Ectotherm

An ectotherm is an animal whose body temperature depends mainly on the temperature of its surroundings rather than on heat it makes itself. Most fish, amphibians, reptiles and invertebrates, including insects, are ectotherms, in contrast to the endothermic birds and mammals.

Ectotherms control their temperature largely through behavior. A lizard such as the green iguana basks in the sun to warm up and retreats to shade or burrows to cool down. Some have adaptations like changing the color of their skin, orienting their bodies to the sun or, in a few cases, generating heat with muscles. Their metabolism is slow when cold, so they need much less food than endotherms of the same size, often about a tenth.

The speed of the chemical reactions in the body rises with temperature, following the Arrhenius equation, so an ectotherm's running speed, digestion and growth increase with warmth up to an optimum, then fall rapidly as enzymes fail. The shape of this thermal performance curve sets which habitats a species can live in. Some, like the American bullfrog, tolerate a wide range, while tropical species often have narrow limits.

Warming can speed development and increase the range of insect vectors such as Aedes aegypti, but extremes can exceed an ectotherm's limits. Dormancy such as diapause lets many survive the cold. The relationship between temperature and survival is a key issue in understanding how climate change affects wildlife, and in studies of homeostasis.