Studies have shown that darted and tagged female bears consistently produce smaller litters and lighter cubs. If tagged in the den area, many pregnant females abandon the site. The world's densest concentration of polar bear birth dens lies off the Siberian coast of Wrangel Island. About 500 have been mapped. Females in the Low Arctic wean their cubs as they approach their second birthday, while those in the High Arctic, where conditions are more demanding, care for their cubs an additional year.
Biologists believe that starvation is the leading cause of death for subadult bears.
Those polar bears that manage to survive to adulthood have learned to master the challenges of arctic life. An adult polar bear's only enemies are human hunters and, on rare occasions, other bears. In the wild, adult polar bears live an average of 15 to 18 years, though biologists have tagged a few bears in their early 30s. In zoos, many captive bears live until their mid- to late 30s. One individual in London lived to the ripe old age of 41.