Population density affects male mate choosiness and morphology in the mate-guarding amphipod Gammarus roeselii (Crustacea: Amphipoda)

Abstract

The extent to which males exert mate choice (mate choosiness) depends on various environmental factors associated with the costs of mate acquisition and mate quality assessment, and the risk of remaining unmated. When low population densities translate into low mate encounter rates, this should result in reduced mate choosiness. In a system with overall high mating costs for males (due to precopulatory mate guarding, called amplexus), we investigated amplexus re-establishment after separating mating pairs and presenting focal males with a novel, size-matched female. Male Gammarus roeselii from a low-density (LD) population spent significantly less time until amplexus re-establishment than males from a high-density (HD) site, suggesting reduced choosiness in LD males. We experimentally ruled out alternative explanations such as differences in locomotor activity or mate finding abilities. To further demonstrate independent evolutionary trajectories of our study populations, we examined divergence in morphology and found males to differ in several traits involved in mate assessment and mate guarding. Together, we report phenotypic divergence in behavioural and morphological traits that is probably driven by contrasting selective regimes arising from different population densities. Our study exemplifies how population parameters contribute to phenotypic diversification of sexually selected traits during local adaptation along ecological gradients.