Sexual Size Dimorphism in Anurans: Ontogenetic Determination Revealed by an Across-Species Comparison

Abstract

Ever since Darwin, why and how sexual size dimorphism (SSD) arises among the animal kingdom has puzzled evolutionary biologists. While comparative analyses focus on ultimate causes of this phenomenon, few attempts have been made to quantify its ontogenetic basis. Here we deal with the latter issue using the skeletochronology-based demographic data gathered from the literature for anurans, a group of indeterminately growing vertebrates typically exhibit female-biased SSD. Both non- and phylogenetically corrected analyses showed (1) SSD was correlated positively with the sex difference in adult mean age but negatively with the corresponding difference in annual growth rate, (2) the sex difference in adult mean age tended to be negatively correlated with that in growth rate, and (3) the sex difference in adult mean age was positively correlated with that in age at maturity. We conclude that in anuran lineages, the degree of SSD bias towards females is mediated by sexual bimaturation: females mature later than males, and thus live longer and attain larger despite growing slower. Stronger fecundity selection benefits to females, despite incurring growth cost, in combination with the benefits to males from increased reproductive opportunities resulting from earlier maturity, should be the primary evolutionary drivers of the typically female-biased SSD.