The polar bear (Ursus maritimus) is the largest of the extant bear species and is classified as a marine mammal. Polar bears are protected under the Endangered Species Act and Marine Mammal Protection Act. The Marine Mammal Protection Act prohibits the take, which is defined as harass, hunt, capture, or kill, of all marine mammals, with an exception in place for harvest by Alaska Native peoples.
The global polar bear population is estimated to be 26,000 individuals as of 2023. Largely dependent on sea ice, their native range lies throughout the circumpolar Arctic, primarily above the Arctic Circle, which is one of the largest ranges for an extant large carnivore. Polar bears occur in 19 subpopulations throughout the seasonally and permanently ice-covered marine waters of Arctic and Subarctic regions of Canada, Greenland, Norway, Russia and the United States. Population status varies for each subpopulation. The U.S. contains portions of two subpopulations: the Chukchi Sea and the Southern Beaufort Sea.
Genetic research has confirmed that polar bears evolved from brown bears (U. arctos) roughly 500 thousand years ago. Polar bears and brown bears overlap in regions of northern Canada and Alaska, as well as eastern Russia.
Polar bears are top predators in the Arctic marine ecosystem. Polar bears prey heavily on ringed seals (Phoca hispida) and, to a lesser extent, bearded seals (Erignathus barbatus). They occasionally take larger animals such as walruses (Odobenus rosmarus) and belugas (Delphinapterus leucas). The remains of bowhead whales (Balaena mysticetus) from subsistence harvest, strandings or orca (Orcinus orca) predation have been an important and seasonally reliable food source in some regions, including the Southern Beaufort Sea, Svalbard and Chukotka, Russia. In general, polar bears have a similar dependence upon sea ice habitats, rely upon similar prey and exhibit similar life history characteristics throughout their range.
Polar bears are a K-selected or slow species, characterized by late sexual maturity, small litter sizes and extended parental investment in raising young. All these factors contribute to the low reproductive rate of the species. Reproduction in the female polar bear is similar to that in other Ursids, or bears. Females generally mature and breed for the first time at 4 to 5 years and give birth at 5 to 6 years of age. Litter size typically ranges between one to three cubs, with litters of two cubs being most common. Cubs typically remain with their mother until they are 2 years old. Consequently, the minimum reproductive interval for adult females with a successful litter is 3 years. Most males reach sexual maturity around 6 years of age, although some may breed as young as 3 or 4 years old.
Denning is a critical period in polar bear life history. Females enter dens between September and December. A successful denning period requires adequate thermal protection, time for cub maturation, a lack of natural or human disturbance and security from predation. Most black (U. americanus) and brown bears of all sex and age classes spend several winter months inside a den in a state of metabolic dormancy to conserve energy. However, only pregnant female polar bears overwinter in dens.
