Between November and February, cubs are born blind, covered with a light down fur, and weighing less than , but in captivity they might be delivered in the earlier months. The earliest recorded birth of polar bears in captivity was on 11 October 2011 in the Toronto Zoo. On average, each litter has two cubs. The family remains in the den until mid-February to mid-April, with the mother maintaining her fast while nursing her cubs on a fat-rich milk. By the time the mother breaks open the entrance to the den, her cubs weigh about . For about 12 to 15 days, the family spends time outside the den while remaining in its vicinity, the mother grazing on vegetation while the cubs become used to walking and playing. Then they begin the long walk from the denning area to the sea ice, where the mother can once again catch seals. Depending on the timing of ice-floe breakup in the fall, she may have fasted for up to eight months. During this time, cubs playfully imitate the mother's hunting methods in preparation for later life.