The polar bear is the largest terrestrial predator in the Arctic, but it is formidable on land, on the ice surface, and in the water. It is closely related to the brown bear (Ursus arctos) and may actually be a subspecies of that bear. Anatomically, however, it is quite different than the brown bear, a result of its Arctic, semi-aquatic lifestyle. One of the polar bear's adaptations is the very large size of its feet, useful for walking across snow and for swimming. The only bear that grows to larger sizes is the kodiak bear, a subspecies of the brown bear. Adult male polar bears reach weights over 1500 pounds (700 kg).
Polar bears are aggressive predators, known for taking a variety of prey. They spend most of the year associated with Arctic sea ice, where they hunt ringed seal and bearded seal pups. Polar bears have an excellent sense of smell and can locate pups even when they are buried in snow dens. Polar bears can easily use their size and strength to break open dens and feed on the helpless pups. Adult males have also been known to eat large aquatic animals, like beluga whales, that they attack from the ice and pull onto the ice surface. When on land (as opposed to surface ice), polar bears scavenge for dead animal matter. This species has no natural predator. It is one of the few species that is even considered to be higher on the food web than humans.
Courtship and mating take place on the ice surface, but birth generally takes place on land. Females give birth to (typically) two cubs, which weigh no more than a few pounds (~one kilogram). Cubs nurse for at least two and a half years, and as in all bears, the mother polar bear is aggressively protective of her offspring. Polar bears reach sexual maturity between ages four and six.
Polar bear populations are decreasing throughout their geographic range, and some populations have been depleted significantly. Scientists now believe the species to be vulnerable to extinction. While hunting for this species does occur, climate change is the primary threat to the polar bear's existence. This bear is intimately tied to Arctic sea ice and to seals that require ice for reproduction, and that ice is expected to continue thinning and melting faster each summer in the face of climate change. Scientists are not sure if polar bears will be able to adapt to these rapidly changing conditions.