The mother-in-law effect.

Individuals often derive considerable evolutionary benefit from manipulating others. In the majority of cases, manipulation involves direct interactions between individuals. In the dung beetle, Onthophagus taurus, females mated with large males provide more resources to their offspring. Here, we demonstrate, however, that this may result in manipulation that extends across generations: the care that a mother provides to a developing son influences the parental effort of his mate (the mother's daughter-in-law (DIL)). Maternal care associated with constructing heavier brood masses has previously been shown substantially to influence offspring size, male mating success and female survival and fecundity in this species. The mother-in-law effect that we document here is, however, the ability to produce large sons from relatively lighter brood masses. Our results demonstrate not only that females are able to manipulate the parental effort of DILs that they do not directly encounter, but that provisioning relatively lighter brood masses may have evolutionary benefits that trade off against the considerable benefits of producing heavy brood masses.