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*Transfers of energy in biological systems are inefficient. In both ectotherms and endotherms, in every transfer of energy—from food molecules to ATP, from ATP to biological work—some of the energy is lost as heat. Working muscles produce heat, as do all of the metabolic activities of all tissues.

Why do endotherms produce more heat than ectotherms? The surprising answer is that the cells of endotherms are less efficient at using energy than are the cells of ectotherms. The cells of endotherms are more “leaky” to ions than are the cells of ectotherms. Therefore Na⁺ ions are constantly diffusing into the cells, and K⁺ ions are constantly diffusing out. Even an endotherm at rest must spend considerable amounts of energy to transport Na⁺ out of the cells and transport K⁺ back in. Because of their constant need to actively transport ions, endotherms expend more energy than do ectotherms just to maintain the ion concentration gradients across their cell membranes. This situation is analogous to a leaky boat: the faster water comes in (i.e., the faster ions diffuse down their concentration gradients), the more metabolic energy has to be expended to bail the water out (i.e., pump ions back up their concentration gradients). Since endotherms expend more energy than ectotherms do to maintain ion concentration gradients, they produce more internal heat.

We can speculate that a mutation resulting in seemingly faulty or leaky ion channels may underlie the evolution of endothermy. Such a mutation in a small ectotherm would have increased its energy expenditure and therefore its heat production. Increased heat production would have enabled the animal to be active earlier in the morning or for a longer time after sunset. Being active in twilight, and eventually at night, would open up a new world of ecological opportunities—a world in which there was less competition from similar-sized ectotherms and less danger of predation.

Two major differences between endotherms and ectotherms are (1) their resting metabolic rates—the sum total of all energy expenditures in their bodies when at rest—and (2) their responses to changes in environmental temperature.