A poikilotherm (/ˈpɔɪkələˌθɜːrm, pɔɪˈkɪləˌθɜːrm/) is an animal (Greek poikilos – 'various', 'spotted', and therme – 'heat') whose internal temperature varies considerably. Poikilotherms have to survive and adapt to environmental stress. One of the most important stressors is outer environment temperature change, which can lead to alterations in membrane lipid order and can cause protein unfolding and denaturation at elevated temperatures. Poikilotherm is the opposite of homeotherm – an animal which maintains thermal homeostasis. In principle, the term could be applied to any organism, but it is generally only applied to vertebrate animals. Usually the fluctuations are a consequence of variation in the ambient environmental temperature. Many terrestrial ectotherms are poikilothermic. However some ectotherms seek constant-temperature environments to the point that they are able to maintain a constant internal temperature, and are considered actual or practical homeotherms. It is this distinction that often makes the term poikilotherm more useful than the vernacular "cold-blooded", which is sometimes used to refer to ectotherms more generally.
Poikilothermic animals include types of vertebrate animals, specifically some fish, amphibians, and reptiles, as well as many invertebrate animals. The naked mole-rat and sloths are some of the rare mammals which are poikilothermic.
