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. 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

An agile bobcat (Lynx rufus) leaps across rocks. (Photo by Hans Reinhard. Bruce Coleman, Inc. Reproduced by permission.)

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-mass ratio, the greater the rate of heat loss. Fur helps to insulate a small mammal to some degree, but often it is not enough to prevent the high rate of heat loss. Small mammals often compensate by obtaining more calories per unit of time by continuously eating foods that are quickly digested and absorbed. Larger mammals' surface area-to-mass ratio decreases as their mass increases, and they lose heat at a lower rate.

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