Polar bears have two layers of fat and fur to keep warm in the cold climate. They can grow up to 3 meters long and weigh 350 kilograms. Polar bears are carnivores who primarily prey on seals but also eat other small polar bears, whales, sea birds, fish, and caribou. In the summer they may also eat berries and plants, and in winter they have been known to eat mice, rabbits, and bird eggs. Polar bears have few natural predators besides other polar bears and humans, and baby cubs are also at risk from other polar bears.