, females with cubs often move away from areas where other bears are active, probably to reduce the risk of predation on the cubs by adult males.

Food Habits

Polar bears feed on ringed seals and to a lesser degree on bearded seals. About half of the ringed seals killed during the spring and early summer are the young of the year. These young seals are up to 50% fat by weight and are probably easy to catch because they are vulnerable and inexperienced. Less frequently taken prey include walrus (Odobenus rosmarus), white whales (Delphinapterus leucas), narwhals (Monodon monoceros), and harp seals (Pagophilus groenlandicus). Polar bears also eat small mammals, bird eggs, sea weed, grass, and other vegetation, although these food sources are much less common and probably not significant.

Polar bears are curious animals and will investigate human settlements and garbage. They have been observed to ingest a wide range of indigestible and hazardous materials, such as plastic bags, styrofoam, car batteries, ethylene glycol, and hydraulic fluid.

Range

Figure 2. Polar bear distribution map

Polar bears are distributed throughout the circumpolar Arctic. In North America, their range extends from the Canadian Arctic Islands and the permanent multiyear pack ice of the Arctic Ocean to the Labrador coast and southern James Bay. The southern limit of their distribution in open ocean areas such as the Bering Sea or Davis Strait varies depending on how far south seasonal pack ice moves during the winter (Fig. 2).

Habitat

From freeze up in the fall, through the winter, and until breakup in the spring, polar bears are dispersed over the annual ice along the mainland coast of continental North America, the inter-island channels, and the shore lead and polynia systems associated with them. Polar bears are not abundant in areas of extensive multiyear ice, probably because of the low density of seals there.

Polar bears use a variety of habitats when hunting seals, including stable fast-ice with deep snowdrifts along pressure ridges that are suitable for seal birth lairs and breathing holes, the floe edge where leads are greater than 1 mile wide (1.6 km), and areas of moving ice with seven-eighths or more of ice cover. Bears may be near the coast or far offshore, depending on the distribution of these habitats. Ringed seals (Phoca hispida) and sometimes bearded seals (Erignathus barbatus) maintain their breathing holes from freeze up in the fall to breakup in the spring. Bears can hunt more successfully in areas where wind, water current, or tidal action cause the ice to continually crack and subsequently refreeze.

During winter, bears are less abundant in deep bays or fiords in which expanses of flat annual ice have consolidated through the winter. In places where the snow cover in the fiords is deep, large numbers of ringed seals give birth to their pups in subnivean lairs in the spring. Consequently, polar bears in general, but especially females with newborn cubs, move into such areas in April and May to hunt seal pups.

During summer, the response of the bears to the annual ice melts varies depending on where they live. Bears in the Beaufort and Chukchi seas may move hundreds of miles to stay with the ice. Bears in the Canadian arctic archipelago make seasonal movements of varying distances depending on ice conditions. Polar bears travel seasonally to remain where ice is present because they depend on the sea ice for most of their hunting.

In Hudson Bay, James Bay, parts of Foxe Basin, and the southeastern coast of Baffin Island, the ice melts completely in the summer and there are no alternate areas with ice close enough to migrate to. In these areas the bears may be forced ashore as early as the end of July to fast on land until November. Some bears remain along the coast while others move inland to rest in pits in snow banks or in earth dens in areas of discontinuous permafrost. By late September or early October, bears that spent the summer on land tend to move toward the coast in anticipation of freeze up. Many conflicts with people occur in the fall when bears are waiting along coastal areas for the sea ice to form.

Peter L. Clarkson. Wolf/Grizzly Bear Biologist. Renewable Resources. Government of the N.W.T. Inuvik, Northwest Territories. Canada X0E 0T0

Ian Stirling. Polar Bear Biologist. Canadian Wildlife Service. Edmonton, Alberta Canada T6H 2S5