Male and female brain evolution is subject to contrasting selection pressures in primates

The claim that differences in brain size across primate species has mainly been driven by the demands of sociality (the "social brain" hypothesis) is now widely accepted. Some of the evidence to support this comes from the fact that species that live in large social groups have larger brains, and in particular larger neocortices. Lindenfors and colleagues (BMC Biology 5:20) add significantly to our appreciation of this process by showing that there are striking differences between the two sexes in the social mechanisms and brain units involved. Female sociality (which is more affiliative) is related most closely to neocortex volume, but male sociality (which is more competitive and combative) is more closely related to subcortical units (notably those associated with emotional responses). Thus different brain units have responded to different selection pressures

Hominin cognitive evolution: identifying patterns and processes in the fossil and archaeological record

As only limited insight into behaviour is available from the archaeological record, much of our understanding of historical changes in human cognition is restricted to identifying changes in brain size and architecture. Using both absolute and residual brain size estimates, we show that hominin brain evolution was likely to be the result of a mix of processes; punctuated changes at approximately 100 kya, 1 Mya and 1.8 Mya are supplemented by gradual within-lineage changes in Homo erectus and Homo sapiens sensu lato . While brain size increase in Homo in Africa is a gradual process, migration of hominins into Eurasia is associated with step changes at approximately 400 kya and approximately 100 kya. We then demonstrate that periods of rapid change in hominin brain size are not temporally associated with changes in environmental unpredictability or with long-term palaeoclimate trends. Thus, we argue that commonly used global sea level or Indian Ocean dust palaeoclimate records provide little evidence for either the variability selection or aridity hypotheses explaining changes in hominin brain size. Brain size change at approximately 100 kya is coincident with demographic change and the appearance of fully modern language. However, gaps remain in our understanding of the external pressures driving encephalization, which will only be filled by novel applications of the fossil, palaeoclimatic and archaeological records. </jats:p

Communicative roots of complex sociality and cognition: preface to the theme issue

Primates live in stable social groups in which they form differentiated relationships with group members and use a range of communication including facial expressions, vocalizations and gestures. However, how these different types of communication are used to regulate social interactions, and what cognitive skills underpin this communication, is still unclear. The aim of this special issue is to examine the types of cognitive skills underpinning the flexible and complex communication that is used to maintain the bonded social relationships found in primates and humans. This article is part of the theme issue 'Cognition, communication and social bonds in primates'

Sociability in a non-captive macaque population is associated with beneficial gut bacteria

The relationship between social behaviour and the microbiome is known to be reciprocal. Research in wild animal populations, particularly in primate social groups, has revealed the role that social interactions play in microbial transmission, whilst studies in laboratory animals have demonstrated that the gut microbiome can affect multiple aspects of behaviour, including social behaviour. Here we explore behavioural variation in a non-captive animal population with respect to the abundance of specific bacterial genera. Social behaviour based on grooming interactions is assessed in a population of rhesus macaques (Macaca mulatta), and combined with gut microbiome data. We focus our analyses on microbiome genera previously linked to sociability and autistic behaviours in rodents and humans. We show in this macaque population that some of these genera are also related to an individual’s propensity to engage in social interactions. Interestingly, we find that several of the genera positively related to sociability, such as Faecalibacterium, are well known for their beneficial effects on health and their anti-inflammatory properties. In contrast, the genus Streptococcus, which includes pathogenic species, is more abundant in less sociable macaques. Our results indicate that microorganisms whose abundance varies with individual social behaviour also have functional links to host immune status. Overall, these findings highlight the connections between social behaviour, microbiome composition, and health in an animal population

Evidence of assortative mating for theory of mind via facial expressions but not language.

Peer reviewed: TrueFunder: NIHR Cambridge Biomedical Research Centre; FundRef: https://doi.org/10.13039/501100018956Funder: Autism Research TrustFunder: Autistica; FundRef: https://doi.org/10.13039/501100008161Funder: Templeton World Charity Foundation; FundRef: https://doi.org/10.13039/501100011730Funder: NIHR Collaboration for Leadership in Applied Health Research and Care East of England at Cambridgeshire and Peterborough NHS Foundation TrustFunder: Medical Research Council; FundRef: https://doi.org/10.13039/501100000265Assortative mating is a phenomenon in which romantic partners typically resemble each other at a level greater than chance. There is converging evidence that social behaviours are subject to assortative mating, though less is known regarding social cognition. Social functioning requires the ability to identify and understand the mental states of others, i.e., theory of mind. The present study recruited a sample of 102 heterosexual couples via an online survey to test if theory of mind as measured using facial expressions (Reading the Mind in the Eyes Test) or language (Stiller-Dunbar Stories Task) is associated with assortative mating. Results provide evidence of assortative mating for theory of mind via facial expressions, though there was no such effect for theory of mind via language. Assortative mating for theory of mind via facial expressions was not moderated by length of relationship nor by partner similarity in age, educational attainment, or religiosity, all variables relevant to social stratification. This suggests assortative mating for theory of mind via facial expressions is better explained by partners being alike at the start of their relationship (initial assortment) rather than becoming similar through sustained social interaction (convergence), and by people seeking out partners that are like themselves (active assortment) rather than simply pairing with those from similar demographic backgrounds (social homogamy)