The
polar bear
is a bear native to the Arctic Ocean and its surrounding seas. The
world's largest predator found on land, an adult male weighs around
400–680 kg (880–1,500 lb), while an adult female is about half that
size.
Although it is closely related to the brown bear,
it has evolved to occupy a narrow ecological niche, with many body
characteristics adapted for cold temperatures, for moving across snow,
ice, and open water, and for hunting the seals which make up most of its
diet. As it can hunt consistently only from sea ice, the polar bear
spends much of the year on the frozen sea, although most polar bears are
born on land.
The
polar bear is classified as a vulnerable species, with 5 of the 19
polar bear subpopulations in decline. For decades, unrestricted hunting
raised international concern for the future of the species; populations
have rebounded after controls and quotas began to take effect. For
thousands of years, the polar bear has been a key figure in the
material, spiritual, and cultural life of Arctic indigenous peoples, and
the hunting of polar bears remains important in their cultures.
The IUCN
now lists global warming as the most significant threat to the polar
bear, primarily because the melting of its sea ice habitat reduces its
ability to find sufficient food. The IUCN states, "If climatic trends
continue polar bears may become extirpated from most of their range
within 100 years." On May 14, 2008, the United States Department of the
Interior listed the polar bear as a threatened species under the
Endangered Species Act.
The
polar bear is often regarded as a marine mammal because it spends many
months of the year at sea. Its preferred habitat is the annual sea ice
covering the waters over the continental shelf and the Arctic
inter-island archipelagos. These areas, known as the "Arctic ring of
life", have relatively high biological productivity
in comparison to the deep waters of the high Arctic. The polar bear
tends to frequent areas where sea ice meets water, such as polynyas and
leads (temporary stretches of open water in Arctic ice), to hunt the
seals that make up most of its diet. Polar bears are therefore found
primarily along the perimeter of the polar ice pack, rather than in the
Polar Basin close to the North Pole where the density of seals is low.
Annual
ice contains areas of water that appear and disappear throughout the
year as the weather changes. Seals migrate in response to these changes,
and polar bears must follow their prey. In Hudson Bay, James Bay, and
some other areas, the ice melts completely each summer (an event often
referred to as "ice-floe breakup"), forcing polar bears to go onto land
and wait through the months until the next freeze-up. In the Chukchi and
Beaufort seas, polar bears retreat each summer to the ice further north
that remains frozen year-round.
