Dominance hierarchy

In the zoological field of ethology, a dominance hierarchy (formerly and colloquially called a pecking order) is a type of social hierarchy that arises when members of animal social groups interact, creating a ranking system. Different types of interactions can result in dominance depending on the species, including ritualized displays of aggression or direct physical violence.

In social living groups, members are likely to compete for access to limited resources and mating opportunities. Rather than fighting each time they meet, individuals of the same sex establish a relative rank, with higher-ranking individuals often gaining more access to resources and mates. Based on repetitive interactions, a social order is created that is subject to change each time a dominant animal is challenged by a subordinate one.

In eusocial animals, whether mammals or insects, aggressive interactions often lead to the suppression of reproduction in non-dominant individuals. Such interactions may be ritualised, and an individual's resulting rank in the dominance hierarchy may be advertised to other individuals by visual or chemical cues. Suppression operates in some species on the reproductive hormones of non-dominant individuals. Dominance hierarchies exist in many bird species, first observed in the domestic chicken, where the hierarchy is maintained by pecking with the beak.

There is a spectrum of social organisations in different species, from a full despotic hierarchy to a relatively egalitarian system in species with little intraspecific competition. Dominance varies, too, depending on the context or resource, and on group size.

Dominance is an individual's preferential access to resources over another based on coercive capacity based on strength, threat, and intimidation, compared to prestige (persuasive capacity based on skills, abilities, and knowledge). A dominant animal is one whose sexual, feeding, aggressive, and other behaviour patterns subsequently occur with relatively little influence from other group members. Subordinate animals are opposite; their behaviour is submissive, and can be relatively easily influenced or inhibited by other group members.

For many animal societies, an individual's position in the dominance hierarchy corresponds with their opportunities to reproduce. In hierarchically social animals, dominant individuals may exert control over others. For example, in a herd of feral goats it is a large male that is dominant and maintains discipline and coherence of the flock. He leads the group but shares leadership on a foraging expedition with a mature she-goat who will normally outlast a succession of dominant males. However, earlier work showed that leadership orders in goats were not related to age or dominance. In sheep, position in a moving flock is highly correlated with social dominance, but there is no definite study to show consistent voluntary leadership by an individual. In birds, dominant individuals preferentially select higher perches to put themselves in the best position to detect and avoid predators, as well as to display their dominance to other members of their own species. It has been suggested that decision-taking about the actions of the group is commonly dissociated from social dominance.

Given the benefits and costs of possessing a high rank within a hierarchical group, there are certain characteristics of individuals, groups, and environments that determine whether an individual will benefit from a high rank. These include whether or not high rank gives them access to valuable resources such as mates and food. Age, intelligence, experience, and physical fitness can influence whether or not an individual deems it worthwhile to pursue a higher ranking in the hierarchy, which often comes at the expense of conflict. Hierarchy results from interactions, group dynamics, and sharing of resources, so group size and composition affect the dominance decisions of high-ranking individuals. For example, in a large group with many males, it may be difficult for the highest-ranking male to dominate all the mating opportunities, so some mate sharing is likely to exist. These opportunities available to subordinates reduce the likelihood of a challenge to the dominant male: mating is no longer an all-or-nothing game and the sharing is enough to placate most subordinates. Another aspect that can determine dominance hierarchies is the environment. In populations of Kenyan vervet monkeys, high-ranking females have higher foraging success when the food resources are clumped, but when food is distributed throughout an area they lose their advantage, because subordinate females can acquire food with less risk of encountering a dominant female.

A benefit to high-ranking individuals is increased foraging success and access to food resources. During times of water shortage the highest-ranking vervet females have greater access than subordinates females to water in tree holes. In chacma baboons, the high-ranking males have the first access to vertebrate prey that has been caught by the group, and in yellow baboons the dominant males feed for longer without being interrupted.