Cold-blooded
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 falled into disfavor because animals 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).
Categories of cold-bloodedness
Cold-bloodedness generally refers to three separate areas of thermoregulation.
- Ectothermy refers to control of an animal's temperature through external means (Greek: ecto = "outside," therm = "heat"), such as the sun, or flowing air/water. For more on this, see below.
- Poikilothermy refers to variation in the internal temperatures of animals, whereby core temperature often matches the ambient temperature of the immediate environment (Greek: poikilos = "varied," therm = "heat").
- *Bradymetabolism 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 favorable conditions return (see hibernation and estivation).
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 maintain elevated body temperatures to varying degrees. These include teleosts (bony fishes) in the suborder Scombroidei and billfishes, tunas, and one species of "primitive" mackerel (Gasterochisma melampus). All sharks in the family Lamnidae—shortfin mako, long fin mako, white, porbeagle, and salmon shark—are known to have this capacity, and evidence suggests the trait exists in family Alopiidae (thresher sharks). The degree of being able to have elevated temperatures varies from the billfish, which warm only their eyes and brain, to bluefin tuna and porbeagle sharks, which can elevate body temperatures in excess of 20 °C above ambient water temperatures. In many cases, 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. This ability to have elevated temperatures allows fish to be active in colder waters and to have enhanced swimming ability because of the warmer muscles. In general, most fish can survive only at a relatively small range of body temperatures, but may adjust their depth in large bodies of water in order to find preferable ranges.
Types of temperature control
Examples of temperature control in cold-blooded animals 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 (inertial homeothermy), has been observed in sea turtles and great white sharks, 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.
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 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.
See also
- Warm-blooded for organisms that fall in between both categories.
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