Hair of polar bear insulates it from cold because it has low emissivity in infrared.
"Polar bears are masters at conserving energy. During their eight-month fast, breeding females can lose as much as 45 percent of their weight. So heat retention is important, to avoid spending too much energy to keep warm. The bears' fur is dense, made of clear hairs that scatter light, creating a white effect. Underneath the fur lies black skin that absorbs the sun's rays…polar bears are nearly invisible in the far infrared, the frequency range in which bodies radiate heat. Researchers from Berkeley's Department of Mechanical Engineering, led by professors Boris Rubinsky and Ralph Greif, found that the conventional explanation, that the bears are so well-insulated that their surfaces are the same temperature as the snow, is correct but incomplete: the hair's emissivity in the infrared is also nearly equal to that of snow and that this low emissivity could help to insulate the bears by lowering the amount of infrared heat that the bears radiate away." (Courtesy of the Biomimicry Guild)
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