Summary: In the complex world of primate politics, some species are “despots” while others are “democrats.” A new study reveals that this level of social tolerance is physically etched into the brain.
By analyzing MRI scans from 12 different macaque species, the team discovered that more tolerant, “friendly” species possess a larger amygdala than their more aggressive, intolerant counterparts. Traditionally viewed as the brain’s “fear and aggression” center, the study redefines the amygdala as a sophisticated social hub that helps primates manage complex relationships, regulate impulsive behavior, and adapt to fluid social environments.
Key Facts
- The Amygdala Link: Species with high social tolerance grades showed significantly larger amygdala volumes than low-tolerance (intolerant) species.
- Redefining a Region: While often associated with aggression, the amygdala appears to act as a “multifunctional hub” for processing complex social information and facilitating peaceful interactions.
- Early Development: Tolerant macaques are born with larger amygdalae that decrease in size with age, whereas intolerant species start with smaller amygdalae that grow throughout their lifespan.
- No Hippocampus Effect: The study found no consistent link between social tolerance and the volume of the hippocampus, suggesting the amygdala is the primary neuroanatomical driver of social style.
- Primate “Grades”: The researchers compared 12 species representing four grades of social tolerance, including rare and previously unscanned species.
Source: eLife
Researchers have found that the size of the amygdala – a region of the brain involved in processing emotions – could be linked to social tolerance in macaque monkeys.
Their research, published today in eLife as the final Version of Record after appearing previously as a Reviewed Preprint, is described by the editors as important work with a convincing methodological approach, offering new insights into the neural basis of social and emotional processing.
Navigating social life in primate societies requires substantial cognitive resources. Individuals must track multiple relationships, regulate their own behaviour and adapt flexibly to changing social contexts. The macaque genus includes 25 species with diverse social systems, ranging from low to high social tolerance grades.
“Links between the size of brain structures and social environment have been reported in humans and non-human primates,” says lead author Sarah Silvère, a PhD student at the Laboratoire de Neurosciences Cognitives et Adaptatives (LNCA), Strasbourg, France, and le Centre de Primatologie (CdP) of the University of Strasbourg, Niederhausbergen, France. “We wanted to understand how variation in brain structure volume is linked to social tolerance, and whether this is an innate feature or acquired through socialisation within more or less tolerant social environments.”
The team used a database of magnetic resonance imaging (MRI) scans of macaque brains – built from opportunistically collected samples, including individuals housed in zoos and animals from the Simian Laboratory Europe (SILABE) platform of the University of Strasbourg – to compare the size of the amygdala and the hippocampus across 12 species. They organised 18 well-known behavioural traits into three core aspects of social tolerance.
The first, socio-cognitive demands, refer to the cognitive resources needed to process, monitor and adapt to complex social environments. The second – behavioural inhibition – includes regulating impulsive behaviour, aggression or inappropriate responses during social interactions.
And the third category, predictability of the social environment, reflects how structured and foreseeable social interactions are in a community, with tolerant species having more fluid relationships.
For each category, the team produced hypotheses for how the volume of the amygdala and hippocampus might vary and tested them using MRI data for 12 macaque species that represent four grades of social tolerance, including two that had never been scanned before and another rarely studied species.
The team found that the volume of the amygdala – but not the hippocampus – was positively linked to the grade of social tolerance, with high-tolerance species showing larger amygdala than low-tolerance species.
This differs from previous studies that say the amygdala primarily supports aggressive behaviours and suggests instead that the amygdala also acts as a multifunctional hub embedded within complex social networks in the brain.
“Larger amygdala in socially tolerant species may reflect an enhanced capacity to process complex social information, facilitate better social interactions and manage conflicts,” explains Silvère.
For the hippocampus, a smaller volume was observed in socially intolerant species compared with tolerant species between the age of 13 and 18 years but, besides this, there was inconclusive evidence of any difference.
The team also found, to their surprise, that the developmental trajectories of the amygdala differed. Grade 1 intolerant species showed an increase in amygdala volume across their lifespan, while grade 4 tolerant species had a significantly higher amygdala volume at the start of their lives, which decreased with age.
In accordance with the team’s hypotheses, their results showed that social tolerance is rooted in neuroanatomical differences that can be observed early in life, and that these structural differences are influenced by social styles throughout individual lifespans.
“To our knowledge, our study is the first to report neuroanatomical links to social tolerance grades based on MRI data, and provides novel insights into the relationship between amygdala volume and social tolerance in macaques,” concludes senior author Sébastien Ballesta, Associate Professor at LNCA and CdP of the University of Strasbourg.
“These findings have profound implications for our understanding of social brain evolution, as well as underscoring the importance of developmental stage and the social environment in shaping neuroanatomical adaptations.
“Our work highlights that comparing closely related macaque species represents a valuable natural framework for investigating the impact of the social environment on brain development, paving the way for future research to unravel the complexities of brain evolution and sociality.”
Key Questions Answered:
A: That’s the old way of thinking! While the amygdala is involved in fear and aggression, this study shows it’s actually about processing social nuance. In a tolerant society, you need a bigger “computer” (amygdala) to track complex friendships, manage conflicts peacefully, and understand subtler social cues.
A: It’s both. The study found that tolerant species start life with a significantly higher amygdala volume, suggesting an innate foundation. However, the different growth trajectories throughout their lives show that the social environment also shapes how the brain develops over time.
A: While the study focused on macaques, the principles of the “Social Brain Hypothesis” often bridge the gap between primates and humans. In humans, amygdala size has also been linked to the size and complexity of social networks, suggesting that our ability to “play nice” with others is deeply rooted in our neurobiology.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
About this social neuroscience research news
Author: Emily Packer
Source: eLife
Contact: Emily Packer – eLife
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Toward neuroanatomical and cognitive foundations of macaque social tolerance grades” by Sarah Silvère, Julien Lamy, Chrystelle Po, Mathieu Legrand, Jérôme Sallet, and Sébastien Ballesta. eLife
DOI:10.7554/eLife.106424.3
Abstract
Toward neuroanatomical and cognitive foundations of macaque social tolerance grades
The macaque genus includes 25 species with diverse social systems, ranging from low to high social tolerance grades. Such interspecific behavioral variability provides a unique model to tackle the evolutionary foundation of primate social brain. Yet, the neuroanatomical correlates of these social tolerance grades remain unknown.
To address this question, we expressed social tolerance grades within a novel cognitive framework and analyzed post-mortem structural scans from 12 macaque species. Our results show that amygdala volume is a subcortical predictor of macaques’ social tolerance, with high tolerance species exhibiting larger amygdala than low tolerance ones.
We further investigated the developmental trajectory of amygdala across social grades and found that intolerant species showed a gradual increase in relative amygdala volume across the lifespan.
Unexpectedly, tolerant species exhibited a decrease in relative amygdala volume across the lifespan, contrasting with the age-related increase observed in intolerant species—a developmental pattern previously undescribed in primates.
Taken together, these findings provide valuable insights into the cognitive, neuroanatomical, and evolutionary basis of primates’ social behaviors.
