Should attractive individuals court more? Theory and a test.

This study incorporates individual differences in attractiveness into the theory of condition-dependent sexual selection. This leads to predictions about relationships between sexually selected traits, particularly whether attractive animals should have high or low levels of courtship. The "differential-cost" hypothesis focuses on differences among individuals in costs of display, whereas the "differential-benefit" hypothesis focuses on differences in benefits of display. To demonstrate how these can be distinguished, I examine male courtship in relation to female preference for body size in Trinidadian guppies, Poecilia reticulata. A simple change in the environment (low vs. high light level) reversed a trend of attractive (large) males courting more often than small ones. When large (attractive) males reduced their courtship under high light levels, they also lost their former twofold mating advantage over small males. This reduced mating success supports the differential-cost hypothesis and not the differential-benefit hypothesis for correlations between sexually selected traits: in this example the correlation may become negative when light-mediated risks of predation outweigh benefits of courtship by conspicuous males. The theory and data suggest that positive or negative correlations between sexually selected traits will depend on how costs and benefits interact with one another. These results also suggest how changing environmental conditions could influence the distribution of matings within a population and impede the coevolution of mate choice and individual sexually selected traits.