Brown bears and black bears spend the winter in dens, living in a state of hibernation with reduced heart rates and body temperatures. Polar bears, on the other hand, do not actually hibernate, but female polar bears dig snow dens to give birth, emerging three months later. During this period, they live off their fat reserves, but they don’t achieve deep hibernation -- the mother polar bear needs to maintain a higher body temperature to cope with the demands of pregnancy, birth, and nursing.
After some serious eating in August and September, a pregnant polar bear will dig a small cave, just large enough so that she can turn around in it. Snow will ultimately clog the entrance, and the bear will give birth to up to three cubs in November or December.
Cold, hard facts about polar bears:
What about the males?