Polar bears have black skin and clear, hollow fur.

Polar bear fur is made of guard hairs and a dense undercoat. Each guard hair is a hollow tube that lacks pigment. These tubes scatter light, which makes the bear look white or yellow to the human eye. This effect is called structural coloration.Under the fur, the skin is dark because it contains a lot of melanin. This black surface absorbs almost all the heat from the sun. A layer of fat called blubber sits under the skin. This fat can be up to 11.4 centimeters thick.This insulation is so good that polar bears are hard to see with infrared cameras. These cameras find animals by detecting the heat they give off. Polar bears trap so much heat that very little leaks out of their bodies. This helps them stay warm in temperatures as low as -40 degrees Celsius.Because they are so well-insulated, polar bears can overheat when they run. They often move slowly to keep their body temperature steady. The hollow hairs also help the bear dry off quickly. After a swim, they can shake away water and ice easily.