Summary: New research shows that differences in social tolerance among macaque species are reflected in brain anatomy. MRI comparisons across multiple species reveal that more socially tolerant macaques tend to have larger amygdalae, suggesting this structure plays a central role in processing complex social information rather than solely mediating fear and aggression.
Researchers analyzed post-mortem and in vivo MRI scans from 12 macaque species representing a range of social tolerance grades. Their findings indicate a clear positive relationship between amygdala volume and social tolerance: species described as more tolerant exhibit larger amygdala volumes than less tolerant species. By contrast, hippocampus volume showed no consistent association with social tolerance. The study also uncovered distinct developmental trajectories: tolerant species are born with relatively larger amygdalae that tend to shrink with age, while less tolerant species start with smaller amygdalae that increase in volume over the lifespan.
Key Facts
- Amygdala and social tolerance: Across 12 macaque species, higher social tolerance correlates with larger amygdala volume.
- Functional reappraisal: Rather than serving only fear or aggression, the amygdala appears to act as a multifunctional social processing hub that helps manage complex relationships and regulate behavior.
- Developmental patterns: Tolerant species are born with larger amygdalae that decline with age; intolerant species show increasing amygdala volume over their lifespan.
- Hippocampus: No robust link between hippocampal volume and social tolerance was found, apart from a limited age-specific difference in some comparisons.
- Comparative coverage: The study included 12 species spanning four social tolerance grades, incorporating rarely scanned and previously unscanned species.
Source: eLife
Overview
Primate social life requires tracking many relationships, controlling impulses, resolving conflict, and adapting to changing social contexts. The macaque genus, with about 25 species exhibiting diverse social systems, provides a powerful comparative model for studying how social environments and brain anatomy co-evolve. This study frames social tolerance within a cognitive model and links observable behavioral grades to underlying neuroanatomy.
Lead author Sarah Silvère and colleagues used a curated MRI database assembled from opportunistically collected samples, including individuals from captive collections and the Simian Laboratory Europe (SILABE) platform. They quantified amygdala and hippocampus volumes and related these measures to an index of social tolerance derived from 18 behavioral traits grouped into three cognitive dimensions:
- Socio-cognitive demands: the cognitive effort needed to monitor and respond to complex social networks.
- Behavioral inhibition: the capacity to regulate impulses, suppress inappropriate reactions, and manage aggression.
- Predictability of social environment: the degree to which relationships and interactions are structured and foreseeable; more tolerant groups typically have more fluid and less predictable interactions.
Testing hypotheses about how subcortical volumes vary with these dimensions, the team found a consistent association between amygdala size and social tolerance grade. High-tolerance species demonstrated larger amygdala volumes than low-tolerance species, supporting the interpretation that the amygdala is engaged in nuanced social processing: integrating social cues, mediating conflict resolution, and supporting flexible social behavior.
The hippocampus did not show a clear pattern across social grades, suggesting that the amygdala is a primary neuroanatomical correlate of social tolerance in macaques. An age-related comparison indicated a modest hippocampal difference between tolerant and intolerant species in the 13–18 year range, but overall results for the hippocampus were inconclusive.
A notable and unexpected result was the contrasting developmental trajectories of the amygdala. Intolerant species (grade 1) showed gradual increases in amygdala volume with age, while tolerant species (grade 4) started life with relatively large amygdalae that decreased through development. These opposing patterns suggest that innate neuroanatomical differences combine with lifelong social experiences to shape the brain.
“Larger amygdalae in socially tolerant species may reflect enhanced capacity to process complex social information, support peaceful interactions, and manage conflicts,” explains Silvère. Senior author Sébastien Ballesta highlights the broader implications: comparing closely related macaque species offers a natural framework to study how social environments influence brain development and the evolution of social cognition.
Key Questions Answered:
A: Not necessarily. While the amygdala has roles in fear and aggression, this study emphasizes its broader function as a social information processor. A larger amygdala in tolerant species likely supports tracking nuanced relationships, negotiating conflicts, and interpreting complex social cues.
A: Both factors matter. Tolerant species tend to be born with larger amygdalae, suggesting an innate predisposition, but the different developmental trajectories indicate that social environment and experience further shape brain structure over time.
A: The study focuses on macaques, but it supports broader ideas in the Social Brain Hypothesis. In humans, amygdala anatomy has also been associated with social network size and complexity, so similar principles may help explain aspects of human social cognition without implying direct equivalence.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- The journal paper was reviewed in full.
- Additional context was contributed by editorial staff.
About this research
Author: Emily Packer
Source: eLife
Contact: Emily Packer – eLife
Image credit: Neuroscience News
Original research (open access): Toward neuroanatomical and cognitive foundations of macaque social tolerance grades — 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
Macaque species vary widely in social organization, providing an opportunity to investigate the evolutionary roots of the primate social brain. This study establishes a cognitive framework for social tolerance and analyzes structural MRI scans from 12 macaque species. Results identify amygdala volume as a reliable neuroanatomical predictor of social tolerance: high-tolerance species have larger amygdalae. Developmental analyses reveal divergent age-related patterns between tolerant and intolerant species, offering new insights into how innate neuroanatomy and life-long social experience interact to shape social cognition. Together, these findings advance understanding of the cognitive, neuroanatomical, and evolutionary foundations of primate social behavior.