Ectotherm

An ectotherm takes most of its body heat from the environment, from sunshine, warm water, air or a warm surface, instead of generating it internally. Most animals are ectotherms, including nearly all invertebrates, fishes and amphibians and all reptiles except birds. They are often called cold-blooded, but the label misleads: a lizard basking in a desert can be warmer than a mammal.

Because their tissues warm and cool with their surroundings, ectotherms regulate temperature mainly through behavior. A lizard shuttles between sun and shade, a snake lies on a warm road at dusk, and honeybees huddle together to keep their hive warm in winter. Temperature sets the pace of their enzymes, so they slow down in the cold, and many become dormant through winter. The payoff is economy: an ectotherm burns only a small fraction of the energy an endotherm of the same size needs, so it can go a long time on little food.

Ectothermy is not the same as poikilothermy, which means having a body temperature that varies; a fish in the deep sea is an ectotherm, yet its temperature barely changes. Because each species tolerates only a limited range of temperatures, ectotherms are a major focus of research on how climate change affects animals.