POLAR BEAR'S SECRET
J. F. L. MD
Polar bears can remain active throughout the year in air temperatures that range from +25°C to -50°C. Eskimos have long known that polar bear fur is both warm and hard-wearing, but exactly how it is so effective has recently been the subject of fierce debate. The outer hairs are straight and tough: over most of the body, they stand erect like a brush over the dense, woolly underfur. Snow does not melt on the outer hairs although the skin feels warm to the touch—cold hands soon warm up if you bury them in the bear's fur. The long, straight outer hairs of polar bear pelt are hollow—as are those of deer—and their white colour arises from light scattering within the hair shaft, not from the presence of a pigment. Much of the visible and infrared light from the Sun is transmitted through the fur and is absorbed by the skin, which is black. All black bodies radiate heat when warm, and polar bear skin is no exception, but the heat does not escape because the white hairs reabsorb it. This recycled warmth heats up the thick layer of air trapped around the wooly underfur, which insulates the body from cooling by conduction and convection of heat to the cold wind around it. Thus on sunny days, the long outer hair enable bears to use solar radiation as well as body heat for warming their underfur. The mechanism is so effective that sometimes the skin is warmer than the middle of the body, even when the air temperature is well below zero. Resting or slowly moving bears lose so little heat across their surface that they are difficult to locate using infrared thermal imaging apparatus. Indeed, from a distance, the only detectable heat is a little patch in front of the nose—the bear's expired breath.
