Polar bears are the heftiest of the planet's existing terrestrial carnivores, though their size may be matched by an Alaskan subspecies of brown bear, the Kodiak bear. Exceptional polar-bear boars (males) may approach a ton in weight and exceed 12 feet in length. This is especially impressive when a cub’s typical weight—one or two pounds—is considered.
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(2) Sexual Dimorphism
Polar bears are dramatically sexually dimorphic, which means the genders exhibit obvious physical differences. Male polar bears are often twice–even three times–as heavy as females, which typically weigh 400 to 700 pounds (making them still larger than most other terrestrial carnivores). This is true of many large mammals.
Scientists conjecture that polar bears descended from brown bears, their closest modern-day genetic relatives, that may have begun venturing out onto polar sea-ice in search of prey. They are the most recently evolved of the bears, having potentially arisen 100,000 years or more ago.
(4) Most Carnivorous Bear
One basic fact about polar bears is that they are also the most carnivorous of bears. Most ursids (members of the bear family) are devoted omnivores, feeding as heavily on plant material as meat. Mainly due to the nature of their environment, however, polar bears are almost exclusively meat-eating, though they will occasionally eat vegetation if the opportunity arises.
(5) Major Prey
Their size, power and prowess allow polar bears to tackle a broad range of prey. They specialize in marine mammals, particularly seals, which they may ambush beside breathing holes. But they can also successfully hunt larger animals, including walrus and small whales stranded in ice leads.
(6) Form & Appearance
Compared with the stockier brown bears from which they likely evolved, polar bears—despite their enormous size—are sleek and streamlined in appearance, an adaptation for their semi-marine lifestyle. Their white coats (which may also appear somewhat yellowish) help them blend in against the blankness of their vast, snowy surroundings. Despite their fur color, polar bears actually have very dark skin underneath.
(7) Swimming Ability
Polar bears are sometimes classified as marine mammals–like seals, sea lions, dolphins or whales–because of their swimming ability and preferred habitat. They may cover broad areas of open water with powerful strokes, assisted by their broad, lightly webbed paws. As sea-ice extent shrinks, polar bears may be forced to swim longer and longer distances to access ice-covered hunting grounds.
Female polar bears bring forth their young in late winter, denning in snow caves. These may be situated on sea-ice or along riverbanks or bluff-sides in coastal areas like Alaska’s North Slope. Male bears and females without cubs generally don’t retreat to dens for long periods during the winter. Two or three cubs are the norm for a given mother.
(9) Distinct Populations
Another interesting fact about polar bears is that they're a circumpolar species–that is, their range roughly encompasses the whole of the Arctic Circle. Within this species distribution are sub-populations: The Alaska Department of Fish and Game notes some 19 such distinct groups scattered through the polar regions, making long-distance seasonal movements. Alaska itself supports at least two such populations that, respectively, also venture into the Canadian and Russian Arctic.
(10) Climate Change Effects
Polar bears are one of the more prominent species discussed in speculations about potential planetary warming. If average global temperatures rise to the extent many scientists believe, polar bears may find their range greatly reduced, as sea-ice extent shrinks dramatically. Grizzly bears in Alaska and Canada have been showing up more and more frequently in polar-bear territory on the sea-ice, suggesting they may be opportunistically expanding their range and potentially competing or interbreeding with their white-coated brethren.
(1) Alaska Dept. of Fish & Game – Wildlife Notebook Series (J. Lentfer): Polar Bear
(2) "Mammals of North America"; R.W. Kays, D.E. Wilson; 2002
(3) PBS Nature: Arctic Bears – How Grizzlies Evolved into Polar Bears
(4) "BBC News"; Polar Bear's Epic Nine Day Swim in Search of Sea Ice; E. Davies; Jan. 2011