“Hibernation” is defined as a state of inactivity over an extended period. By this definition, the term can be applied to a myocardium (heart muscle) that has become dysfunctional, or even a computer that is “sleeping” but not completely turned off.
The definition as applied to animals, however, should be referable to physiological response to environmental change. Most people are aware that when an animal sleeps (the response) through winter (the environmental change), it is, in effect, hibernating. Physiologically, “sleep” is a state of reduced activity of muscle and nerve function accompanied by reduced cell metabolism; that is, reduced chemical reactions in the body cells.
Specifically, hibernation is sleep and also:
- a response to weather conditions
- accompanied by a substantial decline in body temperature; and
- generally measured in months or seasonality rather than hours.
By these criteria, human sleep is clearly not hibernation; nor is a bear (despite popular belief) that “sleeps” through winter, hibernating because its body temperature is decreased only slightly (6°C). Many true hibernators can depress their body temperature by more than 30°C.
Hibernation Versus Aestivation
When hibernation is induced by events other than winter, the term “aestivation” is commonly applied instead. Aestivation, however, can be shorter than winter hibernation because environmental conditions (dry and/or hot spells) that bring about the physiological state are often less predictable. Snails aestivate during the hot hours of the day, but the lungfish may aestivate for years buried in dried-up lakes.
Both “hibernation” and “aestivation” can be interchangeable with “dormancy”, which is inactivity over a long period of time. When a snake is hibernating, it is basically in a state of dormancy. Dormancy in insects that hibernate through winter, during which their growth and development are suspended, is termed “diapause”.
Ectotherms and Endotherms
Snails, lungfish, insects, snakes (which are reptiles), and frogs (amphibians) are ectotherms, meaning their body temperature increases when the environmental temperature is high but decreases when cold outside. When the external temperature is low for a long period, an ectotherm could become inactive for the duration of that (that is, hibernate) because low body temperatures decrease the body’s enzyme activity, resulting in minimal muscle movement.
Mammals and birds are endotherms that can maintain a constantly high body temperature despite changes in the surrounding temperature. A healthy human, for instance, maintains a body temperature of 37°C regardless of whether it is hot or cold outside. Endotherms can do this because metabolizing cells in their body produce a sufficient amount of heat. Cell metabolism in ectotherms does not produce enough heat for body-temperature maintenance.
Torpor and Adaptive Hypothermia
“Torpor” occurs when body temperature, metabolic rate, and other body functions are markedly lower than the normal condition of a given animal species. Hibernation, aestivation, and dormancy are simply a prolonged form of torpor.
Unlike ectotherms which are reliant on increasing external temperature to emerge from torpor, endotherms use internal heat production to arouse from torpor. This ability allows many endothermic species to enter torpor for less than a day, instead of months. This is known as “daily torpor”. Many small mammals and birds use this strategy to their advantage in variable environments.
In all cases, because cell metabolism requires energy, a reduction in metabolic rate is a measure to save energy. Not all animals can save energy this way because a lowered body temperature causes hypothermia, which for many species (human is an example) is lethal if prolonged. Torpor is “adaptive hypothermia” because survival in animals adopting this physiological condition is not negatively affected but is in fact enhanced.
References
F. Geiser (2004) Metabolic rate and body temperature reduction during hibernation and daily torpor. Annual Review of Physiology 66: 239-274.
C. Roots (2006) Hibernation. Greenwood Press.
Join the Conversation