Phenotypic plasticity, sexual dimorphism and Rensch's rule in turtles

Rensch's rule (RR) states that sexual size dimorphism (SSD) increases with body size in male-biased SSD taxa, and decreases with body size in female-biased SSD taxa. This pattern implies that males change body size faster than females through evolutionary time, and that sexual selection might be the main evolutionary force behind this pattern. Phenotypic plasticity has been proposed as a proximate potential mechanism of RR by affecting body growth in a differential manner in males and females. In this dissertation I studied the role of phenotypic plasticity as a mechanism affecting sexual size and shape dimorphism in two turtle species exhibiting contrasting SSD: Chelydra serpentina that exhibits male-biased SSD, and Podocnemis expansa that exhibits female-biased SSD. I found that males of the male-biased SSD species exhibited increased growth plasticity relative to females, thus supporting the differential plasticity hypothesis as a mechanism that facilitates body size enhancement and thus the evolution of SSD in a manner consistent with RR. Whether differential plasticity enhances body size of the larger sex in general, or of males in particular, was addressed in the second experiment. In this second experiment I found no differences in growth plasticity between males and females, thus failing to support the differential plasticity hypothesis in the female-biased SSD species. The combined evidence from both studies suggest that differential plasticity is neither specific to males nor to the larger sex, but instead that it is species-specific. Whether turtles follow Rensch's rule or not was tested in a third analysis by correlating body size of males and females of 138 turtle species in a phylogenetic context and at multiple taxonomic levels. I found no support for Rensch's rule in turtles at the order level and in most families. Instead, results suggest that SSD has evolved independently of the species average body size in those clades. I also found that the family Chelydridae and one subclade within the Testudinidae family follow Rensch's rule, while the Podocnemididae family follows a pattern exactly opposite to Rensch's rule. These results are in agreement with the result of the common garden experiments. Namely, I found support for sex-specific growth plasticity as a mechanism facilitating RR in C. serpentina, a species belonging to the family Chelydridae that follows RR. Furthermore, no support for sex-specific growth plasticity as a mechanism facilitating RR was detected in P. expansa, a species belonging to the family Podocnemididae that does not follow RR. Further research is warranted to test the role of the sex-specific growth plasticity in female-biased SSD species belonging to clades that follow Rensch's rule. Likewise, similar studies should be conducted in male-biased SSD species belonging to clades that do not follow Rensch's rule.