Meeting Abstract
We investigated large-scale patterns of the evolution of sexual shape dimorphism (SShD) and sexual size dimorphism (SSD) in turtles, through the following hypotheses: H1) Female and male turtles consistently differ in shape in all turtle species. H2) Female and male turtles show additional differences within certain habitats. H3) Rates of evolution of SSD and SShD vary among turtle families. H4) Rates of evolution of SSD and SShD vary among habitats. These hypotheses were tested using a data set of 53 3-dimensional landmarks digitized at scute triple-junctions of 2397 specimens of 255 turtle species (of which 226 had both males and females). We tested our first hypothesis by creating separate phylomorphospaces for both females and males, and analyzing the differences using phylogenetic MANOVA. We tested the first parts of our third and fourth hypotheses, that rates of evolution of SSD will vary among turtle families and among different habitats, using a Brownian model of rate variation, implemented in the R package phytools. We tested for differences in the rates of SShD evolution using a modification of the compare.evol.rates function in the R package geomorph. There was no evidence of consistent shape differences between females and males across all turtles. SSD in aquatic species evolved significantly faster than in terrestrial species. SSD in aquatic species may evolve faster because of a greater variety of selective pressures that may be encountered in water as opposed to land, or different modes of sexual selection in different environments. The lack of consistent differences between sexes was surprising, but females and males of different species may achieve the same functional result with a variety of different shapes. Future studies will analyze SSD and SShD in turtles of different sizes, latitudes, and ecosystems.
