Why Is a Polar Bear Not Actually White?

A polar bear appears white against Arctic snow, but its body contains very little white material. Beneath its coat, the bear has dark skin, while the individual hairs above it contain no white pigment.

Polar bear fur has two main layers. A dense undercoat traps air close to the body, while longer guard hairs protect it from water and wind. Each hair is largely transparent and free of pigment.

The coat looks white because visible light enters this complicated forest of hairs and scatters in many directions. Different wavelengths return toward the observer together, producing the appearance of white. Snow looks white through a related process, even though an individual ice crystal is transparent.

The effect changes with conditions. Polar bears may appear yellowish when oils and dirt collect on the fur. They can take on warmer colors in certain light, and bears living in artificial environments have occasionally appeared green when algae grew inside damaged or hollow portions of their hairs.

The dark skin beneath the coat contains melanin. It may help protect the bear from ultraviolet radiation. Dark surfaces can also absorb solar energy efficiently, although the importance of sunlight reaching the skin through the thick coat is often overstated.

The bear's main insulation comes from its thick fur and a substantial layer of body fat. Together they conserve heat so effectively that an adult polar bear can overheat during prolonged running.

Polar bear fur does not work like white paint covering a dark body. It works more like a dense transparent optical material, scattering the Arctic light until a black skinned animal nearly disappears into a white landscape.