How Do We Measure the Cost of Whole-Organism Performance Traits?

Whole-organism performance traits, such as maximal speed and endurance capacity are undoubtedly costly, but we know little about how or when all of the costs associated with performance are paid to individuals or how to measure them. To understand how performance traits might be involved in trade-offs with other life-history traits it is critical to determine the development, production, and maintenance costs of performance traits, as well as how each of these changes with increased or decreased use of the performance trait. We discuss the advantages and disadvantages of several potential phenotypic measures of dynamic whole-organism performance that may be used in life-history studies, including direct performance measures; metabolic rates; ecological cost of transport; and changes in metabolic rate after training. We use the first approach, direct performance measures, to show trade-offs between endurance capacity and several traditional life history variables in phrynosomatid lizards. The largest problem currently in determining the costs of performance traits and how those costs might lead to life-history trade-offs is that there are estimates of performance costs in very few taxa, and when there are, those species typically are not studied with respect to "traditional" life-history traits.