Males and females often have different roles in reproduction, although the origin
of these differences has 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 that reversed sex roles are associated with both male-skewed
adult sex ratios and high breeding densities. Furthermore, phylogenetic path analyses
provide general support for sex ratios driving sex role variations rather than being
a consequence of sex roles. Together, these important results open future research
directions 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.
