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Endotherms have higher metabolic rates than ectotherms. In endotherms, body temperature is kept constant whereas in ectotherms, body temperature fluctuates according to the temperature of their environment. To maintain a constant internal body temperature, an endotherm such as a mouse has to use about thirty times more energy than an ectotherm such as a lizard that has the same body weight. To increase their body temperatures, ectotherms must move themselves to a warmer area rather than raising their metabolic rates.