You searched for: “poikilotherm
poikilotherm (s) (noun), poikilotherms (pl)
An animal whose temperature varies with the temperature of the surrounding environment; allotherm; ectotherm; heterotherm: Invertebrates, fish, or amphibious animals are all considered to be poikilotherm.

To a degree, behavioral responses help to maintain stability of body temperature and to counter the loss of body heat.