What determines the basal rate of metabolism?

The basal rate of metabolism (BMR) is the most reported estimate of energy expenditure in endotherms. Its principal determinant is body mass, but BMR also correlates with a variety of behavioral and ecological factors that do not determine basal rate: they are byproducts of the mechanisms that are its determinate. In mammals, mass-independent BMR increases when muscle mass is >40% of total body mass and BMR is then >= 100% of the value expected from body mass. Mammals with muscle masses <30% of body mass have lower BMRs, a diminished capacity to regulate body temperature and often have reduced activity levels. At muscle masses <42% of body mass, birds have body temperatures and basal rates higher than mammals with the same muscle mass. Their high basal rates derive from fast blood flow and increased mitochondrial density in their pectoral muscles. These enhancements also occur in the flight muscles of bats. Oxygen transport to the pectoral muscles of birds is facilitated by an increase in heart mass and hematocrit. This arrangement avoids transporting a large muscle mass to fuel flight, thereby reducing the cost of flight. Pectoral muscle masses <9% of body mass correlate with a flightless condition in kiwis, rails and ducks but some fruit pigeons have BMRs as low as those measured in kiwis, while remaining volant. The mass-independent BMRs of endotherms principally reflect changes of muscle activity and mass. An increase in muscle mass may have contributed to the evolution of endothermy.