Thermoregulation

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Thermoregulation is the ability of an organism to control its body temperature within certain limits, even when the surrounding temperature is different.[1] This is an aspect of homeostasis: the keeping of a constant internal environment.

Endotherms[change | edit source]

So-called warm-blooded animals control the temperature of their body at quite a high level. This ability is called endothermy. All mammals and birds are endotherms (homeotherms or homoiotherms). The basic source of the heat is chemical energy from the body's metabolism. they have a number of temperature-control devices:

  1. When they are cold they can do things to make themselves warmer. For example, they "shiver", or shake, or run about, or move into a warmer place. This is because all movement by animals creates heat from the chemical reactions of respiration. So polar bears do not freeze because their metabolism produces heat, and movement produces more heat. During hibernation bears exist several degrees lower that they do in active life. Of course, warm-blooded animals have thermal insulation: hair, feathers, or in water blubber (thick fat). These adaptations help keep heat energy inside the animal.
  2. When they are warm they sweat to become cooler, or pant or open their feathers, or move to a less hot place and lie down.

Hibernation[change | edit source]

In the colder climates many mammals hibernate or aestivate. This means they have a two-level metabolism. Their temperature is held at a high level when they are active, and at a lower level when they are hibernating. This has the advantage of saving energy during times when they cannot get enough food to keep up the higher temperature.

Humans[change | edit source]

The human body has automatic responses to help regulate temperature.[1] When the external environment heats up, arterioles leading to the capillary loops in the dermis dilate (widen).[2] This increases blood flow to the surface of the skin where its heat can more easily radiate away.[2] This process is called vasodilation. Sweat glands also produce greater amounts of sweat. This liquid is secreted onto the surface of the skin. Sweat needs energy to turn from a liquid into a gas and evaporate. This energy is called the latent heat of vaporisation.[2] The body supplies this heat and so it cools down as the sweat evaporates.[2]

If the body is unable to maintain a normal temperature and it increases significantly above normal, a condition known as hyperthermia occurs. This occurs when the body is exposed to temperatures of approximately 55°C; any exposure longer than a few hours at this temperature or up to around 70°C kills. The opposite condition, when body temperature decreases below normal levels, is known as hypothermia.

Fossil groups[change | edit source]

Palaeontologists are fairly certain that some fossil groups were endotherms. Obvious examples are pterosaurs, whose bodies were covered by hair (or hair-like filaments),[3] and the smaller carnivorous dinosaurs which evolved feathers ("dinobirds").[4][5] It is thought that feathers originally functioned to keep body temperature higher. They appear on bird-like dinosaurs before there were any flying birds (Anchiornis huxlei), and they appear on dinosaurs which are too heavy to fly at all.

Ectotherms[change | edit source]

Pseudemys turtles (shown here basking for warmth) are ectothermic.
Junonia lemonias is basking under the sun.

Ectotherms are what is often called "cold-blooded". To function properly they need some heat, and usually this is heat from their surroundings.[6] Many need heat in order to digest their food.[7][8]

Many supposedly cold-blooded animals do things to keep their temperature higher than the surrounding air or water. Fast-moving fish, like tunny, and some sharks, keep their temperature above the level of the surrounding water. Reptiles such as lizards and snakes sit in the sun when they are cold and in the shade when they are hot. Some insects control their temperature, especially colonial insects. Termite mounds have 'air conditioning': their system of openings and galleries allows currents of air to flow through the mound. Honey bees fan their wings to cool their colony. However, many smaller invertebrates do not control their body temperature at all.

The correct scientific word for cold-blooded is poikilotherm (noun), poikilothermic (adjective).

References[change | edit source]

  1. 1.0 1.1 "What is thermoregulation?", How it Works (Imagine Publishing) (40): p. 36, 2012-11-01
  2. 2.0 2.1 2.2 2.3 Bradfield, Phil; Potter, Steve (2009). Edexcel IGCSE Biology: Student Book. Pearson Education. pp. 92-93. ISBN 9780435966881.
  3. The discovery is discussed by Wellnhofer P. Pterosaurs of the Triassic, in Norman D. & Wellnhofer P. The illustrated encyclopedia of dinosaurs. London: Salamander. p281/2. ISBN 1-84065-204-7
  4. Dong Zhiming 1988. Dinosaurs from China. transl. Angela Milner, BM(NH)/China Ocean.
  5. Benton M.J. et al. 2008. The remarkable fossils from the Early Cretaceous Jehol biota of China and how they have changed our knowledge of Mesozoic life. Proceedings of the Geological Association 119, p209–228.
  6. Davenport, John. Animal life at low temperature. Springer 1991. ISBN 978-0412403507
  7. Jay M. Savage ; with photographs by Michael Fogden and Patricia Fogden. (2002). The amphibians and reptiles of Costa Rica : a herpetofauna between two continents, between two seas. Chicago, Ill.: University of Chicago Press. p. 409. ISBN 0-226-73538-9.
  8. Hildebrand, Milton & Goslow G.E.Jr. (2001). Analysis of vertebrate structure. New York: Wiley. p. 429. ISBN 0-471-29505-1.

Further reading[change | edit source]

  • Schmidt-Nielsen, Knut 1972. How animals work. Cambridge University Press. ISBN 0-521-09692-8. Basic information on heat balance and heat exchange in animals.