A eurytherm is an organism, often an endotherm, that can function at a wide range of ambient temperatures.[1] To be considered a eurytherm, all stages of an organism's life cycle must be considered, including juvenile and larval stages.[2] These wide ranges of tolerable temperatures are directly derived from the tolerance of a given eurythermal organism's proteins.[3] Extreme examples of eurytherms include Tardigrades (Tardigrada), the desert pupfish (Cyprinodon macularis), and green crabs (Carcinus maenas), however, nearly all mammals, including humans, are considered eurytherms.[4][5][6] Eurythermy can be an evolutionary advantage: adaptations to cold temperatures, called cold-eurythemy, are seen as essential for the survival of species during ice ages.[7] In addition, the ability to survive in a wide range of temperatures increases a species' ability to inhabit other areas, an advantage for natural selection.