Locomotion is the most obvious and most characteristic of animal activities. Like any activity, movement requires energy and, therefore, increases an animal's metabolic rate. From our own experience with walking, running, or swimming, we know that the power demands for movement can be very large. The demands of locomotion may, therefore, be a large component of respiration in the balanced energy equation and a significant energetic cost for the moving animal. This raises a quantitative question: How important are locomotive costs?

This chapter examines the interrelations between the metabolic costs of movement, body size, velocity, distance traveled, time spent traveling, and mode of locomotion – flying, swimming, or running. It provides the basic information required to estimate the metabolic rate of a moving animal. This is achieved by first considering the results of empirical studies relating metabolism to body mass and velocity for terrestrial, aquatic, and aerial locomotion. Because such data are largely derived in the laboratory, equations describing these data have limited ecological relevance when considered in isolation. To use them, we also require some estimate of an organism's speed in nature. Available allometric descriptions of average velocity are, therefore, introduced and then used to calculate the metabolic rates of moving animals. Whenever possible, the ecological importance of these relations is considered by comparing speeds and costs with the demands of existence.