Differential effects of yolk hormones on maternal and paternal contribution to parental care

In species with biparental care, a female gains fitness benefits from the joint reproductive investment of herself and her partner, but pays only the costs of her own care. Selection thus favours mechanisms that allow females to elicit a higher paternal investment from their partners. In oviparous species, the allocation of maternal yolk androgens to the eggs might represent such a female adaptation to sexually antagonistic selection. To test this hypothesis, we experimentally blocked the effects of maternal yolk androgens by an injection of the antiandrogen flutamide or a control substance in the eggs of great tits, Parus major. We subsequently manipulated the food demand of the brood in a brood size manipulation experiment, and recorded the parental feeding rates. The males' food-provisioning rates were not significantly influenced by the actions of maternal yolk androgens, whereas females adjusted their parental effort to androgen-mediated nestling signals, in particular in enlarged broods. These results show that female great tits do not exploit the male's contribution to parental care by allocating high concentrations of yolk androgens to their eggs. However, they indicate that variation in yolk androgen allocation among females has evolved through a process of coadaptation that matches maternal food provisioning and offspring demand.