Evolutionary Significance of Sexual Dimorphism in Amphibians

Abstract

Sexual dimorphism in amphibians -- consistent morphological differences between males and females of the same species -- is among the most phylogenetically variable and mechanistically diverse phenomenon in vertebrate sexual selection, spanning from female-biased body size (the dominant pattern in anurans, where fecundity selection favours larger females) to male-biased body size (characteristic of species with intense male-male combat) and from sexually monochromatic to highly dichromatic species. This study presents the most comprehensive phylogenetic comparative analysis of sexual size dimorphism (SSD) and sexual colour dimorphism (SCD) in amphibians yet conducted, quantifying 12 morphological and chromatic dimorphism metrics on 8,470 specimens from 847 species (614 anurans; 183 salamanders; 50 caecilians) from 84 families, integrated with a time-calibrated phylogeny and ecological trait data. Female-biased SSD was confirmed in 67.4% of anurans and 54.7% of salamanders; male-biased SSD in 18.4% and 28.4% respectively. Rensch's rule -- predicting positive allometry of SSD with body size -- was supported across Anura (slope = 1.24; p < 0.001) but not Caudata. Clutch size was the strongest predictor of female size advantage in anurans (partial R2 = 0.54), and male home range size was the strongest predictor of male size advantage (R2 = 0.47). Sexual colour dimorphism evolved independently at least 84 times across amphibian families and was significantly associated with visual microhabitat (diurnal activity; elevated calling sites) rather than with SSD magnitude.