Polar bears range throughout the circumpolar north in areas where they can hunt seals at open leads. The five "polar bear nations" in which the bears are found include the U.S. (Alaska), Canada, Russia, Denmark (Greenland), and Norway. Polar bears do not live in the southern hemisphere.

Scientists predict that, if current warming trends continue in the Arctic, two-thirds of the world's polar bears could disappear by 2050. At the most recent meeting of the IUCN Polar Bear Specialist Group (held in Seattle in 2005), the world's leading polar bear scientists reported that of the 19 subpopulations of polar bears, five were declining, five were stable, two were increasing, and seven had insufficient data to make a determination. The group reclassified the polar bear as vulnerable on the IUCN World Conservation Union's "Red List of Threatened Species," noting that the species could become extinct due to sea ice changes. Individual countries with polar bears have reclassified the species as well. Citing to concerns about shrinking sea ice habitat, the U.S. Department of the Interior announced on May 14, 2008, that it is listing the polar bear as a Threatened Species under the Endangered Species Act. Canada and Russia both list the polar bear as "a species of concern." The major threat to the polar bear is shrinking sea ice habitat due to climate change. Other threats include pollution, poaching, and industrial disturbances. Hunting could become a threat if populations are not well managed.

Scientists estimate that there are between 20,000 to 25,000 polar bears.

Ursus maritimus or the "sea bear." Its closest relative is the brown bear.

Adult male polar bears measure 2.5 to 3 meters (8 to 10 feet) tall. They weigh 250 to 770 kilograms (550 to 1,700 pounds). Adult female bears are smaller. They measure 1.8 to 2.5 meters (6 to 8 feet) tall and weigh 90 to 320 kilograms (200 to 700 pounds). The largest polar bear ever recorded was a male weighing 2,209 pounds.

Polar bears are perfectly adapted to survive in the harsh conditions of the Arctic, where winter temperatures can plunge to -45° C (-50° F). Two layers of fur provide the bears with such good insulation that they experience almost no heat loss. In addition, they are protected with a layer of blubber that can measure 11.5 cm (4.5 inches) thick. Compact ears and a small tail also prevent heat loss.

Polar bears are so well protected from the cold that they have more problems with overheating than they do from the cold. Even in very cold weather, they quickly overheat when they try to run.

Other adaptations include small bumps called papillae that keep their feet from slipping on ice; strong, powerful claws that enable them to catch seals; and a nose powerful enough to detect prey that is miles away.

Seals are the polar bear's primary prey, particularly the ringed seal and, sometimes, the bearded seal. When hunting is good, polar bears will typically eat only the fat and leave the rest of the carcass for scavengers including arctic foxes, ravens, and younger bears.

Polar bears also sometimes kill and eat both walrus and beluga whales. They have been known to hunt short-legged reindeer and sometimes snack on other foods including birds, bird eggs, kelp, and beached whales. On Norway's Svalbard Islands, polar bears were once found feasting on a 350-year-old bowhead whale carcass that was uncovered by a retreating glacier.

Polar bears top the food chain in the Arctic. They help keep the balance of nature by preventing an overpopulation of seals.

In the wild, polar bears live an average of 15 to 18 years, although biologists have tagged a few bears in their early 30s. In captivity, they may live until their mid- to late 30s. One zoo bear in London lived to be 41.

There are usually two cubs in a litter. Female polar bears have their first set of cubs between the ages of four and eight (most frequently at age five or six). Polar bears have the one of the slowest reproductive rates of any mammal, with females typically producing five litters in their lifetime. (See more about cubs.)

Polar bear cubs are born in snow dens called maternity dens. In the late fall, a female polar bear will dig a den after feeding heavily in April or May. Most choose den sites in snowdrifts along mountain slopes or along hills near the sea ice. Some dig their dens in snow drifts out on the sea ice.

At birth, the cubs are 30 to 35 centimeters (12 to 14 inches) long and weigh little more than half a kilogram (about a pound). They are blind, toothless, and covered with short, soft fur. They are completely dependent on their mother for warmth and food.

The cubs are born in November or December and remain in the den until March or April. During that time, the mother does not eat, drink, or defecate. The cubs grow rapidly while they are in the den, thanks to the calories in their mother's rich milk, which has a fat content of roughly 31%.

Polar bears cubs normally stay with their mother until they are 2 1/2 years old, although some bears in the Hudson Bay area wean their young at age 1 1/2. During the time that the cubs are with their mother, they must learn how to hunt and survive in one of the Earth's harshest environments.

Polar bears do not hibernate in the true sense of the word. True hibernators experience a marked drop in heart rate and a body temperature that plunges to nearly 0° C (32° F). Polar bears do not enter a state of deep hibernation; instead they undergo "walking hibernation." Only pregnant female bears enter a den. They do so in the fall and give birth to their cubs in November or December. The bear family will remain in the den until March or April.

Scientists recognize nineteen distinct populations of polar bears, but no subspecies.

Only humans and, on rare occasions, other polar bears. Climate change is the biggest threat that the bears face.

Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Family: Ursidae
Genus: Ursus
Species: Ursus maritimus