Humans and alcohol share a deep evolutionary history: our capacity to convert alcohol into useable sugars is a trait we share with the African great apes (gorillas and chimpanzees) and is unique to this taxonomic family among the primates. Although the archaeological record only allows us to date the production of alcohol back about 9,000 years (by which time it is already on an industrial scale), a cottage industry of alcohol production must date back a great deal further. With the exception of where its consumption has been prohibited on religious grounds, alcohol use occurs in every culture and society. Notwithstanding its hedonic properties, its real functional benefit is primarily social, playing an important role in rituals and group bonding. I review studies that demonstrate its functional consequences in terms of social bonding, mediated by alcohol’s ability to trigger the brain’s endorphin system. The endorphin system is the central basis for social bonding in primates. The health and other benefits that arise from social bonding are considerable.

The mammalian strategy of internal gestation followed by lactation imposes severe constraints on what males can do in terms of parental investment. This asymmetry in the processes of reproduction results in male mammals placing disproportionate emphasis on mating strategies and less on parental care than is typical of most other taxa. Although male mammals can contribute indirectly to parental care (e.g. by defending a feeding territory for the females(s) or acting as an anti-predator defence), this only remains an option where females can gain a net benefit from associating with males relative to the foraging costs that additional animals must inevitably impose on them.

This asymmetry in reproductive biology has one important implication for mammalian social systems. In birds, it is not uncommon for males to arrive first on the breeding ground.Having partitioned the available space among themselves, they settle on their territories to await the arrival of the females who then choose among them. While a similar pattern can be observed in those mammals that adopt a lek mating system (notably some deer, antelope and pinnipeds), the reverse is probably more typical of many mammals (and most primates). In a classic experiment, Charles- Dominique (1977) released a number of dwarf galago (Galago demidorvii) into a forest in Gabon. He found that the population developed its own natural structure in a surprisingly ordered manner. First, the females established their ranges; once they had decided how to distribute themselves, the males then mapped themselves onto the female distribution.

Introduction

In many species, the process of mate finding begins with advertising. As such, advertising is an opening bid that is often made in the absence of any information about the quality (and sometimes even the identity) of potential mates. These bids will, however, often reflect a trade-off between the individual's assessment of its own preferences and its experience of what the marketplace in general has to offer. This is as true of human beings as it is of other species (Grammer 1989). In humans, bids of this kind are often contingent on the advertiser's perception of his/her own bargaining hand; in consequence, the level of demands that an advertiser makes may vary according to what he/she has to offer (Waynforth & Dunbar 1995).

Advertisements in personal columns provide one outlet where human mate choice criteria are explicitly elaborated. Analyses of the content of such advertisements have consistently shown that the words used in these advertisements reflect a small number of key dimensions that have strong evolutionary valency (Kenrick & Keefe 1992; Wiederman 1993; Greenlees & McGrew 1994; Waynforth & Dunbar 1995; Bereczkei et al. 1997). All these studies concur that the single most important criteria by which men assess the mate quality of women is age (or, as a surrogate of this, physical attractiveness), while women tend to emphasise a wider range of criteria, including wealth/status and commitment (Borgerhof Mulder 1988a; Buss 1989; Voland & Engel 1990; Kenrick & Keefe 1992; Waynforth & Dunbar 1995; for a review of the more general literature, see Grammer 1989). Age in women is significant because it is a direct correlate of both fertility and future reproductive potential (Fisher's reproductive value).

Humans seem to lie on a different cognitive plane to the other primates. My aim in this chapter is to try to provide an explanation as to why and when this might have come about. First, however, I want to identify what seem to me to be crucial cognitive differences between humans and other primates: these I characterise in terms of theory-of-mind ability, though I do not want to suggest that this is all there is to the human-ape difference. I shall then go on to offer an explanation as to why humans should need these additional cognitive abilities and conclude by offering a suggestion as to when they might have evolved.

What makes us human?

While we undoubtedly share a great deal in cognitive terms with our primate cousins, there must be some features that set us apart. At least, this is the implication of our intuition that we are in some sense ‘smarter’ and socially more complex than our nearest relatives, the great apes. Although we do not have a particularly clear handle on this problem as yet, there are nonetheless some features that we can point to as promising candidates. One of these is Theory of Mind (ToM: the ability to understand another's mental state, sometimes also known as ‘mind-reading’) (Perner and Wimmer, 1985). Although I shall focus exclusively on ToM here, I want to emphasise the fact that there may be other features of our cognitive machinery that may also be important.

Introduction

Individual animals seek to maximise their fitness by pursuing strategies that optimise their current opportunities. One of the factors that determines the opportunities available to an individual is the size of group it lives in, because group size constrains the choice of social partners available (Dunbar, 1996). Group size in turn is in part determined by environmental conditions, three sets of which are likely to be especially important. First, resource availability and patchiness will set an upper limit on group size and density. These will be determined by broad climatic and geophysical variables, but indirect competition from both conspecifics and other ecological competitors will also be important. Second, resource quality and thermoregulatory considerations will add an additional constraint on group size through their impact on time budgets. Both resource quality and thermoregulation will be directly influenced by climate. Finally, predation risk (and perhaps direct competition from conspecifics) will set lower limits on group size below which animals will not be able to resist predation or competitive exclusion. Over evolutionary time, these constraints will give rise to cognitive mechanisms designed to facilitate the cohesion of groups of the size that is typical for the species as a whole, given the ecological niche it occupies. These cognitive mechanism will impose upper limits on group size that are species specific.

This chapter presents a general approach to the determinants of group size in primates that helps us to understand why group sizes vary across habitats both within and between species. The general approach is illustrated in Figure 12.1 which shows a linear programming model for baboon (Papio spp.) group sizes in relation to one environmental variable (rainfall).

Introduction

The social intelligence hypothesis posits that the large brains and distinctive cognitive abilities of primates (in particular, anthropoid primates) evolved via a spiralling arms race in which social competitors developed increasingly sophisticated ‘Machiavellian’ strategies (Byrne & Whiten, 1988). The idea stemmed originally from observations suggesting that the polyadic interactions and relationships typical of anthropoid societies place exceptional demands on the ability of individuals to process and integrate social information. This hypothesis has been contrasted with foraging niche hypotheses, which propose that it is the need to find food in patchy, unpredictable environments, or even to extract it from awkward substrates, that required large brains and cognitive specialisation (e.g. Gibson, 1986; Milton, 1988). A major task in the study of primate cognition is to devise tests of these hypotheses.

In trying to establish the nature of cognitive adaptation, studies of the behaviour of individual species (whether experimental or based on field observations) are inevitably of limited value. While such studies may suggest what cognitive skills are used in meeting the demands of a particular lifestyle (e.g. Milton, 1988; Cheney & Seyfarth, 1990; Chapter 8), they do not in themselves provide tests of the evolutionary hypotheses. Indeed, they occasionally lead to the circular logic that primate species are good at the things that primatologists have chosen to study (such as complex sociality, or difficult foraging). This problem may also be compounded by ‘primatocentric’ reasoning, in which primate behaviour is automatically assumed to be more complex and clever, and more demanding of evolutionary explanation, than is non-primate behaviour.