Poikilothermism

2008-12-18T19:40:01Z

SitboCbota: cnataelbovar

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'''Cold-blooded''' is a common term used to describe organisms that maintain their body temperatures in ways different from [[mammal|mammals]] and [[bird|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 preconceived notions of what [[Warm blooded|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|Breaking down Cold-Bloodedness]])

==Breaking down Cold-Bloodedness==

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

# Ectothermy
# Poikilothermy
# Bradymetabolism

*'''Ectothermy''' - This refers to creatures that control their body temperature through external means (Greek: ''ecto'' = "outside," ''therm'' = "heat"), such as the sun, or flowing air/water. For more on this see [[#Types of temperature control|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 said 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 areas that experience extreme winters, are capable of "shutting down" their metabolism and 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. Their 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 criteria listed above. Most animals use a combination of these three aspects of thermophysiology, along with their counterparts ([[Endothermic|endothermy]], [[Homeothermic|homeothermy]] & [[Tachymetabolic|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.

==Types of temperature control==

Examples of this temperature control include:
* [[Snake]]s and [[lizard]]s 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.
* [[Insect]]s that warm their flight muscles by vibrating them in place.
* Dilating or constricting peripheral [[blood vessel]]s 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 [[genome]]s than homeotherms in the same [[ecological niche]]. [[Frog]]s 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 [[metabolism]]s 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 [[metabolism|metabolic]] rates. This phenomenon, known as [[gigantothermy]], has been observed in [[sea turtle]]s and [[great white shark]]s, and was most likely present in many [[dinosaurs]] and [[ichthyosaurs]].

==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 [[bioregion]]s (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.

==See also==
*[[Warm-blooded]] for organisms that fall in between both categories.

==External links==

*[http://www.dinosauria.com/jdp/misc/blood.htm Dinosauria.com: What is Warm-bloodedness anyway?]
*[http://reptilis.net/cold-blood.html The Reptipage: What is cold-blooded?]

Original Wiki source: [http://en.wikipedia.org/ Wikipedia]

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Poikilothermism