A female polar bear typically gives birth to twins, though singles and triplets have been recorded. At birth, a cub weighs only 1.3 pounds (about half a kilogram), but they grow very quickly. Cubs depend on their mothers for warmth and fattening milk, which is 36 percent fat, according to the San Diego Zoo. By spring, the cubs are outside the den, exploring, and at two years of age they are fully mature. Polar bears live around 15 to 20 years.