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If there were to be a ranking of the most formidable female bullies in the animal kingdom, lemurs would likely find themselves near the top. In these unique primates, it is often the females who take on the leadership role, employing physical aggression to assert their dominance and maintain control over males.
However, not every lemur species is characterized by this female-led hierarchy. Within certain branches of the lemur lineage, some species have transformed over the past million years to foster more balanced interactions between males and females.
Recent research indicates that this more harmonious evolution in lemurs may be partly influenced by changes in the role of the “love hormone” oxytocin within their brains.
A study published in the journal Biology Letters by researchers from Duke University examined seven closely related lemur species belonging to the genus Eulemur, distinguishing those with dominant females from those exhibiting a more equitable social structure.
For instance, the blue-eyed black lemurs exemplify this aggressive female behavior, where females prioritize access to food and preferred resting spots, often resorting to smacking, biting, or chasing the males to assert their dominance.
According to Christine Drea, a senior author and professor of evolutionary anthropology at Duke, this aggression is not necessarily protective in nature. Rather, it can occur without provocation, serving as a reminder to others of a female’s authoritative position.
“Males typically concede priority access to whatever the females desire,” Drea noted.
In contrast, collared lemurs illustrate a more peaceful and equitable dynamic, with males and females enjoying comparable status. “The social structure resembles a more level playing field,” remarked Allie Schrock, the study’s first author and a Ph.D. graduate from Drea’s lab.
Although the lemurs studied passed away from natural causes some time ago, their tissues are preserved in a cryogenic bank at the Duke Lemur Center. By employing autoradiography, the researchers could map the brain’s oxytocin binding sites, a hormone integral to social behaviors such as trust and bonding.
The findings uncovered a noteworthy pattern: the recently evolved egalitarian species displayed a higher concentration of oxytocin receptors compared to their hierarchical counterparts, increasing the potential for oxytocin’s effects on social interactions.
The most significant changes appeared in the amygdala, the brain region often associated with emotional responses like fear and anger.
This trend was consistent across both male and female lemurs, implying that egalitarian species may have reached a state of gender equality by reducing overall aggression, rather than males merely escalating their aggressive behavior to compete with dominant females, as explained by Drea.
The implications of this research extend beyond lemurs. Issues with oxytocin signaling have been connected to aggression, personality disorders, and autism across various species, including humans and rodents.
Moving forward, the researchers intend to investigate further correlations between hormone receptors and other facets of social behavior in lemurs, such as solitary versus social living patterns.
“There is a wealth of knowledge still to uncover regarding how the brain influences behavior, particularly from studies on lemurs,” Schrock concluded.
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