Sexual size dimorphism, reproductive ecology and antipredator coloration in poison frogs: Rensch's rule revisited

Juan D Carvajal-Castro, Maribel Rojas-Montoya, Fernando Vargas-Salinas, Bibiana Rojas

Body size differences are the most commonly studied form of sexual dimorphism. The extent of sexual size dimorphism (SSD) varies between species and in function of e.g. ecological factors and phylogenetic constraints. Rensch’s rule (RR) states that SSD is larger in species with male-biased SSD and smaller in species with female-biased SSD. RR assessments in amphibians, which tend to have female-biased SSD, have found no consensus in favour or against its validity. Using poison frogs, a group with diverse reproductive ecology and antipredator coloration, we test how the rates of evolution of male and female body size vary over evolutionary time, and investigate whether SSD follows RR and is influenced by antipredator coloration and tadpole-deposition site types. We show that SSD in poison frogs does not follow RR in general, but the tadpole-deposition site type used shapes SSD in a way opposite to RR, likely because fecundity selection favours larger females and clutches. Aposematism, likely through decreased predation pressure on both sexes, favours the evolution of larger body sizes and less pronounced (or absent) SSD. We underscore the importance of integrating ecologically relevant traits into SSD studies to better understand the selective pressures underlying sex size differences in nature.