Social hierarchies (zoology)
Social hierarchies in zoology refer to structured relationships among individuals within animal populations, primarily established to manage competition for scarce resources such as food, mates, and nesting sites. These hierarchies often arise from initial conflicts, leading to a system of dominance where individuals are ranked based on their ability to assert control over others. The simplest form is despotism, where a single individual dominates the group, while more complex hierarchies can involve multiple ranks, such as alpha, beta, and omega individuals.
Democratic behaviors are also observed in various species, where group decisions are made collectively rather than through direct competition, promoting stability and cooperation. Visual, acoustic, and chemical signals play key roles in establishing and maintaining these hierarchies, as dominant individuals often display specific behaviors or signals to assert their status.
While dominant individuals typically gain priority access to resources, subordinates may still benefit from group living and have opportunities to rise in rank over time. Overall, social hierarchies enhance the efficiency of animal groups by reducing conflict and promoting cooperation, proving essential for survival and reproductive success across diverse species.
Social hierarchies (zoology)
All animal species strive for their share of fitness. In this struggle for reproductive success, there is often competition among the individuals that make up the population. This competition is generally for some essential resource, such as food, mates, or nesting sites. In many species, competition over resources may lead to actual fighting among the individuals. Fighting, however, can be costly to the individuals involved. The loser may suffer real injury or even death, and the winner has to expend energy and still may suffer an injury. To prevent constant fighting over resources, many animal species have adopted a system of dominance, or what sociobiology scholars call a dominance hierarchy or social hierarchy. The dominance hierarchy is a set of aggression-submission relationships among the animals of a population. With an established system of dominance, the subordinate individuals will acquiesce rather than compete with the dominant individuals for resources.
General Characteristics of Hierarchies
The simplest possible type of hierarchy is despotism, where one individual rules over all other group members, and no rank distinctions are made among the subordinates. Despotism is observed in groups of lemurs, macaques, and baboons. Hierarchies more frequently contain multiple ranks in a more or less linear fashion. An alpha individual dominates all others, a beta individual is subordinate to the alpha but dominates all others, and so on, down to the omega individual at the bottom, who is dominated by all others. Sometimes, the network is complicated by triangular or other circular relationships where two or three individuals might be at the same dominance level. Such relationships are typically less stable than despotisms or linear orders. However, a democratic structure is most effective for peaceful populations of many species. Democratic decision-making is usually less extreme. For example, in red deer and African buffalo populations, if the group is lying down in a field, the group will only stand and begin to move as a herd if around 60 percent of the group members stand up. These types of democratic decisions are typically motivated by what is best for the group.
Hierarchies are formed during the initial encounters between animals through repeated threats and fighting, but once the issue of dominance has been determined, each individual gives way to their superiors with little or no hostile exchange. Life in the group may become so peaceful that ranking is hidden from the observer until a crisis occurs to force a confrontation. For example, a troop of baboons can go for hours without engaging in sufficient hostile exchanges to reveal their ranking, but in a moment of crisis, such as a quarrel over food, the hierarchy will suddenly be evident. Some species are organized in absolute dominance hierarchies in which the rank orders remain constant regardless of the circumstances. Status within an absolute dominance hierarchy changes only when individuals move up or down in rank through additional interaction with their rivals. Other animal societies are arranged in relative dominance hierarchies. In these arrangements, such as with crowded domestic house cats, even the highest-ranking individuals acquiesce to subordinates when the latter approach a point that would normally be too close to their personal sleeping space.
The stable, peaceful hierarchy is often supported by status signs. In other words, the mere actions of the dominant individual advertise his dominance to the other individuals. The leading male in a wolf pack can control his subordinates without a display of excessive hostility in most cases. He advertises his dominance by how he holds his head, ears, and tail and by the confident, face-forward manner in which he approaches other pack members. In a similar manner, the dominant rhesus monkey advertises his status by an elaborate posture which, includes an elevated head and tail, lowered testicles, and slow, deliberate body movements accompanied by an unhesitating but measured scrutiny of other monkeys he encounters. Animals not only utilize visual signals to advertise dominance, but they also use acoustic and chemical signals. For example, dominant European rabbits use a mandibular secretion to mark their territory. Secretions denote differences in gender and age and are more prevalent among young adult males.
Special Properties of Dominance Hierarchies
A stable dominance hierarchy presents a potentially effective united front against strangers. Since a stranger represents a threat to the status of each individual in the group, he is treated as an outsider. When expelling an intruder, cooperation among individuals within the group reaches a maximum. Chicken producers have long been aware of this phenomenon. If a new bird is introduced to the flock, it will be attacked for many days and be forced down to the lowest status unless it is exceptionally vigorous or attempts are made at effectively socializing the animal. Sometimes, it will simply die with very little show of fighting back. An intruder among a flock of Canada geese will be met with the full range of threat displays and repeated mass approaches and retreats.
In some primate societies, the dominant animals use their status to stop fighting among subordinates. This behavior has been observed in rhesus, pig-tailed macaques, and spider monkeys. This behavior has been observed even in animal societies, such as squirrel monkeys, that do not exhibit dominance behavior. Because of the power of the dominant individual, relative peace is observed in animal societies organized by despotisms, such as hornets, paper wasps, bumblebees, and crowded territorial fish and lizards. Fighting increases significantly among the equally ranked subordinates as they vie for the dominant position when the dominant animal is removed.
Young males are routinely excluded from the group in a wide range of aggressively organized mammalian societies, such as baboons, langur monkeys, macaques, elephant seals, and harem-keeping ungulates. At best, these young males are tolerated around the fringes of the group, but many are forced out of the group and either join bachelor herds or wander as solitary nomads. As would be expected, these young males are the most aggressive and troublesome members of the society. They compete with one another for dominance within their group and often unite into separate bands that work together to reduce the power of the dominant males. Males in the two groups show different behaviors. Among the Japanese macaques, the dominant males stay calm and aloof when introduced to a new object to not risk the loss of their status, but the females and young males will explore new areas and examine new objects.
Nested hierarchies are often observed in some animal species. Societies that are divided into groups can display dominance both within and between the various components. For example, white-fronted geese establish a rank order of several subgroups, including parents, mated pairs without young, and free juveniles. These hierarchies are superimposed over the hierarchy within each of the subgroups. In wild turkeys, brothers establish a rank order among their brotherhood, but each brotherhood competes for dominance with other brotherhoods on the display grounds before mating.
Scientists studying the evolution of human and animal hierarchies have noted parallels between human and animal social structures and how they are established. Humans and animals assume social roles in their hierarchy. The alpha has particular roles and responsibilities, just like the beta, in animal and human dynamics. Social hierarchies increase productivity in animals in terms of reproduction, protection, foraging, and other advantages based on the species. Similarly, human productivity increases as individuals in a social group increase their effort, education, and skills or modify their social behaviors to match those of the most successful group members. Similarities in social groups across species are attributed to evolutionary adaptations and common ancestors' similarities.
Dominants and Subordinates
To be dominant is to have the priority of access to the essential resources of life and reproduction. In almost all cases, superior, dominant animals will displace the subordinates from food, mates, and nest sites. In the matter of obtaining food, for example, wood pigeons are flock feeders. The dominant pigeons are always found near the center of the flock when feeding, and they feed more quickly than the subordinate birds at the edge of the flock. The birds at the edge of the flock accumulate less food and often obtain just enough to sustain them through the night. Among sheep and reindeer, the lowest-ranking females are also the worst-fed animals and among the poorest of mothers. Baby pigs compete for a teat position on the mother and, once established, will maintain that position until weaning. Those piglets that gain access to the most anterior teats will weigh more at weaning than those that settle for posterior teat positions. In gaining access to mates, one study with laboratory mice has shown that while the dominant males constituted only one-third of the male population, they sired 92 percent of the offspring.
Life is not hopeless for subordinates. Often, the loser in the battle for dominance is given a second chance, and in some social species, the subordinate only has to await its turn to rise in the hierarchy. In some species, cooperation among subordinate groups, especially kin groups, can lead to the formation of a new colony and a new opportunity to establish dominance. In other species, it may be advantageous for the subordinate to stay with the group. For example, individual baboons and macaques will not survive very long if they are away from the group’s sleeping area, and they will have no opportunity to reproduce. It has been shown that even a low-ranking male eats well if he is part of a troop, and he may occasionally have the opportunity to mate. In addition, the dominant male will eventually lose prowess, and the subordinate will have a chance to move up in the dominance hierarchy.
Principal Terms
Adaption: in evolutionary biology, any structure, physiological process, or behavior that gives an organism an advantage in survival or reproduction in comparison with other members of the same species
Aggression: a physical act or threat of action by one individual that reduces the freedom or genetic fitness of another
Competition: the active demand by two or more organisms for a common resource
Dominance: the physical control of some members of a group by other members, initiated and sustained by hostile behavior of a direct, subtle, or indirect nature
Fitness: in the genetic sense, the contribution to the next generation of one genotype in a population relative to the contributions of other genotypes
Sociobiology: the study of the biological basis of the social behavior of animals
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