Data from: The evolution of sex roles: The importance of ecology and social environment

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.