The energetics of movement, from exercise to ecology and evolution

Movement is energetically costly, requiring muscle activity to support and propel the animal as it walks, runs, climbs, swims or flies. In this Review, I examine the metabolic impact of locomotion over different timescales. During locomotion, whole-body energy expenditure can rise by more than an order of magnitude above resting, and these costs arise from activating muscle to exert force as well as the work that muscles perform. Over hours or days, locomotion can dominate daily energy expenditure (i.e. field metabolic rate), particularly when provisioning young, migrating, or during other periods of intense activity. The relationship between muscle force and metabolic cost means that locomotor costs and daily expenditures can be tracked using wearable accelerometers. Over longer timescales, however, the relationship between locomotion and daily expenditure becomes more tenuous. Compensatory trade-offs with other physiological activity, including thermoregulation, growth, maintenance and reproduction, obscure the relationship between daily locomotor activity and daily energy expenditure. Over evolutionary timeframes, variation in daily locomotor activity does not predict variation in daily expenditure. The apparent trade-offs between locomotor expenditure and other physiological tasks suggests that, over long timescales, the cost of locomotion might be best measured by its effects on other systems. The impact of locomotion on growth, reproduction and maintenance should be incorporated into ecological and evolutionary assessments of the costs of movement.