Endotherms generate most of the heat they need internally, unlike ectotherms. When it’s cold out, they increase metabolic heat production to keep their body temperature constant. Because of this, the internal body temperature of an endotherm is more or less independent of the temperature of the environment. Endotherms have considerably higher metabolic rates than ectotherms. That’s because they have to burn large quantities of fuel—food—to maintain their internal body temperature.
This pattern is shown on the graph below: the mouse maintains a steady body temperature close to across a wide range of external temperatures.
