Hibernation: Endotherms

Abstract

The main function of hibernation and daily torpor in heterothermic mammals and birds (i.e. species capable of expressing torpor) is to conserve energy and water and thus to survive during adverse environmental conditions or periods of food shortage no matter if they live in the arctic or the tropics. However, the reduced energy requirements also permit survival of bad weather during reproduction to prolong gestation into more favourable periods, conservation of nutrients for growth during development, and overall result in reduced foraging needs and thus exposure to predators, which appear major contributing reasons why heterotherms are often long lived and have lower extinction rates than strictly homeothermic species that cannot use torpor. Known heterothermic mammals and birds are diverse with about 2/3 of mammalian orders and 1/3 of avian orders containing heterothermic species, and their number continues to grow.

Key Concepts:

  • Hibernation and daily torpor are the most effective means of energy conservation available to mammals and birds and are crucial for survival of adverse conditions of many species.

  • Use of torpor often is enhanced by low ambient temperatures and limited food.

  • Because torpor reduces energy requirements, its opportunistic use allows extension of gestation, nutrient sparing during development, and permits survival in modified and degraded habitats and also reduces the need for foraging and thus exposure to predators.

  • As the rate of extinction in heterothermic mammals is much lower than in homeotherms, thermal energetics are of concern to conservation biologists because mammals and birds that can use and cannot use torpor differ enormously in their energy requirements and thus foraging needs.

Keywords: hibernation; daily torpor; heterothermic endotherms; body temperature; metabolic rate; torpor bouts; body mass; mammals; birds

Figure 1.

Body temperatures as a function of ambient temperature during normothermia (solid horizontal line, about 38°C) and torpor in a hibernator (H, solid line) and a daily heterotherm (DH, broken line) of similar size. The diagonal dotted line represents body temperature=ambient temperature. Averages from Geiser and Ruf .

Figure 2.

Schematic diagram of metabolism, measured indirectly as oxygen consumption and expressed as per gram body tissue, during normothermia (solid line) and torpor (dashed or dotted line) of a 25 g hibernator (H) and a 25 g daily heterotherm (DH). In the temperature range in which torpid animals are thermoconforming (not using thermoregulatory heat production), the metabolic rate decreases curvilinearly with ambient temperature and thus body temperature. In the temperature range in which animals are thermoregulating during normothermia and torpor, the metabolic rate increases with decreasing ambient temperature to compensate for heat loss (see Figure ). Values from Geiser and Ruf and Geiser . BMR, basal metabolic rate.

Figure 3.

A hibernating pygmy‐possum (Cercartetus concinnus) curled into a tight ball to minimise the body surface area. Note: the animal has been turned over to show the position of appendages. Photo Geiser F.

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Further Reading

Barnes BM and Carey HV (eds) (2004) Life in the Cold: Evolution, Mechanisms, Adaptation, and Application. Twelfth International Hibernation Symposium. Biological Papers of the University of Alaska #27. Fairbanks: Institute of Arctic Biology, University of Alaska.

Carey C, Florant GL, Wunder BA and Horwitz B (eds) (1993) Life in the Cold: Ecological, Physiological, and Molecular Mechanisms. Boulder, CO: Westview Press.

Geiser F, Hulbert AJ and Nicol SC (eds) (1996) Adaptations to the Cold: Tenth International Hibernation Symposium. Armidale, Australia: University of New England Press.

Heldmaier G and Klingenspor M (eds) (2000) Life in the Cold: Eleventh International Hibernation Symposium. Heidelberg: Springer Verlag.

Heller HC, Musacchia XJ and Wang LCH (eds) (1986) Living in the Cold: Physiological and Biochemical Adaptations. New York: Elsevier.

Kayser C (1961) The Physiology of Natural Hibernation. Oxford: Pergamon.

Lovegrove BG and McKechnie AE (eds) (2008) Hypometabolism in Animals: Torpor, Hibernation and Cryobiology. 13th International Hibernation Symposium. Pietermaritzburg: University of KwaZulu‐Natal.

Malan A and Canguilhem B (eds) (1989) Living in the Cold, La Vie au Froid. London: John Libbey Eurotext.

Mrosovsky N (1971) Hibernation and the Hypothalamus. New York: Appleton‐Century‐Crofts.

Wang LCH (ed.) (1989) Animal Adaptation to Cold. Heidelberg: Springer Verlag.

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Geiser, Fritz(Oct 2011) Hibernation: Endotherms. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0003215.pub2]