What is Hibernation?

The warmer months of the year are busy for animals; a time to feed and breed. Wildlife seems to be everywhere, even in urban and suburban areas. But as the days grow shorter and the temperature drops, the insects, birds and mammals that seemed to be everywhere are suddenly gone. Where'd they go, and why'd they leave?

Resources Are Scarce in Winter

Winter is a time when resources are scarce. Snow on the ground and freezing temperatures make it challenging for creatures to find enough food and usable water. During winter warm bodied endotherms (animals that regulate their own body temperature) also have to use valuable calories, just to keep from freezing.

Animals' Winter Survival Strategies

Many animals that can fly, or, otherwise travel large distances, migrate to warmer climates. Birds fly toward the equator; even reindeer make the journey from really cold to not-quite-as-cold territories. Migration is a survival strategy, but is certainly not the only option. Some animals are instead able to reduce their needs and stay put when resources are scarce, entering into something like a state of suspended animation. They hibernate.

What Happens During Hibernation?

Hibernation generally means that, during the cold months of winter, the animal goes dormant in a way that resembles deep sleep. But hibernation is more than sleep; it is a type of dormancy where metabolism slows down and body temperature drops, so that far less energy is required than if the animal was active.

Animals that hibernate must first build up their body fat reserves; compiling surplus energy that can be used to fuel the body while the animal sleeps. Bears, for example, can lose 15 - 30% of their body weight during hibernation.

Hibernation is not the same for all animals. Different species that do go dormant during winter may be unresponsive for months at a time (like bears) or wake frequently to snack between naps (like squirrels and chipmunks).

Other Types of Animals Dormancy

Squirrels and chipmunks, mentioned above, don't officially hibernate. Their sporadic state of dormancy is known as torpor. During torpor, metabolism slows and body temperature drops, but the animal is only dormant for relatively short periods of time.

Animals go dormant for reasons other than avoiding extreme cold. Estivation, also known as aestivation, is the term that applies to animals that go dormant to avoid high temperatures and drought. Many reptiles do this, and during estivation, may use only 5% of the energy that they would otherwise expend if active.

Many insects can also go dormant when resources become scarce. This type of dormancy is known as diapause. During diapause, the insect stops growing, uses very little energy and essentially waits out the bad times.

To learn more about hibernation and other forms of animal dormancy, see ThinkQuest's pages on The Deep Sleep or the How Stuff Works article How Hibernation Works.