Sex differences in group-joining decisions in social fish

In social animals, decisions about which group to join can have important fitness consequences and may be particularly critical when groups have a strict dominance hierarchy that relates to reproductive success. Choosing a large group may maximize safety, but choosing to join a small group can minimize the delay until a dominant reproductive position can be reached. We explored this trade-off between safety and rank using Neolamprologus pulcher, a cichlid fish in which individuals conform to a rigid within-group dominance hierarchy. In this species, females typically inherit dominant positions by ascending in rank, while males commonly take over a dominant breeding position by dispersing into a new group. Because females have fewer opportunities to switch groups, we predicted that females would place higher value on social rank within their group than would males. To test this, we examined male and female N. pulcher’s preferences for joining large groups at a low rank versus joining small groups at a high rank. Males showed clear preferences for larger (presumably safer) groups, while females showed no such preferences. In a second experiment, we held joining rank constant, and found that both males and females showed a strong preference for large groups. Our results suggest that when joining a group, females consider both rank and safety whereas males are primarily concerned with safety. Our results help to elucidate the factors underlying social decision making in a cooperative breeder.