Neuroanatomical foundations of social tolerance across macaque species
The macaque genus includes 25 species with hugely diverse social systems, ranging from low to high tolerance social organization. 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 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. To tackle the issue of nature versus nurture origin of the social tolerance effect on amygdala volume, we investigated the development of amygdala across species with different social grades. Intolerant species showed a gradual increase in relative amygdala volume across the lifespan. Unexpectedly, tolerant species exhibited an opposite trend, previously undescribed in primates. Taken together, these findings provide valuable insights into the neuroanatomical and evolutionary basis of primates social behaviors.