Other forms: ectotherms
Unlike a bird or mammal that regulates its own internal body temperature, an ectotherm relies on its environment to keep its body warm, like a lizard basking in the sun.
Any animal that can't produce its own warmth or internally regulate its body temperature is an ectotherm. Some examples of ectotherms are reptiles, amphibians, and fish. For example, a snake might lie on a rock in the sun to raise its body temperature, and then lounge in the shade to lower it. While ectotherms are often referred to as "cold-blooded," this term can be a little misleading, as ectotherms' blood isn't actually cold; it's just that their body temperature changes with the environment.