Oxytocin: The Secret Behind Fierce Leaders, Especially Women

Researchers have found that lemurs, a species where males and females live more equally, have more oxytocin receptors in their brains, especially in the area linked to emotions such as fear and anger. This suggests that oxytocin helped reduce aggression, promoting more peaceful relationships, a finding that could also have implications for understanding social behavior in humans.

Aggression and the brain chemicals that control it are typically studied in males. As a result, scientists know much less about how female aggression works. To better understand how competitive aggression is regulated in the female brain, researchers studied lemurs, small primates from Madagascar, comparing males and females from different species.

Some of these species are predominantly female, meaning that females are more aggressive and control access to food and roosting sites, while others have more equitable relationships between males and females.

In this study, scientists analyzed brain tissue from seven closely related species of lemurs in the genus Eulemur. They focused on how two hormones involved in social behavior, oxytocin (often called the "love hormone") and vasopressin, work in the brain.

Using a technique called autoradiography, they mapped where these hormones bind to brain cells.

Autoradiography is an imaging technique used to visualize the distribution of specific molecules in tissues (such as the brain) using radioactive material. In simple terms, scientists “label” a substance of interest (such as a hormone or receptor) with a tiny radioactive trace, and then detect where that substance binds to or accumulates in the tissue.

Here’s how it works, step by step:

• Radiolabeling:

First, researchers take a molecule that specifically binds to what they want to study (such as the oxytocin receptor) and modify it so that it contains a radioactive atom. A common example of an isotope used is tritium or iodine-125.

• Incubation:

Next, they place very thin slices of tissue (such as brain slices) in contact with this labeled molecule. The radioactive substance binds to the sites where the receptor of interest is present.

• Washing:

Then, they wash away the excess molecules that did not bind, to ensure that the signal comes only from the places where the signal they wanted to detect was actually present.

• Exposure on Photographic Film:

The samples are then placed in contact with a special photographic film (or a plate sensitive to radiation). The radioactive particles emitted by the marked substance impress the film.

• Final Image:

After a period of time (which can be days to weeks, depending on the intensity of the radiation), the film is developed, as in an old photograph. The dark places on the film correspond to the areas where there were more bound molecules, visually showing the distribution of the receptors or substance in the tissue.

Autoradiography was used in this study because of its high sensitivity, which allows it to detect even very small amounts of substances. It has high spatial precision, which shows exactly where in the tissue (and sometimes even inside which cells) the substance is.

It is also especially useful for studying how hormone receptors, neurotransmitters or drugs are distributed in the brain and other organs.

The researchers used autoradiography to map where in the brain there were the most oxytocin receptors. They were able to compare different species and correlate the amount of receptors with social behavior (such as dominance and aggression).

They discovered something very interesting: species with more peaceful and egalitarian relationships between males and females had higher levels of oxytocin receptors, especially in a part of the brain called the amygdala, which controls emotions such as fear, anger and anxiety.

A clear example is the blue-eyed black lemur. In this species, females dominate by biting, chasing and hitting males to assert their status. This aggressive behavior is not limited to protecting the young; it is often unprovoked and appears to be a way of reminding the males who is in charge.

Males will often back off and let the females have first access to food and the best resting places.

Blue-Eyed Black Lemur

On the other hand, species like the collared lemur are much more peaceful. Males and females share status equally, and aggression is much rarer. According to lead author Allie Schrock, in these species, there is a "more level playing field" between the sexes.

Collared Lemur

The lemurs studied died naturally years ago, but their brain tissue has been preserved at the Duke Lemur Center. By studying these samples, the researchers found that the more even-tempered species evolved to have more oxytocin receptors in their brains.

This suggests that greater sensitivity to oxytocin helped reduce aggression, making male-female relationships more even-tempered over time.

Interestingly, this pattern was seen in both males and females. Rather than males becoming more aggressive to match dominant females, both sexes appeared to have become less aggressive overall. This suggests that the increasing peace between the sexes involved reducing ancestral female aggression, rather than increasing male aggression.

The implications of this research may extend beyond lemurs. Problems with oxytocin signaling in the brain have been linked to aggression, personality disorders, and autism in humans and other animals.

By studying lemurs, researchers hope to better understand how brain chemistry shapes social behavior, not just in animals, but possibly in people as well.

READ MORE:

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

Allie E. Schrock, Mia R. Grossman, Nicholas M. Grebe, Annika Sharma, Sara M. Freeman, Michelle C. Palumbo, Karen L. Bales, Heather B. Patisaul, and Christine M. Drea

The Royal Society, Biology Letters, March 2025. Volume 2. Issue 3

https://doi.org/10.1098/rsbl.2024.0647

Abstract

Aggression and its neurochemical modulators are typically studied in males, leaving the mechanisms of female competitive aggression or dominance largely unexplored. To better understand how competitive aggression is regulated in the primate brain, we used receptor autoradiography to compare the neural distributions of oxytocin and vasopressin receptors in male and female members of female-dominant versus egalitarian/codominant species within the Eulemur genus, wherein dominance structure is a reliable proxy of aggression in both sexes. We found that oxytocin receptor binding in the central amygdala (CeA) was predicted by dominance structure, with the members of three codominant species showing more oxytocin receptor binding in this region than their peers in four female-dominant species. Thus, both sexes in female-dominant Eulemur show a pattern consistent with the regulation of aggression in male rodents. We suggest that derived pacifism 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 data establish Eulemur as a robust model for examining neural correlates of male and female competitive aggression, potentially providing novel insights into female dominance.