Unavoidable limits on group size in a body size-based linear hierarchy

Competition between individual group members is a key force in shaping social group structure and size. In animal societies, within-group competition may be structured by linear dominance hierarchies, which can be stable if there are minimum differences in competitive ability between adjacently ranked individuals. This requirement constrains maximum group size because only a certain number of clearly differentiated ranks can be fit into the range of competitive abilities between the top and bottom of the hierarchy. We investigated this hypothesis in the body size-based linear dominance hierarchy of the angelfish Centropyge bicolor. Unlike in previous studies, we found that maximum group size in natural C. bicolor groups was not always strictly limited by the range of possible body sizes and the average size difference between adjacent ranks. Oversized groups displayed a compressed body size hierarchy with smaller size differences between adjacent ranks, less effective regulation of subordinate foraging rates to maintain size differences, and greater spatial segregation between adjacently ranked individuals. Our results suggest that when spatial segregation compromises the regulatory mechanisms that maintain clear size hierarchies, groups can become larger than expected by slotting more individuals into a compressed size hierarchy. However, we also found that oversized groups tended to fission into smaller groups, suggesting that they are transient entities and that ultimately the group size limits imposed by the need to maintain a well-defined hierarchy are unavoidable. Copyright 2010, Oxford University Press.