Polar bears possess a number of adaptations for the cold that keep them warm, including thick fur, thick fat and increased body size compared to other bear species. Due to these adaptations, polar bears are more likely to overheat than to experience hypothermia.
A polar bear's sheer size prevents loss of heat. Larger animals have less surface area compared to their volume to lose heat through. This is why toy dogs have a much harder time keeping warm than larger dogs. With weights of up to 1500 pounds, polar bears are the largest bears in the world.
The polar bear's thick fur is doubly adapted to frigid temperatures. The sheer thickness helps polar bears stay warm. The fur is also water repellent. This is extremely important, as polar bears are great swimmers. Water repellent fur coats prevent polar bears from experiencing heat loss from evaporation.
Polar bears have thick fat reserves covering their bodies. Fat is an excellent insulator and also provides the bears with the bursts of energy they need to hunt during lean times. A diet high in fat allows polar bears to generate large amounts of heat energy from the sheer number of calories they consume.
Another adaptation for heat retention is the small size of the bears' ears and tail. Other animals, such as fennec foxes and elephants, use their enormous ears as a way to release excess heat via the ears' large surface area. Compact ears and tails reduce surface area and also streamline the bear for swimming.
Polar bear hairs are hollow, making them more buoyant. However, the oft-repeated theory that hollow white hairs channel the sun's heat to the skin is a myth, as proven by Daniel V. Koon's 1998 paper, "Is Polar Bear Fur Fiber Optic?".