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.