Warm-blooded is a term referring to animal species whose bodies maintain a temperature higher than that of their environment. In particular, homeothermic species (including and ) maintain a stable body temperature by regulating metabolic processes. Other species have various degrees of thermoregulation.
Because there are more than two categories of temperature control utilized by animals, the terms warm-blooded and Ectotherm have been deprecated in the scientific field.
Terminology
In general, warm-bloodedness refers to three separate categories of
thermoregulation.
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Endotherm is the ability of some creatures to control their body temperatures through internal means such as muscle shivering or increasing their metabolism. The opposite of endothermy is ectothermy.
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Homeotherm maintains a stable internal body temperature regardless of external influence and temperatures. The stable internal temperature is often higher than the immediate environment. The opposite is poikilothermy. The only known living homeotherms are and , as well as one lizard, the Argentine black and white tegu. Some extinct reptiles such as , , and some non-avian are believed to have been homeotherms.
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Tachymetabolism maintains a high "resting" metabolism. In essence, tachymetabolic creatures are "on" all the time. Though their resting metabolism is still many times slower than their active metabolism, the difference is often not as large as that seen in bradymetabolic creatures. Tachymetabolic creatures have greater difficulty dealing with a scarcity of food.
Varieties of thermoregulation
A significant proportion of creatures commonly referred to as "warm-blooded," like birds and mammals, exhibit all three of these categories (i.e., they are endothermic, homeothermic,
and tachymetabolic). However, over the past three decades, investigations in the field of animal thermophysiology have unveiled numerous species within these two groups that do not meet all these criteria. For instance, many bats and small birds become poikilothermic and bradymetabolic during sleep (or, in nocturnal species, during the day). For such creatures, the term
heterothermy was introduced.
Further examinations of animals traditionally classified as Ectotherm have revealed that most creatures manifest varying combinations of the three aforementioned terms, along with their counterparts (ectothermy, poikilothermy, and bradymetabolism), thus creating a broad spectrum of body temperature types. Some fish have warm-blooded characteristics, such as the opah. Swordfish and some have circulatory mechanisms that keep their and above ambient temperatures and thus increase their ability to detect and react to predation.[ Hot Eyes for Cold Fish – Wong 2005 (110): 2 – ScienceNOW]
Heat generation
Body heat is
thermogenesis by
metabolism.
This relates to the chemical reaction in cells that break down
glucose into water and
carbon dioxide, thereby producing adenosine triphosphate (ATP), a high-energy compound used to power other cellular processes. Muscle contraction is one such metabolic process generating heat energy,
and additional heat results from friction as blood circulates through the vascular system in premise to their specialized fat cells which produce heat through uncoupled respiration, contributing to
thermoregulation.
All organisms metabolize food and other inputs, but some make better use of the output than others. Like all energy conversions, metabolism is rather inefficient, and around 60% of the available energy is converted to heat rather than to ATP.
In warm environments, these animals employ evaporative cooling to shed excess heat, either through sweating (some mammals) or by panting (many mammals and all birds)—mechanisms generally absent in poikilotherms.
Defense against fungi
It has been hypothesized that warm-bloodedness evolved in mammals and birds as a defense against
. Very few fungi can survive the body temperatures of warm-blooded animals. By comparison, insects, reptiles, and amphibians are plagued by fungal infections.
[Aviv Bergman, Arturo Casadevall. 2010. Mammalian Endothermy Optimally Restricts Fungi and Metabolic Costs. mBio Nov 2010, 1 (5) e00212-10. ][Vincent A. Robert, Arturo Casadevall. 2009. Vertebrate Endothermy Restricts Most Fungi as Potential Pathogens. The Journal of Infectious Diseases, Volume 200, Issue 10, 15 November 2009, Pages 1623–1626. ][Casadevall A (2012) Fungi and the Rise of Mammals. PLoS Pathog 8(8): e1002808. ] Warm-blooded animals have a defense against pathogens contracted from the environment, since environmental pathogens are not adapted to their higher internal temperature.
See also
Footnotes
Citations
External links
