Hibernation

Hibernation is a special type of deep sleep that enables an animal to survive the extreme winter cold. Hibernation lowers an animal's energy needs and allows it to live off stored fat and not have to search for scarce food. Hibernation is a form of cyclic behavior and is triggered by different cues in the animal's environment.

All animals have different survival tactics that allow them to live through difficult or life-threatening situations. The steady, severe cold that comes with winter poses a problem to animals who do not escape it (by migrating or leaving) nor adapt to it (such as by growing an extra-thick coat of fur or a layer of fat). Winter is difficult for all warm-blooded animals (those that maintain a constant internal temperature despite their environment), since they must spend most of their energy just keeping warm. When the temperature falls below freezing, these animals must eat even more than they usually do simply to produce enough internal heat to stay alive. Nature makes things even more difficult in winter, since at a time when warm-blooded animals need to increase their intake of food, it has suddenly become very scarce.

For certain animals, hibernation is a simple way to solve the particular problems posed by winter cold, since they basically sleep through winter and wake when the weather has become mild. However, hibernation is a fairly complicated physiological event. When an animal hibernates, its body processes such as breathing and heartbeat slow down, sometimes to the point that the animal appears dead. While hibernation could be described simply as a very deep sleep, it is anything but simple. True hibernators are usually small mammals like woodchucks, mice, or ground squirrels. When the time comes to hibernate, the animal responds to one of several environmental "cues," such as a certain low temperature or a reduction in the hours of daylight. These cues trigger the release of a hormone (a chemical messenger) called hibernation induction trigger (HIT) that causes major changes in the animal's body. Its heartbeat becomes slow and weak and its body temperature drops many degrees. It takes a few breaths every minute and it makes hardly any waste (urine). As a true hibernator, the animal falls into such a deep sleep that it looks dead and sometimes cannot even be awakened if picked up. All of these reactions triggered by the hormone allow the animal to maintain its necessary body processes while using far less energy than if it were awake.

Before they fall asleep for the season, hibernators usually develop huge appetites that allow them to store as much fat as possible to be burned later while sleeping. They also usually prepare the den or burrow where they sleep to comfortably insulate themselves from the cold. As spring approaches, different cues in their environment, like warmer temperatures or lengthening daylight, awaken them and they soon resume their normal level of activity. This does not happen immediately, however. As their heart rate increases, along with their blood pressure and respiration, hibernators usually begin to shiver, which slowly raises their body temperature. After readjusting to this now-high rate of metabolism (the chemical processes that take place in an organism), they are ready for normal activity.

Like their warm-blooded counterparts, cold-blooded animals (whose temperature changes with the surroundings) also hibernate. Animals like frogs, turtles, and snakes bury themselves in the mud where their slowed-down systems find just enough trapped oxygen to stay alive. Some insects like butterflies also hibernate in the open, and their systems produce chemicals that act as antifreeze.

Other animals like bears, raccoons, and skunks are not true hibernators, although they do very much the same thing. Rather, they are considered "light sleepers" since their bodily functions do not fall to such low rates as those of true hibernators. These "light sleepers" also sometimes wake up and eat something that they may have stored or even go outside to eliminate waste. Brown bears can awaken very quickly, and often give birth during a long, cold winter. Even true hibernators have built-in mechanisms to awaken them under unusual conditions. For example, if temperatures fall to such a low that even the hibernating animal is in danger of freezing to death, its body will automatically switch to actively producing heat.

Another form of hibernation occurs in the summer. This is called "estivation," and is a process similar to hibernation. Certain desert animals estivate underground when they are threatened by prolonged, extreme heat or drought. Estivation is what enables many desert dwellers to survive during the very hot summer months.

[See alsoMetabolism ]