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The main purpose of hibernation is probably to reduce the amount of energy that animals use up during periods when there is little food available. Bears are superbly adapted to survive winter food shortages. Mortality of bears during winter is much lower than that of many other animals. According to scientific research, the common assertion that bears are not true hibernators is incorrect. Whilst it is true that they do not lower their body temperature as much as "deep sleepers" such as rodents, their breathing rate and heart beat slow greatly. The fact that their body temperature remains relatively high can be an advantage because, if threatened, they can wake quickly and have a chance to escape or defend themselves. Unlike "deep sleepers", which must become active every few days, bears are the only mammals which can sleep for weeks or even months without eating, drinking or defecating.

Photograph: S. Ondruš

To achieve this feat, bears burn the reserves of fat they accumulated by intensive feeding during summer and autumn. This yields almost twice as much energy as protein or carbohydrates. The by-products are carbon dioxide, which the bear breathes out, and water, which can be used in other metabolic processes. Another by-product is glycerine, which is combined with waste nitrogen from protein metabolism to rebuild amino acids for the bear to use. This prevents urea from building up to poisonous levels. Bears can thus not only avoid having to urinate, and hence can remain inactive, but also their rate of protein metabolism remains high during hibernation and so they do not lose any muscle mass or otherwise weaken.

When other mammals, including humans, are inactive for very long periods, they lose calcium from their bones and eventually risk osteoporosis. This does not happen to bears. Hibernating bears have very high levels of blood cholesterol and yet they do not get gallstones or show signs of vascular disease. Bears also somehow avoid the potentially dangerous build-up of toxic by-products from fat metabolism, even while living purely from their body fat.

Studying how bears do things has already led to treatments to help people with gallstones and kidney disease. Our understanding of bear hibernation is still very incomplete. Unlocking its secrets could have benefits to people ranging from further advances in health care to space travel. For more information about this fascinating and important subject, see:-

"A bear in its lair" by Lynn L. Rogers
"Bear hibernation" by Gerald Eugene Smith