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Social hierarchy

Contributed by: Michael D. Breed Publication year: 2014

A fundamental aspect of social organization that is established by fighting or display behavior and results in a ranking of the animals in a group. Social, or dominance, hierarchies are observed in many different animals, including insects, crustaceans, mammals, and birds. In many species, size, age, or sex determines dominance rank. Dominance hierarchies often determine first or best access to food, social interactions, or mating within animal groups.

Dominance and agonistic behavior

When two animals fight, several different behavioral patterns can be observed. Aggressive acts (such as biting, kicking, or stabbing with horns or antlers) and submissive acts (such as a dog rolling on its back and exposing its stomach) are both parts of a fight. Aggression and submission, together, are known as agonistic behavior.

Agonistic encounters often result in a winner, or dominant animal, and a loser. The losing individual may simply retreat or leave the area, so that it is not part of the winner’s social group. If the loser stays in the vicinity of the winner, it usually displays submissive behavior when the winner approaches in order to prevent further attacks. The submissive individual is subordinate in the relationship.

An agonistic relationship in which one animal is dominant and the other is submissive is the simplest type of dominance hierarchy. In nature, most hierarchies involve more than two animals and are composed of paired dominant-subordinate relationships. For example, if a young female bighorn sheep enters a herd of other females, she will have agonistic encounters with other females and will establish dominant-subordinate relationships with the other herd members that determine her rank in the group.

The simplest dominance hierarchies are linear and are known as pecking orders. In such a hierarchy, the top individual (alpha) dominates all others. The second-ranked individual (beta) is submissive to the dominant alpha but dominates the remaining animals. The third animal (gamma) is submissive to alpha and beta but dominates all others. This pattern is repeated down to the lowest animal in the hierarchy, which cannot dominate any other group member. Linear hierarchies can be observed in chickens.

Other types of hierarchies result from variations in these patterns. If alpha dominates beta, beta dominates gamma, but gamma dominates alpha, a dominance loop is formed. In some species, a single individual dominates all members of the social group, but no consistent relationships are formed among the other animals. In newly AccessScience from McGraw-Hill Education Page 2 of 4 www.accessscience.com

formed hierarchies, loops or other nonlinear relationships are common, but these are often resolved over time so that a stable linear hierarchy is eventually observed.

Hierarchies in social insects

In some social wasps and bees, the queen has agonistic relationships with her workers, and a linear dominance hierarchy is established. In paper wasps (Polistes sp.), the queen lunges at her workers and opens her mandibles in a threatening manner; in extreme cases, biting, grappling, and stinging occur. The workers submit by crouching down to the comb. The dominance hierarchy assures that the queen lays most of the eggs in the colony, while the workers build comb and bring food to the queen and her offspring. Males are not a part of the societies of stinging social insects (ants, bees, and wasps) and do not participate in these hierarchies.

In some of the more advanced social insects, such as the honeybee, the queen uses chemical signals to dominate her workers, rather than aggressive behavior. If a honeybee queen dies or is removed from the colony, the workers are able to lay eggs; fights and agonistic behavior over dominance then occur among the workers. See also: CHEMICAL ECOLOGY; SOCIAL INSECTS.

Dominance and mating

Males often fight over access to females and to mating with them. Male dominance hierarchies are seen in many hooved mammals (ungulates). Horns or antlers are usually adaptations used in agonistic encounters among males, and body size is often the determinant of the dominant male. Males may come together in bachelor herds during the nonmating season; when the females are ready to mate, agonistic behavior among the males intensifies and the dominant male is determined. See also: REPRODUCTIVE BEHAVIOR.

Dominance and feeding

Herds of females use dominance hierarchies to determine access to food. A female occupying a favored feeding site will yield to a dominant female but will displace a subordinate female. Agonistic interactions among females are often not as overtly aggressive as those among males, but the effects of the dominance hierarchy can easily be observed by recording which female is able to displace which others. In female dairy cattle, the order of entry into the milking barn is determined by dominance hierarchy, with the alpha female entering first.

Dominance hierarchies occur among very young siblings in many species; dominance determines which nipple is available for suckling. Piglets prefer the foremost nipples, which produce the most milk. Piglets relegated to the hindmost nipples receive less food and grow more slowly. Similar nipple-order dominance appears among domestic kittens. AccessScience from McGraw-Hill Education Page 3 of 4 www.accessscience.com

Genetic factors

Because dominant animals may have advantages in activities such as feeding and mating, they will have more offspring than subordinate animals. If this is the case, then natural selection will favor genes for enhanced fighting ability; a striking example is seen in males with armament used in fighting for access to mates. Large horns or antlers will be be favored by natural selection, as will ferocity in fighting. In some cases, such selection results in seemingly bizarre or disproportionate weapons. However, practicalities, such as ability to move in the environment or to evade predators, may place a limit on such selection. Heightened aggressive behavior may be counterselected by the necessity for amicable social interactions in certain circumstances, such as parent-offspring relationships.

Artificial selection has been used by humans to raise (pit bull terriers, fighting cocks) or to lower (most breeds of dogs, horses, cattle) fighting tendencies and dominance behavior. Domesticated animals are generally easier to handle and manipulate if they are less likely to fight and injure one another or human handlers. They may also be kept under more crowded conditions if fighting tendencies are reduced. Selection for passivity toward humans usually results in relaxation of dominance within animal social groups. See also: BREEDING (ANIMAL).

Complex social hierarchies

Many higher primates live in large groups of mixed sex and exhibit complex social hierarchies. In these groups, intra- and intersexual dominance relationships determine many aspects of group life, including feeding, grooming, sleeping sites, and mating. In rhesus macaques, the rank of an individual’s mother is very important in determining rank. The cultural and genetic history of a macaque may be more important in determining social rank than its body size. Macaque, baboon, and chimpanzee societies are characterized by cooperative alliances among individuals that are more important than individual fighting ability in maintaining rank.

Significance of hierarchies

Social hierarchies provide a means by which animals can live in groups and exploit resources in an orderly manner. In particular, food can be distributed among group members with little ongoing conflict. Although social hierarchies can be viewed as a group characteristic, they have evolved as a result of the effects of natural selection on individual animals. The dominant animals gain first access to food, mates, or other important environmental features without needing to drive all other members of the species away; the selective benefits to the dominant are obvious. Subordinate animals reduce their risk of injury in combat by submitting, but by doing so they also give up priority of access to resources. However, subordinates are close to resources that may be taken if there is a surplus and, perhaps most important, if the alpha animal dies they may compete for its position.

Another motivation for group living is mutual defense. Even though subordinates receive less food or have fewer opportunities to mate, they may have greatly increased chances of escaping predation. This is particularly striking AccessScience from McGraw-Hill Education Page 4 of 4 www.accessscience.com

in herds of ungulates, such as antelopes, in which a few animals are always in a vigilant posture (head up, looking for predators) while the others feed. Thus social hierarchies make group living orderly, allocate resources, and encourage mutual defense; the peaceful coexistence of competing individuals is facilitated by such hierarchies. See also: BEHAVIORAL ECOLOGY; POPULATION ECOLOGY; SOCIAL MAMMALS; TERRITORIALITY. Michael D. Breed

Bibliography

J. Archer, The Behavioural Biology of Aggression, 1988

I. Eibl-Eibesfeldt, Human Ethology, 1989

J. L. Gould, Ethology, 1982

D. H. Morse, Behavioral Mechanisms in Ecology, 1990

E. O. Wilson, Sociobiology: The New Synthesis, 1990

Additional Readings

G. Manteuffel et al., Social hierarchy affects the adaption of pregnant sows to a call feeding learning paradigm, Appl. Anim. Behav. Sci., 128(1):30–36, 2010 DOI: http://doi.org/10.1016/j.applanim.2010.10.002

D. S. Mills et al. (eds.), The Encyclopedia of Applied Animal Behavior and Welfare, CABI, Cambridge, UK, 2010

F. Wang et al., Bidirectional control of social hierarchy by synaptic efficacy in medial prefrontal cortex, Science, 334(6056):693–697, 2011 DOI: http://doi.org/10.1126/science.1209951

D. E. Westneat and C. W. Fox (eds.), Evolutionary Behavioral Ecology, Oxford University Press, Oxford, UK, 2010

E. M. Zitek and L. Z. Tiedens, The fluency of social hierarchy: The ease with which hierarchical relationships are seen, remembered, learned, and liked, J. Pers. Soc. Psychol., 102(1):98, 2012 DOI: http://doi.org/10.1037/a0025345