R. I. M. Dunbar

The social brain hypothesis

Conventional wisdom over the past 160 years in the cognitive and neurosciences has assumed that brains evolved to process factual information about the world. Most attention has therefore been focused on such features as pattern recognition, color vision, and speech perception. By extension, it was assumed that brains evolved to deal with essentially ecological problem‐solving tasks.

Coevolution of neocortical size, group size and language in humans

Group size is a function of relative neocortical volume in nonhuman primates. Extrapolation from this regression equation yields a predicted group size for modern humans very similar to that of certain hunter-gatherer and traditional horticulturalist societies. Groups of similar size are also found in other large-scale forms of contemporary and historical society. Among primates, the cohesion of groups is maintained by social grooming; the time devoted to social grooming is linearly related to group size among the Old World monkeys and apes. To maintain the stability of the large groups characteristic of humans by grooming alone would place intolerable demands on time budgets. It is suggested that (1) the evolution of large groups in the human lineage depended on the development of a more efficient method for time-sharing the processes of social bonding and that (2) language uniquely fulfills this requirement. Data on the size of conversational and other small interacting groups of humans are in line with the predictions for the relative efficiency of conversation compared to grooming as a bonding process. Analysis of a sample of human conversations shows that about 60% of time is spent gossiping about relationships and personal experiences. It is suggested that language evolved to allow individuals to learn about the behavioural characteristics of other group members more rapidly than is possible by direct observation alone.

Neocortex size as a constraint on group size in primates

Abstract Two general kinds of theory (one ecological and one social) have been advanced to explain the fact that primates have larger brains and greater congnitive abilities than other animals. Data on neocortex volume, group size and a number of behavioural ecology variables are used to test between the various theories. Group size is found to be a function of relative neocortical volume, but the ecological variables are not. This is interpreted as evidence in favour of the social intellect theory and against the ecological theories. It is suggested that the number of neocortical neurons limits the organisms information-processing capacity and that this then limits the number of relationships that an individual can monitor simultaneously. When a groups size exceeds this limit, it becomes unstable and begins to fragment. This then places an upper limit on the size of groups which any given species can maintain as cohesive social units through time. The data suggest that the information overload occurs in terms of the structure of relationships within tightly bonded grooming cliques rather than in terms of the total number of dyads within the group as a whole that an individual has to monitor. It thus appears that, among primates, large groups are created by welding together sets of smaller grooming cliques. One implication of these results is that, since the actual group size will be determined by the ecological characteristics of the habitat in any given case, species will only be able to invade habitats that require larger groups than their current limit if they evolve larger neocortices.

Evolution in the social brain.

The evolution of unusually large brains in some groups of animals, notably primates, has long been a puzzle. Although early explanations tended to emphasize the brains role in sensory or technical competence (foraging skills, innovations, and way-finding), the balance of evidence now clearly favors the suggestion that it was the computational demands of living in large, complex societies that selected for large brains. However, recent analyses suggest that it may have been the particular demands of the more intense forms of pairbonding that was the critical factor that triggered this evolutionary development. This may explain why primate sociality seems to be so different from that found in most other birds and mammals: Primate sociality is based on bonded relationships of a kind that are found only in pairbonds in other taxa.

Social network size in humans

This paper examines social network size in contemporary Western society based on the exchange of Christmas cards. Maximum network size averaged 153.5 individuals, with a mean network size of 124.9 for those individuals explicitly contacted; these values are remarkably close to the group size of 150 predicted for humans on the basis of the size of their neocortex. Age, household type, and the relationship to the individual influence network structure, although the proportion of kin remained relatively constant at around 21%. Frequency of contact between network members was primarily determined by two classes of variable: passive factors (distance, work colleague, overseas) and active factors (emotional closeness, genetic relatedness). Controlling for the influence of passive factors on contact rates allowed the hierarchical structure of human social groups to be delimited. These findings suggest that there may be cognitive constraints on network size.

Primate social systems

1. Primates and their Societies.- Asking the Right Questions.- The Primate Heritage.- Primate Social Systems.- 2. Theory of Reproductive Strategies.- An Evolutionary Perspective.- Optimal Strategy Sets.- A Question of Ontogeny.- Structure and Function in Primate Society.- 3. Survival Strategies.- Nutritional Requirements.- Dietary Strategies.- Optimal Foraging.- Exploiting the Habitat.- Economics of Territoriality.- The Problem of Predation.- 4. Demographic Processes: (1) Lifehistory Variables.- Lifehistory Variables.- Variance in Birth Rates.- Mortality Rates.- 5. Demographic Processes: (2) Population Parameters.- Population Dynamics.- Migration and Fission.- Sex Ratio.- 6. Time Budgets and Other Constraints.- Time Budgets.- Demographic Constraints on Behaviour.- 7. Evolution of Grouping Patterns.- Why Form Groups?.- Costs of Group-living.- Evolution of Groups.- Evolution of Social Structure.- 8. Mating Strategies.- Reproductive Behaviour in Primates.- Gaining Access to Mates.- Male Lifetime Reproductive Success.- Alternative Strategies of Mate Acquisition.- 9. Rearing Strategies.- Primate Rearing Patterns.- Ecology of Motherhood.- Social Aspects of Rearing.- Male Parental Investment.- 10. Conflicts and Coalitions.- The Constraint-free Strategy.- Economics of Coalition Formation.- Demographic Considerations.- Acquisition of Rank.- 11. Mechanics of Exploitation.- Processes of Negotiation.- Dynamics of Social Relationships.- Role of Communication.- Exploiting Others.- Infanticide as a Reproductive Strategy.- 12. Socio-ecological Systems.- Models as Descriptive Tools.- Models as Analytical Tools.- The Problem of Monogamy.- Comment:The Function of Territoriality.- A Note on the Use of Modelling.- 13. Evolution of Social Systems.- Social Evolution in Baboons.- Social Evolution in the Great Apes.- Conclusion.- Scientific and Common Names.- References.

Fission-Fusion Dynamics: New Research Frameworks

Renewed interest in fission‐fusion dynamics is due to the recognition that such dynamics may create unique challenges for social interaction and distinctive selective pressures acting on underlying communicative and cognitive abilities. New frameworks for integrating current knowledge on fission‐fusion dynamics emerge from a fundamental rethinking of the term “fission‐fusion” away from its current general use as a label for a particular modal type of social system (i.e., “fission‐fusion societies”). Specifically, because the degree of spatial and temporal cohesion of group members varies both within and across taxa, any social system can be described in terms of the extent to which it expresses fission‐fusion dynamics. This perspective has implications for socioecology, communication, cognitive demands, and human social evolution.

Intergroup aggression in chimpanzees and humans.

The occurrence of fatal attacks during intergroup encounters among chimpanzees suggests that certain aspects of chimpanzee and human intergroup aggression may be explicable in similar ways. This paper addresses three questions: What conditions favor the evolution of lethal raiding in intergroup aggression? Why is intergroup aggression in these two species predominantly the domain of males? Under what circumstances do groups compete over access to females as opposed to material resources? Examination of comparative data on nonhuman primates and crosscultural study of foraging societies suggests that attacks are lethal because where there is sufficient imbalance of power their cost is trivial, that these attacks are a male and not a female activity because males are the philopatric sex, and that it is resources of reproductive interest to males that determine the causes of intergroup aggression.

Functional Significance of Social Grooming in Primates

Frequencies of social grooming recorded from 44 species of free-living primates correlate with group size but not body size. This is interpreted as evidence for the social function of grooming and against the purely hygienic function. However, there is some evidence to suggest that body size is a more important determinant of grooming time among platyrrhine primates. This might imply that there has been a shift in the functional system governing grooming during primate evolution.

Time: a hidden constraint on the behavioural ecology of baboons

SummaryData from wild populations of baboons are used to derive functional equations relating time budget components, day journey length and group size to environmental variables. This set of equations predicts both time budgets in an independent sample of populations and the geographical distribution of baboon populations extremely well. I then use these equations to examine the maximum ecologically tolerable group size for baboons occupying different habitats. Groups which exceed this value exhibit signs of ecological stress: they spend less time resting and in social activity than would be expected for their size and environment, they are more likely to fragment during foraging and they travel faster. Populations living in poor quality (low rainfall) habitats are more likely to live in groups that are stressed in this way.