Cold-blooded organisms, more technically known as poikilothermic, are animals that have no internal metabolic mechanism for regulating their body temperatures. Some (usually smaller) animals have unregulated temperatures, but most have sophisticated physiological and behavioral techniques for obtaining their desired core body temeprature from the environment. Cold-blooded animals are often referred to as ectotherms.

Ectotherms depend largely on external sources of heat, such as solar radiation. As the environmental temperature increases, the animal's metabolic rate will increase. Lizards, fish, and amphibians are examples of ectotherms. Whereas an endotherm, or warm-blooded animal will use up to 98% of its energy for heat production, an ectotherm has all this energy available for activity, growth, repair and reproduction.

Types of temperature control

Examples of this temperature control 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.

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 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.