Natural variation in social conditions affects male mate choosiness in the amphipodGammarus roeselii

Abstract

The extent of male mate choosiness is driven by a trade-off between various environmental factors associated with the costs of mate acquisition, quality assessment and opportunity costs. Our knowledge about natural variation in male mate choosiness across different populations of the same species, however, remains limited. In the current study, we compared male mate choosiness across ten natural populations of the freshwater amphipod Gammarus roeselii (Gervais, 1835), a species with overall high male mating investments, and evaluated the relative influence of population density and sex ratio (both affecting mate availability) on male mate choosiness. We investigated amplexus establishment after separating mating pairs and presenting focal males with a novel, size-matched female from the same population. Our analysis revealed considerable effects of sex ratio and (to a lesser extent) population density on time until amplexus establishment (choosiness). Male amphipods are able to perceive variable social conditions (e.g. sex ratio) and modify their mating strategy accordingly: We found choosiness to be reduced in increasingly male-biased populations, while selectivity increases when sex ratio becomes female biased. With this, our study expands our limited knowledge on natural variations in male mate choosiness and illustrates the importance of sex ratio (i.e. level of competition) for male mating decisions in natural environments. Accounting for variation in sex ratios, therefore, allows envisioning a distinctive variation of choosiness in natural populations and highlights the importance of considering social background information in future behavioural studies.

Small-scale population structuring results in differential susceptibility to pesticide exposure

“…Both antennae are characterized by sexual dimorphism (longer antennae in males), with the second pair of antennae being used to locate potential mating partners [99]. In female-biased populations, males become more selective and choose partners of higher reproductive potential [99][100][101]. The variation in length of their antennae is probably a response to differences in population densities or sex-ratios that alter the need to find and assess mating partners [99], and not miss the short period after moulting in which copulation is possible [100].…”