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

Abstract

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.

Keywords: Amphibians; comparative methods; macroevolution; phylogeny; sexual selection.

Figure 1

Differences in body size (log₁₀(female body size (mm))-log₁₀(male body size(mm))) between species that provide parental care (presence) and species that do not (absence). On average it seems that males and female of species that provide care are slightly more similar to one another (mean difference = 2.35 mm) than species that do not (mean difference = 4.50). However, this difference between species that provide care and those that do not is negligible (two-sided Welch t-test: t = −0.55, DF = 66.14, P-value = 0.58).

Figure 2

Log₁₀ body size of females and males of species that do (presence) and do not (absence) provide parental care. Females and males that provide care to their offspring seem to be smaller than those males and females of species that do not provide care. Both males and females that provide care are significantly smaller than females and males that do not provide care (ANOVA: DF = 1, F-value = 61.99, P-value ≤0.001). Male and female symbols are marked directly on boxplots to represent male or female body size.