Fig. (b) This giant ground sloth would have had a lower metabolic rate than smaller animals.

A metabolic rate reflects how quickly organisms process energy and thus sets limits on how fast they can grow and reach maturity, or how much energy they can allocate to reproduction. The basal rate, which is the lowest rate of metabolism in an endotherm such that energy expenditure is at a minimum, is proportional to the mass of the muscle (McNab, 1990). Metabolic rate is proportional to (mass)^<1.0. Therefore, an increase in body size involves an increase in the rate of energy expenditure, and in the amount of food and energy required. An animal can, thus, respond to a shortage in energy by reducing its mass or by reducing its metabolic rate at its fixed mass.

In general, large animals have low metabolic rates compared to small animals (McGowan, 1994). If they do not fulfill their energy requirements, they will not survive. Metabolic rate therefore, is a limiting factor on how large a body can grow.

Metabolism is lower for animals living in colder climates, so these taxa often live much longer and grow much larger (Radford, 1997). Depending on what latitude an organism lives at, it must adjust its diet and activities to store enrgy and survive (McNab, 1990).