Parental-care parasitism: how do unrelated offspring attain acceptance by foster parents?

In this review, we describe a new term, parental-care parasitism, that we define as the interaction in which an individual (the parasite) obtains reproductive benefits while reducing or completely eliminating its own costs of parenting by exploiting any type of offspring care provided by other individuals (the hosts). Parental-care parasitism comprises parasitic behaviors ranging from interactions in which just the nest is taken over to those where various combinations of nest, food and offspring care are parasitized. We subdivide parental-care parasitism into 3 categories depending on the strategy used by the parasite to reach host nest: 1) the parasite approaches the nest during host absence, 2) parasite and host adults meet at the nest but no aggression is carried out, or 3) the host tries to evict the parasite at the nest. We also discuss the costs and benefits for both parents and offspring, as well as for hosts and parasites, placing different forms of parental-care parasitism in an evolutionary context within the frameworks of both parental investment theory and coevolutionary arms race theory. Herein, we thoroughly discuss the lack of offspring discrimination found in some species, some populations of the same species and some individuals within the same population on the basis of the coevolutionary arms race theory, and the fact that unrelated offspring attain acceptance by foster parents, contrary to the general predictions of parental investment theory. This review offers a conceptual framework that seeks to link parental investment theory with coevolutionary arms race theory.