Load lightening and negotiation over offspring care in cooperative breeders

In cooperative breeders, parents that receive help with offspring care may either maintain their own effort or reduce it in compensation so that offspring gain little. Here, I build on an approach developed by McNamara et al. (McNamara, Gasson, and Houston 1999. Incorporating rules for responding into evolutionary games. Nature. 401:368-371 and McNamara, Szekely, Webb, and Houston 2003. Should young ever be better off with one parent than with two? Behav Ecol. 14:301-310.) to model behavioral negotiation over care in a cooperatively breeding species. I show that parental responsiveness to helper effort should vary with the shape of the parental cost function and that sex differences in the costs of care can give rise to sex differences in response to helpers. Such differences in responsiveness have major implications for the evolution of helping effort. When load lightening occurs, help may benefit mothers or fathers as much as offspring and to different degrees depending on the behavior of the 2. Consequently, when considering the influence of relatedness on helping effort, it is not only the relatedness of helpers to the young they assist that matters. Whether maternal or paternal kin help more, for instance, should reflect the relative degree of compensation by female and male parents. To understand helping, one must consider the whole pattern of relatedness among family members, as well as the ways in which they respond to one another.