Abstract
The question as to why primates have evolved unusually large brains has received much attention, with many alternative proposals all supported by evidence. We review the main hypotheses, the assumptions they make and the evidence for and against them. Taking as our starting point the fact that every hypothesis has sound empirical evidence to support it, we argue that the hypotheses are best interpreted in terms of a framework of evolutionary causes (selection factors), consequences (evolutionary windows of opportunity) and constraints (usually physiological limitations requiring resolution if large brains are to evolve). Explanations for brain evolution in birds and mammals generally, and primates in particular, have to be seen against the backdrop of the challenges involved with the evolution of coordinated, cohesive, bonded social groups that require novel social behaviours for their resolution, together with the specialized cognition and neural substrates that underpin this. A crucial, but frequently overlooked, issue is that fact that the evolution of large brains required energetic, physiological and time budget constraints to be overcome. In some cases, this was reflected in the evolution of ‘smart foraging’ and technical intelligence, but in many cases required the evolution of behavioural competences (such as coalition formation) that required novel cognitive skills. These may all have been supported by a domain-general form of cognition that can be used in many different contexts.This article is part of the themed issue ‘Physiological determinants of social behaviour in animals’.
Highlights
Primate evolution has been dominated, as much as anything, by unusually large brains [1]
Explanations that apply only to special cases may be true, but they cannot be general explanations. To highlight how these issues have obstructed our understanding of primate brain evolution, we evaluate each of the major explanations and specify exactly what assumptions and potential limitations underlie each in turn
We identified primate sociality as a key element in this story: so what is it about primate sociality that is so cognitively demanding as to require a large brain? We address this question in more detail elsewhere, but for present purposes we highlight two key features of primate sociality that we see as being crucially different from most other mammals and birds, and which are likely to be cognitively very demanding
Summary
Primate evolution has been dominated, as much as anything, by unusually large brains [1]. The test we must apply to any prospective explanation is that it can explain six key empirical findings: (i) that primates have larger brains relative to their body size than all other animals [1]; (ii) that some primates have larger brains than other primates [1]; (iii) that there is a remarkably robust quantitative relationship between brain (and especially neocortex) size and group size in primates (but not other mammals or birds) [5,6]; (iv) that primates have a peculiar form of bonded sociality that seems to be very different to that of other mammals [7,8,9,10,11,12], reflected in the fact that primate societies are highly structured in network terms (whereas those of most other mammals and birds are not) [13,14,15,16]; (v) that pairbonded monogamy in birds and mammals is associated with larger than average brain size for their orders [7,8]; and (vi) that some (but not all) species of primates exhibit novel technical competences [17,18]. There are alternative approaches that can be used, and we will argue that that these need to be given more attention
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