Spatial variation in sex ratio and density explains subtle changes in the strength of size-assortative mating in Edessa contermina (Hemiptera: Pentatomidae)

Abstract

Size-assortative mating (SAM) is usually explained by the mate-choice hypothesis. However, this hypothesis does not consider the effects of variations in the intensity of mating competition on mate choice and SAM. For example, in populations with male-biased operational sex ratios (OSR) and high densities, large males will have more advantages during mating competition and will have access to the most mating opportunities with large, preferred females (i.e. mating competition hypothesis). In this study, we investigated the effects of mating competition and mate choice on the strength of SAM in Edessa contermina (Hemiptera: Pentatomidae). We also described sexual size dimorphism and the mating system of this Neotropical stink bug in a conservation area of the Brazilian savanna. We used density as a proxy for mating competition intensity because it was positively correlated with OSR. Size-assortative mating was more consistent under intense mating competition. However, males copulated with relatively larger females as population density increased. Males assortatively mated based on size in the first and second copulation events observed, while both female matings were random. Thus, male mate choice may promote SAM in E. contermina, but its strength is affected by variations in the intensity of mating competition. Hence, we provided empirical support for mating competition as a potential mechanism promoting subtle changes in SAM. We discussed potential implications of this mechanism on the mating patterns of pentatomids.