Limits to the exaggeration and diversification of a male sexual trait in the false blister beetle Oedemera sexualis

Differentiation in sexual traits among populations is one of the major predictions of theories of sexual selection and sexual conflict. A balance between maximizing reproductive benefits and minimizing survival costs could explain variation in sexual traits within and between populations. The false blister beetle Oedemera sexualis (Coleoptera: Oedemeridae) has exaggerated sexual traits, that is, sexually dimorphic hindlegs. In this study, we characterized scaling relationships in populations of O. sexualis to evaluate the determinants of sexual trait variation in the species. We quantified sexual dimorphism in body size and hindleg sizes in three representative populations based on distance measurements and an elliptical Fourier analysis. We found significant variation in the degree of sexual dimorphism for body and hindleg sizes among populations. In particular, differentiation in the male hind femur shape, especially the femur width, was conspicuous. Scaling relationships between male hind femur width and body size were best described by logistic models, showing that increases in male hind femur width were constrained for large individuals in all three populations. The degree of constrained growth of the hind femur width differed among populations in accordance with the population means, while the basal growth rates did not. Populations with smaller mean values for sexual traits showed more limited sexual trait exaggeration, contrary to the predictions based on resource competition among body parts. The latitudinal cline in femur widths suggests that environmental constraints on exaggeration might be responsible for sexual trait diversification in O. sexualis.

Automated summary

This study characterized sexual dimorphism in populations of the false blister beetle Oedemera sexualis, finding significant variation in both body size and hindleg sizes among populations. It was observed that the degree of constrained growth of male hind femur widths differed among populations, with smaller mean values for sexual traits showing more limited sexual trait exaggeration within populations. Environmental constraints on exaggeration may be responsible for sexual trait diversification in O. sexualis.