Homeotherm Doi (identifier) ISBN (identifier)
The common frog is a poikilotherm and is able to function over a wide range of body core temperatures.

A poikilotherm (/ˈpɔɪkələˌθɜːrm, pɔɪˈkɪləˌθɜːrm/) is an animal whose internal temperature varies considerably. It is the opposite of a homeotherm, an animal which maintains thermal homeostasis. While the term in principle can apply to all organisms, it is generally only applied to animals, and mostly to vertebrates. Usually the fluctuations are consequence of variation in the ambient environmental temperature. Many terrestrial ectotherms are poikilothermic.[1] However some ectotherms remain in temperature-constant environments to the point that they are actually able to maintain a constant internal temperature (i.e. are homeothermic). It is this distinction that often makes the term "poikilotherm" more useful than the vernacular "cold-blooded", which is sometimes used to refer to ectotherms more generally.

Poikilothermic animals include types of vertebrate animals, specifically some fish, amphibians, and reptiles, as well as many invertebrate animals. The naked mole-rat[2][3] and sloth[4] are some of the rare mammals which are poikilothermic.


The term derives from Greek poikilos (ποικίλος), meaning "varied," ultimately from a root meaning "dappled" or “painted,” and thermos (θερμός), meaning "heat".


Sustained energy output of a poikilotherm (a lizard) and a homeotherm (a mouse) as a function of core body temperature. The homeotherm has a much higher output, but can only function over a very narrow range of body temperatures.

Poikilotherm animals must be able to function over a wider range of temperatures than homeotherms. The speed of most chemical reactions vary with temperature, and in order to function poikilotherms may have four to ten enzyme systems that operate at different temperatures for an important chemical reaction.[5] As a result, poikilotherms often have larger, more complex genomes than homeotherms in the same ecological niche. Frogs are a notable example of this effect, though their complex development is also an important factor in their large genome.[6]

Because their metabolism is variable and generally below that of homeothermic animals, sustained high-energy activities like powered flight in large animals or maintaining a large brain is generally beyond poikilotherm animals.[7] The metabolism of poikilotherms favors strategies such as sit-and-wait hunting over chasing prey for larger animals with high movement cost. As they do not use their metabolisms to heat or cool themselves, total energy requirement over time is low. For the same body weight, poikilotherms need only 5 to 10% of the energy of homeotherms.[8]

Adaptations in poikilotherms


It is comparatively easy for a poikilotherm to accumulate enough energy to reproduce. Poikilotherms at the same trophic level often have much shorter generations than homeotherms: weeks rather than years.[citation needed] Such applies even to animals with similar ecological roles such as cats and snakes.

This difference in energy requirement also means that a given food source can support a greater density of poikilothermic animals than homeothermic animals.[10] This is reflected in the predator-prey ratio which is usually higher in poikilothermic fauna compared to homeothermic ones. However, when homeotherms and poikilotherms have similar niches, and compete, the homeotherm can often drive poikilothermic competitors to extinction, because homeotherms can gather food for a greater fraction of each day.

In medicine

In medicine, loss of normal thermoregulation in humans is referred to as "poikilothermia". This is usually seen with sedative and hypnotic drugs or in 'compartment syndrome'. For example, barbiturates, ethanol, and chloral hydrate may precipitate this effect.[citation needed] REM sleep is also considered a poikilothermic state in humans.[11]


  1. ^ Milton Hildebrand; G. E. Goslow, Jr. Principal ill. Viola Hildebrand. (2001). Analysis of vertebrate structure. New York: Wiley. p. 429. ISBN 0-471-29505-1.CS1 maint: multiple names: authors list (link)
  2. ^ Daly, T.J.M., Williams, L.A. and Buffenstein, R., (1997). Catecholaminergic innervation of interscapular brown adipose tissue in the naked mole-rat (Heterocephalus glaber). Journal of Anatomy, 190: 321-326. doi:10.1046/j.1469-7580.1997.19030321.x
  3. ^ Sherwin, C.M. (2010). The Husbandry and Welfare of Non-traditional Laboratory Rodents. In "UFAW Handbook on the Care and Management of Laboratory Animals", R. Hubrecht and J. Kirkwood (Eds). Wiley-Blackwell. Chapter 25, pp. 359-369
  4. ^ Britton, S. W.; Atkinson, W. E. (1938). "Poikilothermism in the Sloth". Journal of Mammalogy. 19 (1): 94. doi:10.2307/1374287. JSTOR 1374287.
  5. ^ Cavalier-Smith, T. "Coevolution of vertebrate genome, cell, and nuclear sizes". Symposium on the Evolution of Terrestrial Vertebrates: 51–86.
  6. ^ Ryan Gregory, T. (1 January 2002). "Genome size and developmental complexity". Genetica. 115 (1): 131–146. doi:10.1023/A:1016032400147. PMID 12188045.
  7. ^ Willmer, P., Stone, G., & Johnston, I. A. (2000): Environmental physiology of animals. Blackwell Science, London. 644 pages, ISBN 0-632-03517-X.
  8. ^ Campbell, N. A., Reece, J. B., et al. (2002). Biology. 6th edition. Benjamin / Cummings Publishing Company.
  9. ^ Hill, Richard (2016). Animal Physiology. Sunderland, MA: Sinauer Associates. p. 270. ISBN 978-1605354712.
  10. ^ Steen, J.B, Steen, H. & Stenseth, N.C. (1991): Population Dynamics of Poikilotherm and Homeotherm Vertebrates: Effects of Food Shortage. OICOS Vol. 60, No 2 (March, 1991), pp 269-272. summary
  11. ^ Leon Rosenthal (2009). "3". In Teofilo Lee-Chiong (ed.). Sleep Medicine Essentials. Wiley-Blackwell. p. 12.