Helping Relatives Survive and Reproduce: Inclusive Fitness and Reproductive Value in Brood Parasitism.

Costly help can raise a relative's reproduction, survival, and reproductive value and increase the inclusive fitness of the donor of help. Donor fitness is explored here in conspecific brood parasitism. In this alternative reproductive tactic, some females, "parasites," lay eggs in nests of other females of the same species, "hosts," suppliers of help that alone take care of the offspring. Modeling shows that hosts can gain inclusive fitness if parasitized by relatives whose reproduction or survival is thereby increased. These predictions are explored in waterfowl with frequent brood parasitism, female-biased philopatry, and neighbor relatedness. Approximate estimates based on waterfowl reproductive and life-history data show that host inclusive-fitness gain is often possible with related parasites. The largest gains can be achieved through increased reproduction, but gain is also possible through higher survival of parasites that avoid increased predation and other risks of nesting. Inclusive fitness depends on parasite reproductive value and can be highest for a host parasitized by her mother and for old, senescent hosts with low fecundity, helping young related parasites. These results and observed levels of host-parasite relatedness suggest that being "parasitized" in waterfowl is sometimes neutral or even advantageous because of inclusive-fitness benefits, contributing to evolution of frequent conspecific brood parasitism in this group.