Models of hierarchy formation in animal societies

Groups in many species of mammals and birds form hierarchies: these hierarchies are frequently linear or near linear in moderate size groups. Two models have been proposed, both implicitly and explicitly, in the literature to explain the process of hierarchy formation. One explanation contends that animals win their places in a hierarchy through a round robin competition, and the other states that there is a high statistical correlation between position and rank on some trait or composite of traits thought to predict dominance. It is demonstrated here that both models require stringent mathematical conditions to predict linear and near linear hierarchies and that available data indicate that these conditions are not met. Most of the data cited are from studies of dominance in chickens, but the same mathematical conditions are necessary to generate linear and near linear hierarchies by the round robin and correlational models in any species. Thus, the findings presented here have application to any species of animals forming linear and near linear hierarchies.