In earlier chapters we were concerned with the scaling of structures; in this and later chapters we shall deal mostly with function. The first subject will be the metabolic rates of animals. Energy is needed for maintenance and for all the normal functions of the living animal: for moving about, for feeding, for escaping, and so on. The energy an animal needs for all this comes from the chemical energy contained in food. The total use or turnover of chemical energy is frequently referred to as the metabolic rate, and for reasons that we shall not discuss here, it is convenient as well as reasonably accurate to measure the rate of energy metabolism as the rate of oxygen consumption.

The determination of oxygen consumption is technically easy, and it is so commonly used for estimation of metabolic rate that the two terms often are used interchangeably. This is not correct; for example, an anaerobic organism that depends on nonoxidative metabolic processes has zero oxygen consumption, but it certainly does not have a zero metabolic rate. In the following, however, we shall be concerned mostly with the rate of oxidative metabolism as it almost universally is measured as the rate of oxygen consumption.

In the absence of external activity, metabolism, or oxygen consumption, continues at a rate that can be called the resting or maintenance rate.