Why don't polar bears freeze?

In the early Eighties, clever scientists thought they had found the ultimate answer to the question of why polar bears appear to be immune to the harsh climate and the incredible cold in the Arctic: polar bears' hairs act like fibre glass cables and transport the ultraviolet (UV) light of the sun directly to the animal's black skin, where it is converted into useful heat.

A claim that was very soon propagated as a scientific fact, even though it was only based on a few observations: to the human eye, polar bears' hairs seem to be transparent; minute bubbles of air are arranged in a row inside them like pearls on a chain. In addition to which, polar bears' fur absorbs some of the UV light that hits it rather than reflecting it all.

The scientist Daniel W. Koon from St. Lawrence University in the US state of New York finally decided he wanted to know for sure and was probably the first research scientist ever to investigate the light conductivity of polar bears' hairs - with the result that the theory was disproved like a chewing gum bubble bursting.

Koon tried to conduct the spectrum of white light along the longitudinal axis through polar bears' hairs three centimetres long - with no great success: the hairs allowed less than 0.001 per cent of the red light and less than one billionth of the violet light through. The results with UV light were even worse; Koon suspects that this is probably because it is absorbed by the horny substance in the hairs.

Professor Stephan Steinlechner from Hanover Veterinary University in Germany is convinced that there is only one explanation why polar bears do not freeze - and that is the classic one: polar bear's black skin acts like a light or heat trap; the UV light absorbed by the bears' fur could intensify this effect.

The hollow hairs, on the other hand, have solely an insulating effect. They reduce the amount of heat bears lose and help to make sure in this way that they do not cool down too quickly. Steinlechner: "Polar bears' hairs basically have the same function as double-glazed windows." (GJD)