Males and females often have different roles in reproduction, although the origin of these differences have remained controversial. Explaining the enigmatic reversed sex roles where males sacrifice their mating potential and provide full parental care is a particularly long-standing challenge in evolutionary biology. While most studies focused on ecological factors as the drivers of sex roles, recent research highlights the significance of social factors such as the adult sex ratio. To disentangle these propositions, here we investigate the additive and interactive effects of several ecological and social factors on sex role variation using shorebirds (sandpipers, plovers and allies) as model organisms that provide the full spectrum of sex role variation including some of the best known examples of sex role reversal. Our results consistently show that social factors play a prominent role in driving sex roles. Importantly, we show for the first time that reversed sex roles are associated with both male-skewed adult sex ratios and high breeding densities. Furthermore, phylogenetic path analyses provide the first general support for sex ratios driving sex role variations rather than being a consequence of sex roles. Together, these novel results open a new research direction by showing that different mating opportunities of males and females play a major role in generating the evolutionary diversity of sex roles, mating system and parental care.

All data were collected from published literature or open databases. We extensively searched the primary literature reporting the behavioral, demographic, and ecological variables used in this study through Web of Knowledge and Google Scholar. We used the keywords ‘shorebird’, ‘wader’, or the English and Latin name of species in combination with ‘sex ratio’, ‘sex role’, ‘mating system’, ‘polygamy’, ‘parental care’, ‘breeding’, or ‘population monitoring’. We also attempted to locate and screen-specific publication on shorebirds not available through the above databases (e.g., annual population reports, taxon specialists' group reports). First, because ASR was usually the limiting information, we searched for data on this variable to update and extend the dataset used in Liker et al. Nature (2013). Then, for the species with ASR estimates, we also screened the sources for other variables (mating system, parental care, hatching success, breeding density, see below). Data collection was conducted between January 2019 and June 2020. The total number of records and populations and their references are given in Supplementary Table S5 of the paper.