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Review
, 362 (1480), 649-58

Understanding Primate Brain Evolution

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Review

Understanding Primate Brain Evolution

R I M Dunbar et al. Philos Trans R Soc Lond B Biol Sci.

Abstract

We present a detailed reanalysis of the comparative brain data for primates, and develop a model using path analysis that seeks to present the coevolution of primate brain (neocortex) and sociality within a broader ecological and life-history framework. We show that body size, basal metabolic rate and life history act as constraints on brain evolution and through this influence the coevolution of neocortex size and group size. However, they do not determine either of these variables, which appear to be locked in a tight coevolutionary system. We show that, within primates, this relationship is specific to the neocortex. Nonetheless, there are important constraints on brain evolution; we use path analysis to show that, in order to evolve a large neocortex, a species must first evolve a large brain to support that neocortex and this in turn requires adjustments in diet (to provide the energy needed) and life history (to allow sufficient time both for brain growth and for 'software' programming). We review a wider literature demonstrating a tight coevolutionary relationship between brain size and sociality in a range of mammalian taxa, but emphasize that the social brain hypothesis is not about the relationship between brain/neocortex size and group size per se; rather, it is about social complexity and we adduce evidence to support this. Finally, we consider the wider issue of how mammalian (and primate) brains evolve in order to localize the social effects.

Figures

Figure 1
Figure 1
Basal metabolic rate (BMR), controlling for body size (residuals from a linear regression of basal metabolic rate versus log-transformed body size), plotted against relative brain size. Species with higher metabolic rates than expected for their body size also have larger than expected brain size.
Figure 2
Figure 2
Path diagram of predicted relationship between life history, ecology, brain size and group size in primates. Group size is pushed upwards by external factors such as predation, but is limited by ecological and cognitive constraints. Brain size is, in turn, limited by energetic and life-history constraints.
Figure 3
Figure 3
Relative neocortex size (as measured by a linear regression of log-transformed neocortex volume over log-transformed volume of the rest of the brain) in species that do and do not form coalitions with other group members (solitary and monogamous species excluded from analysis). We use Plavcan et al.'s (1995) categorization of coalition-forming species.
Figure 4
Figure 4
Volumes of primary visual cortex (V1) and rest of neocortex (non-V1) plotted against total brain volume for primates. Source: data from Stephan et al. (1981).

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