An ectotherm, from the Greek εκτός (ektós) "outside" and θερμός (thermós) "hot", is an organism in which internal physiological sources of heat are of relatively small or quite negligible importance in controlling body temperature. Some refer to these organisms as "cold blooded". Such organisms (for example frogs) rely on environmental heat sources, which permits them to operate at very economical metabolic rates. Some of these animals live in environments where temperatures are practically constant, as is typical of regions of the abyssal ocean. In contrast, in places where temperature varies so widely as to limit the physiological activities of other kinds of ectotherms, many species habitually seek out external sources of heat or shelter from heat; for example, many reptiles regulate their body temperature by in the sun, or seeking shade when necessary in addition to a whole host of other behavioral thermoregulation mechanisms. In contrast to ectotherms, endotherms rely largely, even predominantly, on heat from internal metabolic processes.