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Eating very little in the presence of food or failure to search for food has been documented in various species during the hibernation season, incubation, molting, and defense of the territory or harem. At these times feeding competes with other, more important activities. One way to avoid conflicts between feeding and these other activities is to lower the programmed weight or set-point for body fat. Experiments on mammalian hibernators and incubating birds provide evidence that set-points are indeed lowered. Failure to eat in these two examples depends on anorexia, loss of appetite. A review of other examples suggests that conceptualization in terms of lowered set-points provides a unified and testable way of understanding many naturally occurring instances of fasting in the animal kingdom. Finally, spontaneous animal anorexias are contrasted with attempts by people to lose weight.