How can we best measure the energetic trade-offs faced by animals?

Energy is commonly presumed to be a limiting resource for most animals, forcing them to trade off the benefits of allocating energy to competing activities or processes. However, empirical studies have generally not found strong evidence for such trade-offs: increased energy expenditure on one activity often appears to have no impact on other activities. In this Commentary, I examine this conundrum by first exploring whether there are real limits to energy expenditure (which would make trade-offs more likely), and whether these limits are fixed or flexible (the concept of 'plastic floors' and 'plastic/concrete ceilings'). Calculating the investment of energy into different bodily activities or processes is surprisingly hard. The existence of some species that can expend energy at rates far higher than is typical, with seemingly no adverse consequences, poses challenges for our understanding of the limitations on energy consumption. However, it is clear that we need to think beyond simply measuring energy, as the trade-off may involve other currencies such as oxidative damage, immune function or sexual signalling. Moreover, the immediate benefits of an increased allocation of energy to an activity now may be traded off against a cost paid (in a different currency) far in the future. I conclude by suggesting approaches that may more conclusively test for the existence of these trade-offs, emphasising the need for experimental manipulations that disrupt typical patterns of energy expenditure and measure potential impacts over multiple traits and currencies and appropriate time scales.