ectothermic

/ˌɛktoʊˈθɜrmɪk/
adjective
  1. Describing an animal that depends on outside heat sources to control its body temperature; cold-blooded.
    • Turtles are ectothermic reptiles that need sunlight to stay active.
    • In ectothermic animals, metabolism slows down when the environment gets cold.
    • The aquarium's ectothermic fish became sluggish when the heater broke.