(Web Desk) - Scientists have discovered that macaque species with more tolerant social systems possess a larger emotion-processing center in the brain.
The finding reframes a brain region long tied to aggression as a core component of how primates manage complex social relationships.
Comparisons across macaque brains reveal that species known for more flexible and tolerant social interactions carry a larger amygdala, a brain region that processes emotions and social signals, relative to overall brain size.
Examining these differences, Sarah Silvère at the University of Strasbourg (Unistra) connected variations in amygdala volume to the distinct social styles observed across macaque species.
Patterns across the genus show that tolerant species begin life with larger amygdalae than their more combative relatives.
That early contrast raises a deeper question about whether these neural differences arise from inherited biology, lifelong social experience, or both.
Primatologists already sort macaques into four social grades based on how often fights end with friendly contact.
Across the roughly 25 species in the macaque genus, some groups run on strict rank rules and quick retaliation.
More tolerant species show looser hierarchies, more repair after clashes, and social ties that change depending on the moment.
The team organized these social traits into three themes: maintaining relationships, controlling impulses, and navigating the unpredictability of everyday social life.
Deep in the brain, the amygdala, a cluster that tags emotion and social cues, helps primates judge faces and intentions.
Signals from the amygdala extend to memory and control circuits, allowing the region to either dampen reactions or intensify them.
In humans, larger amygdala volume has been linked to bigger social networks, tying anatomy to everyday relationship load.
Results from captive macaques showed that social group size reshaped circuits connected to the amygdala. Based on 42 brain MRI scans, Silvère ranked each species by social grade and compared volumes between the ends of the scale.
Species at the tolerant end carried more amygdala tissue relative to brain size, even when overall brains were larger or smaller.
Even after accounting for age and sex, the link between amygdala size and social tolerance remained consistent across species.
The pattern suggests this brain region may help primates interpret complex social signals – rather than serving only as a center for aggression.