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Why Do Some Animals Hibernate?

Certain animals hibernate because food supplies become scarce during the winter months. By going into a long deep sleep, they bypass this period completely, waking up when food becomes more plentiful.

Bears are most commonly associated with hibernation (although they are not considered true hibernators, because their body temperature doesn’t lower significantly), but a variety of animals take advantage of this evolved adaptation, including the jumping mouse, little brown bat, eastern chipmunk, woodchuck, and certain species of ground squirrels. At least one bird is known to be a hibernator—the poorwill, which lives in western North America.

It is believed that a compound in the blood of hibernators known as HIT (Hibernation Induction Trigger) lets animals know when it’s time to prepare for hibernation. Shorter days, diminishing food supplies, and colder temperatures all appear to influence HIT, though the exact mechanism remains a mystery.

Hibernators tend to eat a lot of extra food in the fall in anticipation of their winter slumber and build significant stores of white and brown body fat to tide them over. Brown fat provides extra body heat as well as needed energy when the animal wakes up. Some animals also store food in their dens to consume during brief periods of wakefulness.

Hibernation is nothing like a regular night’s sleep. In fact, significant physical changes occur within the body during the hibernation period. An animal’s body temperature plunges, and its respiration and heart rate slow dramatically. The effect can be so extreme that a hibernating animal may appear dead.

Several factors can influence hibernation. In regions where the winter is relatively mild, animals may hibernate for only brief periods or not at all. During periods of long hibernation, many animals will rouse briefly to go to the bathroom and eat a snack before falling asleep again. Certain species of bears are unique in that they do not awaken, eat, drink, or go to the bathroom during their hibernation period, which can last several months.