Cold-blooded animals, unlike warm-blooded organisms, do not maintain thermal homeostasis; that is, they do not keep their core body temperature at a nearly constant level regardless of the temperature of the surrounding environment. Rather, cold-blooded animals have a variable body temperature, which reflects the environmental temperature. Invertebrates, fish, amphibians, and reptiles are considered to be cold-blooded, while birds and mammals are defined as warm-blooded animals.
Cold-blooded creatures initially were presumed to be incapable of maintaining their body temperatures at all. Whatever the environmental temperature was, it was thought that so too was their body temperature. However, they do have various mechanisms for raising their body temperature, and thus their core temperature may be higher than the ambient temperature.
Cold-bloodedness and warm-bloodedness are terms that have fallen into disfavor because animals, reflecting the great diversity in nature, do not fit neatly into these two categories. 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 (see below: Categories of cold-bloodedness). Many species fit more in line with a graded spectrum from one extreme (cold-blooded) to another (warm-blooded). This diversity adds external value in fostering greater ecological stability and exploiting of varied ecological niches, but it also adds to the inner value of enhancing the beauty of creation for humans.
Cold-bloodedness generally refers to three separate areas of thermoregulation.
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.
Although cold-blooded animals may fit all three of the above criteria, many do not. Instead, many animals use a combination of these three aspects of thermophysiology, along with their warm-blooded counterparts (endothermy, homeothermy, and 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, such as "warm-blooded" bats or small birds that are poikilothermic and bradymetabolic when they sleep for the night, or day.
While fish are defined as poikilothermic in that they do not maintain constant internal temperatures and the temperature often mirrors the ambient temperature, certain species of fish have body temperatures elevated above the ambient environment to varying degrees. Among these are teleosts (bony fishes) in the suborder Scombroidei, and billfishes and tunas. All sharks in the family Lamnidae, such as the shortfin mako, have this capacity. Some, such as the billfish, only have elevated temperatures in their eyes and brain, but bluefin tuna and probeagle sharks can elevate their core body temperature to more than 20 °C above ambient water temperatures. For some fish, this phenomena has been traced to heat exchange, as warmer blood being returned to the gills in small veins runs close to colder, oxygenated blood in narrow arteries leaving the gills. By having elevated temperatures, the fish can be more active in colder water and have better swimming ability due to the warmer muscles.
Examples of temperature control in cold-blooded animals include:
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, such as the swimming muscles of tuna, which are warmed through heat exchange.
In general, poikilothermic animals do not use their metabolisms to heat or cool themselves. For the same body weight poikilotherms need one-third to one-tenth of the energy of homeotherms. They therefore eat only one-third to one-tenth 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 (inertial homeothermy), has been observed in sea turtles and great white sharks, and was most likely present in many dinosaurs and ancient sea reptiles (such as ichthyosaurs and plesiosaurs). For example, some species of sea turtles maintain a constant temperature some of the time despite the variation in environmental temperature. They float on the surface of the ocean to absorb heat and then, after submerging again, stay homeothermic for periods of time because of their sheer size. During long periods of time underwater, their body temperature may decrease, depending on the temperature of the surrounding water. Their body temperature may also decrease when they float on the surface of the ocean at night, depending on the surrounding temperature.
It is comparatively easy for a poikilotherm to accumulate enough energy to reproduce. Poikilotherms in the same ecological niche often have much shorter lifetimes 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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