Hibernation. With the exception of pregnant females, polar bears do not overwinter in dens like brown and black bears. Instead adult male polar bears and non-pregnant females remain active throughout the year.
Pregnant female polar bears dig a snow den, give birth, and emerge three months later. During this time, they live off their fat reserves. But they don't hibernate in the strict sense of the word.
True hibernators experience a marked drop in heart rate and body temperature. Mother bears do not enter a state of deep hibernation because they need a higher body temperature in order to meet the demands of pregnancy, birth, and nursing.
Walking hibernation? It was once thought that polar bears may reduce body condition losses when food deprived in summer by entering a hibernation-like state. If real, this “walking hibernation” response could help polar bears prolong survival even as global warming-induced sea ice declines progressively reduce foraging opportunities. Recent research, however, suggests that, although polar bears can conserve energy by minimizing activity level, their core body temperature remains well above levels of hibernation. This indicates lower limits on fasting than may have been possible if polar bears could enter a true hibernation like state while food deprived in summer. Polar bears, therefore, apparently have no special physiological escape from contiuing sea ice decline. As previously predicted, unabated climate warming and associated sea ice decline will cause declines in body condition of individual bears, and ultimately lead to population reductions throughout the polar bear’s current range.
Denning. Pregnant polar bears den in the fall after feeding in August and September. They choose den sites in snowdrifts along coastal and river bluffs, in hills near sea ice, or in banks of snow on the frozen sea. Along southern and western Hudson Bay, mother bears dig into raised peat soils found in palsa formations or along lakeshores and rivers.
To build her den, the female excavates a small snow cave—just large enough for her to turn around. She then waits for the snow to close the entrance tunnel. She gives birth to one, two, or three cubs in November or December. Twins are most common.
Polar bear families generally emerge from their den in March or April when the cubs are strong enough to survive outside and ready to make the trek to the sea ice.