Evolutionary Origins and Patterns of Diversification in Animal Brood Parasitism

AbstractBrood parasitism involves the exploitation of host parental care rather than the extraction of resources directly from hosts. We identify defining characteristics of this strategy and consider its position along continua with adjacent behaviors but focus on canonical brood parasites, where parasitism is obligate and hosts are noneusocial (thereby distinguishing from social parasitism). A systematic literature survey revealed 59 independently derived brood parasitic lineages with most origins (49) in insects, particularly among bees and wasps, and other origins in birds (seven) and fish (three). Insects account for more than 98% of brood parasitic species, with much of that diversity reflecting ancient (≥100-million-year-old) brood parasitic lineages. Brood parasites usually, but not always, evolve from forms that show parental care. In insects, brood parasitism often first evolves through exploitation of a closely related species, following Emery's rule, but this is less typical in birds, which we discuss. We conducted lineage-level comparisons between brood parasitic clades and their sister groups, finding mixed results but an overall neutral to negative effect of brood parasitism on species richness and diversification. Our review of brood parasites reveals many unanswered questions requiring new research, including further modeling of the coevolutionary dynamics of brood parasites and their hosts.