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Biological Overview of the Polar Bear

Sarah R. Morgan


Animal Legal & Historical Center
Publish Date:
2007
Place of Publication: Michigan State University College of Law
Printable Version

Biological Overview of the Polar Bear

 

Polar bears, ursus maritimus, are the largest land carnivores in North America and top predator in the arctic habitat. Having evolved from the grizzly bear during the Pleistocene, its white coat permits it to blend in with the ice and snow of the Arctic. This serves as a perfect camouflage enabling the polar bear to remain undetected by its prey while it hunts. Polar bears are massive animals. Although there are regional differences in size between polar bears, they are quite bigger than humans, weighing as much as 1,500 pounds and up to 11 feet in length.

Polar bears may forage in garbage dumps when food is scarce; however they are generally strict carnivores, and do not consider humans their prey. Generally polar bears prefer to eat animals such as seals, beluga whales, walruses, other small mammals, birds and occasionally even other bears. Interestingly, polar bears eat the fat and skin of their prey, leaving the meat for scavengers. Its diet consisting primarily of fat (approximate 2kg, 4.4lbs per day), serves to create an 11cm thick layer of fat beneath the skin that insulates the polar bear’s body from the cold and also provides for an energy reserve in case food becomes scarce. From late April until mid-July, the polar bear’s experiences its largest annual caloric intake. This period is critical to the polar bear’s survival during the ice-free season when it fasts sometimes up to four months. This is possible because the polar bear can “switch from a normal metabolic state to a slowed-down hibernation-like condition in about 7-10 days at any time of the year when food is scarce.” [i]

The polar bear is well insulated such that it must move slowly as to avoid overheating. Other characteristics that help it stay warm are its fur and skin. The polar bear’s fur, which is water-repellent, is made of individual hairs that, given their structure, permit the channeling of solar energy directly to the polar bear’s skin. Here, the polar bear’s black skin allows for better absorption of the energy helping the polar bear stay warm. This has allowed the polar to live in the Arctic where temperatures typically fall to -22˚F (-30˚C) during winter.

For the rugged terrain, the polar bear has big paws that act as snowshoes preventing it from breaking through ice and snow.

Polar bears in the wild are found in the Arctic, in the United States, Russia, Greenland, Canada, and Norway. In Canada their range is limited to the provinces of Ontario, Quebec and Manitoba and the territories of Yukon, Northwest Territories and Nunavut while in the United States the polar bears are limited to Alaska.  See Figure. Prior to the 1960s the polar bears were thought to be of a single population and their distribution was explained by circumpolar migration (known as Perdersens’ theory ). This was later disproved when it was discovered that polar bears belong to several separate populations that stay generally in the same area from one year to another. Worldwide, the population is estimated to be around 22,000-27,000 individuals distributed in about 19 subpopulations; five of these subpopulations are in decline, while five are stable and there exists insufficient data to predict population trends for the other subpopulations. Dividing polar bears in such populations makes it easier from a management standpoint simplifying communications and jurisdictional issues.

As it stands, sixty percent of polar bears live in Canada while 3,000-5,000 of them live in Alaska. However, estimating polar bear populations is inherently difficult. This is especially true because they occur at “low densities in remote regions” and a “potential for bias [is therefore] introduced by heterogeneity in recapture probability (e.g. due to differences in distribution, behavior, sightability).”[ii] It is important to note that the southernmost range of the polar bear is in the Southern Hudson Bay and Akimiski Island on James Bay.

Polar bears are said to have “low reproductive rate” because they only become sexually mature at around five-years old and do not have many young. Also, females below 189kg cannot successfully reproduce. Generally, female polar bears give birth in January and around March/April emerge with their cubs from the maternal den. The young then accompany their mother for approximately 2.5 years before they go their own way. There are four age classes: COYs, yearlings, subadults (2-4 yr), and adults (5+yr).

The maternal dens are usually constructed in November when the pregnant females dig in deep snow drifts on land.[iii] Recent studies show a trend whereby bears are denning more inshore than in the early years of polar bear studies. Some have attributed this shift to the changing sea ice conditions due to climate change which have become less suitable for denning.

Polar bears are nomadic and have been known to swim more than 100 miles and walk an average of 5,500 miles a year, equivalent to 15 miles a day. These movements are generally correlated with the artic ice melting and freezing cycles. Generally, in summer when the artic ice cap melts, some polar bears may follow the retreating ice north while others spend their summers on land. In the fall, when the ice freezes polar bears on land resume their life on the sea ice while those that ventured north descend.



[i] Stefan Norris, Lynn Rosentrater and Pal Martin Eid, Polar Bears at Risk: A WWF Status Report, World Wildlife Federation, May 2002, http:///assets.panda.org/downloads/polarbearsatrisk.pdf at 8.

[ii] Eric V. Regehr, Steven C. Amstrup & Ian Stirling, Polar Bear Population Status in the Southern Beaufort Sea, Open-File Report 2006-1337, U.S. Geological Survey, http://pubs.usgs.gov/of/2006/1337/pdf/ofr20061337.pdf at 12.

[iii] World Wildlife Federation, The family life of polar bears, www.panda.org (search “polar bear tracker;” then follow “family” hyperlink) (last visited Sept. 25, 2007).

 

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