Hibernating animals pass the winter in a state of low biological activity. Their body temperatures drop, and metabolism slows dramatically. In deep hibernation, the condition is called torpor, but hibernating animals do not remain in a state of torpor for the entire winter. At intervals, and for reasons not fully understood, they warm up to normal body temperatures before returning again to torpor.
In the past, many scientists did not consider bears to be true hibernators because their body temperatures remain relatively high during their times of dormancy. Other metabolic changes associated with hibernation do occur, however. For example, a bear's metabolic rate reduces by 50 to 60%, its respiration drops to about three breaths every two minutes, and the animal's heart rate drops to less than twenty beats per minute (less than half of the usual summer heart rate). For these reasons, many biologists today do consider bears to be hibernators.
Hibernation helps animals survive. By entering the deep sleep of hibernation, animals are able to conserve energy because they experience reduced metabolic needs. By stretching how long stores of body fat last, hibernating animals can live through long periods of extreme weather during which food is scarce.
Examples of hibernators include some warm-blooded mammals such as bears, chipmonks, ground squirrels, woodchucks, prairie dogs, and several kinds of bats. A type of bird, the common poorwill (a western nightjar) is also known to hibernate. In cold-blooded animals, such as reptiles and amphibians, a dormant state similar to hibernation is called brumation.
Another type of inactivity is called aestivation (also sometimes spelled estivation). Aestivation involves a period of dormancy during the summer. It serves as a means to survive periods of extreme heat and dryness. Examples of aestivators include snails and amphibians.
The original version of this article, along with a list of additional resources for more information, appeared in the January 2016 issue of Observations: A Pier Press® Newsletter. You can access the archived issue here, and if you're not already a subscriber, click here to sign up. It's free.