Social Brain: Evolution 21

Social Brain: Evolution 21 Social Brain: Evolution R I M Dunbar, University of Oxford, Oxford, UK large neocortices are clearly a primate speciality – so ã 2009 Elsevier Ltd. All rights reserved. much so, in fact, that when one asks why primates have big brains, one is really in effect asking why they have big neocortices. In the following sections, the selective advantages of large brains are first discussed, in order to identify why Explaining Brain Evolution large brains have evolved in various animal lineages. Although neuroscientists do not often consider the Then the role that cognition plays in these processes is evolutionary history of the brain, understanding how considered. Finally, the question of whether these and why brains have evolved may help explain why forms of social cognition relate to specific brain struc- they have come to have the structure they do. Verte- tures is briefly explored. brate species, for example, vary enormously in both absolute brain size and brain size relative to body size, Theories for the Evolution of Large Brains but some species (notably primates) have dispropor- tionately large brains. Why should that be? Why do all We might expect large-bodied species to have large mammals not have primate-like brains? A second issue brains simply because they have larger muscle masses is that brain researchers have conventionally assumed to coordinate if they are to survive effectively. How- that the brain’s main function is to acquire and process ever, it has long been recognized that some groups of information about the environment, usually informa- species (e.g., primates) have unusually large brains for tion directly relevant to the business of survival (i.e., their body size compared to all other groups of verte- finding food and avoiding predators). While obviously brates. Why this should remain so is unclear, although broadly correct, this view contains an implicit assump- it has been generally assumed that it probably has tion that may be less defendable: the assumption that something to do with the ecology of these species. ‘environment’ here means the physical environment. Initial Explanations Of course, a great deal of brain activity is about the physical environment, and, in the final analysis, it is Initial attention was drawn to the fact that, within the physical properties of the environment that consti- many mammalian taxa, species with larger brains tute all perceptual inputs. have larger territories, or have diets that are more However, for more advanced vertebrates (birds and difficult to locate or process (e.g., diets of ephemeral mammals), and in particular the more intensely social fruits as opposed to more widely available leaves, or of these species, the environmental behavior that of invertebrates that have to be extracted from hiding needs to be processed by the brain is that of the places). Others have pointed to correlations with ‘social’ world. In the limiting case, the social world altriciality (giving birth to babies that are not mature is a virtual world of behavior, the nature of which is enough at birth to move and forage for themselves) understood and manipulated only in the animal’s and the scaling of maternal energy demands, and have mind. Because the dynamic complexities of this virtu- argued that it was spare maternal energy capacity al social world vastly exceed the complexities of the that made large brains possible. By the late 1980s, physical world, it is inevitably much harder to under- however, this assumption began to be questioned: it stand. Hence, intensely social species (such as pri- eventually became clear that the kinds of ecological mates) have brains that are specialized for dealing tasks faced by primates do not differ much from those with this aspect of their world. faced by other species. This realization begged the The emphasis on primates in the early literature on question as to why primates should need a substan- comparative brain evolution led to a natural focus on tially larger brain (in effect, more computing power) neocortex volume. The scaling relationships among to solve the same foraging problems as, say, cows. mammalian brain units are such that neocortex vol- The social brain hypothesis was proposed as an ume increases much faster than that of all other parts explanation. of the brain, and especially among the primates. The initial proposal, known as the ‘Machiavellian Although the neocortex accounts for as little as 10% intelligence hypothesis,’ was intended to explain why of total brain volume in some of the more primitive primates had larger brains for body size than all insectivores and at most 40% in nonprimate mam- other species. It was suggested that this might have mals, it accounts for a minimum of 50% in primates something to do with the fact that primates live in and rises to as high as 80% in humans. Thus, while the unusually complex social systems. Primate sociality is neocortex may be viewed as a mammalian invention, characterized by kinds of behaviors (for example, 22 Social Brain: Evolution tactical deception, the formation of coalitions and left with two main contenders that can broadly be alliances long before they are actually needed) that defined as ecological theories and social theories. are rare or entirely absent in nonprimate species. This Ecological hypotheses argue for a focus on foraging proposal was later renamed the ‘social brain hypo- decisions (how to extract nutrients from the environ- thesis’ (or ‘social intelligence hypothesis’) in order to ment, how to find one’s way about, etc.). In contrast, avoid unnecessary and unintended implications of the social hypotheses argue that the computational sociopolitical scheming and to focus attention more load is created by the demands of tracking social closely on the positive socially integrating aspects that relationships and calculating the pros and cons of really underpin primate sociality. alternative social actions. In both cases, of course, the fundamental issue is Asking the Right Questions the brain’s problem-solving capabilities, but the focus of the problem and the mode of solution are different An important consideration at the outset is to appreci- in each case. However, it is worth emphasizing that, ate that there is an important distinction between either way, the root issue is essentially ecological: all functional hypotheses (those that provide a reason animals have to overcome problems of day-to-day why large brains might be selected for) and explana- survival if they are to contribute offspring to the tions that are essentially about constraints. Most next generation (the root of all evolutionary process- developmental hypotheses fall into the latter category: es). The difference lies in how this is done, and which they identify trajectories along which brain evolution particular computational demand imposes the high- is forced to run, or system limitations that have to be est cognitive load. Ecological hypotheses assume that overcome. Examples of the first include the fact that the basis of an animal’s decision-making in this the way brain tissue is laid down necessarily results in respect is explicitly trial-and-error learning on an unavoidable scaling of all brain parts: if such scaling is individual basis; the social hypotheses assume that very tight, then it may result in all parts of the brain the ecological problems are solved cooperatively enlarging proportionately when evolution imposes (and that the limiting cognitive demand is created by selection on just one component. This might mean the need to coordinate one’s behavior with that of that in order to evolve more capacity in some key others in some way). brain component, it is necessary to evolve everything One way to test between these competing hypo- on a larger scale. Examples of the second kind of theses is to correlate some measure of relative brain limitation would be energetic constraints that limit size with one or more behavioral indices that repre- brain growth: some diets may not provide sufficient sent the hypothesis of interest. Relevant indices that surplus energy that can be channeled into brain have been used include the amount of fruit in the diet growth. Consequently, species that need to evolve a (representing the claim that the cognitive constraint larger brain may only be able to do so if they first lies in finding and monitoring high-quality food change diet in order to bypass the energetic constraint. sources such as fruiting trees), the style of extractive While these kinds of developmental hypotheses are foraging (whether or not food items are extracted perfectly legitimate and tell us something about how from some physical matrix, such as a pod, or dug large brains are evolved, they do not specify why they out of the ground, i.e., an index of complex food- should evolve. Given that brain tissue is among the processing decisions), day journey length and territo- most ‘expensive’ in the body (the so-called expensive ry size (indices of mental mapping capacities), and tissue hypothesis), to merely show that evolutionary social group size (a simple index of social complexity). change in a particular direction is possible is not When these indices have been correlated with indices enough. The costs of evolving (and indeed growing) of relative brain size, only group size is correlated a large brain are such that they provide an extremely with brain size (and specifically with neocortex size) steep gradient, up which selection has to push the (see Figure 1). organism. In the absence of any selective benefit (or More recently, a number of new analyses have been advantage), the costs impose strong negative selection undertaken that use new and more powerful statisti- that will constantly favor a reduction in size (or, at cal methods that allow the multivariate influences of best, a stable state).

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