Polar bear (Ursus maritimus)

Polar bears are the largest species of bear in the world. Compared to other bears polar bears have: a long narrow head; a head that is relatively small compared to the body; small, heavily furred ears; claws that are short and strong; canine teeth that are long; and cheek teeth that are sharper. These features have been selected for in polar bears as a consequence of their almost purely carnivorous way of life. The polar bear has a circumpolar distribution, but is largely restricted to areas that have sea ice during a significant part of the year. Polar bears are typically off-white in colour, but vary from grey, through shades of white to yellow.

Polar bearPolar bear. Photo: Ann Kristin Balto / Norwegian Polar Institute

Polar bears are the largest species of bear in the world. They are markedly sexually dimorphic, with males being larger than females. Males weigh between 300-700 kg while females weigh 150-350 kg. The length of adult polar bears varies from 180–260 cm. Weight varies dramatically seasonally, especially among females that can more than double their body weight from the spring to the late summer. Polar bears are typically off-white in colour, but vary from grey, through shades of white to yellow. Adult males can be distinguished from females on the basis of their larger size, but more reliably on their more powerful necks that looks wider than their heads. Compared to other bears polar bears have: a long narrow head; a head that is relatively small compared to the body; small, heavily furred ears; claws that are short and strong; canine teeth that are long; and cheek teeth that are sharper. These features have been selected for in polar bears as a consequence of their almost purely carnivorous way of life.

Photos

Bilder av isbjørn

Distribution

The distribution of the polar bearThe distribution of the polar bear. Map: Norwegian Polar Institute

The polar bear has a circumpolar distribution, but is largely restricted to areas that have sea ice during a significant part of the year. They live in Canada, Alaska (USA), Greenland, the Russian Arctic, the Norwegian Arctic, and on areas of the ice shelf surrounding the North Pole. They occur as far south as 50° N in James Bay, Canada. There are 19 different defined populations of polar bears in the Arctic, that together contain an estimated 20,000– 25,000 bears. Individual bears roam over large areas, and populations are therefore well connected.

In Svalbard, most bears are found in areas with sea ice. The western part of Spitsbergen therefore has low densities of bears most of the year, while higher densities are found along the east coast, and also in the fjords to the north. Many bears hunt at glacier fronts in the fjords in the spring because ringed seal lairs are concentrated in such areas. The most important denning areas for polar bears in Svalbard are located on Kongsøya, Svenskeøya, Edgeøya, Nordaustlandet and Hopen. In Svalbard, some bears have small home ranges of a few hundred square kilometres where they hunt on the sea ice in the spring; during summer these bears wait on land for the sea ice to freeze again. Other bears follow the retreat of the sea ice and move north-east or east in the summer, hunting at the ice edge or in Franz Josef Land in Russia. The Barents Sea population of polar bears includes animals that den in both Svalbard and Franz Josef Land. Movement westwards to Greenland, or from Greenland to Svalbard, seem to be quite restricted.

General Ecology

Polar bearPolar bear. Photo: Jon Aars / Norwegian Polar Institute

A survey of the Barents Sea polar bear population, conducted in August 2004, concluded that there are between 1,900 and 3,600 polar bears in this region. Approximately half of these animals may reproduce in Svalbard. Polar bears are powerful swimmers. Their most important prey by far is ice-living seals, so they spend a lot of their time on the sea ice. Adult bears can spend days at a time in the water and travel very long distances at sea.

In Svalbard, their main prey species are ringed seals, bearded seals and harp seals. However, polar bears are opportunistic feeders and eat almost everything edible that they encounter – including carrion. Some bears take a lot of bird eggs in the summer. Polar bears also hunt sea birds even in steep cliff areas. Predation on reindeer has been recorded occasionally. Stranded corpses of dead whales can attract large numbers of scavenging bears. Polar bears are largely solitary, except for mothers with their young or male–female pairs that are together during brief periods in the mating season. Only pregnant female polar bears den during the winter; all other polar bears are winter active, even during the months with 24 hour darkness. Non-pregnant bears will dig day-beds in the snow in which they rest for a day or two at a time, and in some cases temporary dens where they can stay for days or weeks, e.g. in periods with bad weather.

Life history and reproduction

Denning areasDenning areas for polar bears in Svalbard. Map: Norwegian Polar Institute

Polar bears mate in April or May. At this time of year, males track the smell of females that can be many kilometres away. Females in Svalbard usually mate for the first time when they are five years old. Males reach sexual maturity somewhat later and do not reach full body size until they are 8-10 years old. A large male can mate with several females within the same season, attending each for some days or weeks. Polar bears have delayed implantation; the egg does not begin to develop beyond some few initial steps before the autumn (September-October), although the female mates in the spring. A pregnant female builds a birthing den in late autumn which she enters for about four months. During her time in the den she goes into a winter sleep, dropping her metabolic rate and her body temperature to conserve energy.

Females give birth around New Year. The cubs are very small (approximately half a kilo) when they are born. Most frequently polar bears have two cubs in a litter, but litter size ranges from one to three. Polar bear milk is rich in fat, and the nursing cubs grow quickly in the security of the den. They weigh about 10 kg when the family leaves their winter home in late March or early April. Juveniles follow their mother until the age of 2.5 years, at which time the mother is ready to mate again. Survival of juveniles is low, with only about one out of three reaching the age of two years. Adults have high survival, and typically live to be 15-25 years old.

Management status and monitoring

Polar bear hunting was banned in Svalbard in 1973, after 100 years of intensive exploitation. The population has recovered significantly in recent decades.  few bears are killed on average each year in Svalbard in defence of people or property. Despite the number of encounters between bears and man likely have increased in recent years concomitant with increased human traffic within the archipelago, the number of bears killed have dropped in recent years. Sea ice habitat has decreased in periods of the year from 1978 to recent, and in particular, this has affected the number of pregnant females that have been able to reach Hopen and Kongsøya in autumn. However, it is unknown is loss of habitat yet have led to a decrease in the population, or if it is still recovering from the low population size at the time of protection in 1973. Years will a milder climate influence condition. It affects reproduction locally, but it is not clear to which extent it affects population growth rate (see www.mosj.npolar.no).

According to the Polar Bear Treaty Norway shall "take appropriate action to protect the ecosystem of which polar bears are a part, with special attention to habitat components such as denning and feeding sites and migration patterns, and shall manage polar bear populations in accordance with sound conservation practices based on the best available scientific data".

MOSJ indicators (Environmental Monitoring of Svalbard and Jan Mayen):

Awaiting results from MOSJ…