Polar bears, unlike brown and black bears, do not hibernate. Expecting mothers spend the winter in dens, but the others stay out on the pack ice and do the same things they do in the summer - hunt, eat and sleep.

The bears' snow dens protect newborn cubs from the cold. The world's densest concentration of birth dens is on Wrangel Island, off the Siberian coast of Russia. About 500 dens have been mapped in that area.

Pregnant polar bears usually enter their dens around November and give birth to cubs about two months later. Newborns are 12-14 inches / 30-35 cm long and weigh little more than a 1/2 kilo / 1 pound when born. At about three months old, the cubs are able to leave the den.

For at least 20 months, the cubs depend on their mother's milk for survival, so mom's success at hunting seals directly influences their own well-being.

Female polar bears in the Low Arctic wean their cubs as they approach their second birthday, while those in the High Arctic, where conditions are more severe and demanding, care for their cubs an additional year.

The youngsters must develop their hunting skills quickly. An adult polar bear's only enemies are human hunters and, on rare occasions, other bears, so biologists believe that starvation is the leading cause of death for young bears.