|Thermoregulation in animals|
|Ectotherm • Endotherm • Poikilotherm • Homeothermy • Heterothermy • Stenotherm • Eurytherm • Thermolabile • Thermostability • Gigantothermy • Kleptothermy • Bradymetabolism • Tachymetabolism|
The term warm-blooded is a colloquial term to describe animal species that have a relatively higher blood temperature, and maintain thermal homeostasis primarily through internal metabolic processes. These are characteristics of mammals and birds.
Both the terms "warm-blooded" and "cold-blooded" have fallen out of favour with scientists because of the vagueness of the terms and an increased understanding of the field. Body temperature types are not discrete categories. Each term may be replaced with one or more variants (see the next section for examples). Body temperature maintenance (thermoregulation) incorporates a wide range of different techniques that result in a body temperature continuum.
Characteristics of warm-bloodedness
In general, warm-bloodedness refers to three separate aspects of thermoregulation.
- Endothermy is the ability of some creatures to control their body temperatures through internal means such as muscle shivering or increasing their metabolism (Greek: endon = "within", thermē = "heat"). Some writers[who?] restrict the meaning of endothermy to mechanisms that directly raise the animal's metabolic rate in order to produce heat. The opposite of endothermy is ectothermy.
- Homeothermy is thermoregulation that maintains a stable internal body temperature regardless of external influence. This temperature is often, though not necessarily, higher than the immediate environment (Greek: homoios = "similar", thermē = "heat"). The opposite is poikilothermy.
- Tachymetabolism is the kind of thermoregulation used by creatures that maintain a high "resting" metabolism (Greek: tachys/tachus = "fast, swift", metabolēn = "throw beyond"). 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.
Reasons for term falling into disuse
A large proportion of the creatures traditionally called "warm-blooded", such as mammals and birds, fit all three of these categories (i.e., they are endothermic, homeothermic, and tachymetabolic). However, over the past 30 years, studies in the field of animal thermophysiology have revealed many species belonging to these two groups that do not fit all these criteria. For example, many bats and small birds are poikilothermic and bradymetabolic when they sleep for the night, or day, as the case may be. For these creatures, the term heterothermy was coined.
Further studies on animals that were traditionally assumed to be cold-blooded have shown that most creatures incorporate different variations of the three terms defined above, along with their counterparts (ectothermy, poikilothermy, and bradymetabolism), thus creating a broad spectrum of body temperature types. Even some fish have warm-blooded characteristics. Swordfish and some sharks have circulatory mechanisms that keep their brains and eyes above ambient temperatures, and thus increase their ability to detect and react to prey. Tunas and some sharks have similar mechanisms in their muscles, improving their stamina when swimming at high speed.
Body "heat" is generated by the metabolism. This refers to the chemical reactions cells use to break down glucose into water and carbon dioxide and in so doing generate ATP (adenosine triphosphate), a high-energy compound used to power other cellular processes.
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 most organisms, this heat is simply lost to the environment. However, endothermic homeotherms (the animals generally characterized as "warm-blooded") both produce more heat and have better ways to retain and regulate it than other animals. They have a higher basal metabolic rate, and also a greater capacity to increase their metabolic rate when engaged in strenuous activity. They usually have well-developed insulation in order to retain body heat, fur in the case of mammals and feathers in birds. When this insulation is insufficient to maintain body temperature, they may resort to shivering--rapid muscle contractions that quickly use up ATP, thus stimulating cellular metabolism to replace it and consequently produce more heat. In general, in hot environments, they use evaporative cooling to shed excess heat, either by sweating (some mammals) or by panting (many mammals and all birds)- in general, mechanisms not present in poikilotherms.
Defence against fungi
It has been hypothesized that mammals and birds evolved warm-bloodedness as a defence against fungal infections. Very few fungi can survive the body temperatures of warm-blooded animals. By comparison, insects, reptiles, and amphibians are plagued by fungal infections.
- Hot Eyes for Cold Fish -- Wong 2005 (110): 2 -- ScienceNOW
- Block, B.A., and Carey, F.G. (March 1985). "Warm brain and eye temperatures in sharks". Journal of Comparative Physiology B: Biochemical, Systemic, and Environmental Physiology (Springer) 156 (2): 229. doi:10.1007/BF00695777.
- "Warm eyes give deep-sea predators super vision". University of Queensland. 11 January 2005.
- McFarlane, P. (January 1999). "Warm-Blooded Fish". Monthly Bulletin of the Hamilton and District Aquarium Society.
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