Relatedness and the evolution of conspecific brood parasitism

In conspecific brood parasitism (CBP), a parasitic female takes advantage of the parental care performed by a host female by laying eggs in the nest of the host. The host female raises the offspring of the parasitic female as well as her own. In species where local females are related, direct costs for the host might be more than compensated for by gains in inclusive fitness through increased reproduction of a related parasite, but the role of relatedness in CBP is debated. This inclusive-fitness model of parasitism, structured as a game between host and parasite, suggests that both females can gain inclusive fitness and that host-parasite relatedness can therefore facilitate the evolution of CBP. Crucial assumptions are that there is kin discrimination and a potential for host resistance to parasitism by unrelated females but close relatives are accepted. The cost of parasitism in terms of reduced clutch size or offspring survival for the host must not be large; otherwise, parasitism will reduce her inclusive fitness. Therefore, if these costs are high, it does not benefit a host to accept a parasite, even if the parasite is closely related. The secondary female may still have higher fitness from parasitism, but if the costs are high, she should parasitize an unrelated host, not a relative. This requires that the reduction in parasite success that a host can cause by resistance is not too large; otherwise, it will be better for the secondary female to parasitize an accepting related host or to nest solitarily. For these reasons, host-parasite relatedness is most likely to occur in animals where costs of being parasitized are low and host resistance can markedly reduce the success of an unrelated parasite. When costs are higher, parasitism of unrelated hosts may be better, and if host resistance strongly reduces parasite success, solitary breeding is preferable. In some cases, CBP is directly advantageous for the host, and it may sometimes evolve in close connection with cooperative breeding, which is also considered in the model. Some but not all empirical results support these ideas, and more detailed studies of behavior, relatedness, and reproduction of host and parasite are needed for critical tests.