In the wild, polar bears normally live to be 20-30 years old. Both males and females mature at the age of four to five years. Females often give birth to their first litter of cubs when they reach maturity, while males do not usually start mating until they are between eight to ten years old. Mating takes place in April-May, but delayed implantation (the fertilized egg stops developing at an early stage) means that fetus development does not commence until September-October. During late autumn, females go into hibernation and remain there for approximately four months. At about the beginning of January, between one and three cubs are born (depending on the mother's age and condition).The newborn cubs are extremely small, weighing only about half a kilo, but they grow rapidly due to the high fat content of their mother's milk. The cubs stay with the mother until they are two-and-a-half years old. The cub infant mortality rate is high and can exceed 70%. Only about a third of cubs reach the age of two. On Svalbard, polar bears live mainly in areas where there is sea ice, and most of them are therefore found along the eastern coast and in the fjords in the north. The most important hibernation areas on Svalbard are located on the islands of Kongsøya, Svenskøya, Edgeøya, Nordaustlandet and Hopen. However, because the sea ice is retreating as a result of climate change, polar bears have stopped hibernating on Hopen. This is an indication of the fact that these hugely important hibernation areas are under direct threat from global warming. In 2006 the polar bear was categorized as being vulnerable (VU) on the IUCN’s (the World Conservation Union) Red List. Because of the polar bear’s specialization for living on the marine sea ice, it is particularly vulnerable to large-scale changes in its habitat. A low reproduction rate and a long generation time means that this species is poorly equipped for tackling rapid changes in its habitat like those currently being experienced in the Arctic. The area covered by sea ice has decreased considerably during the last fifty years. 2007 was a record year in this respect and the extent of the sea ice during the summer has not been lower since measurements started in 1979.
The chief threat to the polar bear is the loss of its sea ice habitat due to global warming. As suggested by its specific scientific name (Ursus maritimus), the polar bear is actually a marine mammal that spends far more time at sea than it does on land. It is on the Arctic ice that the polar bear makes its living, which is why global warming is such a serious threat to its well-being. In southern portions of their range, like Hudson Bay, Canada, there is no sea ice during the summer, and the polar bears must live on land until the Bay freezes in the fall, whereupon they can again hunt on the ice. While on land during the summer, these bears eat little or nothing. In just 20 years the ice-free period in Hudson Bay has increased by an average of 20 days, cutting short polar bears' seal hunting season by nearly three weeks. The ice is freezing later in the fall, but it is the earlier spring ice melt that