Primate sociality is an area of primatology that aims to study the interactions between three main elements of a primate social network: the social organisation, the social structure and the mating system. The intersection of these three structures describe the socially complex behaviours and relationships occurring among adult males and females of a particular species.Lindenfors P 2018 Primate social evolution. In: Callan H (ed.) International Encyclopedia of Anthropology. John Wiley & Sons, Ltd. Cohesion and stability of groups are maintained through a confluence of factors, including: kinship, willingness to cooperate, frequency of agonistic behaviour, or varying intensities of dominance structures.
Primate social organisation exists along a spectrum, with networks ranging from the solitary neighbourhood systems to the multi-individual units to the complex multilevel societies that are composed of hierarchically-organised social units. The evolution of diverse primate social systems is considered to be a naturally selected anti-predation response. Increased resource detection, cooperation and social learning are also considered as co-benefits of group living.
Emergence of group living Similar to genetic traits, behavioural characteristics can similarly result from natural selection processes. In opposition to many animal-decision making strategies which encourage individual fitness, group living (or sociality) prioritises an inclusive group fitness. Socioecological factors are thought to influence primate social organisation. For example, the main benefits of group living are hypothesised to be: Improved predator detection. Predator vigilance (or awareness) and predator defence are thought to increase with group living. More eyes means detection will occur sooner, communication among members will ensure appropriate responses and actions are taken, minimising the primates' susceptibility to predation. Improved resource (water or food) detection. The hypothesis is that more individuals infer a heightened spatial knowledge and an increased ability to detect resources if more landscape is being covered. Opportunity for cooperation. Primate sociality and living in close proximity bolsters cooperative behaviours necessary for participating in activities such as hunting, alloparenting, and/or territory or mate defence. Reduced risk of infanticide. There have been observations from certain baboon populations that suggest a correlation between infant survival and group size: e.g. infants are likelier to survive in larger groups. Increased opportunity for social learning.
The main constraining factors of social group sizes are related to: Resource abundance. Because living in groups requires members to share access to essential resources (like food, water, mates, sleeping sites) there are selective costs that constrain group size. Pathogen transmission. Larger groups increases exposure to pathogens among its members. Competition and aggression. If intra-group competition becomes too high, the associated stress can potentially impose negative health impacts. Cognitive capabilities. There is an assumption that cognitive abilities must be able to interpret the complex information of group living (including information resulting from social relationships).
Interestingly, there are competing hypotheses for the role feeding takes in influencing primate sociality. It is interpreted as having both as a positive (resource detection) and negative (resource competition) effect depending on the analysis.
In order for sociality to have been selected for via natural selection, the collective benefits of group living must outweigh the collective costs. Thus, if intra-group competition becomes too great, the group is likely to fracture into smaller units. A thorough review of the literature suggests that the lower threshold of primate group living is determined by risk of predation while the upper limit of group size is determined by feeding competition among individuals.
Primate social organisation Social organisation refers to the size (number of individuals), composition (variation between the sexes), and cohesion (relating to proximity and bond strengths among individuals) of the society in consideration. The synchronisation of individuals, or lack thereof, also provides insight into relationships among individuals. There are seven types of primate social organisations identified in the literature (discussed below), including: solitary primate systems, pair-bonded systems, one-male-multi-female systems, one-female-multi-male systems, multi-male-multi-female systems, fission fusion societies, and multilevel societies. Interestingly, primate social organisation is not necessarily species-specific. For instance, an example of within species (intra-species) variation would be tamarins and marmosets. These two primates are part of the callitrichidae family and have been observed to demonstrate pair-bonding systems in some populations while others have one-male unit (OMU) systems.
Solitary primate systems, sometimes referred to as neighbourhood systems, occur when an adult male's territory overlaps with one (or more) adult female's territory and individuals conduct activities (most often foraging or offspring care) independently from one another. In this system, solitary does not imply antisocial, but rather behaviour is characterised by this lack of synchronisation among the individuals. In fact, many solitary primates maintain social networks by using vocal or olfactory signals to communicate.
Examples of solitary primates: orangutans, galagos, lorises, some species of lemurs, some tarsiers
Pair-bonded systems, or pair-living primates, are small social units consisting of one adult male and one adult female, and their immature offspring. There are factors of time and space that define this type social system. Firstly, pair-bonds must demonstrate a long-term affiliative partnership for at least one year or one seasonal cycle. Secondly, there must also be a higher frequency of association (spatial proximity) between the bonded-pair individuals than there is with other individuals.
Paternal care of offspring is a relatively uncommon trait in primate social systems; however, the monogamous mating system often observed (though it should not be assumed) in pair-bonding generates an equal variance for offspring success for both pair members. Thus, paternal involvement in off-spring rearing is much likelier to be observed in primate species where pair-living occurs.
Examples of pair-bonded primate species: titi monkeys, owl monkeys, some species of marmosets and tamarins, many species of siamangs and gibbons
One-female-multi-male groups are composed of one reproductive adult female and two or more adult male partners in the group. If there are other associated females within the group, they will likely have their reproductivity suppressed either via agonistic behaviours (aggressive and submissive interactions) or olfactory signals (such as pheromones). This social system promotes cooperative breeding (or alloparenting), where the non-breeding individuals assist in providing care for the off-spring produced by the main breeding female.
Examples of one-female-multi-male structured primate species: many species of tamarin and marmoset
One-male-multi-female groups are usually characterised by a single resident male who defends a group of (often related) adult females against males from outside the group. While tenure ship is held, this form of social organisation allows a male exclusive access to reproductive females for breeding purposes. A resident male often suffers challenges from extra-group males (perhaps belonging to all-male bachelor groups) whereby these males may attempt takeovers with the goal of gaining sole access to the reproductive females. A takeover by a new resident male could lead to infanticide (infant killing). This behaviour is interpreted as a strategic attempt to bring females back into estrous, which allows mating opportunities to occur sooner for the new resident male.
Examples of one-male-multi-female structured primate species: some species of gorillas, numerous colobine and guenon species, patas monkeys, howler monkeys
Multi-male-multi-female social systems are characterised by associations between larger numbers of individuals forming groups. Since individuals are able to mate with multiple partners, paternity is often hidden or skewed which helps ensure the survival of off-spring. A variety of social relationships and bonds exist among multi-male-multi-female group members. For instance, some research has led to observations of dyadic relationships, or friendships. These friendships are more moderate forms of the pair-bonded social structures, existing within the multi-male-multi-female system. For instance, one study of savannah baboons (Papio cynocephalusI) observed that the lactating females in the group would more closely associate with specific adult males. As further research is conducted on primate friendships, three main benefits have been hypothesised: close associations with a specific male (1) tends to discourage infanticide, (2) tends to reduce incidence of harassment of the female, and (3) stimulates paternal investment and care in the offspring. The benefits of friendships within the multi-male-multi-female group systems demonstrate similar advantages as pair-bonded systems.
Examples of multi-male-multi-female structured primate species: many species of macaques, baboons, vervet monkeys, mangabeys, capuchins, squirrel monkeys, woolly monkeys, some colobine species, some lemurs (ring-tailed and sifaka).
Fission fusion societies demonstrate a high degree of fluidity by splitting (fission) and merging (fusion) as the group moves across a landscape. This type of organisation is less cohesive than multi-male-multi-female groups, with patterns often reflecting the local availability of resources. For instance, if foraging patch sizes are small, the larger group will often break apart to forage and later merge in order to sleep. This type of society is typically characterised by female philopatry, where female kin lineages make up the core of the groups and males disperse to other groups. Interestingly, some researchers hypothesise that fission fusion societies may have been socially inherited from the last shared common ancestor of humans, chimpanzees and bonobos.
Examples of primate species with fission fusion societies: humans, chimpanzees, bonobos, spider monkeys
Multilevel societies, sometimes referred to as hierarchical or modular societies, are the largest and most complex form of primate social organisation. Social stratification of these societies is discrete and has at least one stable core unit. Typically, multilevel societies are composed of between two and four levels of social structures: one-male units (OMUs, or harems) nested within clans, which are nested within bands, which are nested within a troop. OMUs are composed of a single-male breeding unit (a leader male), several females, and may even have a follower male. Similar to the resident male of a one-male-multi-female group, the OMU leader male is susceptible to takeovers by outside adult males. In some species, there is an additional level to the society: clans. The clan level is nested between the OMU and the band level. Clans consist of OMUs and of all-male units (AMUs) of bachelor males (either kin-related or not). Finally, a band is a coalesced grouping of OMUs who routinely sleep and forage together. The troop is a temporary aggregation of bands who might also forage or sleep in the same sites depending on environmental constraints.