Abstract
BackgroundThe evolution of sexual size dimorphism (SSD) is likely constrained by life history. Using phylogenetic comparative methods, we examined correlations between SSD among anurans and their life history traits, including egg size, clutch size, mating combat, and parental care behaviour. We used sexual dimorphism index (SDI = Body-sizefemale /Body-sizemale –1) as the measurement for SSD. Body size, life history and phylogenetic data were collected from published literature. Data were analysed at two levels: all anuran species and within individual families.ResultsFemale-biased SSD is the predominant form in anurans. SSD decreases along with the body size increase, following the prediction of Rensch’s rule, but the magnitude of decrease is very small. More importantly, female body size is positively correlated with both fecundity related traits, egg size and clutch size, and SDI is also positively correlated with clutch size, suggesting fecundity advantage may have driven the evolution of female body size and consequently leads to the evolution of female-biased SSD. Furthermore, the presence of parental care, male parental care in particular, is negatively correlated with SDI, indicating that species with parental care tend to have a smaller SDI. A negative correlation between clutch size and parental care further suggests that parental care likely reduces the fecundity selection pressure on female body size. On the other hand, there is a general lack of significant correlation between SDI and the presence of male combat behaviour, which is surprising and contradictory to previous studies.ConclusionsWe find clear evidence to support the ‘fecundity advantage hypothesis’ and the ‘parental care hypothesis’ in shaping SSD in anurans. Nevertheless, the relationships of both parental care and combat behaviour to the evolution of SSD are complex in anurans and the extreme diversity of life history traits may have masked some potential interesting relationships. Our study represents the most comprehensive study of SSD in anurans to date.
Highlights
The evolution of sexual size dimorphism (SSD) is likely constrained by life history
Parental care is likely associated with the body size of the care-providing parent, and it may lead to dimorphism between two sexes
Egg size and clutch size are potentially correlated with female body size and size dimorphism index (SDI), and we considered both as explanatory variables in a linear multiple regression analysis
Summary
The evolution of sexual size dimorphism (SSD) is likely constrained by life history. We examined correlations between SSD among anurans and their life history traits, including egg size, clutch size, mating combat, and parental care behaviour. Sexual size dimorphism (SSD), where males and females differ in body size, is the most conspicuous difference between the two sexes in many animals. SSD has been hypothesized as adaptation of males and females to their disparate reproductive roles and associated differences in ecology and life history strategies [2,3]. Parental care is likely associated with the body size of the care-providing parent, and it may lead to dimorphism between two sexes. Parental care activities may result in the small body size of the care-providing parents because the energy expenditure on breeding activities may constrain growth Parental care activities may result in the small body size of the care-providing parents because the energy expenditure on breeding activities may constrain growth (e.g. [7])
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