The evolution of fecundity is associated with female body size but not female-biased sexual size dimorphism among frogs.

Sexual size dimorphism (SSD) is one of the most common ways in which males and females differ. Male-biased SSD (when males are larger) is often attributed to sexual selection favouring large males. When females are larger (female-biased SSD), it is often argued that natural selection favouring increased fecundity (i.e. larger clutches or eggs) has coevolved with larger female body size. Using comparative phylogenetic and multispecies regression model selection approaches, we test the hypothesis that among-species variation in female fecundity is associated with the evolution of female-biased SSD. We also ask whether the hypothesized relationship between SSD and fecundity is relaxed upon the evolution of parental care. Our results suggest a strong relationship between the evolution of fecundity and body size, but we find no significant relationship between fecundity and SSD. Similarly, there does not appear to be a relationship between fecundity and the presence or absence of parental care among species. Thus, although female body size and fecundity coevolve, selection for increased fecundity as an explanation for female-biased SSD is inconsistent with our analyses. We caution that a relationship between female body size and fecundity is insufficient evidence for fecundity selection driving the evolution of female-biased SSD.