Dominance hierarchy
In the zoological field of ethology, a dominance hierarchy 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.
Definitions
Dominance is an individual's preferential access to resources over another based on coercive capacity based on strength, threat, and intimidation, compared to prestige. 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.Dominance
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.When individuals seek high rank
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.Benefits
Foraging success
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.In many bird species, the dominant individuals have higher rates of food intake. Such species include dark-eyed juncos and oystercatchers. The dominant individuals in these groups fill themselves up first and fill up more quickly, so they spend less time foraging, which reduces the risk of predation. Thus they have increased survival because of increased nutrition and decreased predation.
Reproductive success
In primates, a well-studied group, high rank brings reproductive success, as seen in a 1991 meta-analysis of 32 studies. A 2016 study determined that higher status increased reproductive success amongst men, and that this did not vary by type of subsistence. This contradicts the "egalitarian hypothesis", which predicts that status would affect reproductive success more amongst foragers than amongst nonforagers.High-ranking bonnet macaque males have more access to fertile females and consequently partake in most of the matings within the group; in one population, three males were responsible for over 75% of matings. In this population, males often vary in rank. As their rank improves, they gain more exclusive time with fertile females; when their rank decreases, they get less time. In many primates, including bonnet macaques and rhesus monkeys, the offspring of high-ranking individuals have better fitness and thus an increased rate of survival. This is most likely a function of two factors: The first is that high-ranking males mate with high-ranking females. Assuming their high rank is correlated with higher fitness and fighting ability, this trait will be conferred to their offspring. The second factor is that higher-ranking parents probably provide better protection to their offspring and thus ensure higher survival rates. Amongst rhesus macaques, higher-ranking males sired more offspring, though the alpha male was never the one to sire the most offspring, with that instead being a high-ranking but not top male. The complex relationship between rank and reproduction in this species is likely explained by the fact that rhesus macaques queue for dominance, rather than fighting for it, meaning that the alpha male is not necessarily the strongest or most attractive male.
In rodents, the highest-ranking male frequently sires the most offspring. The same pattern is found in most carnivores, such as the dwarf mongoose. The dwarf mongoose lives in a social system with one dominant pair. The dominant female produces all or almost all of the offspring in the living group, and the dominant male has first access to her during her oestrus period. In red deer, the males who experienced winter dominance, resulting from greater access to preferred foraging sites, had higher ability to get and maintain larger harems during the mating season.
In many monogamous bird species, the dominant pairs tend to get the best territories, which in turn promote offspring survival and adult health. In dunnocks, a species of bird that experiences many mating systems, sometimes individuals will form a group that will have one dominant male who achieves all of the mating in the group.
In the monogynous bee species Melipona subnitida, the queen seeks to maintain reproductive success by preventing workers from caring for their cells, pushing or hitting them using her antennae. Workers display aggression towards males, claiming priority over the cells when males try to use them to place eggs.
Costs of being dominant
There are costs to being of a high rank in a hierarchical group which offset the benefits. The most common costs to high-ranking individuals are higher metabolic rates and higher levels of stress hormones. In great tits and pied flycatchers, high-ranking individuals experience higher resting metabolic rates and therefore need to consume more food in order to maintain fitness and activity levels than do subordinates in their groups. The energetic costs of defending territory, mates, and other resources can be very consuming and cause high-ranking individuals, who spend more time in these activities, to lose body mass over long periods of dominance. Therefore, their physical condition decreases the longer they spend partaking in these high-energy activities, and they lose rank as a function of age.In wild male baboons, the highest-ranking male, also known as the alpha, experiences high levels of both testosterone and glucocorticoid, which indicates that high-ranking males undergo higher levels of stress which reduces fitness. Reduced health and longevity occurs because these two hormones have immunosuppressant activity, which reduces survival and presents opportunities for parasitic infestation and other health risks. This reduced fitness due to the alpha position results in individuals maintaining high rank for shorter periods of time and having an overall reduced health and longevity from the physical strain and costs of the position.