Summary: New research shows that in some lemur species, the evolution of greater gender equality is associated with changes in the brain’s oxytocin system. By comparing seven closely related species in the genus Eulemur, researchers found that the more egalitarian species have higher densities of oxytocin receptors, especially in the amygdala, a brain region linked to emotion and aggression.
These findings indicate that egalitarian social structures in these lemurs arose through a reduction in overall aggression for both sexes, rather than through males increasing their aggressiveness to match females. The work sheds light on how neuromodulators like oxytocin can shape social behavior across species and suggests broader links between oxytocin signaling, emotional regulation, and social organization.
Key Facts:
- Brain chemistry shift: Egalitarian Eulemur species show greater oxytocin receptor binding in the amygdala, a change associated with reduced aggression.
- Path to gender parity: Equality between males and females appears to result from lowered aggression in both sexes rather than increased male aggression.
- Wider significance: Altered oxytocin signaling has been linked to aggression and certain social or emotional disorders in a range of animals, including humans.
Source: Duke University
If there were a contest for the most domineering females in the animal kingdom, some lemur species would rank high. In several Eulemur species, females assert priority access to resources and maintain dominance through physical aggression.
That said, not all Eulemur societies are dominated by aggressive females. Within this genus, a group of species that diverged within the last million years display more peaceful, egalitarian relationships between males and females.
A new paper published in Biology Letters from researchers at Duke University suggests that this shift toward peaceful gender relations is linked to differences in how oxytocin operates in the brain. The team examined seven closely related Eulemur species, categorizing them by social structure—female-dominant versus egalitarian—and mapped oxytocin receptor distributions across brain regions.
Blue-eyed black lemurs provide a vivid example of female dominance: females take first access to food and preferred resting spots and use physical behaviors such as slapping, biting, and chasing to maintain priority. According to senior author Christine Drea, a professor of evolutionary anthropology, this aggression is not limited to maternal defense but includes unprovoked acts that reinforce social rank.
By contrast, species such as collared lemurs exhibit more balanced social dynamics, where males and females share access to resources and social status. First author Allie Schrock, who completed her Ph.D. in Drea’s laboratory, describes these groups as operating on a more level playing field.
The study relied on tissue samples preserved in the Duke Lemur Center’s tissue bank. The lemurs had died of natural causes, and their frozen samples allowed the researchers to investigate neurochemical architecture without disturbing living animals.
Using receptor autoradiography, a laboratory imaging method that visualizes where hormones and neuropeptides bind in the brain, the team quantified oxytocin receptor sites. The comparison revealed a clear pattern: species with egalitarian social structures exhibited higher oxytocin receptor binding overall, with a particularly strong signal in the central amygdala (CeA).
Importantly, this pattern was consistent for both males and females. That suggests the pathway to egalitarianism in these species involved the suppression of ancestral female aggression across the population rather than a selective increase in male aggression. The authors propose that increased oxytocin receptor binding in the CeA may have facilitated this behavioral shift by dampening aggressive tendencies.
The implications of this research extend beyond lemurs. Dysregulation of oxytocin and its receptors has been implicated in aggressive behavior, personality disorders, and conditions affecting social cognition in humans and other animals. By identifying a clear neurochemical correlate of social structure in Eulemur, the study highlights the value of primate models in understanding the neural regulation of social behavior.
Future work from the group will explore how distributions of hormone receptors relate to other social traits, such as the tendency to live solitarily or in larger social groups, and will continue to use lemurs as a model for unraveling the neural bases of male and female competitive behavior.
“There is much to learn from lemurs about how the brain regulates behavior,” said Schrock.
About this aggression and neuroscience research news
Author: Robin Smith
Source: Duke University
Contact: Robin Smith – Duke University
Image: Image credit: Neuroscience News
Original Research: Closed access. “Neuropeptide Receptor Distributions in Male and Female Eulemur Vary Between Female-Dominant and Egalitarian Species” by Christine Drea et al., Biology Letters. DOI: 10.1098/rsbl.2024.0647
Abstract
Neuropeptide Receptor Distributions in Male and Female Eulemur Vary Between Female-Dominant and Egalitarian Species
Aggression and its neurochemical modulators are typically studied in males, leaving the mechanisms underlying female competitive aggression and dominance less explored. To address this gap, the researchers used receptor autoradiography to compare distributions of oxytocin and vasopressin receptors in male and female individuals from female-dominant and egalitarian/codominant Eulemur species. Dominance structure in this genus reliably reflects levels of aggression in both sexes.
The study found that oxytocin receptor binding in the central amygdala (CeA) was associated with dominance structure: members of the three codominant species showed greater oxytocin receptor binding in this region than members of the four female-dominant species. This pattern was observed in both sexes.
The authors propose that the derived pacifism observed in some Eulemur species results from selective suppression of ancestral female aggression through increased oxytocin receptor binding in the CeA, rather than from enhanced male aggression. This interpretation implies there may be fitness costs to persistent female aggression or benefits to its attenuation.
Overall, these data establish Eulemur as a valuable model for studying neural correlates of competitive aggression in both males and females and provide new insight into the neurobiology of female dominance and social evolution.