At the risk of flogging a dead, er, penguin, why don鈥檛 鈥� feet freeze?
鈥� Unlike the penguin with its fancy internal plumbing, the reason that polar bears鈥� feet do not freeze is good insulation, pure and simple.
Polar bears (Ursus maritimus) are just about the best-insulated animals on the planet, certainly among those species of mammal that do not live primarily immersed in water. An adult bear has 10 centimetres of blubber beneath its skin which, in turn, is covered by a thick coat of fur. This fur relies not only on its density, but also on its unique structure: each hair is a hollow tube, so that air is trapped inside the hairs as well as between them. Even without covering its nose with its paws (as it is reputed to do, although the evidence is very limited) a polar bear is almost invisible to heat-sensitive infrared photography or the latest military image-intensification technology.
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The polar bear also has very hairy pads on its feet, and the tough skin is extremely callused on the underside of the paws, so there is a sturdy layer of dead tissue between the ice and any blood vessels.
There may also be another factor at work. The underside of a polar bear鈥檚 paw is dotted with dozens of papillae 鈥� small nipple-shaped extrusions of even-more callused skin 鈥� which provide extra grip in the same way as the studs on a footballer鈥檚 boot. It is these papillae that enable a polar bear to accelerate to a very respectable pace on the ice and overcome its awesome inertia. They also prevent it skating out of control, past a potential meal.
鈥淎 polar bear鈥檚 paw is dotted with dozens of papillae, which provide extra grip like footballers鈥� studs鈥�
On really compacted ice, the bears tend to lift part of the underside of the paw clear of the surface. The papillae enable an additional cushion of insulating air to be trapped between most of the pad and the ice.
Such highly developed thermal adaptations can, however, be a double-edged sword. A bear attempting a brisk trot in ambient temperatures of 10 掳C or greater would succumb, almost immediately, to a fatal attack of heat stroke. During the Arctic summer it can often be far hotter than that, limiting the polar bear鈥檚 ability to function as a hunter.
鈥淎 bear trotting in temperatures around 10 掳C would succumb to a fatal attack of heat stroke鈥�
This potential cramping of the polar bear鈥檚 style may prove as fatal to the species鈥� chances of survival as the actual destruction of its territory. If global warming causes the polar bear to die out, it would surely be the most terrible irony that this was because it had mastered the art of conserving the very energy that a profligate humanity has squandered so obscenely.
Hadrian Jeffs, Norwich, Norfolk, UK