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Life history is a robust correlate of relative brain size: large-brained mammals and birds have slower life histories and longer lifespans than smaller-brained species. One influential adaptive hypothesis to account for this finding is the Cognitive Buffer Hypothesis (CBH). The CBH proposes that large brains permit greater behavioural flexibility and thereby buffer the animal from unpredictable environmental challenges, allowing reduced mortality and increased lifespan. In contrast, the Developmental Costs Hypothesis (DCH) suggests that life-history correlates of brain size reflect the extension of maturational processes needed to accommodate the evolution of large brains. The hypotheses are not mutually exclusive but do make different predictions. Here we test novel predictions of the hypotheses in primates: examining how the volume of brain components with different developmental trajectories correlate with relevant phases of maternal investment, juvenile period and post-maturational lifespan. Consistent with the DCH, structures with different allocations of growth to pre-natal versus post-natal development exhibit predictably divergent correlations with the associated periods of maternal investment and pre-maturational lifespan. Contrary to the CBH, adult lifespan is uncorrelated with either whole brain size or the size of individual brain components once duration of maternal investment is accounted for. Our results substantiate and elaborate on the role of maternal investment and offspring development in brain evolution, suggest that brain components can evolve partly independently through modifications of distinct developmental mechanisms, and imply that postnatal maturational processes involving interaction with the environment may be particularly crucial for the development of cerebellar function. They also provide an explanation for why apes have relatively extended maturation times, which relate to the relative expansion of the cerebellum in this clade.

Much attention has focused on the dramatic expansion of the forebrain, particularly the neocortex, as the neural substrate of cognitive evolution. However, though relatively small, the cerebellum contains about four times more neurons than the neocortex. I show that commonly used comparative measures such as neocortex ratio underestimate the contribution of the cerebellum to brain evolution. Once differences in the scaling of connectivity in neocortex and cerebellum are accounted for, a marked and general pattern of correlated evolution of the two structures is apparent. One deviation from this general pattern is a relative expansion of the cerebellum in apes and other extractive foragers. The confluence of these comparative patterns, studies of ape foraging skills and social learning, and recent evidence on the cognitive neuroscience of the cerebellum, suggest an important role for the cerebellum in the evolution of the capacity for planning, execution and understanding of complex behavioural sequences—including tool use and language. There is no clear separation between sensory–motor and cognitive specializations underpinning such skills, undermining the notion of executive control as a distinct process. Instead, I argue that cognitive evolution is most effectively understood as the elaboration of specialized systems for embodied adaptive control.

Several theories have been proposed to explain the evolution of species differences in brain size, but no consensus has emerged. One unresolved question is whether brain size differences are a result of neural specializations or of biological constraints affecting the whole brain. Here I show that, among primates, brain size variation is associated with visual specialization. Primates with large brains for their body size have relatively expanded visual brain areas, including the primary visual cortex and lateral geniculate nucleus. Within the visual system, it is, in particular, one functionally specialized pathway upon which selection has acted: evolutionary changes in the number of neurons in parvocellular, but not magnocellular, layers of the lateral geniculate nucleus are correlated with changes in both brain size and ecological variables (diet and social group size). Given the known functions of the parvocellular pathway, these results suggest that the relatively large brains of frugivorous species are products of selection on the ability to perceive and select fruits using specific visual cues such as colour. The separate correlation between group size and visual brain evolution, on the other hand, may indicate the visual basis of social information processing in the primate brain.

The enlargement of cetacean brain size represents an enigmatic event in mammalian evolution, yet its genetic basis remains poorly explored. One candidate gene associated with brain size evolution is the abnormal spindle-like microcephaly associated (ASPM), as mutations in this gene cause severe reductions in the cortical size of humans. Here, we investigated the ASPM gene in representative cetacean lineages and previously published sequences from other mammals to test whether the expansion of the cetacean brain matched adaptive ASPM evolution patterns. Our analyses yielded significant evidence of positive selection on the ASPM gene during cetacean evolution, especially for the Odontoceti and Delphinoidea lineages. These molecular patterns were associated with two major events of relative brain size enlargement in odontocetes and delphinoids. It is of particular interest to find that positive selection was restricted to cetaceans and primates, two distant lineages both characterized by a massive expansion of brain size. This result is suggestive of convergent molecular evolution, although no site-specific convergence at the amino acid level was found.

The neocortex is widely held to have been the focus of mammalian brain evolution, but what selection pressures explain the observed diversity in its size and structure? Among primates, comparative studies suggest that neocortical evolution is related to the cognitive demands of sociality, and here I confirm that neocortex size and social group size are positively correlated once phylogenetic associations and overall brain size are taken into account. This association holds within haplorhine but not strepsirhine primates. In addition, the neocortex is larger in diurnal than in nocturnal primates, and among diurnal haplorhines its size is positively correlated with the degree of frugivory. These ecological correlates reflect the diverse sensory-cognitive functions of the neocortex.

How brains have evolved in response to particular selection pressures is illuminated by ecological correlates of differences in brain structure among contemporary species. The focus of most comparative studies has been on the overall size of brains relative to body size, hence ignoring the ways in which selection operates on specific neural systems. Here we investigate evolutionary radiations in the size of visual and olfactory brain structures within three orders of mammals: primates, bats and insectivores. The comparative relationships within these three orders show both similarities and differences. After removal of the allometric effect of overall brain size, the sizes of different structures within each sensory modality are positively correlated in all three orders. Correlations between visual and olfactory structures, however, are negative in primates, negative but non-significant in insectivores, and positive in bats. In both primates and insectivores, nocturnal lineages tend to have larger olfactory structures than do diurnal or partly diurnal lineages, and among the primates diurnal lineages have larger striate visual cortexes. Hence the apparent trade-off between vision and olfaction in primates seems to be related to the divergence of nocturnal and diurnal forms. However, negative correlations between visual and olfactory structures were also found when nocturnal strepsirhines and diurnal haplorhines were analysed separately, suggesting that ecological variables in addition to activity timing may be significant. Indeed, there were also associations with diet: frugivory was associated with enlargements of the geniculostriate visual system in diurnal primates, enlargements of olfactory structures in nocturnal primates, and possibly enlargements of both in bats. Further ecological associations were found within insectivores: aquatic lineages had smaller olfactory structures than in their non-aquatic counterparts, and fossorial lineages had smaller optic nerves than in non-fossorial forms. We conclude that activity timing, diet and habitat have each played a role in the evolutionary radiation of mammalian sensory systems, but with varying effects in the different taxa. Some of the associations between ecology and sensory systems suggest alternative explanations for correlates of overall brain size, which have in the past commonly been interpreted in terms of selection on intelligence.

The evolution of cognitive and sensory specializations must involve concomitant modifications of neural substrates. Ecological correlates of species differences in brain structure are intriguing sources of evidence about such evolutionary specialization but, to date, these have been identified only for gross parameters, such as overall brain size and the size of major brain regions. Here we show that a behavioural specialization in mammals, predation, is associated with species differences in the fine structure of a single neural pathway, the tectospinal tract. Both the relative number of neurons in this pathway and the relative size of their cell bodies were greater in more predatory species than in their less predatory counterparts within each of four separate mammalian orders. Expansion of these analyses to consider comparisons between taxa at a variety of taxonomic levels gave further support to the idea of a relation between predatory habits and the evolution of the tectospinal tract. In addition, within the primates, the number of neurons in the tectospinal tract was significantly correlated with the proportion of prey in the diet. These results therefore appear to provide an example of correlated evolution between a specific neural system and behaviour which applies generally within the mammals. They also help to unify findings from physiological and anatomical studies on a wider range of vertebrate taxa, including reptiles and amphibians.

As large-bodied savannah primates, baboons have long been of special interest to students of human evolution: many different populations have been studied and dietary comparisons among them are becoming possible. Baboons’ foraging strategies can be shown to combine high degrees of flexibility and breadth with selectivity. In this paper we develop and test multivariate models of the basis of diet selection for populations of montane and savannah baboons. Food selection is positively related to protein and lipid content and negatively to fibre, phenolics and alkaloids. Seasonal changes in dietary criteria predicted by these rules are tested and confirmed. Although nutritional bottlenecks occur at intervals, a comparison between long-term nutrient intakes in four different populations indicates convergence on lower degrees of variation than exist in superficial foodstuff profiles.

The presence and intensity of red coloration correlate with male dominance and testosterone in a variety of animal species, and even artificial red stimuli can influence dominance interactions. In humans, red stimuli are perceived as more threatening and dominant than other colours, and wearing red increases the probability of winning sporting contests. We investigated whether red clothing biases the perception of aggression and dominance outside of competitive settings, and whether red influences decoding of emotional expressions. Participants rated digitally manipulated images of men for aggression and dominance and categorized the emotional state of these stimuli. Men were rated as more aggressive and more dominant when presented in red than when presented in either blue or grey. The effect on perceived aggression was found for male and female raters, but only male raters were sensitive to red as a signal of dominance. In a categorization test, images were significantly more often categorized as ‘angry’ when presented in the red condition, demonstrating that colour stimuli affect perceptions of emotions. This suggests that the colour red may be a cue used to predict propensity for dominance and aggression in human males.