Aggression, rank and power: why hens (and other animals) do not always peck according to their strength.

Thorlief Schjelderup-Ebbe's seminal paper on the 'pecking' order of chickens inspired numerous ethologists to research and debate the phenomenon of dominance. The expansion of dominance to the broader concept of power facilitated disentangling aggression, strength, rank and power. Aggression is only one means of coercing other individuals, and can sometimes highlight a lack of power. The fitness advantages of aggression may only outweigh the costs during periods of uncertainty. Effective instruments of power also include incentives and refusals to act. Moreover, the stability of the power relationship might vary with the instruments used if different means of power vary in the number and types of outcomes achieved, as well as the speed of accomplishing those outcomes. In well-established relationships, actions or physiological responses in the subordinate individual may even be the only indicator of a power differential. A focus on strength, aggression and fighting provides an incomplete understanding of the power landscape that individuals actually experience. Multiple methods for constructing hierarchies exist but greater attention to the implications of the types of data used in these constructions is needed. Many shifts in our understanding of power were foreshadowed in Schjelderup-Ebbe's discussion about deviations from the linear hierarchy in chickens. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.