Many animals use varying degrees and lengths of dormancy to conserve energy and survive periods of limited food supply. Dormancy is a period of time when growth, development and (in animals) physical activity are stopped.
There are several types of dormancy. They include diapause, aestivation, brumation and hibernation.
Diapause is used often by insects that stop developing from fall until spring. For instance, they may overwinter as a larval (caterpillar) stage. When environmental conditions are improved, they continue to develop. Some mammals use diapause, too, to delay development of their young.
Aestivation is a form of dormancy used by animals that need to escape very hot and/or dry conditions. They become inactive in a place where they are cooler and/or can stay moist.
Brumation is a type of dormancy in reptiles. It starts in late fall and continues until the outside conditions are favorable for the reptile and its food source to be active. During the brumation period, reptiles wake often to drink water then return to their dormant state.
Other animals may become inactive for a part of every day to reduce their energy requirements. Ruby-throated hummingbirds (Archilochus colubris) and little brown bats (Myotis lucifugus) are examples of such animals.
True hibernation involves greatly reducing an animal’s bodily functions and activity. Body temperature drops dramatically, as do heart and breathing rates. From a normal body temperature of 95°F, the temperature of a hibernating mammal may be as low as 36°F. A normal heart rate of more than 100 beats per minute may drop to only four or five beats per minute, and breathing may slow to less than one breath per minute. True hibernation can only occur in animals that generate their body heat internally. They are able to maintain a fairly constant body temperature at all times. Birds, mammals and some fishes fit this definition, but fishes cannot hibernate, although some may become dormant.
Not all activity stops during hibernation, though. Scientists have discovered that even during hibernation there are periods of wakefulness that become more frequent as the hibernation period comes to an end. External temperature is a factor in these periods of sporadic activity. For each species there is a critical temperature above which they will waken, and all will waken temporarily if the temperature drops so low that they are in danger of freezing. Wakening allows mammals to move to a deeper, warmer chamber or to warm up a little—by shivering or moving around—until the temperature moderates.