Dominance hierarchy

Dominance hierarchy is a type of social hierarchy that arises when members of a social group interact, to create a ranking system. In social living groups, members are likely to compete for access to limited resources and mating opportunities. Rather than fighting each time they meet, relative rank is established between members of the same sex. 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.[1]

Dominance

When to seek high rank

Given the benefits and costs of possessing a high rank within a hierarchal group, there are certain characteristics of individuals, groups, and environments that determine whether an individual will benefit from a high rank. Individual characteristics include whether or not high rank gives them access to valuable resources such as mates and food. Individuals will often weigh the cost of the resource against factors including their age, intelligence, experience, and physical fitness, which can determine the costs to gaining rank.[citation needed]

Hierarchy results as an accumulation of individual interaction, group dynamics, and sharing of resources, therefore group size and composition can affect the dominance decisions of high-ranking individuals and hierarchy type. For example, in a large group with many males, it may be very challenging for the highest-ranking male to dominate all the mating opportunities, so some mate sharing probably exists. These occasional mating opportunities available to subordinates reduce the likelihood of subordinates challenging the dominant male - mating is no longer an all-or-nothing game and those shared are enough to placate most subordinates. Another aspect that can determine dominance hierarchies is the environment. In populations of Kenyan vervets, 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.[2]

Benefits

Foraging success

Another 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.[citation needed]

In many bird species the dominant individuals have higher rates of food intake including 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.[2]

Reproductive success

In primates, one of the most widely studied hierarchical groups, many studies have found a positive relationship between high rank and reproductive success. In baboons, higher-ranking males have the highest reproductive success due to increased female acquisition. Also, female baboons benefit from increased rank because high-ranking females produce more surviving offspring.[2] A 1991 meta-analysis of 32 studies determined there was a positive relationship between dominance rank and reproductive success amongst primates.[3] A 2016 study determined that higher status increased the reproductive success amongst men and the relationship between status and reproductive success did not depend upon the type of subsistence (forager, horticulture, pastoralism, agriculture), contrary to the "egalitarian hypothesis", which predicted status would have a weaker effect on reproductive success amongst foragers than amongst nonforagers.[4]

Bonnet macaques gain increased reproductive success with high rank. High-ranking 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 their rank, and as they gain rank, they gain more time spent exclusively with fertile females; the opposite is seen as males drop in rank.[5] 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.[2] 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, rather than fight, for dominance, meaning that the alpha male is not necessarily the strongest or most attractive male.[6][7]

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.[2]

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 birds 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.[2]

In the monogynous bee species Melipona subnitida it is noted that the queen wants to maintain reproductive success, and does so by preventing workers from caring for their cells, pushing or hitting them using her antennae. Workers additionally have been noted to display aggression towards males, claiming priority over the cells when males try to use them to place eggs. This species further demonstrates the conflict between hierarchy and reproductive success.[8][9]

Costs

Despite the benefits to being of a high rank in a hierarchal group, there are also costs which offset these benefits. The most common costs to high-ranking individuals are higher metabolic rates and higher levels of stress hormones.[2] 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 compared to 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.[2]

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.[10]