Proceedings of the British Academy, Volume 88

Evolution of Social Behaviour Patterns in Primates and Man

edited by W G Runciman, John Maynard Smith & R I M Dunbar

Details of the volume

Social Evolution in Primates: The Role of Ecological Factors and Male Behaviour

Keywords: socio-ecology; social strategies; competition, scramble, contest; predation; infanticide.

In order to explain the variation in primate social systems, socio-ecology has focussed on the role of ecological factors to explain female associations and relationships and on the spatio-temporal distribution of mating opportunities to explain male associations and relationships. While this approach has been quite successful, it ignores male–female associations and relationships and ignores the possibility that male behaviour modifies other aspects of the social system. In this paper, the ecological approach is complemented by consideration of a social factor found to limit fitness, namely infanticide by males. Infanticide risk is proposed to have selected for male–female associations and relationships, and to have modified female–female relationships in some cases. It is also hypothesized to have selected for the unusual male bonding by species such as chimpanzees. Finally, its possible impact on between-group relations is examined. The findings suggest that infanticide is of equal importance to ecological factors, with which it may interact in sometimes complex ways, in shaping primate social systems.

Determinants of Group Size in Primates: A General Model

Keywords: group size; systems model; time budgets; baboons; chimpanzees.

Significant constraints are placed on group size by local habitat conditions as a consequence of both the selection pressures that act on the animals and the design of their physiological systems. I use a linear programming approach to develop a model of habitat-specific minimum and maximum group sizes for baboons. Three main variables define the state space of realizable group sizes. These are the maximum group size within which the animals can still balance their time budgets (the maximum ecologically tolerable group size), the minimum group size that reduces predation risk to some (undefined) acceptable level (the minimum permissible group size) and the maximum group size that animals' neocortex size will allow them to maintain as a coherent stable social entity (the cognitive group size). Similar models have also been developed for gelada and chimpanzees. Once group size can be determined for a particular habitat, a number of other behavioural patterns can be determined as a consequence of well-understood general principles. I illustrate this with the example of male mating strategies.

Function and Intention in the Calls of Non-Human Primates

Keywords: non-human primate; communication; mental state attribution; semantic signalling; reconciliation; contact calls.

Many of the vocalizations produced by non-human primates are functionally semantic, in the sense that they denote objects and events in the external world. Moreover, at least some monkey species appear to assess and compare calls on the basis of their meanings. In their social interactions, non-human primates also use their calls in ways that are functionally analogous to the ways that humans use language. The grunts given by free-ranging baboons, for example, serve to facilitate social interactions and to reconcile opponents following fights. The mental mechanisms underlying the vocalizations of non-human primates, however, appear to be fundamentally different from those that underlie human speech, because monkeys do not apparently call to one another with the intent of modifying or influencing each other's mental states. The alarm and contact calls of monkeys provide information about the signaller's current physical and mental states, but they are not deliberately given to inform or instruct others. Instead, listeners appear to extract relevant information about a call's function based on behavioural contingencies and their own experiences.

Why Culture is Common, but Cultural Evolution is Rare

Keywords: cultural evolution; social learning; dual inheritance; imitation.

If culture is defined as variation acquired and maintained by social learning, then culture is common in nature. However, cumulative cultural evolution resulting in behaviours that no individual could invent on their own is limited to humans, song birds, and perhaps chimpanzees. Circumstantial evidence suggests that cumulative cultural evolution requires the capacity for observational learning. Here, we analyse two models the evolution of psychological capacities that allow cumulative cultural evolution. Both models suggest that the conditions which allow the evolution of such capacities when rare are much more stringent than the conditions which allow the maintenance of the capacities when common. This result follows from the fact that the assumed benefit of the capacities, cumulative cultural adaptation, cannot occur when the capacities are rare. These results suggest why such capacities may be rare in nature.

An Evolutionary and Chronological Framework for Human Social Behaviour

Keywords: human evolution; social evolution; environment of evolutionary adaptedness; human behavioural ecology.

Human social behaviour is the product of millions of years of evolution. The details of the chronological and phylogenetic context in which human behaviour evolved can provide information about both the historical depth of specific behaviours and the reasons underlying their evolution. The chronological framework is described, and the ecological basis for human social evolution discussed. Eight key 'events' and time periods are identified: 35 million years ago (35 Myr), 25 Myr, 15 Myr, 5 Myr, 2 Myr, 300,000 years ago (300 Kyr), 100 Kyr and 30 Kyr. Critical developments occur in these periods when such attributes as compulsive sociality, male kin-bonding and changes in life history strategy and parenting behaviour occur. It is argued that a key factor in hominid social evolution is the conjunction of male kin-bonding and selection for energetically expensive offspring; that the shift to modern human behaviour occurs over a prolonged period in excess of 200 Kyr; and that the human evolutionary heritage (the EEA) is not unitary.

Friendship and the Banker’s Paradox: Other Pathways to the Evolution of Adaptations for Altruism

Keywords: reciprocity; altruism; co-operation; social exchange; reciprocal altruism; evolutionary psychology.

The classical definition of altruism in evolutionary biology requires that an organism incur a fitness cost in the course of providing others with a fitness benefit. New insights are gained, however, by exploring the implications of an adaptationist version of the 'problem of altruism', as the existence of machinery designed to deliver benefits to others. Alternative pathways for the evolution of altruism are discussed, which avoid barriers thought to limit the emergence of reciprocation across species. We define the Banker's Paradox, and show how its solution can select for cognitive machinery designed to deliver benefits to others, even in the absence of traditional reciprocation. These models allow one to understand aspects of the design and social dynamics of human friendship that are otherwise mysterious.

The Early Prehistory of Human Social Behaviour: Issues of Archaeological Inference and Cognitive Evolution

Keywords: prehistoric archaeology; social behaviour; evolution of the mind.

Unlike the social behaviour of non-human primates, that of human foragers pervades all domains of behaviour. The natural world, material culture and spatial positioning all play an active role in the social interactions of humans. This pervasiveness of social behaviour is readily apparent in the ethnographic record and can be traced in the archaeological records of the Upper Palaeolithic and Mesolithic periods, for which archaeologists can reconstruct complex patterns of social behaviour. For the Early Palaeolithic, however, the archaeological evidence for social behaviour implies that groups were uniformly small and lacking in social structure. This conflicts with evidence from the fossil and palaeoenvironmental records which suggest social complexity and variability. A resolution of these contradictory lines of evidence is offered in terms of a high degree of domain specific mentality for the Early Humans of the Lower and Middle Palaeolithic. The Early Human mind appears to be one in which social behaviour was relatively isolated from interaction with the natural world and material culture.This is in marked contrast to the pervasiveness of social behaviour among behaviourally modern humans arising from a dramatic increase in cognitive fluidity that becomes apparent in the archaeological record at c. 50,000 years ago.

The Emergence of Biologically Modern Populations in Europe: A Social and Cognitive ‘Revolution’?

Keywords: archaeology; Palaeolithic; human evolution; demography; modern humans; Neanderthal.

The appearance of anatomically modern populations in Europe around 40–45,000 years ago appears to reflect a major population dispersal, which replaced the preceding Neanderthal populations. Closely associated with this population dispersal there is archaeological evidence for a range of dramatic cultural innovations, including the appearance of more complex forms of stone and bone technology, personal ornaments, larger and more highly structured living sites, and remarkably sophisticated representational art and other forms of visual symbolism. There is also evidence for a major increase in human population densities, marked by an increase in the numbers of occupied sites in many regions. It is argued here that several other social transformations, including the appearance of larger residential group sizes, increased separation and specialization of personal roles within these groups, more sharply bounded territorial and demographic groupings, and more complex forms of descent and kinship structures, may be attributable at least in part to this increase in human population densities. A further critical factor in these social and cultural transformations was almost certainly the appearance of more complex and highly structured language patterns, associated with the dispersal of the anatomically modern populations. While the origins of these changes must be sought outside Europe, it was probably this range of behavioural innovations which allowed the biologically modern populations to compete with, and eventually replace, the pre-existing Neanderthal populations of Europe.

Responses to Environmental Novelty: Changes in Men’s Marriage Strategies in a Rural Kenyan Community

Keywords: social change; Kenya; marriage payments; adaptation; behavioural ecology; Kipsigis.

This chapter examines the use of behavioural ecological models as applied to humans in conditions of environmental novelty. On the assumption that individuals pursue behavioural strategies that maximize their fitness, predictions can be made concerning how social and ecological conditions generate a variety of optimal responses. With environmental novelty, the question arises: For how much of human history (or prehistory) can we assume that the environment remained sufficiently unchanged for appropriate behaviour to be elicited and genuine functional outcomes to be observed? Data from rural Kenya show how Kipsigis men vary their allocations to mating effort, as measured through bridewealth payments, consistent with predictions from an optimality model. The pattern of men paying large bridewealth payments for women of high reproductive value and high labour value disappeared in the 1980s. This shift may reflect the changing reproductive and economic roles of women, contingent on incipient demographic transition in Kenya and an increasing involvement of men in food production and the cash economy. Despite some problems with the interpretation of data such as these, a generally positive appraisal is made of the appropriateness of using behavioural ecological theory in the study of contemporary human populations, both because it provides an empirical measure of the extent to which adaptive responses are still generated, and because it focuses attention on variability. These results challenge the view held by some evolutionary social scientists that there is no a priori reason to suppose that any specific modern cultural or behavioural practice is adaptive. At the same time these findings point to a potentially important area of congruence between behavioural ecology and evolutionary psychology, by highlighting the need to investigate decision-making rules that, on account of their sensitivity to new socioecological conditions, might contibute to the generation of cultural change.

Genetic Language Impairment: Unruly Grammars

Keywords: specific language impairment; language instinct; compensatory mechanism; automatic procedural rule; cross-linguistic investigation; linguistics.

Though most children acquire language quickly and easily, some children have great difficulty in acquiring their native language. Over the past several years family studies and epidemiological studies have shown that this disorder aggregates in families. It has also been shown that monozygotic twins are significantly more concordant with respect to this disorder than dizygotic twins. This evidence suggests that at least some cases of this disorder may be heritable.

This paper will report on the linguistic properties of this disorder in English, Japanese and Greek. The cumulative data from several years of testing across several languages show that these subjects do not construct productive rules for such linguistic features as tense or case. They are able to use compensatory mechanisms such as conceptual selection, memorization and explicitly learned rules in place of automatic, procedural rules that normally govern language. These data suggest that this genetic disorder affects normal language learning mechanisms.

The Emergence of Cultures among Wild Chimpanzees

Keywords: chimpanzees; culture; imitation; social learning; social norms.

Culture has been granted by primatologists to the chimpanzee, on the base of the many population-specific behaviour patterns they possess. Psychologists tend to disagree arguing that individual learning constrained by ecological factors could produce the same results. After setting up some rigorous criteria to differentiate between these two opposite positions, I show that social canalization, including imitation, is important in explaining the acquisition of nut-cracking behaviour in wild chimpanzees. Then, I argue that a culture requires not only a social learning process to produce a faithful transmission of information, but also a mechanism that guarantees the permanence of the information between transmission events. Leaf-clipping, leaf-grooming, knuckle-knocking and a symbolic drumming communication system are proposed to be examples of behaviour patterns fixed within chimpanzee populations by social norms. The stringent criteria that have to be fulfilled to grant a behaviour cultural properties in an animal species strongly limit the possible candidates. Despite these restrictions, the repertoire of the wild chimpanzee includes many cultural behaviour patterns.

Terrestriality, Bipedalism and the Origin of Language

Keywords: evolution of language; cognition; human evolution; evolution of the brain.

Language is unique to humans, but in the context of the long time span of human evolution it is a fairly recent innovation. All evidence suggests that human brain size and inferred cognitive and linguistic abilities reached their modern norms only within the last quarter of a million years. Foundations for human linguistic and cognitive evolution, however, lie much further back in evolutionary history. Arguments are presented suggesting that these unique human abilities are the legacy of our ancestors' terrestrial and bipedal adaptations. Both terrestriality and bipedalism are directly associated with the unique human propensity for vocal communication, including the range and quality of sound that all humans are capable of producing. Furthermore, terrestriality and bipedalism can also be directly associated with an increase in brain size and cognition. Increases in group size accompanying a committed terrestrial adaptation would have put a premium on social, or Machiavellian intelligence while bipedalism would have been associated with the increased neural circuity involved in enhanced speed and co-ordination of hand and arm movements. The constricted bipedal pelvis would have also necessitated the birth of less mature offspring, exposing them to a rich environment while the brain was still rapidly growing and developing. A larger brain is not without its costs, however. Energetic arguments are also presented which suggest that a large brain can only evolve in concert with a change to a high quality diet, resulting directly in lifestyle changes for our early ancestors.

All of these features were in place by the appearance of early Homo erectus about 1.8 million years ago and underpinned an apparently stable hominine adaptation that lasted for well over 1.5 million years. Modern human cognitive and linguistic talents are rooted in this earlier Homo erectus adaptation and may have begun to develop in response to further need for increased group size. Both the costs and the benefits of this later increase in brain size are considered.