Sexual selection operates by acting on variation in mating success. However, since selection acts on whole-organism manifestations (i.e., performance) of underlying morphological traits, tests for phenotypic effects of sexual selection should consider whole-animal performance as a substrate for sexual selection. Previous studies have revealed positive relationships between performance and survival, that is, natural selection, but none have explicitly tested whether performance may influence reproductive success (through more matings), that is, sexual selection. Performance predicts dominance in some species, implying the effects of sexual selection, but how it does so has not been established, nor is it certain whether performance might be a by-product of selection for something else, for example, elevated circulating testosterone levels. We investigated the potential for sexual selection on sprint speed performance in collared lizards (Crotaphytus collaris), considering the potential mediating effects of circulating hormone levels. Among territorial, adult male collared lizards, only sprint speed significantly predicted territory area and number of offspring sired as determined by genetic paternity analysis. Body size, head size, and hind limb length had no effect. Neither plasma testosterone levels nor corticosterone levels correlated with sprint speed, territory area, or number of offspring sired. Thus, our results provide a direct link between whole-animal performance and reproductive success, suggesting that intrasexual selection can act directly on sprint speed performance and drive the evolution of underlying morphological traits.

The evolution of exaggerated structures that function as weapons in sexually dimorphic species is often explained by intra-sexual selection related to male combat, as these structures are used in fights among males and can determine dominance during such interactions. In many lizard species, males have a larger head than females, a condition attributed to intra-sexual selection. Although head size has been shown to predict dominance in lizards, the way that head size influences dominance remains unclear. We staged interactions between body size-matched male Venerable Collared Lizards (Crotaphytus antiquus) in the laboratory to test the hypothesis that harder-biting males would be dominant over males with weaker bite-force performance. Winners of staged interactions bit significantly harder than losers, but no measured morphological trait was significantly different between winners and losers. This result indicates the strong role of weapon performance, as opposed to weapon morphology, in determin...

For almost 40 years, studies of whole-organism performance have formed a cornerstone of evolutionary physiology. Although its utility as a heuristic guide is beyond question, and we have learned much about morphological evolution from its application, the ecomorphological paradigm has frequently been applied to performance evolution in ways that range from unsatisfactory to inappropriate. More importantly, the standard ecomorphological paradigm does not account for tradeoffs among performance and other traits, nor between performance traits that are mediated by resource allocation. A revised paradigm that includes such tradeoffs, and the possible ways that performance and fitness-enhancing traits might affect each other, could potentially revivify the study of phenotypic evolution and make important inroads into understanding the relationships between morphology and performance and between performance and Darwinian fitness. We describe such a paradigm, and discuss the various ways that performance and key life-history traits might interact with and affect each other. We emphasize both the proximate mechanisms potentially linking such traits, and the likely ultimate factors driving those linkages, as well as the evolutionary implications for the overall, multivariate phenotype. Finally, we highlight several research directions that will shed light on the evolution and ecology of whole-organism performance and related life-history traits.

Summary: While it is generally accepted that the evolution of morphological traits is mediated by selection on whole‐animal performance, few studies have examined how variation in performance explains variation in fitness. Most studies to date have studied how performance predicts survival, and these typically examine only a single age class. Maximal sprint speed was measured for hatchling and adult Collared Lizards (Crotaphytus collaris), in addition to morphological measurements, in order to determine how well annual survival could be predicted by performance and whether the relationship between performance and survival differed between age classes. Logistic regression revealed that no morphological trait or sprint speed performance predicted survival in adults. However, sprint speed predicted survival in hatchlings. Further, hindlimb length differed between survivors and non‐survivors and correlated with sprint speed, suggesting that the evolution of hindlimb length in hatchlings may be driven by selection on sprint speed performance. Selection on sprint speed may be mediated directly by predators where hatchlings cannot attain speeds sufficient to escape predators or indirectly where slower individuals use a greater proportion of their maximal capacity than faster individuals while escaping predators, perhaps causing elevated stress levels or a higher expenditure of energy stores.

1 Natural selection is generally thought to operate on organisms’ maximal abilities to perform ecological tasks in nature (i.e. whole-animal performance). However, selection may instead operate on the manner in which that performance trait is used (i.e. ‘ecological performance’). 2 I tested whether survival of adult Collared Lizards (Crotaphytus collaris) depended on maximal sprint speed capacity or on the speed at which they actually performed two important ecological tasks: chasing a prey item and escaping a predator. 3 Maximal sprint speed did not significantly predict annual survival as determined by daily censuses of the site the following season, nor did speed while foraging, but speed while escaping a predator did. Survival also was positively related to the proportion of maximal capacity used while escaping. 4 These results suggest that selection may operate on ecological performance that is constrained, but not necessarily determined, by maximal performance capacity, suggesting that researchers should consider how organisms utilize maximal performance in nature when testing for a performance–survival relationship.