A poikilotherm is an animal (Greek poikilos – 'various, spotted', and therme – 'heat) whose internal temperature varies considerably. Poikilotherms have to survive and adapt to environmental stress.[1] One of the most important stressors is temperature change, which can lead to alterations in membrane lipid order and can cause protein unfolding and denaturation at elevated temperatures. It is the opposite of a homeotherm, an animal which maintains thermal homeostasis. While the term in principle can apply to all organisms, it is generally only applied to animals, and mostly to vertebrates. Usually the fluctuations are a consequence of variation in the ambient environmental temperature. Many terrestrial ectotherms are poikilothermic.[2] However some ectotherms remain in temperature-constant environments to the point that they are actually able to maintain a constant internal temperature and are considered homeothermic.[3] It is this distinction that often makes the term "poikilotherm" more useful than the vernacular "cold-blooded", which is sometimes used to refer to ectotherms more generally.
Poikilothermic animals include types of vertebrate animals, specifically some fish, amphibians, and reptiles, as well as many invertebrate animals. The naked mole-rat[4] [5] and sloth[6] are some of the rare mammals which are poikilothermic.
The term derives from Greek poikilos (Greek, Modern (1453-);: ποικίλος), meaning "varied," ultimately from a root meaning "dappled" or "painted," and thermos (Greek, Modern (1453-);: θερμός), meaning "heat".
Poikilotherm animals must be able to function over a wider range of temperatures than homeotherms. The speed of most chemical reactions vary with temperature, and in order to function poikilotherms may have four to ten enzyme systems that operate at different temperatures for an important chemical reaction.[7] 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, though their complex development is also an important factor in their large genome.[8]
Because their metabolism is variable and generally below that of homeothermic animals, sustained high-energy activities like powered flight in large animals or maintaining a large brain is generally beyond poikilotherm animals.[9] The metabolism of poikilotherms favors strategies such as sit-and-wait hunting over chasing prey for larger animals with high movement cost. As they do not use their metabolisms to heat or cool themselves, total energy requirement over time is low. For the same body weight, poikilotherms need only 5 to 10% of the energy of homeotherms.[10]
It is comparatively easy for a poikilotherm to accumulate enough energy to reproduce. Poikilotherms at the same trophic level often have much shorter generations than homeotherms: weeks rather than years. Such applies even to animals with similar ecological roles such as cats and snakes.
This difference in energy requirement also means that a given food source can support a greater density of poikilothermic animals than homeothermic animals.[12] This is reflected in the predator-prey ratio which is usually higher in poikilothermic fauna compared to homeothermic ones. However, when homeotherms and poikilotherms have similar niches, and compete, the homeotherm can often drive poikilothermic competitors to extinction, because homeotherms can gather food for a greater fraction of each day and in more effective, specialized ways (e.g. chimpanzees actively seeking out and collecting army ants with sticks versus the typical poikilotherm sit-and-wait strategy).
In medicine, loss of normal thermoregulation is referred to as poikilothermia. This can be seen in compartment syndrome and with use of sedative-hypnotics like barbiturates, ethanol, and chloral hydrate. REM sleep is considered a poikilothermic state in humans.[13] Poikilothermia is one of the signs of acute limb ischemia.