Love Hormone Turns Mean Girl Lemurs Nice

Researchers have discovered why some female lemurs have abandoned their bullying ways, pointing to changes in how the “love hormone” oxytocin affects their brains. The findings could have implications for understanding aggression in other species, including humans.

In most lemur societies, females reign supreme through physical intimidation. But within one branch of the lemur family tree, certain species have evolved more peaceful social structures where males and females share relatively equal status—a rare phenomenon among these Madagascar primates.

Lemurs are among the animal world’s most notable female bullies, according to researchers at Duke University who recently published their findings in the journal Biology Letters. The team examined seven closely related lemur species in the genus Eulemur, comparing those with domineering females to the more egalitarian species.

Using tissue samples from lemurs that had died of natural causes at the Duke Lemur Center, they mapped brain binding sites for oxytocin using a technique called autoradiography.

In blue-eyed black lemurs, one of the female-dominant species, females aggressively maintain their social position. Females get first dibs on food and prime resting spots, often smacking, biting and chasing males to get their way. This behavior isn’t protective mothering but appears to be about maintaining dominance.

“Males let females have priority access to whatever they want,” said senior author Christine Drea, a professor of evolutionary anthropology at Duke.

By contrast, collared lemurs demonstrate a more harmonious social arrangement. “It’s more of an even playing field,” said first author Allie Schrock, who earned her Ph.D. in the Drea lab.

The brain mapping revealed that more recently evolved, egalitarian lemur species have significantly more oxytocin receptors in the amygdala—a brain region associated with emotions like fear and anger—compared to their female-dominant cousins. The pattern appeared in both males and females, suggesting that species with gender parity achieved this balance by reducing overall aggression rather than by males becoming more combative.

This evolutionary shift appears to be relatively recent, occurring within the last million years. The findings challenge assumptions about aggression in primates and suggest that oxytocin plays a crucial role in modulating competitive behavior.

The researchers found that the pattern held up for both sexes, suggesting that egalitarian species achieved gender parity by becoming less aggressive overall, rather than males increasing their aggression to match females. This indicates that evolution favored reduced aggression in these lemur societies.

The researchers suggest that their findings could offer insights beyond the world of lemurs. Problems with oxytocin signaling have been linked to aggression, personality disorders, and autism in humans and other animals.

These endangered primates offer a unique window into brain chemistry and behavior. With their tissues preserved in a bank at the Duke Lemur Center, scientists can continue investigating the neurological underpinnings of social behavior in these fascinating animals.

“There’s a lot more that we can learn from lemurs about how the brain regulates behavior,” Schrock said.

The research team plans to expand their investigation to examine links between hormone receptors and other aspects of social behavior in lemurs, such as whether they are solitary or social. As humans continue to grapple with understanding the biological basis of social behaviors and disorders, these peculiar primates may hold valuable clues about the evolution of cooperation and aggression.

Thank you for catching this error. I’ve now ensured that only text that appears as direct quotes in the source material is presented in quotation marks, with proper attribution.RetryClaude can make mistakes. Please double-check responses.