Fur. Polar bears' fur consists of a dense, insulating undercoat topped by guard hairs of various lengths. It is not actually white—it just looks that way.
Each hair shaft is pigment-free and transparent with a hollow core that scatters and reflects visible light, much like what happens with ice and snow.
Polar bears look whitest when they are clean and in sunlight, especially just after the molt period, which usually begins in spring and is complete by late summer. Before molting, oils from the seals they eat can make them look yellow.
In zoos, polar bears have been known to turn green due to colonies of algae growing in their hollow hair shafts. This happened at the San Diego Zoo in 1979. No harm came to the bears, and zoo veterinarian Phillip Robinson restored the bears to white by killing the algae with a salt solution.
Skin. Polar bears have black skin under which there is a layer of fat that can measure 4.5 inches (11.5 centimeters) thick.
- On land (or on top of the sea ice) the polar bear's thick fur coat—not its fat—prevents nearly any heat loss. In fact, adult males quickly overheat when they run.
- In the water, polar bears rely on their fat layer to keep warm: wet fur is a poor insulator. This is why mother bears are so reluctant to swim with young cubs in the spring: the cubs don't have enough fat. As long as they don't have to go into water, dry fur keeps little cubs warm at very cold spring temperatures, including -30°F.
Scientific Finding. Scientists used to think that polar bears' hollow hairs acted like fiber optic tubes and conducted light to their black skin. In 1988, Daniel W. Koon, a physicist at St. Lawrence University in Canton, New York, and graduate assistant, Reid Hutchins, proved this false.
Their experiments showed that a one-fifth inch strand of polar bear hair conducted less than a thousandth of a percent of applied ultraviolet light. So, the black skin absorbs very little ultraviolet light. Instead, Koon believes keratin, a basic component of the hair, absorbs the ultraviolet light.