Ectotherms are animals that primarily gains heat through the environment.
Most mammals, including humans, as well as most birds are endothermic homeotherms, while most fish, invertebrates, reptiles, and amphibians are ectothermic poikilotherms.
Ectothermic do not require energy to heat themselves, and can spend their energy on reproduction, which in some environments is more valuable than staying active in cold weather.
