Poikilothermic organisms, commonly referred to as 'cold-blooded' organisms, are animals that have no internal mechanism for regulating their body temperatures. Therefore, they either have unregulated temperatures, or their behavior controls their temperature.
Examples of this temperature control include:
- Snakes 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.
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.
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 exinction because homeotherms can gather food at more times than poikilotherms.
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.