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What Happens Inside an Animal When It's Hibernating?

During the winter at higher latitudes, various birds and mammals such as bears go into hibernation, an incredible adaptation that helps them survive this annual period of scarce food and bitter cold. By so doing, a hibernating animal conserves a considerable amount of energy that would otherwise have been required to stay active. Although hibernation may sound simple, it actually involves a complex series of changes inside the animal's body.

Temperature
- Ordinarily, warm-blooded animals or endotherms maintain a fairly constant internal temperature. During hibernation, however, they allow their body temperatures to fall to much lower levels -- in some cases as low as a couple degrees above freezing. Even at these low temperatures, their cardiovascular, respiratory and central nervous systems continue to function, albeit at greatly decreased rates. Many other functions, however, are temporarily suppressed or inactive. The animal's metabolic rate may be as much as a few hundred times lower than normal metabolic rates.
Suppression
- Hibernation suppresses the activity of the immune system and the renal system (the kidneys), although both become reactivated during periodic arousals when the animal briefly reawakens. At the cellular level, the rate of transcription and translation decreases dramatically, as does DNA synthesis and cell division. Shivering and other automatic responses to colder temperatures are suppressed, since the animal is now allowing its body temperature to fall rather than trying to keep it constant.
Fats
- Before hibernation, animals store up on extra food, putting on weight in the form of fat. This fat later becomes fuel during hibernation. Consequently, while metabolism of carbohydrates is dramatically reduced, metabolism of fats becomes the main source of energy. This shift appears to be associated with increased activity for two genes. Much of the fat the animal consumes is used up during the brief arousals. Ultimately, hibernating animals lose a substantial amount of their body weight during the winter months -- even though they use far less energy than they would if they remained constantly active.
Ongoing Research
- Levels of insulin in the animal's bloodstream decrease markedly until late in the hibernation season, when they start to increase preparatory to reawakening in spring. Aside from the two genes that appear to be associated with the switch from carbohydrate to fat metabolism, the activity of various other genes also increases. Scientists are still working to understand the molecular mechanisms that enable animals to survive under these conditions -- body temperatures at which other nonhibernating animals would die.

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