Fluctuating selection strength and intense male competition underlie variation and exaggeration of a water strider’s male weapon

Sexually selected traits can reach high degrees of expression and variation under directional selection. A growing number of studies suggest that such selection can vary in space, time and form within and between populations. However, the impact of these fluctuations on sexual trait expression is poorly understood. The water strider Microvelia longipes displays a striking case of exaggeration and phenotypic variation where males display extreme differences in the size of their rear legs. To study the origin and maintenance of this exaggerated trait, we conducted comparative behavioral and morphometric experiments in a sample of Microvelia species. We uncovered differences both in the mating behavior and the degree of sexual dimorphism across these species. Interestingly, M. longipes evolved a specific mating behavior where males compete for egg-laying sites, consisting of small floating objects, to intercept and copulate with gravid females. Through male-male competition assays, we demo...