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Primate Sociality and Social Systems

By: Larissa Swedell (Queens College, City University of New York; New York Consortium in Evolutionary Primatology) © 2012 Nature Education 
Citation: Swedell, L. (2012) Primate Sociality and Social Systems. Nature Education Knowledge 3(10):84
Why be social? And, why not be? What are the costs and benefits of sociality, and what types of sociality characterize nonhuman primates?
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Why Be Social?

Many primates and other animals live in social groups. In social groups, individual members coordinate their activities, communicate with one another, and interact in both affiliative (friendly) and agonistic (aggressive or submissive) ways. In many social groups, individuals are gregarious; that is, they interact with one another frequently, engage in a variety of types of social interactions, and typically form and maintain social bonds (strong social relationships) with other individuals (Dunbar 1988, Silk 2007). These bonds are often expressed in the form of grooming, a common social activity among primates in which one individual carefully picks through the fur of another and removes any debris or ectoparasites (Figure 1). Grooming serves an important social function for most primates in addition to its hygienic function (Henzi & Barrett 1999).

Two female geladas (<i>Theropithecus gelada</i>) grooming one another.
Figure 1: Two female geladas (Theropithecus gelada) grooming one another.
© 2012 Nature Education Courtesy of L. Swedell. All rights reserved. View Terms of Use

Group life carries with it inevitable conflict and competition. Individuals must share food resources, water resources, sleeping sites, and mates. While usually mediated by dominance hierarchies in which higher-ranking individuals have priority of access to limited resources, aggressive competition over food and mates is common in primates, and is not only energetically costly but can lead to injury and even death (Mason & Mendoza 1993). In addition to energy costs and risk of injury, high levels of aggression-given or received-can lead to chronic psychological stress. Chronic stress can adversely affect health and reproduction in numerous ways (Sapolsky 2002). Other socially-induced events may also increase stress levels: in chacma baboons, for example, infanticidal (infant killing) behavior by males-or even simply the immigration of a potentially infanticidal male into a social group-increases stress hormone levels in females with infants (Engh et al. 2006). In addition to these social costs of group living, close social contact also increases the potential transmission of pathogens, which increases each individual's risk of contracting infectious diseases. Finally, groups may be disadvantageous because they are more easily detectable by predators compared to solitary individuals.

Given all of these potential costs of group living, why do so many primates-and other mammals-bother living in groups at all? One might especially wonder about this for the lowest-ranking individuals in a social group (i.e., those who are last in line for the available resources).

Despite the costs, sociality is important to these animals for several reasons. Probably most importantly, living in a group likely decreases one's risk of falling victim to predation (van Schaik 1983). There are three reasons for this. First, in social groups there are more individuals looking out for predators and thus predators will be detected more quickly. Second, living in a group decreases each individual's chance of being preyed upon due to an effect called "geometry for the selfish herd" (Hamilton 1971): this states that the larger the group (e.g., 100 versus 10), the lower each individual's chance (1/100 versus 1/10) of becoming prey. Third, individuals in groups can collectively mob predators and successfully drive them away, whereas lone individuals cannot.

Sociality also benefits animals via access to food and other resources. In groups, there are many individuals looking for food simultaneously and thus detection of good food resources (e.g., ripe fruit; Figure 2) will inevitably be communicated to others simply because group members are usually in close proximity to one another. Group members will also benefit from cooperation over defense of food-or other limited resources such as water holes and sleeping sites-as groups can out-compete individuals, and larger groups can out-compete smaller groups (Wrangham 1980).

A moor macaque (<i>Macaca maurus</i>) feeding on ripe fruit in Sulawesi.
Figure 2: A moor macaque (Macaca maurus) feeding on ripe fruit in Sulawesi.
© 2012 Nature Education Courtesy of L. Swedell. All rights reserved. View Terms of Use

In addition, sociality is beneficial to group-living animals in that it makes it easier for them to find mates. Animals that do not live in groups must either search for mates or opportunistically mate when they encounter other individuals. Group-living animals simply choose mates within their social group.

Moreover, sociality allows cooperative socialization of offspring. In social groups, infants and juveniles play with one another, which develops motor skills as well as the social skills necessary to survive and reproduce in a social setting. For example, during social play juveniles receive reinforcement from adults about how dominance hierarchies work, and what it means to have a given rank within a social group of a given species. Thus when they reach adulthood they will have learned what they can and cannot do, and thereby fit into the social fabric of their social group (Pereira & Fairbanks 1993).

Finally, sociality in and of itself appears to carry benefits for individuals. As noted above, one of the costs of group living is the potentially high level of conflict and aggression that occurs among group members, which involves greater energy expenditure, risk of injury, and chronic stress. In baboons, this socially-induced stress appears to be alleviated by the receipt of affiliative vocalizations, as well as the maintenance of grooming relationships with a small network of close associates (Crockford et al. 2008). Ultimately, female baboons with strong social bonds (i.e., social relationships characterized by frequent proximity and grooming; Figure 3) experience greater offspring survival and even longer lifespans than females with weaker bonds (Silk et al. 2003, 2009, 2010). These studies demonstrate that strong social relationships within groups, beyond group living alone, can carry important fitness benefits for individuals.

Olive baboons (<i>Papio anubis</i>) grooming in Gombe Stream National Park, Tanzania.
Figure 3: Olive baboons (Papio anubis) grooming in Gombe Stream National Park, Tanzania.
© 2012 Nature Education Courtesy L. Swedell All rights reserved. View Terms of Use

Primate Social Systems

The social system of a given species is an outcome of (1) its social structure, the size and composition of a typical group of that species, and (2) its social organization, how those individuals are organized (i.e., the patterns of spacing, agonistic and affiliative social interactions, philopatry [whether one or both sexes remain in their natal group], and dispersal [whether one or both sexes move to a new group to reproduce]) that typify that species. Each species also tends to have a characteristic mating system (i.e., the pattern of mating among members of each sex). All of these aspects of primate societies vary widely across the primate order.

The least gregarious primates have what is often referred to as a solitary dispersed social system. In these primates, an adult male's territory overlaps the territory of one or more adult females, but each individual forages alone and maintains social contact mainly through vocal and/or olfactory communication. These primates are typically nocturnal, foraging at night and sleeping in trees during the day. The mating system in these primates is usually polygynous (i.e., each male mates with multiple females). This type of social system characterizes galagos, lorises, some lemurs, some tarsiers, and orangutans. Notably, orangutans are the only anthropoid primates with a solitary social system.

Titi monkeys, owl monkeys, some callitrichids (marmosets and tamarins), and many hylobatids (gibbons and siamangs) are characterized by a pair-bonded social system. Here, one adult male and one adult female form a small social group and defend a territory from other pairs. The mating system in these groups is usually monogamous (only one male mates with only one female), though extra-pair copulations have been observed (Palombit 1994), and the male usually participates in offspring care, which is unusual for male mammals (Fuentes 2002).

Many marmosets and tamarins live in one-female multi-male groups characterized by cooperative breeding. In this type of system, usually only one female breeds, and that female suppresses the reproduction of any subordinate females via aggression and/or pheromonal (olfactory) signals. Usually there is more than one breeding male, thus the mating system is polyandrous, a rarity among mammals. Some or all of the individuals in these groups participate in offspring care and this social system is thus often called cooperative polyandry.

One of the most common primate social systems is the one-male group, which characterizes most colobine monkeys, most guenons, patas monkeys, howler monkeys, and some gorillas. Here, a single resident adult male defends a group of (usually) philopatric, related females from other males and, while his tenure lasts, enjoys exclusive mating access to those females (i.e., polygyny). Sometimes called harems, these groups are always at risk of takeover by non-resident males, who typically form all-male groups while awaiting their chance to become a resident male. Often, takeovers are accompanied by infanticide, in which the new resident male kills the young infants in the group. This behavior has the effect of bringing the mothers back into estrus (sexual receptivity) sooner than they would have otherwise.

Also common among primates are multi-male multi-female groups, in which multiple individuals of each sex form large social groups in which the mating system is usually polygynandrous (i.e., both males and females are polygamous in that they mate with multiple members of the opposite sex). These are the largest groups of primates, and usually quite complex socially, with differentiated social and kin relationships among group members. This type of social system characterizes many monkeys, including macaques, most baboons, vervet monkeys, mangabeys, capuchins, squirrel monkeys, woolly monkeys, and some colobine monkeys, as well as some lemurs-most notably the ringtailed lemur and sifaka. In most of these species, females are philopatric and males disperse.

Similar to multi-male multi-female groups are the fission-fusion communities of chimpanzees, bonobos, spider monkeys, and some other ateline monkeys. Fission-fusion communities are less cohesive than typical multi-male multi-female groups. These groups occupy very large home ranges in which temporary foraging parties cleave and coalesce over time with changes in resource availability and female reproductive condition. These social systems are typically characterized by female dispersal and male philopatry.

The most complex type of social system found in primates, and in mammals as whole, is the multi-level society (also known as a hierarchical or modular society) characterizing hamadryas baboons (Figure 4), geladas, snub-nosed monkeys, and a few other mammals such as elephants. In this type of system, there are at least three levels of social structure: the one-male unit (OMU), the band, and the troop or herd. The OMU is the reproductive unit and consists of one "leader" male, sometimes a follower male, and several females; the band is the ecological unit that forages and sleeps together, and the troop or herd is a temporary aggregation at a sleeping site or foraging area. In hamadryas baboons, there is a fourth layer between the OMU and the band, the clan, which consists of OMUs and bachelor males linked by social bonds and possibly kinship among males. In geladas, bachelor males join together to form all-male groups. Reproduction in these societies is usually polygynous, and OMUs are always at risk of takeover by bachelor males, who may commit infanticide after taking over females with young infants.

A hamadryas baboon (<i>Papio hamadryas</i>) one-male unit (OMU) at Filoha, Ethiopia.
Figure 3: A hamadryas baboon (Papio hamadryas) one-male unit (OMU) at Filoha, Ethiopia.
© 2012 Nature Education Courtesy M. Pines All rights reserved. View Terms of Use

It is important to note that the social organization of a species might not be immediately apparent from its social structure. For example, both geladas (Figure 1) and hamadryas baboons (Figure 4) are characterized by multiple layers of society, and their social structure is almost identical. However, the social organization of these two species could not be more different (Kummer 1967, Dunbar 1983). In geladas, cohesion of OMUs is maintained by philopatric females, as each OMU is a female kin group and these kin groups are taken over as entire units by males. In hamadryas, by contrast, cohesion of OMUs is maintained by philopatric males who take over females one by one, often exchanging them with other males in their clans (Pines et al. 2011), and aggressively condition those females to remain in their OMU. Thus, hamadryas social organization includes strong male-female bonds (the glue that holds together an OMU) and strong male-male bonds (as males are philopatric and thus related to one another within bands and clans), whereas gelada social organization is characterized by strong female-female bonds (which hold together OMUs) but weak bonds between the sexes.

Moreover, one must be careful not to make assumptions about a species' mating system simply upon observing its social system. For example, multi-male multi-female groups of gorillas may have an age-graded dominance structure in which only the oldest, highest-ranking (alpha) silverback male is allowed to copulate; in this case, the mating system is not polygynandrous (as one might expect in a multi-male group) but instead polygynous (Robbins 2011). Another example can be found in guenons, which live in one-male groups. During the mating season multi-male influxes occur in which outside males come into the group, copulate with the females, and then leave again (Cords 1988). The mating system in this case is not polygynous, which one would expect, but polygynandrous. Finally, gibbons and siamangs were first thought to be monogamous, but then observations of extra-pair mating (i.e., copulations with individuals outside of the pair-bond) confirmed that they are not always monogamous but sometimes polygynous, polyandrous, or both (Palombit 1994).


Affiliative: Friendly or non-threatening.

Age-graded dominance structure: A pattern in which the oldest and most dominant male in a group prevents the younger, subordinate males from copulating with females and thus achieves a reproductive monopoly.

Agonistic: Involving aggression, or the threat of aggression, and submission.

All-male group: A social group consisting exclusively of males, usually occurring in species with one-male groups or multi-level societies (i.e., social systems in which single males monopolize several females and there are extra males without females).

Band: A layer of social structure in the multi-level societies of geladas, hamadryas baboons, and snub-nosed monkeys that consists of multiple one-male units and generally remains relatively cohesive over space and time, analogous to the group or troop of other monkeys.

Clan: A layer of social structure in the hamadryas social system between the one-male unit and the band, linked by social bonds (and likely kinship) among males.

Competition: A state in which something is sought by more than one individual at the same time.

Conflict: A state of incompatibility or opposing interests.

Cooperative breeding: A social system in which individuals care for offspring other than their own, usually at the expense of their own reproduction.

Cooperative polyandry: A mating system in which one female copulates with more than one male, and all individuals in the group care for offspring, including the non-parent(s).

Dominance: A power relationship among individuals.

Dominance hierarchy: A ranking of dominance relationships within social groups.

Dispersal: Departure from one social group (usually one's natal group) typically followed by immigration into another, usually at reproductive maturity.

Estrus: Period of increased sexual receptivity, attractivity, and proceptivity around the time of ovulation, accompanied in some species by external morphological cues such as sexual swellings.

Extra-pair mating: Copulations with individuals outside of the pair bond.

Fission-fusion community: A large social group that is less cohesive than a typical primate social group, in which sub-units of variable composition break apart and come together over time.

Female bonded: Characterized by strong bonds among females.

Female-female bonds: Strong social relationships among females, often deriving from close kinship as a result of female philopatry.

Gregarious: Characterized by a high motivation to interact socially; engaging in frequent social interactions with other members of one's species.

Grooming: A characteristic primate social behavior in which the hands are used to search through the fur for ectoparasites and debris, which are then removed.

Harem: A term sometimes used for a one-male unit, usually of geladas or hamadryas baboons.

Herd: A temporary aggregation of one-male units of geladas, similar to the hamadryas troop.

Hierarchical society: See multi-level society.

Infanticide: Deliberate killing of infants, usually by males, and often in the context of immigration or takeovers.

Male-female bonds: Strong social relationships between males and females.

Male-male bonds: Strong social relationships among males, often deriving from close kinship among those males as a result of male philopatry.

Mating system: The patterns of sexual behavior characterizing a given species.

Modular society: See multi-level society.

Monogamy: A mating system in which only one male copulates with only one female.

Multi-level society: A social system characterized by multiple layers of social structure in which the smaller sub-units (usually one-male units of consistent composition) break apart and come together over time, also known as a hierarchical society or modular society.

Multi-male influx: Temporary immigration into a group by multiple males, usually during the mating season.

Multi-male multi-female group: A social group consisting of multiple adult males and multiple adult females.

Nocturnal: Active at night.

Olfactory: Involving the sense of smell.

One-female multi-male group: A social group consisting of multiple adult males but only one adult female.

One-male group: A social group consisting of one adult male and multiple adult females.

One-male unit: The smallest layer of social structure in the multi-male societies of hamadryas baboons, geladas, and snub-nosed monkeys, consisting of one "leader male" and one or more females.

Pair-bonded: Characterized by strong bonds between the sexes.

Pathogen: A disease-carrying agent.

Philopatry: A pattern in which individuals remain in the social group in which they were born and do not disperse.

Play: Behavior characteristic of infants and juveniles in which normal elements of the behavioral repertoire are used in non-normal contexts and sequences.

Polyandry: A mating system in which one female copulates with multiple males.

Polygyny: A mating system in which one male copulates with multiple females.

Polygynandry: A mating system in which members of both sexes copulate with multiple members of the opposite sex.

Predation: The consumption of one animal by another as a source of food.

Social bond: A term used for a social relationship, usually referring to relatively strong social relationships.

Social group: An aggregation of individuals that is cohesive in space and time in which individuals tolerate one another and interact with one another.

Social organization: The patterns of spacing, interactions, dispersal, and philopatry characterizing a given species.

Social relationship: A pattern of social interaction over time between two individuals.

Social structure: The typical size and composition of a social group for a given species.

Social system: The specific pattern of group living and social interaction that characterizes a given species, including its social structure and social organization.

Sociality: The state of being social or living in a social group.

Socialization: The process of learning the social norms of a particular group or social system.

Solitary dispersed: A social system in which individuals spend most of their time alone, especially during foraging, but usually maintain contact with other individuals via vocalizations and olfactory communication (scent-marking).

Stress: A state of internal tension in which the body copes by producing stress hormones which, if chronic, can lead to suppression of the immune system.

Takeover: The defeat of a resident male by a new male and the subsequent acquisition of the former resident male's female(s).

Territory: An area that is defended by members of a species (usually a social group) against other members of that same species.

Troop: A temporary aggregation of bands of hamadryas baboons, similar to the gelada herd. Also used as a general term referring to a social group of monkeys (especially baboons).

References and Recommended Reading

Cords, M. "Mating systems of forest guenons: A preliminary review," in A Primate Radiation: Evolutionary Biology of the African Guenons, eds. A. Gautier-Hion et al. (Cambridge, UK: Cambridge University Press, 1988) 323-339.

Crockford C. et al. Social stressors and coping mechanisms in wild female baboons. Hormones and Behavior 53, 885-890 (2008).

Dunbar, R. I. M. "Relationships and social structure in gelada and hamadryas baboons," in Primate Social Relationships: An Integrated Approach, ed. R. A. Hinde (Sunderland, MA: Sinauer Associates, 1983) 299-307.

Dunbar, R. I. M. Primate Social Systems. Ithaca, NY: Cornell University Press, 1988.

Engh, A. E. et al. Female hierarchy instability, male immigration, and infanticide increase glucocorticoid levels in female chacma baboons. Animal Behaviour 71, 1227-1237 (2006).

Fuentes, A. Patterns and trends in primate pair bonds. International Journal of Primatology 23, 953-978 (2002).

Hamilton, W. D. Geometry for the selfish herd. Journal of Theoretical Biology 31, 295-311 (1971).

Henzi, S. P. & Barrett, L. The value of grooming to female primates. Primates 40, 47-59 (1999).

Kummer, H. Dimensions of a comparative biology of primate groups. American Journal of Physical Anthropology 27, 357-366 (1967).

Mason, W. A. & Mendoza, S. P. eds. Primate Social Conflict. Albany, NY: SUNY Press, 1993.

Pereira, M. E. & Fairbanks, L. A. eds. Juvenile Primates: Life History, Development, and Behavior. New York, NY: Oxford University Press, 1993.

Robbins, M. M. "Gorillas: Diversity in ecology and behavior," in Primates in Perspective, 2nd ed. eds. C. J. Campbell et al. (New York, NY: Oxford University Press, 2011) 326-339.

Palombit, R. A. Extra-pair copulations in a monogamous ape. Animal Behaviour 47, 721-723 (1994).

Pines, M., Saunders, J. & Swedell, L. Alternative routes to the leader male role in a multi-level society: Follower versus solitary male strategies and outcomes in hamadryas baboons. American Journal of Primatology 73, 679-691 (2011).

Sapolsky, R. M. "Endocrinology of the Stress Response," in Behavioral Endocrinology, eds. J. Becker et al. (Cambridge, MA: MIT Press, 2002) 409-450.

Silk, J. B. Social components of fitness in primate groups. Science 317, 1347-1351 (2007).

Silk, J. B., Alberts, S. C. & Altmann, J. Social bonds of female baboons enhance infant survival. Science 302, 1231-1234 (2003).

Silk, J. B. et al. The benefits of social capital: Close social bonds among female baboons enhance offspring survival. Proceedings of the Royal Society B: Biological Sciences 276, 3099-3104 (2009).

Silk, J. B. et al. Strong and consistent social bonds enhance the longevity of female baboons. Current Biology 20, 1359-1361 (2010).

van Schaik, C. P. Why are diurnal primates living in groups? Behaviour 87, 120-144 (1983).

Wrangham, R. W. An ecological model of female-bonded primate groups. Behaviour 75, 262-300 (1980).


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