Sexual size dimorphism is not associated with the evolution of parental care in frogs

Sex differences in parental care are thought to arise from differential selection on the sexes. Sexual dimorphism, including sexual size dimorphism (SSD), is often used as a proxy for sexual selection on males. Some studies have found an association between male‐biased SSD (i.e., males larger than females) and the loss of paternal care. While the relationship between sexual selection on males and parental care evolution has been studied extensively, the relationship between female‐biased SSD (i.e., females larger than males) and the evolution of parental care has received very little attention. Thus, we have little knowledge of whether female‐biased SSD coevolves with parental care. In species displaying female‐biased SSD, we might expect dimorphism to be associated with the evolution of paternal care or perhaps the loss of maternal care. Here, drawing on data for 99 extant frog species, we use comparative methods to evaluate how parental care and female‐biased SSD have evolved over time. Generally, we find no significant correlation between the evolution of parental care and female‐biased SSD in frogs. This suggests that differential selection on body size between the sexes is unlikely to have driven the evolution of parental care in these clades and questions whether we should expect sexual dimorphism to exhibit a general relationship with the evolution of sex differences in parental care. Previous research suggests that sex differences in parental care are associated with the magnitude and direction of differential selection on the sexes. The majority of these studies focus on the correlations between male‐biased sexual size dimorphism (SSD) and female parental care, even though, female‐biased SSD is much more common throughout the animal kingdom. Our study evaluates the correlation between parental care and female‐biased SSD and suggests that unlike male‐biased SSD and its strong correlation with female parental care, there is little to no correlation between female‐biased SSD and parental care, or the type of care provided (biparental, maternal, paternal).