Macaque species that live in more tolerant social groups have larger volumes in brain regions tied to social processing, according to a study published in eLife that compared neuroanatomy across species with starkly different social hierarchies. The finding adds a new dimension to the long-running debate over how group living shapes primate brains, suggesting that the quality of social relationships, not just their quantity, leaves a measurable imprint on brain structure.
Tolerant and Despotic Macaques Show Distinct Brain Differences
Primatologists have long classified macaque species along a spectrum from “tolerant” to “despotic,” based on how rigidly individuals enforce dominance hierarchies and how evenly social interactions are distributed across group members. Tolerant species, such as Barbary macaques, tend to share food more readily, groom across rank lines, and maintain broader social networks. Despotic species, such as rhesus macaques, concentrate social bonds among close kin and enforce steep dominance gradients. A comparative network analysis across multiple macaque groups and species confirmed that these differences in tolerance, dominance steepness, and kinship bias are systematic rather than anecdotal, giving researchers a reliable behavioral framework for cross-species comparison.
The new eLife study builds on that framework by asking whether these behavioral differences correspond to differences in brain anatomy. Researchers used MRI to compare subcortical brain volumes across macaque species that fall at different points on the tolerance spectrum. They found that the size of the amygdala is linked to social tolerance in macaque monkeys, with more tolerant species showing larger volumes in this region, which plays a central role in processing social and emotional information. That link between tolerance grade and amygdala volume is notable because it shifts the conversation from how many social partners an animal has to how it interacts with them, suggesting that managing more equitable, less rigid social environments may require enhanced neural resources for evaluating subtle cues and maintaining diverse relationships.
From Lab Experiments to Free-Ranging Primates
The relationship between social environment and brain structure in macaques is not a new finding, but earlier work approached it from a different angle. A landmark experiment published in Science manipulated the size of social groups in captive macaques and then scanned their brains. Animals housed in larger groups showed increases in gray matter in the mid-superior temporal sulcus and the rostral prefrontal cortex, along with altered functional connectivity between these regions. That study demonstrated a causal link: changing the social environment changed the brain. Yet it left open the question of whether similar patterns hold in animals living under more natural conditions, where social relationships are far more complex than simple group size can capture and where dominance style, kinship, and tolerance vary across populations.
A subsequent study in Science Advances took that step, examining free-ranging rhesus macaques on Cayo Santiago. Researchers found that the number of social partners an individual maintained predicted volumes in the mid-superior temporal sulcus and the ventral-dysgranular insula in adult brains. Critically, these brain volume differences were not clearly present at birth, suggesting they develop over time in response to social experience rather than being hardwired from the start. Together, these studies established that social life reshapes brain anatomy in macaques, both in controlled settings and in the wild, and set the stage for asking whether specific dimensions of social structure, such as tolerance, map onto particular neural systems.
Why Tolerance Matters More Than Group Size Alone
Most prior research focused on a single variable: how many individuals an animal interacts with. The eLife study, detailed in a preprint reviewed by the journal, reframes the question around the character of those interactions. Two macaque species might live in groups of similar size, yet one enforces rigid hierarchies while the other distributes social contact more evenly. If brain volume tracks tolerance grade rather than raw group size, it implies that the cognitive demands of navigating a flexible, egalitarian social world differ meaningfully from those of living under strict dominance rules. Processing ambiguous social signals, reading the intentions of a wider range of partners, and managing conflicts without clear rank-based resolution all place distinct demands on neural circuitry that may be reflected in enlarged subcortical regions involved in social evaluation and emotion.
Behavioral evidence supports this interpretation. Research comparing tolerant Barbary macaques and despotic rhesus macaques found that tolerant species maintain juvenile-like social attention into old age, while despotic species show a decline. Social-attention measures such as gaze-following, which reflect how closely an animal tracks the behavior of others, stayed elevated across the lifespan in tolerant species. In despotic species, older individuals appeared to disengage from monitoring their social surroundings. If tolerant species sustain higher social vigilance throughout life, the neural infrastructure supporting that vigilance may grow or be maintained accordingly, which could explain why their brains show larger volumes in regions tied to social cognition. This behavioral continuity, coupled with anatomical differences, suggests that tolerance is not just a surface-level social style but a deeply embedded feature of primate biology.
Standardized Tools Sharpen the Comparison
One challenge in comparing brain anatomy across species is ensuring that researchers are measuring the same structures consistently. The eLife study relied on standardized neuroanatomical tools, including the Subcortical Atlas of the Rhesus Macaque, known as SARM. This hierarchical parcellation is mapped onto the NIMH Macaque Template and is designed for consistent localization and segmentation of subcortical regions of interest in macaque MRI data. By using a shared anatomical reference, researchers can be more confident that volume differences between species reflect genuine biological variation rather than inconsistencies in how brain regions are defined or measured across different labs, scanners, or analysis pipelines.
That methodological rigor matters because the claim at stake is significant: that evolved differences in social systems are mirrored by evolved differences in the neural substrates supporting social behavior. Earlier comparative work on primate brains, such as analyses of cortical expansion across species, has emphasized how overall brain size and neocortical volume scale with group size and other ecological pressures. The macaque tolerance findings suggest that more fine-grained aspects of social organization, like how evenly affiliation and aggression are distributed, may also leave a detectable signature in subcortical structures. Standardized atlases and templates make it possible to test such hypotheses across datasets and species in a way that is reproducible and transparent.
Implications for Social Brain Evolution
Placing the new results in a broader evolutionary context, the link between social tolerance and amygdala volume dovetails with a growing body of work arguing that the primate “social brain” is shaped by multiple, interacting pressures. Comparative analyses that integrate brain morphology, behavior, and phylogeny, such as studies of variation in social organization among mammals, indicate that factors like mating system, dispersal, and cooperative care can all influence how social groups are structured. The macaque data refine this picture by pointing to specific neural correlates of tolerance within a single primate radiation, suggesting that even relatively subtle shifts in dominance style may be accompanied by measurable anatomical changes.
For humans, who exhibit both strong dominance hierarchies and extensive cooperation, the findings offer a comparative lens rather than a direct blueprint. They highlight that social complexity is not just about living in large groups, but about how power, affiliation, and conflict are negotiated within those groups. In macaques, more tolerant societies appear to recruit or maintain larger subcortical resources for processing social and emotional information, potentially supporting broader, more flexible networks of relationships. Future work that combines standardized neuroimaging, detailed behavioral observation, and cross-species comparisons will be crucial for testing whether similar principles apply across other primates and for clarifying how different dimensions of social life—from tolerance to partner number to kinship structure, jointly shape the evolution of the social brain.
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*This article was researched with the help of AI, with human editors creating the final content.
