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. 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
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