Some species of animal form long-lasting social groups. Most commonly, it is thought, this has the biological function of reducing predation risk. The resulting close association of individuals inevitably increases direct, contest competition: for mating, food, and other resources. For any particular pair of individuals, contests often have a consistent outcome, over time and over different types of resource; when this happens, one individual is said to be dominant to the other. Once a dominance relationship is established, the subordinate will often avoid further contests with the dominant, thus reducing risk of injury in aggressive encounters—whose outcome is predictably unlikely to benefit the subordinate itself. (Confusingly, it is sometimes claimed that dominance benefits subordinates, since they can use avoidance to reduce futile aggression—but note that the main resource benefits accrue to the dominant.)
Dominance may be no more than a reflection of relative physical power, weight or weaponry. However, in some species of PRIMATES at least, other factors are involved. Maternal support of offspring generally enables a dependant to win contests with any adult its mother can dominate. In many Old World monkeys, in which females tend to remain in the troop of their birth all their lives, this effect is so prevalent that daughters effectively ‘inherit’ a rank just below their mother’s, and retain it after the mother’s death.
Among chimpanzees, rank appears unimportant to females, whereas males invest much energy in status interactions. In male chimpanzees, support from other individuals is often crucial for attaining the highest rank, or ALPHA MALE status. In different populations, this support has been noted to come from kin (an elder brother), from a group of several females, or from a single male. In the latter case, this ‘kingmaking’ role gives the supporter an opportunity sometimes to switch allegiance between two powerful rivals, thereby gaining personal benefits during the resulting instability.
While dominance relationships may not be transitive, in many species they are (that is, if A beats B, and B beats C, then A beats C). Then, a linear ordering fully describes the dominance structure of the group. This was originally noticed in chickens and called a PECKING ORDER, but is most commonly termed a LINEAR HIERARCHY OF DOMINANCE, often somewhat confusingly shortened to DOMINANCE HIERARCHY. The explanatory value of this concept depends on its generality: where it holds true for several sorts of interaction (mating supplants, monopolization of prime food resources, choice of favoured sleeping sites, and so on) it is a useful and compact description of social relationships. However, this is not always the case. In chimpanzees, for instance, despite the great energy invested by male chimpanzees in competing for rank, dominance rank does not reliably predict access to resources. Access to the most prized food, meat, is a prerogative of older animals, not high-ranking ones; sleeping sites seem to be uncontested; and while the highest-rank male can monopolize matings with any nearby female, females often exercise their own choice, going off temporarily with a male of lower rank when they are fertile.
