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An ectotherm is an animal that does not control it's body temperature through metabolic regulation. Ectotherms depend largely on external sources of heat, such as solar radiation. As the environmental temperature increases, the animal's metabolic rate will increase. Lizards, fish, and amphibians are examples of ectotherms.

Whereas an endotherm will use up to 98% of its energy for heat production an ectotherm has all this energy available for activity, growth and repair, and reproduction.