- Dominance hierarchy
A dominance hierarchy (in humans: social hierarchy) is the organization of individuals in a group that occurs when competition for resources leads to aggression. Schjelderup-Ebbe, who studied the often-cited example of the pecking order in chickens, found that such social structures lead to more stable flocks with reduced aggression among individuals.
Dominance hierarchies can be despotic or linear. In a despotic hierarchy, only one individual is dominant, while the others are all equally submissive. In a linear hierarchy, for example, in the above cited pecking order of chickens, each individual dominates all individuals below him and not those above him.
Dominance hierarchies occur in most social animal species that normally live in groups, including primates. Dominance hierarchies have been extensively studied in fish, birds, and mammals. Dominance hierarchies can be simple linear structures, which often arise from the physical differences among individuals in a group in relation to their access to resources. They are also influenced by the complex social interactions among individuals in the group.
Mechanisms that regulate the formation of hierarchies in animals
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.
Recently, it has been postulated that paired interactions alone can not account for the emergence of dominance hierarchies. This is because in nature, such paired interactions rarely occur in isolation. Thus, a relatively new concept has now emerged in animal behavior: the study of socially embedded dyads. Such phenomena as the audience effect, the context-dependent audience effect in Betta fish (Betta splendens), the observer effect, and the winner-loser effect, may play important roles in the formation of dominance hierarchies in social groups. Furthermore, it has been argued that the social group forms a complex signaling network: interactions that occur among just two individuals of the group are in turn affected by other signals transmitted by individuals in direct communication with them. In many animals, these putative signals can include postural changes, as well as changes in “state” (such as color changes).
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 it serves as an intrinsic factor for population control, insuring adequate resources for the dominant individuals and thus preventing widespread starvation. Territorial behavior enhances this effect.
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 the top ranked individual (“alpha”) is usually replaced by its direct subordinate (“beta”), that assume its role in the group and gain the same benefits. In eusocial species, decrease of fertility is among the main reasons for ranking displacement.
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 result 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). Accordingly 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) 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 haplodiploidy. 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 usually virgin and only able to lay eggs that are not inseminated. 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 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.
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, the paper wasp Polistes annularis and in the ants Dinoponera australis and D. quadriceps. 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. This stereotyped behavior is common in social insects, especially within ponerine ants, and was also observed in interactions of naked mole rats.
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 usually the clue used by nestmates to recognize each other in the colony and to access information regarding the reproductive status of each one (and therefore its rank in the hierarchy). However, another recent finding suggest that visual cues may also transmit the same information. The paper wasp Polistes dominulus have individual facial marks ("facial badges") that permit them to recognize each other and to identify the hierarchy/reproductive status of each individual. Individuals that have their badges modified by painting were aggressively treated by their nestmates, suggesting that advertising a false ranking status is a costly behavior and it is suppressed in these wasps.
In addition, other behaviors have been demonstrated to be involved in the maintenance of reproductive status in social insects. The removal of a thoracic sclerite in Diacamma ants inhibit ovary development and the only reproductive individual of this naturally queenless genus is the one that retains its sclerite intact. This individual is called 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.
In the honey bee Apis mellifera, pheromone produced by the queen mandibular glands is responsible for inhibiting ovary development in the worker caste. “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. 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. 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.
Female dominance in mammals
Female-biased dominance occurs rarely in mammals, and it is only observed consistently in hyenas, lemurs and the bonobo. 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:
- The Energy Conservation Hypothesis: males subordinate to females to conserve energy for intense male-male competition experienced during very short breeding seasons
- 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.
- 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 hierarchies, though often more subtle, can be observed in human societies and are important for understanding the organization of family, tribe or clan, work organizations, politics, etc. in normal and abnormal social situations. It is not clear how much of dominance hierarchy in humans is due to the intrinsic biology of our brains, derived from evolution, and how much is due to cultural factors.
- Social organization
- Hierarchical organization
- Evolution of intelligence
- Pecking order
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