Variation in sexual selection on male body size within and between populations of the soapberry bug

Abstract

In spite of its evolutionary importance, little is known about intraspecific variation in sexual selection. In the soapberry bug, Jadera haematoloma, absolute adult sex ratio varies within and between populations because of differences in mortality rates, creating natural opportunities for variation in the intensity of sexual selection. Sex ratios in Oklahoma, U.S.A., are variably male biased, but approach 1:1 in the Florida Keys. Comparisons of reproductive aggregations in Oklahoma and the Florida Keys showed a higher large-male mating frequency only in Oklahoma, mainly in aggregations with above-average male:female ratios. There was no evidence of assortative mating by size in either region. In laboratory tests, large and small males were similar in several mating characteristics, including mate-guarding ability, but large males searched for mates with a significantly greater rate of locomotion. To predict how this difference in searching rate interacted with sex ratio, the equilibrium mating frequencies of large and small males were modelled across the observed range of sex ratios. This model predicted the pattern observed in nature: with increasing sex ratio, the proportional large-male mating advantage increased and then asymptoted. Notably, the asymptotic value was close to the ratio of the average mate-searching locomotion rate of large males to that of small males, further indicating that a size-based difference in mate searching is the main cause of the large-male mating advantage. This species thus shows a mosaic pattern in the intensity of sexual selection on body size, which depends on the interaction of sex ratio and male mating competition.