ectotherms

/EH1 K T AH0 TH ER1 M Z/
noun
  1. 1

    An animal, such as an amphibian, fish, reptile, or arthropod, which has a limited ability to regulate its body temperature and whose body temperature thus depends on the ambient temperature.

Source: https://en.wiktionary.org/wiki/ectotherm, https://en.wiktionary.org/wiki/ectotherms