Data from: Macroevolutionary patterns of sexual size dimorphism in copepods

Major theories compete to explain the macroevolutionary trends observed in sexual size dimorphism (SSD) in animals. Quantitative genetic theory suggests that the sex under historically stronger directional selection will exhibit greater interspecific variance in size, with covariation between allometric slopes (male to female size) and the strength of SSD across clades. Rensch’s Rule also suggests a correlation, but one in which males are always the more size variant sex. Examining free-living pelagic and parasitic Copepoda, we test these competing predictions. Females are commonly the larger sex in copepod species. Comparing clades that vary by 4 orders of magnitude in their degree of dimorphism, we find that isometry is widespread and, thus, no support for either Rensch’s Rule or for covariation between allometry and SSD. Our results suggest that selection on both sexes has been equally important. We next test the prediction that variation in the degree of SSD is related to the adult sex ratio. As males become relatively less abundant this leads to a reduction in both inter-male competition and male size. The lack of a correlation across diverse free-living pelagic families provides no support for this hypothesis. However, the fish parasite sea-lice of the family Caligidae, where the males may suffer elevated mortality when they leave the host and roves for sedentary females, provide some qualitatively support with female-biased SSD that is stronger than in many free-living families. As other parasitic copepods in which there is no strong evidence for sex-based differences in mate searching risks also show similar or even more extreme SSD, more likely a range of factors drive these extremes.