The rate of energy metabolism required to maintain a nongrowing, unstressed, endothermic animal at rest. BMR is measured in terms of heat production per unit time and is usually expressed in kilojoules of heat released per square metre of body surface per hour (kJm⁻²h⁻¹). It indicates the energy consumed in order to sustain such vital functions as heartbeat, breathing, nervous activity, active transport, and secretion. Different tissues have different metabolic rates (e.g. the BMR of brain tissue is much greater than that of bone tissue) and therefore the tissue composition of an animal determines its overall BMR. For organisms generally, BMR is proportional to body weight according to Kleiber’s law, which is an example of an allometric equation (see allometric growth); small animals tend to have a higher metabolic rate per unit weight than large ones. Compare standard metabolic rate.