Evolutionary origins of cooperative and communal breeding: Lessons from the crotophagine cuckoos

Most cooperatively breeding birds live in family groups, in which a breeding pair is assisted by genetically related “helpers” who do not reproduce. In some species, however, several breeding adults reproduce in a single nest and cooperatively provide parental care to the mixed brood of nestlings. Recent comparative analyses have convincingly argued that these two forms of sociality—family-based helping and communal breeding—are distinct, since they differ in key respects and have apparently arisen along separate evolutionary trajectories. In this review, I first suggest that the similarities between these systems are also informative for understanding the evolution and persistence of cooperative societies. For example, many communal breeders also have non-reproductive helpers, suggesting that similar selective pressures may help maintain both kin-based helping and communal breeding despite their disparate origins. I next discuss hypotheses for the origins of communal breeding in the anis and allies (Crotophaga and Guira), a lineage of Neotropical cuckoos, focusing on the evolutionary and behavioral links between communal breeding and brood parasitism. Several traits that are ancestral to cuckoos, including rapid offspring development and the ability to lay large, indeterminate clutches, are implicated as evolutionary precursors of both strategies. Finally, I discuss recent research on alternative reproductive tactics in the greater ani (Crotophaga major), in which communal breeding, conspecific brood parasitism, and helping by both relatives and non-relatives are common. The coexistence of all four behaviors within the same population provides a rare opportunity to dissect the costs and benefits of group living and parental care, and, ultimately, to examine the selective pressures favoring cooperation under varying degrees of genetic relatedness.