AbstractSexually 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 striderMicrovelia longipesdisplays 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 ofMicroveliaspecies. We uncovered differences both in the mating behavior and the degree of sexual dimorphism across these species. Interestingly,M. longipesevolved a specific mating behavior where males compete for egg-laying sites, consisting of small floating objects, to intercept and copulate with gravid females. Field observations revealed rapid fluctuation inM. longipeshabitat stability and the abundance of egg-laying sites. Through male-male competition assays, we demonstrated that male rear legs are used as weapons to dominate egg-laying sites and that intense competition is associated with the evolution of rear leg length exaggeration. Paternity tests using genetic markers demonstrated that small males could only fertilize about 5% of the eggs when egg-laying sites are limiting, whereas this proportion increased to about 20% when egg-laying sites become abundant. Furthermore, diet manipulation and artificial selection experiments also showed that the exaggerated leg length inM. longipesmales is influenced by both genetic and nutritional factors. Collectively, our results highlight how fluctuation in the strength of directional sexual selection, through changes in the intensity of male competition, can drive the exaggeration and phenotypic variation in this weapon trait.
Larval competition risk shapes male–male competition and mating behavior in an anuran
