Poikilotherm

A poikilotherm is an organism whose internal temperature varies considerably. It is the opposite of a homeotherm, an organism which maintains thermal homeostasis. Usually the variation is a consequence of variation in the ambient environmental temperature. Many terrestrial ectotherms are poikilothermic. However some ectotherms remain in temperature-constant environments to the point that they are actually able to maintain a constant internal temperature (i.e. are homeothermic). 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 fish, amphibians, and reptiles, as well as a host of invertebrate animals.

Etymology
The term derives from Ancient Greek, poikilos (ποικίλος), meaning "varied"; and thermos (θερμός), meaning "heat".

Physiology
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 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. This favoures e.g. sit-and-wait hunting strategy 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 half to 1/10 of the energy of homeotherms.

Adaptations in poikilotherms

 * Some adaptations are behavioral. Lizards and snakes bask in the sun in the early morning and late evening, and seek shelter around noon.
 * Termite mounds are usually oriented in a north-south direction so that they absorb as much heat as possible around dawn and dusk and minimise heat absorption around noon.
 * Tuna are able to warm their entire bodies through a heat exchange mechanism called the rete mirabile, which helps keep heat inside the body, and minimises the loss of heat through the gills. They also have their swimming muscles near the center of their bodies instead of near the surface, which minimises heat loss.
 * Gigantothermy means using a low ratio of surface area to volume to minimise heat loss, such as in sea turtles.

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. Such applies even to animals with similar ecological roles such as cats and snakes.

This difference in energy requirement also means that a given niche of a given ecology can support a greater density of poikilothermic animals than homeothermic animals. This is reflected in e.g. the predator-prey ratio which is usually higher in poikilothermic fauna compared to homeothermic ones. However, in a given niche, homeotherms often drive poikilothermic competitors to extinction because homeotherms can gather food for a greater fraction of each day, homeotherm predators such as hyenas often hunting at night.

Poikilotherms succeed in some niches, such as islands and hot deserts, or distinct bioregions (such as the small bioregions of the Amazon basin). These biomes 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 subplant homeotherms such as birds and mammals.

In medicine
In medicine, loss of normal thermoregulation in humans is referred to as "poikilothermia". This is usually seen with sedative and hypnotic drugs. For example, barbiturates, ethanol, and chloral hydrate may precipitate this effect.