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Abstract
The origin of sexual size dimorphisms (SSD) has long been a central topic in evolutionary
biology. However, there is little agreement as to which factors are most important
in driving the evolution of SSD, and several hypotheses concerning SSD evolution have
never been tested empirically. Emydid turtles include species with both male and female-biased
SSD, and some emydids exhibit among the most extreme SSD in tetrapods. Here, we use
a comparative phylogenetic approach in emydids to analyze the origins of SSD and test
several hypotheses for the evolution of SSD, some for the first time. We test the
Fairbairn-Preziosi hypothesis for the origin of Rensch's rule, and support it in lineages
with male-biased SSD but not those with female-biased SSD. We also find support for
the secondary ecological dimorphism hypothesis, which proposes that selection for
ecological divergence between sexes exaggerates preexisting SSD. Finally, we find
only equivocal support for the Bolnick-Doebeli hypothesis, which relates intersexual
ecological divergence to interspecific ecological divergence. Our results also illustrate
how global analyses of SSD may mislead in groups in which the factors that drive the
evolution of SSD vary among clades.