Covariation of sexual size dimorphism and adult sex ratio in parasitic nematodes

Abstract

Females are larger than males in most invertebrate taxa, a phenomenon believed to result from the pressures exerted on female body size by size-dependent fecundity. Male-male competition, which can act on male body size, is not thought to play as important a role in the evolution of sexual size dimorphism in invertebrates as it apparently does in some vertebrate groups. Here, using a comparative approach, the relationship between sexual size dimorphism and adult sex ratio is examined across 46 natural populations (41 species) and 30 experimental populations (21 species) of parasitic nematodes. If male-male competition via physical contests is important, relative male size should increase as the sex ratio becomes less female-biased. This is exactly what was found in the analyses, where residuals of male size regressed on female size were used as measures of sexual size dimorphism. This result was independent of any phylogenetic influences, and was obtained for both natural and experimental nematode populations. In addition, there was no evidence of any allometric relationship between male and female body size. The average ratio of male size to female size was roughly constant across all species and independent of body size. The results are consistent with a role for male-male competition in explaining specific deviations from the average ratio of male to female body size, suggesting a significant role for sexual selection in the evolution of nematode body sizes.