A test of Rensch’s rule in dwarf chameleons (Bradypodion spp.), a group with female-biased sexual size dimorphism

Rensch’s rule describes a pattern of allometry in sexual size dimorphism (SSD): when males are the larger sex (male-biased SSD), SSD increases with increasing body size, and when females are the larger sex (female-biased SSD), SSD decreases with increasing body size. While this expectation generally holds for taxa with male-biased or mixed SSD, examples of allometry for SSD consistent with Rensch’s rule in groups with primarily female-biased SSD are remarkably rare. Here, I show that the majority of dwarf chameleons (Bradypodion spp.) have female-biased SSD. In accordance with Rensch’s rule, the group exhibits an allometric slope of log(female size) on log(male size) less than one, although statistical significance is dependent on the phylogenetic comparative method used. In this system, this pattern is likely due to natural selection on both male and female body size, combined with fecundity selection on female body size. In addition to quantifying SSD and testing Rensch’s rule in dwarf chameleons, I discuss reasons why Rensch’s rule may only rarely apply to taxa with female-biased SSD.