Environment-Dependent Sexual Selection: Bateman's Parameters under Varying Levels of Food Availability.

Sexual selection is a potent evolutionary force that has been shown to vary in strength and direction depending on demographic factors such as density and sex ratio. However, the effect of other environmental factors on the mode of sexual selection remains largely unexplored. Here, we tested experimentally how food restriction affects the potential for sexual selection in the male and the female sex function of the simultaneously hermaphroditic freshwater snail Physa acuta. We manipulated food availability and compared Bateman's metrics of sexual selection between groups of five well-fed and five food-restricted snails. Food-restricted individuals had a reduced female reproductive output, suggesting that we successfully manipulated the reproductive resources. Importantly, food restriction reduced the male opportunity for sexual selection (in terms of a lowered variance in male mating success) and led to diminishing returns of mating in both sexes (in terms of nonsignificant Bateman gradients). Furthermore, we observed significant changes in the relative contribution of different fitness components, suggesting stronger postcopulatory selection in the male sex role and stronger fecundity selection in the female sex role under restricted food conditions. This study highlights the need to incorporate ecological factors to better understand how sexual selection operates in the wild.