Ecological, evolutionary, and allometric patterns of sexual size dimorphism in turtles

Background/Question/Methods

Macroevolutionary patterns of sexual size dimorphism (SSD) indicate how sexual selection, natural selection, and genetic and developmental constraints mold sex differences in body size. One putative pattern, known as Rensch’s rule, posits that, among species with female-larger SSD, the relative degree of SSD declines with species’ body size, whereas, among male-larger SSD species, relative SSD increases with size. Turtles provide an interesting group to investigate ecological and evolutionary patterns of body size and SSD, because though the body plan and mode of reproduction is conservative in turtles, overall body size and the direction and degree of SSD varies dramatically. Using a dataset of 196 chelonian species (61% of extant species) including all fourteen families and spanning the range of body sizes and SSD, we used appropriate phylogenetic-comparative methods to investigate the correlation in body size evolution between male and female turtles and the validity of Rensch’s rule for the taxon and within its major clades. We also investigated how female body size and degree of SSD relate to annual fecundity. Finally, we investigated the association between habitat use and the degree and direction of SSD.

Results/Conclusions

Male–female correlations in body size evolution are high in Chelonia, although these correlations differ among turtle families. Overall, SSD scales isometrically with body size; Rensch’s rule is valid for only one family, Testudinidae (tortoises), the only family wholly comprised of terrestrial species. The allometry of SSD is not associated with female annual fecundity, indicating that natural selection on female fecundity is probably not a general driver of SSD evolution in turtles. Habitat use (e.g. aquatic, terrestrial) is associated with the direction and allometry of SSD; this may be due to differences in the form of sexual selection on males in terrestrial vs. aquatic environments, and is likely to be the reason that the exclusively terrestrial Testudinidae is the only family that conforms to Rensch’s rule. Because macroevolutionary patterns of SSD can vary markedly among clades, even in a taxon as morphologically conservative as Chelonia, one must guard against inappropriately pooling clades in comparative studies of SSD.