SEXUAL DIMORPHISM AS A PROXY FOR SEXUAL SELECTION IN CRETACEOUS OSTRACODES FROM THE US GULF COASTAL PLAIN
We digitized the valve outlines of adult specimens and computed area as a measure of body size. Each outline was approximated as an ellipse, and the ratio of the major to minor axes was used as a measure of valve elongation. Likelihood-based mixture models were used to identify male and female clusters in size-shape space and differences between the inferred sexes were used as estimates of the magnitudes of size and shape dimorphism. In nearly all samples with adequate sample size, we find evidence for two clusters consistent with sex differences. Thus far, we have measured nearly 5,000 ostracodes to produce 109 population-level estimates of dimorphism for 77 species and 33 genera.
Populations vary greatly in the strength of size and shape dimorphism. Males are more elongate than females by as little as 2% or as much as 20%. Males are usually the larger sex but can range from 15% smaller to 30% larger than females. Magnitudes of size and shape dimorphism are not correlated, and neither is related to body size, suggesting that dimorphism differences are not simply a manifestation of selection for body size. Estimates of dimorphism are quite stable within species, mostly similar among congeners, but not very conserved at the family level. Thus, although they are subject to evolutionary divergence, the sexual selection regimes that underpin sex differences can be stable enough to be treated as species-level traits for macroevolutionary analysis.