Polar bears are usually born in a maternity den between November and February and spend the next 2.5 years with their mother; by the age of 5 years, they are fully mature and ready to mate. Adult polar bears usually live to the ages of 15 to 18 years, but some polar bears have lived into their 30s.
Polar bears mate on the ice during the months of April and May when the female bear enters estrus for about 3 weeks. During this mating season a group of male polar bears may follow a female around, fighting with each other for the right to mate with her. Once a female polar bear is pregnant, the fertilized egg divides several times before suspending growth until August or September. During the summer months, the female will put on as much weight as possible to support herself and her offspring in winter.
In October, the female polar bear digs a maternity den in the snow and remains there for the winter. She will give birth to between one and four cubs in December or January. The family will remain in the den until about April, when the female leaves with her cubs to start hunting for food. The cubs stay with the mother for 2.5 years, suckling and learning to eat fat and hunt. At this point the cubs are half-grown and may stay with each other for a short period of time. Eventually, they separate and go off on their own. Females are fully mature by 5 years of age and males between 8 to 10 years of age. Polar bears usually have a new litter every 3 to 4 years.