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Introduction

Dominance hierarchy arises when members of a social group interact, often aggressively, to create a ranking system. In social living groups, members are likely to compete for access to limited resources and mating opportunities. Rather than fight each time they meet, relative relationships are formed between members of the same sex. These repetitive interactions lead to the creation of a social order that is subject to change each time a dominant animal is challenged by a subordinate one.

This manifestation of intrasexual conflict can be observed in one of two systems. The social order can be either linear or despotic. In a linear ranking system, every member of the gender is recognized as either dominant or submissive relative to one another, creating a linear distribution of rank. For example, groups of spotted hyenas and brown hyenas both demonstrate linear dominance. In a despotic system, one member is considered dominant while all others members of the living group are equally submissive. Examples of despotic social systems are found in meerkats, wolves, male gorillas, and African wild dogs.

Determining the outcome of conflict

Patterns of animal conflict reveal important insights into the evolution of behavior and the influence of behavior on relationships that develop in a social group. Studies show that pair-wise interactions help develop social hierarchies within groups of animals where individuals with successful agonistic behaviors often achieve dominance. These behaviors, which include aggression, threats, displays, and fighting, are indicative of competition over resources, such as food or mates. The dominant individual, therefore, is awarded increased access to food as well as increased fitness. The outcome of these pair-wise interactions, however, may vary based on the situation and position of the individuals involved.

Animal decisions regarding involvement in conflict are defined by the interplay between the costs and benefits of agonistic behaviors. When initially developed, game theory, the study of optimal strategies during pair-wise conflict, was grounded in the false assumption that animals engaged in conflict were of equal fighting ability. Modifications, however, have provided increased focus on the differences between the fighting capabilities of animals and raised questions about their evolutionary development. These differences are believed to determine the outcomes of fights, their intensity, and animal decisions to submit or continue fighting. The influence of aggression, threats, and fighting on the strategies of individuals engaged in conflict has proven integral to establishing social hierarchies reflective of dominant-subordinate interactions

The asymmetries between individuals have been categorized into three types of interactions. 1. Resource-holding potential: Animals that are better able to defend resources often win without much physical contact. 2. Resource value: Animals more invested in a resource are likely to invest more in the fight despite potential for incurring higher costs. 3. Intruder retreats: When participants are of equal fighting ability and competing for a certain territory, the resident of the territory is likely to end as the victor because he values the territory more. This can be explained further by looking at the example of the common shrews. If one participant believes he is the resident of the territory, he will win when the opponent is weaker or food is scarce. However, if both shrews believe they are the true territory holder, the one with the greater need for food, and therefore, one that values the resource more, is most likely to win.

The individual who emerges triumphant is rewarded with the dominant status, having demonstrated his/her physical superiority. However, the costs incurred to the defeated, which include loss of reproductive opportunities and quality food, can hinder the individual’s fitness. In order minimize these losses, animals generally retreat from fighting or displaying fighting ability unless there are obvious cues indicating victory. These often involve characteristics that provide an advantage during agonistic behavior, such as size of body, displays, etc. Red stags, for example, engage in exhausting roaring contests to exhibit their strength . However, such an activity would impose more costs than benefits for unfit stags, and compel them to retreat from the contest. Larger stags have also been known to make lower-frequency threat signals, acting as indicators of body size, strength, and dominance. This display demonstrates the futility of engaging in conflict, and, as a result, saves both stags from wasting energy.

Since agonistic behavior can be very costly, there are many examples in nature of animals who achieve dominance in more passive ways. In some, the dominance status of an individual is clearly visible, eliminating the need for agonistic behavior. In wintering bird flocks, white-crowned sparrows display a unique white plumage; the higher the percentage of the crown that consists of white feathers, the higher the status of the individual . For other animals, the time spent in the group serves as a determinant of dominance status. Pack members of gray wolves, for example, need the time to reach the top of the ladder. Rank may also be acquired from maternal dominance rank. In rhesus monkeys, offspring gain dominance status based on the rank of the mother: the higher ranked the mother is, the higher ranked the offspring will be . Similarly, the status of a male Canada goose is determined by the rank of his family. Although dominance is determined differently in each case, it is influenced by the relationships between members of social groups .

These observed interactions reflect the battle for resources. The association between resource availability and agonistic behavior suggests that animal conflict is adaptive by enabling competition and exploitation of available food and mates. The outcome of these interactions results in important social patterns that define hierarchies and, therefore, future access to resources .

Dominance

There are numerous instances of hierarchies, and often times the order of rank changes with time, so the study of why it is beneficial to be of a high rank helps to clarify dominance hierarchies.

Benefits

Reproductive Success

In primates, one of the most widely studied hierarchal 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

Bonnet macaques demonstrate another example of increased reproductive success from high rank. High-ranking males have more access to fertile females and consequently partake in most of the mating within the group, demonstrated by one population in which only three males were responsible for over 75% of offspring. 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 relationship is seen as males drop in rank . 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 .

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 resources, preferred foraging sites and were more capable of acquiring and maintaining 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 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 .

Foraging Success

There are many other benefits to high-ranking individuals but the most prevalent is increased foraging success and access to food resources. In vervet monkey females during times of 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.

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 .

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 . 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 .

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 .

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.

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 what type of hierarchy exists. 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. The final aspect that can determine dominance hierarchies is the environment. One example of how environment can seriously affect hierarchy is vervets in Kenya. In this population, high-ranking females have higher foraging success when the food resources are located close together. When the food is distributed throughout an area they do not do better because subordinate females can acquire food with less risk of encountering a dominant female .

Subordinance

Costs

Decreased fitness and reduced access to nutrition

Subordinate individuals suffer a range of costs from dominance hierarchies, one of the most notable being reduced access to food sources. When a resource is obtained dominant individuals are first to feed as well as taking the longest time. Subordinates also lose out in shelter and nesting sites. Brown hyenas, which display defined linear dominance in both sexes, allow subordinate males and females decreased time of feeding at a carcass . In toque monkeys subordinates are often displaced from feeding sites by dominant males. Additionally, they are excluded from sleeping sites, and they suffer reduced growth and mortality .

Decreased reproductive success

Subordinate individuals often demonstrate a huge reproductive disadvantage in dominance hierarchies. Among brown hyenas, subordinate females have less opportunity to rear young in the communal den, and thus had decreased survival of offspring when compared to high-ranking individuals. Subordinate males have far less copulations with females compared to the high-ranking males . In african wild dogs which live in social packs separated into male and female hierarchies, top ranking alpha females have been observed to produce 76-81% of all litters .

Benefits

Direct benefits

There are a number of benefits to being subordinate that come to subordinate individuals. Subordination is beneficial in agonistic conflicts where rank can predict the outcome of a fight. Less injury will occur if subordinate individuals avoid fighting with higher-ranking individuals who would win a large percentage of the time. In hens it has been observed that both dominants and subordinates benefit from a stable hierarchical environment because fewer injuries means more resources can be dedicated to laying eggs . It is also possible that in groups consisting of highly related individuals, kin selection may play a role in the stability of hierarchical dominance. It may be beneficial to a subordinate individual if the dominant individual is related, as his or her genes are still passed along. In a study of male savanna baboons, alpha males exhibited high levels of testosterone and stress. This stress and testosterone over a long period of time can lead to decreased fitness. The lowest ranking males in the hierarchy also demonstrated high stress levels, suggesting that it is the beta males that gain the most fitness, avoiding stress and still maintaining some of reproductive and nutritional benefits of being moderate ranking .

Mitigating costs of being subordinate

Subordinate animals engage in a number of behaviors in order to outweigh the aforementioned costs. Being subordinate may offset the costs of leaving the group because dispersal incurs high mortality rates. In the red fox, it has been shown that subordinate individuals, given the opportunity to desert, often do not due to the risk of death and the low possibility that they would establish themself as a dominant member in a new group . There is also the possibility that a subordinate individual becomes a high-ranking individual at a future time if the alpha male dies or is usurped.

Fighting for occasional success

Fighting with the dominant male(s) is usually a risky behavior resulting in defeat, injury or even death. In bighorn sheep, however, subordinates occasionally win a fight for a female, resulting in subordinates fathering 44% of the lambs born in the population. These sheep live in large flocks, and dominance hierarchies are often restructured each breeding season .

Sneak copulations and female mimicry

Burying beetles, which have a social order involving one dominant male controlling most access to mates, display a behavior known as sneak copulation. While one male at a carcass has a 5:1 mating advantage, subordinate males will tempt females away from the carcass with pheromones and attempt to copulate, before the dominant male can drive them forcefully away . In flat lizards, young males take advantage of their underdeveloped secondary sex characteristics to engage in sneak copulations. These young males mimic all the visual signs of a female lizard in order to successfully approach a female and copulate without detection by the dominant male. This strategy does not work at close range because the chemical signals given off by the sneaky males reveal their true nature, and they are chased out by the dominant .

Ganging up

Savanna baboon display a number of mating tactics correlated with their age. One such tactic attributed to older, subordinate males involves forming alliances to combat higher-ranking males in order to achieve access to females for copulation. These lowest ranking males would get no opportunity to copulate otherwise