Background: Temperature affects essentially every aspect of the biology of poikilothermic animals including the energy and mass budgets, activity, growth, and reproduction.

Although poikilothermic animals do not have the ability to regulate body temperature, whether this pathway is required for cold tolerance remains unknown.

Our findings provide an example of an evolutionarily conserved mechanism for cold tolerance that has persisted in both poikilothermic and homoeothermic animals.

The rest of the poikilothermic species, from teleosts to reptilians, appear to contain a well-developed complement system resembling that of homeothermic vertebrates.