Mammals produce their own body heat (endothermy) as opposed to absorbing energy from the outside environment. This metabolic heat is produced mainly in their mitochondria. This metabolic heat is produced mainly in their mitochondria. Internal organs such as the heart, kidney, and brain are larger in mammals than reptiles and the corresponding increase in mitochondrial membrane surface area adds to their heat production.
Mammals also regulate their body temperature within a stable range, generally between 87 and 103°F (30–39° C). This is called homeothermy. Having a constant temperature allows mammals to maintain warm muscles, which gives them the ability to react quickly, either to secure food or to escape predation. They can also maintain the optimum operating temperature for many enzymes, providing a more effective physiology. Some mammals are heterothermic (able to alter their body temperature voluntarily).
Many insectivorous bats are heterothermic. When in torpor they lower their body temperature to the ambient temperature, conserving calories that would otherwise be used for heat production. To regulate body temperature, mammals must also have the means to retain a certain amount of the heat they produce. Small mammals lose heat more rapidly than larger mammals because they have a greater proportion of surface area to volume (or, equivalently, to their body mass).
Heat is lost through surface area. The higher the surface area–to-