Obligate brood parasites are cheats that exploit the parental care of other species to raise their young. This reproductive strategy has evolved repeatedly among taxa that provide parental care, particularly among birds (e.g., cuckoos, cowbirds, honeyguides) and insects (e.g., socially parasitic hymenopterans). Crucially, parasitic offspring have no evolutionary interest in their foster siblings, or in their foster parents' residual reproductive value. This chapter considers how host parents are duped into tending for an imposter, and how interactions between hosts and parasitic offspring differ from those among genetic family members. It suggests that the key to predicting the host's co-evolutionary response to brood parasitism, and to explaining how selection influences the behaviour of the young parasite, lies in the virulence of parasitic offspring (defined as the fitness costs that the parasite imposes on its host). Such costs of parasitism influence the strength of selection on hosts to defend themselves against parasitism. With the help of mainly avian examples, the chapter argues that this explains some of the vast diversity both in host defences and in subsequent parasite counter-adaptations. Furthermore, the virulence of the young parasite dictates the social environment in which parasitic offspring extract parental care from their hosts. This in turn explains some of the variation in brood parasitic tactics to secure care from foster parents. Finally, since variation in virulence explains so much about the interactions between brood parasites and their hosts, the chapter considers the factors that cause variation in virulence in the first place.
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