Polar Bears Are Not Actually White — They're Transparent

Anyone painting a polar bear reaches for the tube of white, and anyone admiring one praises its bright white coat. Both are slightly off the mark. The San Diego Zoo Wildlife Alliance describes the hairs as translucent, with no pigment at all. There is no white colouring in the hair. The coat looks white for much the same reason snow does: plenty of clear material scattering light back towards your eyes.

The coat has two layers. According to the zoo’s fact sheet, thick underfur about 5 centimetres long sits beneath guard hairs about 15 centimetres long, and the guard hairs are hollow. So the white bear is really wearing a thick layer of clear, air-filled tubes over a woolly base.

Underneath is the real surprise. The skin, including the nose and lips, is black and absorbs heat. The colour of the coat can shift, too. The zoo notes that the coat is whitest just after the moult and turns yellowish in summer, from oxidation by the sun or oils from the bears’ prey. A bear that looks cream-coloured in a photograph is not necessarily dirty; it may simply be due a moult.

For years a much more dramatic story circulated: that the hollow hairs work like fibre-optic cables, channelling sunlight down to the black skin to warm it. The North American Bear Center describes how this idea was finally put to the test. Daniel Koon, a physics professor at St. Lawrence University, made the first actual measurements of light travelling along polar bear hair shafts. He found that less than 0.001 per cent of red light, and less than a trillionth of violet light, travelled an inch down a hair. Whatever the hairs are doing, they are not acting as fibre-optic cables.

The hairs do something else with light instead. The North American Bear Center points out a striking consequence of the hair absorbing ultraviolet light: in ultraviolet light, polar bears appear black.

So the colour of a polar bear depends on how you look at it. To the naked eye it is white, because clear hairs scatter light. Under the fur it is black. Photographed in ultraviolet, it is dark all over. And hair by hair, it has no colour at all. The animal has built one of the best winter coats on the planet out of air, keratin and light, and it needs no borrowed superpowers to be impressive. The painter, meanwhile, can keep the white tube. It is still the best way to paint what our eyes actually see.