Sex-role reversal and the Bateman gradient in coucals—females benefit from mating with multiple partners

Conventional sex roles imply that males compete more vigorously with each other for fertilizations, whereas females are more selective in choosing a mate. As a consequence, mating and reproductive success is typically more variable in males (Bateman's principles). However, Charles Darwin already mused that some species may defy these principles, resulting in stronger sexual selection in females than males, but empirical evidence has been mixed. We studied the potential for sexual selection in two sympatric coucals-a bird family with high sex-role variability. In sex-role-reversed black coucals, females compete for territories while males provide parental care. In white-browed coucals, sex roles are flexible. Females of both species had steeper Bateman gradients than males, suggesting females benefit from multiple mating. Male black coucals benefitted from sneaking copulations with their social mate while she prepared clutches for other mates-a male strategy not conceptually included in Bateman's principles that focuses on the number of mating partners, but not on increasing reproductive success by repeatedly mating with the same partner. Our findings support reversed Bateman's principles, with females having a higher potential for sexual selection than males. Sex-role flexibility allows females to secure additional mates and emancipate themselves from parental care, and it allows males to invest in parental care and emancipate themselves from pre-copulatory competition.