Mammals and birds differ from other endotherms because they can maintain high body temperature at rest. A high basal metabolism is necessary for mammals and birds to elevate their body temperature above environmental temperature even if the animal is resting. Birds and mammals have much higher standard metabolic rates than ectotherms, and the differences in standard metabolism are associated with different molecular, cellular, and organ characteristics. Compared to ectotherms, birds and mammals have higher mitochondrial volume, greater membrane surface per tissue volume, and higher aerobic enzyme activity. Their cellular membranes have greater ion fluxes, such that physiologists describe the cellular membranes of endotherms as being 'leakier' than those of ectotherms. Indeed, the greater leakiness of cellular membranes has been hypothesized to be one of the major processes responsible for the higher standard metabolic rate of endotherms.
Most endothermic reptiles, fishes, and insects support an elevated body temperature with heat produced by working muscle, and they do not maintain endothermy at rest. So their molecular, cellular, and organ characteristics are more similar to ectotherms than they are to endotherms. Besides producing heat by muscle, endother-mic fishes developed vascular countercurrent heat exchangers to reduce heat loss and thereby promote endothermy. Some sea turtles also have countercurrent heat exchangers (in their flippers). In fishes that elevate their brain or eye temperature, the method of heat production varies among species, and the method of heat production is known not for all species. In lamnid sharks, heat is transferred from muscle, whereas billfishes and mackerel produce heat with specialized heater tissues (evolved from ocular muscles). In plants, endothermy is achieved by decoupling ATP generation from heat production via use of an alternative electron-transport mechanism (i.e., cyanide-insensitive respiration).
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