Hibernation. Polar bears do not overwinter in dens like brown and black bears. Only pregnant females enter dens to give birth where they stay until the cubs are big enough to trek.
Most polar bears remain active throughout the year. They have the ability to reduce their metabolic rate when food is scarce and adjust it again when food is abundant.
An example of this are the polar bears that come ashore after the ice melts in Hudson Bay each summer. These bears have no food source and enter a state scientists call walking hibernation. Polar bears in this state appear to maintain normal body temperature but are able to save energy by reducing their metabolic rate and recycling proteins.
One study of Hudson Bay polar bears that fed at a garbage dump during iceless months revealed that they did not enter a state of walking hibernation.
Denning. Pregnant polar bears den in the fall after feeding heavily in August and September. They choose den sites in snowdrifts along mountain slopes or hills near sea ice or in banks of snow on the frozen sea. Along Western Hudson Bay mother bears begin by denning in earthen dens along river banks and later move to snow dens.
To build her den, the female scrapes a tunnel into the snow and digs two chambers. She gives birth to one, two, or three cubs in November or December. Twins are most common.
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.
When the mother bear emerges from her den in March or April, she is in a state similar to that of a denning black bear. Her body temperature, however, is higher, ranging from 35º to 37º C compared with 31º to 35º C in a black bear.