Brain Link to Primacy in Primates

The size of different brain regions relates to our status in the social hierarchies we encounter every day.

The importance of social rank is something we all learn at an early age.

Life on the lowest rung can be stressful, but life at the top also requires careful acts of balancing and coalition forming.

In non-human primates, social dominance influences access to food and mates. In humans, social hierarchies influence our performance everywhere from school to the workplace and have a direct influence on our well-being and mental health. However, we know very little about the relationship between these social ranks and brain function.

New research, conducted at the University of Oxford, has revealed differences between individual primate’s brains which depend on their social status. The more dominant you are, the bigger some brain regions are. If your social position is more subordinate, other brain regions are bigger.

The research, led by Dr MaryAnn Noonan of the Decision and Action Laboratory in the Department of Experimental Psychology, determined the position of 25 macaque monkeys in their social hierarchies. The team then analysed non-invasive scans of the monkey’s brains which had been taken as part of other ongoing University research programs. The findings, published in the open access journal PLOS Biology, show that brain regions in one neural circuit are larger in more dominant animals. Previous research has shown that these regions are involved in learning and in processing social and emotional information.

This image shows an adult Barbary Macaque monkey with two juvenile Macaque monkeys playing.
The macaque groups ranged between one and seven individuals. The research showed that grey matter in regions involved in social cognition correlated with both group size and social status. Previous research has shown that these regions are important for a variety of social behaviours. The image is for illustrative purposes only and shows an adult Barbary Macaque monkey with two juvenile Macaque monkeys playing. Credit PublicDomainPictures.

The MRI scans also revealed that another circuit of brain regions, which collectively can be called the striatum, were found to be larger in more subordinate animals. The striatum is known to play a complex but important role in learning the value of our choices and actions.

The study further reports that the brain’s activity, not just its structure, varies with position in the social hierarchy. The researchers found that the strength with which activity in some of these areas was coupled together also related to social status. Collectively, these results mean that social status is not only reflected in the brain’s hardware, it is also connected to differences in the brain’s software, or communication patterns.

The team also found that the size of another set of brain regions correlated not only with social status but also with the size of the animal’s social group. The macaque groups ranged between one and seven individuals. The research showed that grey matter in regions involved in social cognition correlated with both group size and social status. Previous research has shown that these regions are important for a variety of social behaviours. These include interpreting facial expressions or physical gestures, understanding the intentions of others and predicting their behaviour.

“This finding may reflect the fact that social status in macaques depends not only on the outcome of competitive social interactions but on social bonds formed that promote coalitions,” said Matthew Rushworth, the head of the Decision and Action Laboratory in Oxford. “The correlation with social group size and social status suggests this set of brain regions may coordinate behaviour that bridges these two social variables”.

The reasons for the identified brain differences remain unclear, particularly whether they are present at birth or result from social differences. Dr Noonan said: “One possibility is that the demands of a life in a particular social position use certain brain regions more frequently and as a result those areas expand to step up to the task. Alternatively, it is possible that people born with brains organised in a particular way tend towards certain social positions. In all likelihood, both of these mechanisms will work together to produce behaviour appropriate for the social context”.

Social status also changes over time and in different contexts. Dr Noonan explained: “While we might be top-dog in one circle of friends, at work we might be more of a social climber. The fluidity of our social position and how our brains adapt our behavior to succeed in each context is the next exciting direction for this area of research.”

Notes about this neuroscience research

The study was supported by funding from the Medical Research Council, the Wellcome Trust and the National Institute of Health Research.

Contact: Press Office – Oxford University
Source: Oxford University press release
Image Source: The image is credited to PublicDomainPictures and is in the public domain
Original Research: Full open access research for “A Neural Circuit Covarying with Social Hierarchy in Macaques” by MaryAnn P. Noonan, Jerome Sallet, Rogier B. Mars, Franz X. Neubert, Jill X. O’Reilly, Jesper L. Andersson, Anna S. Mitchell,Andrew H. Bell, Karla L. Miller, and Matthew F. S. Rushworth in PLOS Biology. Published online September 2 2014 doi:10.1371/journal.pbio.1001940

Open Access Neuroscience Abstract

A Neural Circuit Covarying with Social Hierarchy in Macaques

Despite widespread interest in social dominance, little is known of its neural correlates in primates. We hypothesized that social status in primates might be related to individual variation in subcortical brain regions implicated in other aspects of social and emotional behavior in other mammals. To examine this possibility we used magnetic resonance imaging (MRI), which affords the taking of quantitative measurements noninvasively, both of brain structure and of brain function, across many regions simultaneously. We carried out a series of tests of structural and functional MRI (fMRI) data in 25 group-living macaques. First, a deformation-based morphometric (DBM) approach was used to show that gray matter in the amygdala, brainstem in the vicinity of the raphe nucleus, and reticular formation, hypothalamus, and septum/striatum of the left hemisphere was correlated with social status. Second, similar correlations were found in the same areas in the other hemisphere. Third, similar correlations were found in a second data set acquired several months later from a subset of the same animals. Fourth, the strength of coupling between fMRI-measured activity in the same areas was correlated with social status. The network of subcortical areas, however, had no relationship with the sizes of individuals’ social networks, suggesting the areas had a simple and direct relationship with social status. By contrast a second circuit in cortex, comprising the midsuperior temporal sulcus and anterior and dorsal prefrontal cortex, covaried with both individuals’ social statuses and the social network sizes they experienced. This cortical circuit may be linked to the social cognitive processes that are taxed by life in more complex social networks and that must also be used if an animal is to achieve a high social status.

“A Neural Circuit Covarying with Social Hierarchy in Macaques” by MaryAnn P. Noonan, Jerome Sallet, Rogier B. Mars, Franz X. Neubert, Jill X. O’Reilly, Jesper L. Andersson, Anna S. Mitchell,Andrew H. Bell, Karla L. Miller, and Matthew F. S. Rushworth in PLOS Biology, September 2 2014 doi:10.1371/journal.pbio.1001940.

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