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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.
When two animals fight, several different behavioral patterns can be observed. Aggressive acts and submissive acts are both parts of a fight. Aggression and submission, together, are known as agonistic behavior. 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. 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.
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 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.
Males often fight over access to females and to mating with them. Male dominance hierarchies are seen in many hooved mammals (ungulates). Herds of females use dominance hierarchies to determine access to food. 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. In female dairy cattle, the order of entry into the milking barn is determined by dominance hierarchy, with the alpha female entering first. See Reproductive behavior
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. Heightened aggressive behavior may be counterselected by the necessity for amicable social interactions in certain circumstances. 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. Macaque, baboon, and chimpanzee societies are characterized by cooperative alliances among individuals that are more important than individual fighting ability in maintaining rank. See Primates
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. 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. See Behavioral ecology, Population ecology, Social mammals, Territoriality