Actual versus counterfactual fitness consequences of dispersal decisions in a cooperative breeder

In many species, dispersal from the natal group is crucial for reproduction. However, venturing into a new territory and integrating into a novel social environment is associated with risks and costs. In cooperatively breeding species, moreover, an individual’s prospects of future reproduction upon dispersal or upon remaining in the natal group might crucially depend on other group members’ concurrent decisions to disperse or to stay. We developed a methodology for evaluating how the actual decision of a potential disperser to join or not to join a dispersing individual or coalition affects its fitness in comparison with the fitness consequences it would have had if it had taken the reverse decision – to disperse instead of staying, or to stay instead of dispersing. We then examined 64 dispersal events by unisex coalitions of Arabian babblers who could not breed in their group of origin, and aimed to acquire breeding opportunities by joining another group. For each such dispersal event, we compared the fitness consequences for the members of the dispersing coalition as well as for their same-sex siblings who stayed, to the counterfactual consequences of taking the reverse decision for each of them in turn. Fitness consequences were assessed based on breeding success in the ensuing year as the leading criterion, and on social rank as a secondary criterion. We found that 69% of the dispersers and 38% of the individuals who stayed made fitness-enhancing decisions relative to the alternative they faced, and for an additional 10% of dispersers and 21% of those who stayed, their choice yielded fitness consequences on par with those of the alternative choice. These findings suggest that despite the risky and uncertain circumstances in which dispersal decisions are taken, most individuals make informed, fitness-enhancing dispersal choices, taking into account the concurrent choices of their groupmates.