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Food, like water, is in short supply in the desert. We report a specialized mechanism used by a desert mouse for surviving prolonged food shortages. The key element of this adaptation is a large reduction in resting metabolism. After about 2 weeks of restricted food intake (50% of normal), the desert mouse ’’switched down’’ its resting metabolism and was able to survive and maintain its weight indefinitely on these limited rations, When food was again freely available, resting metabolism ’’switched up,’’ returning to normal levels in a single day. The reduced metabolism occurred without a decrease in body temperature or in levels of activity. In marked contrast, metabolism of the laboratory white mouse increased during food restriction, and the experiments had to be terminated to avoid starvation. We think this ’’metabolic switch’’ is common among desert mammals. It may be an amplification of a general metabolic response for coping with food scarcity common to all mammals, including humans.