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. Female polar bears have been known to adopt other cubs. Multiple cases of adoption of wild cubs have been confirmed by genetic testing. Adult bears of either sex occasionally kill and eat polar bear cubs. As of 2006, in Alaska, 42% of cubs were reaching 12 months of age, down from 65% in 1991. In most areas, cubs are weaned at two and a half years of age, when the mother chases them away or abandons them. The Western Hudson Bay subpopulation is unusual in that its female polar bears sometimes wean their cubs at only one and a half years. This was the case for 40% of cubs there in the early 1980s; however by the 1990s, fewer than 20% of cubs were weaned this young. After the mother leaves, sibling cubs sometimes travel and share food together for weeks or months. Later life Females begin to breed at the age of four years in most areas, and five years in the area of the Beaufort Sea. Males usually reach sexual maturity at six years; however, as competition for females is fierce, many do not breed until the age of eight or ten. A study in Hudson Bay indicated that both the reproductive success and the maternal weight of females peaked in their mid-teens. Maternal success appeared to decline after this point, possibly because of an age-related impairment in the ability to store the fat necessary to rear cubs. Polar bears appear to be less affected by infectious diseases and parasites than most terrestrial mammals. Polar bears are especially susceptible to Trichinella, a parasitic