Cold-blooded

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Cold-blooded is a common term used to describe organisms that maintain their body temperatures in ways different from mammals and birds. The term is now archaic in scientific contexts. Cold-blooded creatures were, initially, presumed to be incapable of maintaining their body temperatures at all. They were presumed to be "slaves" to their environments. Whatever the environmental temperature was, so too was their body temperature.

Since that time, advances in the study of how creatures maintain their internal temperatures (deemed: thermophysiology), have shown that many of the earlier notions of what warm blooded and cold blooded mean, were far from accurate. Today scientists realize that body temperature types are not a simple matter of black and white. Most creatures fit more in line with a graded spectrum from one extreme (cold blooded) to another (warm blooded). Because of this, both of these terms have since fallen out of favour. They have been generally replaced with one or more of their variants (see below: Breaking down Cold-Bloodedness).

1 Breaking down Cold-Bloodedness
2 Types of temperature control
3 Ecological niches
4 See also
5 External links

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Breaking down Cold-Bloodedness

Cold-bloodedness generally refers to three separate areas of thermoregulation.

Ectothermy
Poikilothermy
Bradymetabolism

Ectothermy - This refers to creatures that control body temperature through external means (Greek: ecto = "outside," therm = "heat"), such as the sun, or flowing air/water. For more on this, see below.

Poikilothermy - This refers to creatures whose internal temperatures vary, often matching the ambient temperature of the immediate environment (Greek: poikilos = "varied," therm = "heat").

Bradymetabolism - This term refers to the resting metabolism of a creature. If a creature has a low resting metabolism, it is considered to be bradymetabolic (Greek: brady = "slow," metabol = "to change"). Bradymetabolic animals can often undergo dramatic changes in metabolic speed, according to food availability and temperature. Many bradymetabolic creatures in deserts and in areas that experience extreme winters are capable of "shutting down" their metabolisms to approach near-death states, until favourable conditions return.
- Note: It is important to keep in mind that a bradymetabolic animal has a low resting metabolism only. Its active metabolism is often many times higher. As such, a bradymetabolic creature should not be considered slow.

Few creatures actually fit all three of the above criteria. Most animals use a combination of these three aspects of thermophysiology, along with their counterparts (endothermy, homeothermy & tachymetabolism) to create a broad spectrum of body temperature types. Most of the time, creatures that use any one of the previously defined aspects, are usually pigeon-holed into the term cold-blooded.

Physiologists also coined the term heterothermy for creatures that exhibit a unique case of poikilothermy.

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Types of temperature control

Examples of this temperature control include:

Snakes and lizards sunning themselves on rocks.
Fish changing depths in the water column to find a suitable temperature.
Desert animals burrowing beneath the sand during the day.
Insects that warm their flight muscles by vibrating them in place.
Dilating or constricting peripheral blood vessels to adapt more or less quickly to the ambient temperature.

Many homeothermic, or warm-blooded, animals also make use of these techniques at times. For example, all animals are at risk of overheating on hot days in the desert sun, and most homeothermic animals can shiver.

Poikilotherms often have more complex metabolisms than homeotherms. For an important chemical reaction, poikilotherms may have four to ten enzyme systems that operate at different temperatures. 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.

Because their metabolism is so variable, poikilothermic animals do not easily support complex, high-energy organ systems such as brains or wings. Some of the most complex adaptations known involve poikilotherms with such organ systems. One example is the swimming muscles of Tuna, which are warmed by a heat exchanger. In general, poikilothermic animals do not use their metabolisms to heat or cool themselves. For the same body weight poikilotherms need 1/3 to 1/10 of the energy of homeotherms. They therefore eat only 1/3 to 1/10 of the food needed by homeothermic animals.

Some larger poikilotherms, by virtue of their substantial volume to surface area ratio, are able to maintain relatively high body temperatures and high metabolic rates. This phenomenon, known as gigantothermy, has been observed in sea turtles and great white sharks, and was most likely present in many dinosaurs and ichthyosaurs.

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Ecological niches

It is comparatively easy for a poikilotherm to accumulate enough energy to reproduce. Poikilotherms in the same ecological niche often have much shorter generations than homeotherms: weeks rather than years.

This energy difference also means that a given niche of a given ecology can support three to ten times the number of poikilothermic animals as homeothermic animals. However, in a given niche, homeotherms often drive poikilothermic competitors to extinction because homeotherms can gather food for a greater fraction of each day.

Poikilotherms succeed in some niches, such as islands, or distinct bioregions (such as the small bioregions of the Amazon basin). These often do not have enough food to support a viable breeding population of homeothermic animals. In these niches, poikilotherms such as large lizards, crabs and frogs supplant homeotherms such as birds and mammals.

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