Special Evolution Seminar: Christina Riehl, Harvard University

Cheaters and collaborators: the evolution of cooperative animal societies

Most social animals live in family groups, in which cooperation is thought to be partly maintained by kin selection. But how do cooperative interactions evolve among unrelated individuals? Theory predicts that such interactions should be easily undermined by competition and cheating, and that they should arise only under rather restricted circumstances. Recent empirical research, however, shows that cooperative animal societies composed of non-kin are far more widespread than previously thought. In this seminar I will discuss one case study of non-kin cooperation: the Greater Ani, a social bird species in which several unrelated pairs build a single nest and rear a shared clutch of young. Communal nesting comes with benefits – it decreases the likelihood of nest predation – but individual group members also use an array of parasitic and manipulative tactics to increase their own reproductive fitness at the expense of their fellow group members. I will present evidence that the benefits of social nesting are sufficiently high for cooperation to persist despite these selfish behaviors, and I will argue that, in general, direct benefits probably play a more important role in animal societies than previously thought.