Female polar bears reach sexual maturity around four or five years of age. Mating season occurs in the spring, but embryos do not begin to develop until the female enters her den. Polar bears give birth to one to three cubs during the winter;twins are the most common. Newborn cubs are 12 to 14 inches long and weigh a little more than a pound. In the spring, the mother bear and her cubs will emerge from their den. Mother bears are very protective of their cubs, and sometimes have to protect them from male polar bears who are known to kill young of their species. Polar bear cubs grow rapidly and will stay with their mother for a little over two years learning survival skills.