Neuropeptide receptor distributions in male and female Eulemur vary between female-dominant and egalitarian species

Aggression and its neurochemical modulators are most often studied in males, leaving the mechanisms of female competitive aggression largely unexplored. To better understand how female aggression is regulated in the primate brain, we used receptor autoradiography to compare the neural distributions of oxytocin and vasopressin receptors in members of female-dominant versus codominant species within the Eulemur genus, wherein dominance structure is a reliable proxy of female (and male) aggression. We found that oxytocin receptor binding in the central amygdala was predicted by dominance structure, with male and female members of three codominant species showing more oxytocin receptor binding in this region than their peers in four female-dominant species. Based on the rodent literature, this finding is consistent with the presence of a male-like neural circuit regulating competitive aggression in female Eulemur. We suggest that derived peacefulness in Eulemur stems from selective suppression of ancestral female aggression over evolutionary time via a mechanism of increased oxytocin receptor binding in the CeA, rather than from augmented male aggression. This interpretation implies fitness costs to female aggression and/or benefits to its inhibition. These novel data establish Eulemur as a robust model for understanding the neural correlates of female competitive aggression.