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.

Systems

Social order continuum
Spectrum of social orders. In an egalitarian society, all members are equal. In a linear hierarchy (pecking order), each member has a relative rank. In a despotic hierarchy, one member is dominant.

The manifestation of intrasexual conflict can be observed in one of two systems. The social order can be either egalitarian or despotic. In a linear ranking system (often referred to as a pecking order), every member of the gender is recognized as either dominant or submissive relative to every other member, creating a linear distribution of rank. For example, groups of spotted hyenas and brown hyenas both demonstrate linear dominance.[1] In a despotic system, one member is considered dominant while all other members of the living group are equally submissive. Examples of despotic social systems are found in meerkat, gray wolf, gorilla, Neolamprologus pulcher, and African wild dog.[2]

Weeper Capuchin 01 (cropped)
Wedge-capped capuchins have a clear dominance hierarchy

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. Pair-wise interactions have been observed to promote social hierarchies within groups of animals where individuals with successful agonistic behaviors often achieve dominance. These behaviors, which include aggression, threat displays, and fighting, are indicative of competition over resources, such as food or mates. However, they 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.[3]

The asymmetries between individuals have been categorized into three types of interactions.[4]

  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, the one that values the resource more, is most likely to win.

As expected, 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 to 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 (Huntingford). 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.

Engaging in agonistic behavior can be very costly and thus 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.[5] 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, the higher ranked the offspring will be (Yahner). 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.[6]

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.[3]

Dominance

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.[7]

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.[8] 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.[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.[7]

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.[7]

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.[9][10]

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.

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.[7]

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.[7] 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.[7]

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.[11]

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 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 the scraps 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.[7]

Subordinance

Benefits

Direct benefits

There are a number of benefits to being subordinate. Subordination is beneficial in agonistic conflicts where rank predicts 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 - knowledge of the pecking order keeps both parties from incurring the costs of a prolonged fight. In hens it has been observed that both dominants and subordinates benefit from a stable hierarchical environment because fewer challenges means more resources can be dedicated to laying eggs.[12] It is also possible that in groups consisting of highly related individuals, kin selection influences the stability of hierarchical dominance. If a subordinate individual is closely related to the dominant individual, the subordinate may benefit more genetically by assisting the dominant individual, as his or her genes are still passed along in the offspring of the dominant individual. 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 while still maintaining some of reproductive and nutritional benefits of moderate rank.[11]

Mitigating costs of being subordinate

Subordinate animals engage in a number of behaviors in order to outweigh the costs of low rank. Dispersal is often associated with increased mortality and subordination may decrease the potential benefits of leaving the group. 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 themselves as dominant members in a new group.[13] There is also the possibility that a subordinate individual who stays may become a high-ranking individual at a future time if the alpha male dies or is usurped.

Fighting with the dominant male(s) is a risky behavior that may result in defeat, injury or even death. In bighorn sheep, however, subordinates occasionally win a fight for a female, and they father 44% of the lambs born in the population. These sheep live in large flocks, and dominance hierarchies are often restructured each breeding season.[14]

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.[15] 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.[16]

Savanna baboons 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.[17]

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.[18] 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 increased mortality.[19]

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.[18] 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.[20]

Reverse

Boehm, having explored data from 48 societies spread across the globe, ranging from small hunting and gathering bands to more sedentary chiefdoms, suggested that with the advent of anatomically modern humans who continued to live in small groups and had not yet domesticated plants and animals (hunter-gatherer), it is very likely that all human societies practised egalitarianism and that most of the time they did so very successfully.[21]

Boehm writes:

"As long as followers remain vigilantly egalitarian because they understand the nature of domination and leaders remain cognizant of this ambivalence-based vigilance, deliberate control of leaders may remain for the most part highly routinized and ethnographically unobvious."

Boehm identifies the following mechanisms ensuring the Reverse Dominance Hierarchy: Public Opinion, Criticism and Ridicule, Disobedience, and Extreme Sanctions. Further characteristics include ambivalence towards leaders and anticipation of domination.[21]

Interpersonal complementarity

The interpersonal complementarity hypothesis suggests that obedience and authority are reciprocal, complementary processes. That is, it predicts that one group member's behaviours will elicit a predictable set of actions from other group members. Friendly behaviours are predicted to be met with friendly behaviours, and hostile behaviours are predicted to be reciprocated with similar, hostile behaviours. When an individual acts in a dominant, authoritative manner in a group, this behaviour tends to prompt submissive responses from other group members. Similarly, when group members display submissive behaviour, others feel inclined to display dominant behaviours in return. Tiedens and Fragle (2003) found that hierarchical differentiation plays a significant role in liking behaviour in groups.[22] Individuals prefer to interact with other group members whose power, or status behaviour complements their own. That is to say, group members who behave submissively when talking to someone who appears to be in control are better liked, and similarly individuals who display dominant behaviours (e.g., taking charge, issuing orders) are more liked when interacting with docile, subservient individuals.

Regulation mechanisms

The most basic interaction that establishes a Dominance Hierarchy is the dyad, or paired interaction among individuals. To study the formation of hierarchies, scientists have often used the dyadic method, in which two individuals are forced to interact isolated from others. All individuals in the group are paired with each other (i.e. a round-robin), in isolation, until a hierarchy can be deduced. The process of deducing the hierarchy involves the construction of a dominance matrix, in which wins/ties are expressed in relation to each member of the group.

Individuals with greater hierarchical status tend to displace those ranked lower from access to space, to food and to mating opportunities. Thus, individuals with higher social status tend to have greater reproductive success by mating more often and having more resources to invest in the survival of offspring. Hence, hierarchy serves as an intrinsic factor for population control, ensuring adequate resources for the dominant individuals and thus preventing widespread starvation. Territorial behavior enhances this effect.[23][24]

These hierarchies are not fixed and depend on any number of changing factors, among them are age, gender, body size, intelligence, and aggressiveness. For instance, in linear hierarchies, a lost top ranked individual ("alpha") is usually replaced by its direct subordinate ("beta"); this individual assumes its role in the group and gains the same benefits. In eusocial species, decrease of fertility is among the main reasons for ranking displacement.

Brood hierarchy

The blue-footed booby brood of two chicks always has a dominance hierarchy due to the asynchronous hatching of the eggs. One egg is laid four days before the other, and incubation starts immediately after laying, so the elder chick is hatched four days before the younger chick and has a four-day head start on growth. The elder, stronger chick almost always becomes the dominant chick. During times of food shortage, the dominant chick often kills the subordinate chick by either repeatedly pecking or by ousting the younger chick from the nest. The brood hierarchy makes it easier for the subordinate chick to die quietly in times of food scarcity, which provides an efficient system for booby parents to maximize their investment.[25]

Hormones and dominance

It has been shown that modulation of hormone levels after hibernation may be associated with the establishment of dominance hierarchies within the social order of the paper wasp (Polistes dominulus).[26] Former studies have shown that dominance hierarchies within the paper wasp species is dependent on the queen (also known as a foundress), though the mechanism behind the establishment is currently under investigation. Recent research suggests that this manifestation may be dependent on specific hormones contained within the foundress. Laboratory experiments have shown that when foundresses are injected with juvenile hormone (JH), a hormone responsible for regulating growth and development in many insect species including wasps, the foundresses exhibit an increase in dominant behavior compared to those who had not been injected.[26] Further evidence shows that foundresses that have a larger corpora allata, a region of the female wasp brain responsible for the synthesis and secretion of JH, are naturally more dominant.[26] In an effort to determine if JH is indeed responsible for manifesting dominance, a follow up experiment was done utilizing 20-hydroxyecdysone, an ecdysone known to enhance maturation and size of oocytes.[26] The size of the oocytes plays a significant role in establishing dominance within Polistes dominulus and thus researchers used this hormone to compare which condition (either JH or 20-hydroxyecdysone treated) induced higher levels of dominance intensities within foundresses.[27] The results showed that the 20-hydroxyecdysone treated foundresses showed increased dominance compared to those foundresses treated with JH and suggests that 20-hydroxyecdysone, not JH may play a larger role in establishing dominance (Roseler et al., 1984). Subsequent research however, suggests that JH is implicated in the manifestation of dominance, though it only exerts dominance-type effects on certain individuals. By recording the number of mounting attempts between rival foundresses as a measure of dominance, researchers found that when injected with the same amount of JH, larger foundresses showed more mounting behaviors compared to foundresses that were smaller. Additionally, the more dominant foundress tended to show an increased number of oocytes contained within her ovaries. It was concluded that larger foundresses are more reproductively fit and thus JH, which is responsible for the growth and maturation of the ovaries, be more active within these individuals compared to the smaller, less fertile foundresses though the mechanism of action and/or any synergistic effects between JH and other hormones remains unknown.[27]

Wikipedia project hormone
The effect of relative rank on stress hormone levels in savanna baboons[11]

The hormone model of dominance and reproductive capacity has also been demonstrated in the naked mole-rat (Heterocephalus glaber). It has previously been established that the dominance hierarchy within the species is dependent on the highest ranking female (queen) and her ability to suppress critically important reproductive hormones in male and female sub-dominants. In sub-dominant males, appears that lutenizing hormone and testosterone are suppressed while in females it appears that the suppression involves the entire suppression of the ovarian cycle. This suppression reduces sexual virility and behavior and thus redirects the sub-dominant's behavior into helping the queen with her offspring.[28] though the mechanisms of how this is accomplished are debated. Former research suggests that primer pheromones secreted by the queen cause direct suppression of these vital reproductive hormones and functions however current evidence suggests that it is not the secretion of pheromones which act to suppress reproductive function but rather the queen's extremely high levels of circulating testosterone, which cause her to exert intense dominance and aggressiveness on the colony and thus "scare" the other mole-rats into submission.[29] Research has shown that removal of the queen from the colony allows the reestablishment of reproductive function in sub-dominant individuals. To see if a priming pheromone secreted by the queen was indeed causing reproductive suppression, researchers removed the queen from the colony but did not remove her bedding. They reasoned that if a primer pheromones were on the bedding then the sub-dominant's reproductive function should continue to be suppressed. Instead however, they found that the sub-dominants quickly regained reproductive function even in the presence of the queen's bedding and thus it was concluded that primer pheromones do not seem to play a role in suppressing reproductive function.[29]

Evidence also suggests that glucocorticoids, signaling molecules (commonly known as stress hormones) produced by the adrenal glands which stimulate the fight or flight response may also be implicated in the establishment of dominance hierarchies. Field research has shown that higher ranking individuals tend to have much higher levels of circulating glucocorticoids compared to subdominant individuals within the hierarchy.[30] This is in stark opposition to the original suggestion stating that being subdominant was more stressful than being dominant within a hierarchy.[31] Two core hypotheses attempt to explain these unusual findings. The first suggests that higher ranking individuals exert more energy and thus need higher levels of glucocorticoids to more readily mobilize glycogen stores for energy use.[32] This hypothesis is supported by research showing that when food availability is low, cortisol levels tended to increase within the dominant male thus allowing increased glycogen metabolism and subsequent energy production.[31] The second hypothesis suggests that elevated stress hormones are a result of social factors within the hierarchy particularly when the hierarchy is in transition. Proponents of this theory assert that when a hierarchy is unstable, aggressive interaction and confrontations increase within the hierarchy. As a result, the dominant individual ends up fighting much more than when the hierarchy is stable and as consequence of the increased fighting, glucocorticoids are elevated during this period. Field studies of olive baboons in Kenya seem to support this hypothesis as it has been observed that dominant individuals tended to have lower cortisol levels in a stable hierarchy than did subdominant individuals. When the hierarchy shifted to being unstable however, the dominant individual showed much higher levels of cortisol compared to subdominant individuals.[33] Many are quick to point out however, that this is extremely unusual as dominant individuals rarely have low levels of glucocorticoids regardless of hierarchy state.[31] Nonetheless, it appears that being dominant may not be as beneficial as previously thought since chronic levels of glucocorticoids can have a variety of adverse health effects including immunosuppression, increased appetite, and osteoporosis (see Cortisol) and is a possible reason why subdominant individuals are reluctant to challenge the dominant individual for their position during hierarchy transitory periods.

In eusocial insects

In insect societies, only one to few individuals members of a colony can reproduce, whereas the other colony members have their reproductive capabilities suppressed. This conflict over reproduction in some cases results in a dominance hierarchy. Dominant individuals in this case are known as queens and have the obvious advantage of performing reproduction and benefiting from all the tasks performed by their subordinates, the worker caste (foraging, nest maintenance, nest defense, brood care and thermal regulation). According to Hamilton's rule, the reproduction costs of the worker caste are compensated by the contribution of workers to the queen's reproductive success, with which they share genes. This is true not only to the popular social insects (ants, termites, some bees and wasps), but also for the naked mole-rat Heterocephalus glaber. In a laboratory experiment, Clarke and Faulkes (1997)[34] demonstrated that reproductive status in a colony of H. glaber was correlated with the individual's ranking position within a dominance hierarchy, but aggression between potential reproductives only started after the queen was removed.

Social insects above mentioned, excluding termites, are haplodiploid. Queen and workers are diploid, but males develop from haploid genotypes. In some species, suppression of ovary development is not totally achieved in the worker caste, which opens the possibility of reproduction by workers. Since nuptial flights are seasonal and workers are wingless, workers are almost always virgin, and (as gamergate ants or laying worker bees) can only lay unfertilised eggs. These eggs are in general viable, developing into males. A worker that perform reproduction is considered a 'cheater' within the colony, because its success in leaving descendants becomes disproportionally larger, compared to its sisters and mother. The advantage of remaining functionally sterile is only accomplished if every worker assume this 'compromise'. When one or more workers start reproducing, the 'social contract' is destroyed and the colony cohesion is dissolved. Aggressive behavior derived from this conflict may result in the formation of hierarchies, and attempts of reproduction by workers are actively suppressed. In some wasps, such as Polistes fuscatus, instead of not laying eggs, the female workers begin being able to reproduce, but once being under the presence of dominant females, the subordinate female workers can no longer reproduce.[35]

In some wasp species such as Liostenogaster flavolineata there are many possible Queens that inhabit a nest, but only one can be queen at a time. When a Queen dies the next Queen is selected by an age based dominance hierarchy. This is also true in the species Polistes instabilis, where the next queen is selected based on age rather than size.Polistes exclamans also exhibits this type of hierarchy.[36] Within the dominance hierarchies of the Polistes versicolor, however, the dominant-subordinate context in the yellow paper wasps is directly related to the exchange of food. Future foundresses within the nest compete over the shared resources of nourishment, such as protein. Unequal nourishment is often what leads to the size differences that result in dominant-subordinate position rankings. Therefore, if during the winter aggregate, the female is able to obtain greater access to food, the female could thus reach a dominant position.[37]

In some species, especially in ants, more than one queen can be found in the same colony, a condition called Polygyny. In this case, another advantage of maintaining a hierarchy is to prolong the colony lifespan. The top ranked individuals may die or lose fertility and "extra queens" may benefit of starting a colony in the same site or nest. This advantage is critical in some ecological contexts, such as in situations where nesting sites are limited or dispersal of individuals is risky due to high rates of predation. This polygynous behavior has also been observed in some eusocial bees such as Schwarziana quadripunctata. In this species, multiple queens of varying sizes are present. The larger, physogastric, queens typically control the nest, though a "dwarf" queen will take its place in the case of a premature death.[38]

Regulatory mechanisms in eusocial organisms

The suppression of reproduction by dominant individuals is the most common mechanism that maintains the hierarchy. In eusocial mammals this is mainly achieved by aggressive interactions between the potential reproductive females. In eusocial insects, aggressive interactions between sexuals are common determinants of reproductive status, such as in the bumblebee Bombus bifarius,[39] the paper wasp Polistes annularis[40] and in the ants Dinoponera australis and D. quadriceps.[41] In general aggressive interactions are ritualistic and involve antennation (drumming), abdomen curling and very rarely mandible bouts and stinging. The winner of the interaction may walk over the subordinated, that in turn assumes a prostrated posture on the substrate. In order to be effective, these regulatory mechanisms must include traits that make an individual rank position readily recognizable by its nestmates. The composition of the lipid layer present on the cuticle of social insects is the clue used by nestmates to recognize each other in the colony, and to discover each insect's reproductive status (and therefore its rank).[42] Visual cues may also transmit the same information. Paper wasps Polistes dominulus have individual "facial badges" that permit them to recognize each other and to identify the status of each individual.[43] Individuals whose badges were modified by painting were aggressively treated by their nestmates, suggesting that advertising a false ranking status is a costly behavior, suppressed in these wasps.

Other behaviors are involved in maintaining reproductive status in social insects. The removal of a thoracic sclerite in Diacamma ants inhibits ovary development; the only reproductive individual of this naturally queenless genus is the one that retains its sclerite intact. This individual is called a gamergate, and is responsible for mutilating all the newly emerged females, to maintain its social status. Gamergates of Harpegnathos saltator arise from aggressive interactions, forming a hierarchy of potential reproductives.[44]

In the honey bee Apis mellifera, pheromone produced by the queen mandibular glands is responsible for inhibiting ovary development in the worker caste.[45] "Worker policing" is an additional mechanism that prevents reproduction by workers, found in bees and ants. Policing may involve oophagy and immobilization of egg-layers from the worker caste.[46] In some ant species such as the carpenter ant Camponotus floridanus, eggs from queens have a peculiar chemical profile that workers can distinguish from worker laid eggs. When worker-laid eggs are found, they are eaten.[47] In some species, such as Pachycondyla obscuricornis, workers may try to escape policing by shuffling their eggs within the egg pile laid by the queen.[48]

Female dominance in mammals

Female-biased dominance occurs rarely in mammals, and it is observed consistently in hyenas, lemurs and the bonobo.[49] It is also observed in elephants. It occurs when all adult males exhibit submissive behavior to adult females in social settings. These social settings are usually related to feeding, grooming, and sleeping site priority.

There are three basic proposals for the evolution of female dominance:[50]

  1. The Energy Conservation Hypothesis: males subordinate to females to conserve energy for intense male-male competition experienced during very short breeding seasons
  2. Male behavioral strategy: males defer as a parental investment because it ensures more resources in a harsh unpredictable climate for the female, and thus, the male's future offspring.
  3. Female behavioral strategy: dominance helps females deal with the unusually high reproductive demands; they prevail in more social conflicts because they have more at stake in terms of fitness.

Since these original proposals, scientists like Peter Kappeler have modified and integrated other ideas. However, in the case of lemurs, there is no single hypothesis that can fully explain female social dominance at this time and all three are likely to play a role.

Dominance in birds

In dunnocks, two or three males may share one or more females in a polyandrous or polygynandrous mating system. This leads to the males' ranges to overlap so they would cooperate in order to defend the territory. Among the males is a dominance hierarchy: older birds tend to be the alpha male and first year birds are usually the beta males. Studies have not found cases of close male relatives being involved in any mating combinations.

Dominance in fish

Dominance hierarchies can also be observed in many fish. For example, in the dragonet, males form hierarchies that are often exhibited during mating. They can act extremely aggressively towards another male if it intrudes upon courtship and pairing with a female, and fights can be very intense.

See also

References

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Amphiprion leucokranos

Amphiprion leucokranos (White bonnet anemonefish) is a naturally occurring hybrid anemonefish found in the western central Pacific Ocean. Like all anemonefishes it forms a symbiotic mutualism with sea anemones and is unaffected by the stinging tentacles of the host anemone. It is a sequential hermaphrodite with a strict dominance hierarchy, features which are critical to the direction of gene flow.

Amphiprion thiellei

Amphiprion thiellei (Thielle's anemonefish) is thought to be a naturally occurring hybrid anemonefish found in the vicinity of Cebu, Philippines. Like all anemonefishes it forms a symbiotic mutualism with sea anemones and is unaffected by the stinging tentacles of the host anemone. It is a sequential hermaphrodite with a strict dominance hierarchy.

Bonnet macaque

The bonnet macaque also known as zati (Macaca radiata) is a macaque endemic to southern India. Its distribution is limited by the Indian Ocean on three sides and the Godavari and Tapti Rivers along with a related competing species of rhesus macaque in the north. Land use changes in the last few decades have resulted in changes in its distribution boundaries with the rhesus macaque, raising concern for its status in the wild.This Old World monkey is a diurnal animal.

It is 35–60 cm long plus a tail of 35–68 cm. Males weigh 5.5 to 9.0 kg, females 3.5 to 4.5 kg. Captive monkey can live up to 35 years.The bonnet macaque feeds on fruits, nuts, seeds, flowers, invertebrates, and cereals. In southern India, this macaque exists as commensal to humans, feeding on food given by humans and raiding crops and houses.Two subspecies of bonnet macaques have been identified: M. r. radiata and M. r. diluta.

Chacma baboon

The chacma baboon (Papio ursinus), also known as the Cape baboon, is, like all other baboons, from the Old World monkey family. It is one of the largest of all monkeys.

Located primarily in southern Africa, the chacma baboon has a wide variety of social behaviors, including a dominance hierarchy, collective foraging, adoption of young by females, and friendship pairings. These behaviors form parts of a complex evolutionary ecology.

In general the species is not threatened, but human population pressure has increased contact between humans and baboons. Hunting, accidents, and trapping kill or remove many baboons from the wild. This has reduced baboon numbers and disrupted their social structure.

Dominance (ethology)

Dominance in ethology is an "individual's preferential access to resources over another". Dominance in the context of biology and anthropology is the state of having high social status relative to one or more other individuals, who react submissively to dominant individuals. This enables the dominant individual to obtain access to resources such as food or potential mates at the expense of the submissive individual, without active aggression. The absence or reduction of aggression means unnecessary energy expenditure and the risk of injury are reduced for both. The opposite of dominance is submissiveness.

Dominance may be a purely dyadic relationship, i.e. individual A is dominant over individual B, but this has no implications for whether either of these is dominant over a third individual C. Alternatively, dominance may be hierarchical, with a transitive relationship, so that if A dominates B and B dominates C, A always dominates C. This is called a linear dominance hierarchy. Some animal societies have despots, i.e. a single dominant individual with little or no hierarchical structure amongst the rest of the group. Horses use coalitions so that affiliated pairs in a herd have an accumulative dominance to displace a third horse that normally out-ranks both of them on an individual basis.

Gamergate

A gamergate () is a mated worker ant that is able to reproduce sexually, i.e., lay fertilized eggs that will develop as females. Gamergates are restricted to taxa where the workers have a functional sperm reservoir ('spermatheca'). In various species, gamergates reproduce in addition to winged queens (usually upon the death of the original foundress), while in other species the queen caste has been completely replaced by gamergates. In gamergate species, all workers in a colony have similar reproductive potentials, but as a result of physical interactions, a dominance hierarchy is formed and only one or a few top-ranking workers can mate (usually with foreign males) and produce eggs. Subsequently however, aggression is no longer needed as gamergates secrete chemical signals that inform the other workers of their reproductive status in the colony.

Depending on the species, there can be one gamergate per colony (monogyny) or several gamergates (polygyny). Most gamergate species have colonies with a few hundred or fewer workers.

Lasioglossum zephyrum

Lasioglossum zephyrum is a sweat bee of the family Halictidae, found in the U.S. and Canada. It is considered a primitively eusocial bee (meaning that they do not have a permanent division of labor within colonies), although it may be facultatively solitary (i.e., displaying both solitary and eusocial behaviors). The species nests in underground burrows and has been observed forcing open unbloomed flowers of species Xyris tennesseensis to extract the pollen, ensuring first and exclusive access.

Liostenogaster flavolineata

Liostenogaster flavolineata (tropical hover wasp) is an insect that belongs to the wasp family Vespidae. This hairy-faced hover wasp species is predominantly found in South Asian rain forests, especially in Malaysia. Individual colonies of this species are very small, but aggregations of nests allow for interactions between many smaller colonies. Some worker wasps, known as "helpers", will move between multiple nests in an attempt to improve their position in the dominance hierarchy. Its nests are pale-colored and are usually built with mud.Liostenogaster flavolineata is one of the most studied species in the Stenogastrinae.

Maroon clownfish

Premnas biaculeatus, commonly known as spine-cheeked anemonefish or the maroon clownfish, is a species of anemonefish found in the Indo-Pacific from western Indonesia to Taiwan and the Great Barrier Reef. They can grow up to be about 17 cm (6.7 in). Like all anemonefishes it forms a symbiotic mutualism with sea anemones and is unaffected by the stinging tentacles of the host anemone. It is a sequential hermaphrodite with a strict size-based dominance hierarchy; the female is largest, the breeding male is second largest, and the male nonbreeders get progressively smaller as the hierarchy descends. They exhibit protandry, meaning the breeding male changes to female if the sole breeding female dies, with the largest nonbreeder becoming the breeding male. The fish's natural diet includes algae and zooplankton.

Oophagy

Oophagy ( oh-OFF-ə-jee) sometimes ovophagy, literally "egg eating", is the practice of embryos feeding on eggs produced by the ovary while still inside the mother's uterus. The word oophagy is formed from the classical Greek ᾠόν (ōion, egg) and classical Greek φᾱγεῖν (phāgein, to eat). In contrast, adelphophagy is the cannibalism of a multi-celled embryo.

Oophagy is thought to occur in all sharks in the order Lamniformes and has been recorded in the bigeye thresher (Alopias superciliosus), the pelagic thresher (A. pelagicus), the shortfin mako (Isurus oxyrinchus) and the porbeagle (Lamna nasus) among others. It also occurs in the tawny nurse shark (Nebrius ferrugineus), and in the family Pseudotriakidae.

This practice may lead to larger embryos or prepare it for a predatory lifestyle.There are variations in the extent of oophagy among the different shark species. The grey nurse shark (Carcharias taurus) practices intrauterine cannibalism, the first developed embryo consuming both additional eggs and any other developing embryos. Slender smooth-hounds

(Gollum attenuatus), form egg capsules which contain 30-80 ova within which only one ovum develops while all other ova are ingested and packed to an external yolk sac. The embryo then develops normally without ingesting further eggs.Oophagy is also used to refer to more general egg-eating behaviours such as those practised by some snakes. Similarly, the term can be used to describe the destruction of non-queen eggs in nests of certain social wasps, bees, and ants. This is seen in the wasp species Polistes biglumis and Polistes humilis. Oophagy has been observed in Leptothorax acervorum and Parachartergus fraternus, where oophagy is practiced to increase energy circulation and consume more protein. Polistes fuscatus use oophagy as a method to establish a dominance hierarchy; dominant females will eat the eggs of subordinate females such that they no longer produce eggs, possibly due to the unnecessary expending of energy and resources. This behavior has also been observed in some bee species. Bee species include Xylocopa sulcatipes and Bombus ruderatus, where queen bees will eat the larva deposited by workers or ejected them from the nest in order to maintain dominance over the female workers.

Pecking order

Pecking order or peck order is the colloquial term for the hierarchical system of social organization. It was first described by Thorleif Schjelderup-Ebbe in 1921 under the German terms Hackordnung or Hackliste and introduced into English in 1927.The original use of pecking order referred to the expression of dominance in chickens. Dominance in chickens is asserted by various behaviours, including pecking, which was used by Schjelderup-Ebbe as a measure of dominance and leadership order. In his 1924 German-language article, he noted that "defense and aggression in the hen is accomplished with the beak". This emphasis on pecking led many subsequent studies on fowl behaviour to use it as a primary observation, however, it has been noted that roosters tend to leap and use their claws in conflicts.The term dominance hierarchy is often used for this type of social organisation in other animals.

Pecking order is a basic concept in social stratification and social hierarchy that has its counterparts in other animal species, including humans, although the term "pecking order" is often used synonymously.

Polistes annularis

Polistes annularis (P. annularis) is a species of paper wasp which lives throughout the Caribbean and in parts of North America. Its species name is Latin for "ringed" and it is known for its distinct red body color. It builds its nest under overhangs near bodies of water that minimize the amount of sunlight penetration. It clusters its nests together in large aggregations, and consumes nectar and other insects. Its principal predator is the ant, although birds are also known to prey on it. It is a primitively eusocial wasp, meaning that all individuals develop the capacity for reproduction, regardless of social caste. This primitive eusociality has been seen in bees as well, including the sweat bee, Lasioglossum zephyrum. As such, P. annularis demonstrates behavior typical of other polistine wasps, and has a dominance hierarchy, relatively small colony size, and a female-biased sex ratio. Unlike other wasps, P. annularis is relatively robust in winter conditions, and has also been observed to store honey in advance of hibernation. It is closely related to P. major, P. buysonni, and others in the subgenus Aphanilopterus, and slightly less related to the more common P. bellicosus, P. carolina, P. metricus, and P. fuscatus.

Polistes japonicus

Polistes japonicus is a eusocial paper wasp found in Japan. It was first described by Henri Louis Frédéric de Saussure in 1858. It is closely related to Polistes formosanus. This species lives in small colonies with few workers and a foundress queen. Nests of these wasps are sometimes used as a traditional medicine in Korea, China, and Japan.

Power structure

A power structure is an overall system of influence relationships between any individual and every other individual within any selected group of people. A description of a power structure would capture the way in which power or authority is distributed between people within groups such as a government, nation, institution, organization, or a society. Such structures are of interest to various fields, including sociology, government, economics, and business. A power structure may be formal and intentionally constructed to maximize values like fairness or efficiency, as in a hierarchical organization wherein every entity, except one, is subordinate to a single other entity. Conversely, a power structure may be an informal set of roles, such as those found in a dominance hierarchy in which members of a social group interact, often aggressively, to create a ranking system. A culture that is organised in a dominance hierarchy is a dominator culture, the opposite of an egalitarian culture of partnership. A visible, dominant group or elite that holds power or authority within a power structure is often referred to as being the Establishment. Power structures are fluid, with changes occurring constantly, either slowly or rapidly, evolving or revolutionary, peacefully or violently.

Protonectarina

Protonectarina sylveirae, commonly referred to as the Brazilian wasp, is a neotropical swarm-founding wasp species that ranges widely across South America. This species relies heavily on the consumption of animal protein rather than nectar. P. sylveirae preys heavily on agricultural pests to coffee crops, keeping pest populations low.

Spatial organization

Spatial organization can be observed when components of an abiotic or biological group are arranged non-randomly in space. Abiotic patterns, such as the ripple formations in sand dunes or the oscillating wave patterns of the Belousov-Zhabotinsky reaction emerge after thousands of particles interact millions of times. On the other hand, individuals in biological groups may be arranged non-randomly due to selfish behavior, dominance interactions, or cooperative behavior. W. D. Hamilton (1971) proposed that in a non-related "herd" of animals, spatial organization is likely a result of the selfish interests of individuals trying to acquire food or avoid predation. On the other hand, spatial arrangements have also been observed among highly related members of eusocial groups, suggesting that the arrangement of individuals may provide some advantage for the group.

Top Dog

The expression top dog is an idiom for the boss or the leader. In a competition, it is also the favorite or the one expected to win, and the opposite of the underdog. It may be a shorthand reference for a dominance hierarchy.

The name Top Dog may refer to: Vern W

Top Dog (comics), a comic book character from Top Dog series by Star Comics (a Marvel Comics imprint)

The Top Dog, a 1918 British film

Top Dog (1995 film), a 1995 action film with Chuck Norris

Top Dog (2014 film), a 2014 crime film with Leo Gregory

Top Dog (rapper), member of the hip hop group O.G.C.

Top Dog, another name for the card game Big Two

Top Dawg Entertainment, independent record label

Unstriped ground squirrel

The unstriped ground squirrel (Xerus rutilus) is a species of rodent (order Rodentia) in the family Sciuridae. It is found in Djibouti, Eritrea, Ethiopia, Kenya, Somalia, Sudan, Tanzania, and Uganda. Its natural habitats are dry savanna and subtropical or tropical dry shrubland.

Wedge-capped capuchin

The wedge-capped capuchin or weeper capuchin (Cebus olivaceus) is a capuchin monkey from South America. It is found in northern Brazil, Guyana, French Guiana, Suriname, Venezuela and possibly northern Colombia. The genus Cebus is divided into several different species. However, taxonomists argue over the specific divisions within the genus, which are uncertain and controversial. Cebus olivaceus is known to dwell in tall, primary forest and travel over long distances during the day.These primates are medium-sized monkeys with distinctive "wedge cap" markings on their head and slightly longer limbs than other capuchins for jumping through the forest canopy. Similar to other capuchin monkeys, the diet of wedge-capped capuchin primarily consists of fruits, invertebrates, other plant parts, and on rare occasions small vertebrates. They have also been known to rub millipedes against their fur, especially in the rainy seasons, as a potential means of mosquito repellent. Although this species is classified as an animal of least concern by IUCN Red List of Threatened Species, it falls prey to many predators in South America ranging from vultures to jaguars.

Cebus olivaceus is a polygamous species that lives in groups anywhere from 5-30 individuals, with a female biased sex ratios. The group is organized according to a pre-determinded hierarchal system of dominance for both males and females. Although biological lineage is less of a factor of dominance for males than it is for females, due to male migration between groups. Wedge-capped capuchin partake in several behavioral mechanisms to assert and maintain dominance within the group including: infanticide, when an infant is deliberately killed; grooming, used to facilitate social rapport; and alloparenting, which is when members of the group care for offspring that are not their own.

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