The ecological impact of energy expended on an activity stems from its effect on foraging requirements. For locomotion, the effect of moving each additional unit distance probably depends on the proportional increase in energy expenditure. Other common measures of the cost of locomotion do not reflect the impact of energy expenditure on foraging requirements. In terrestrial mammals, both the effect of body mass and the unit cost itself are very small: moving one kilometre requires less than 2% of all other energy expenditures combined. Total locomotor energy expenditure ranges from 1/2% of daily metabolism for a 10 g mammal to 6% for an elephant. Potential sources of bias in the estimation of these costs include systematic bias in estimates of distance traversed and extra energy required for non-linear locomotion. Because larger mammals can readily locomote at greater speeds, the primary locomotor advantage of large size may not be conservation of energy but of time, which can mean greater safety and more or better food.