Hormones influence power and peace in lemur society

In the forests of Madagascar, few animals command attention quite like lemurs – especially when it comes to questions of social hierarchy.

In many lemur species, it is the females that rule the roost. They dominate food, take prime spots, and often chase, bite, or hit males who get in their way.

Yet new evidence shows that this aggressive matriarchy is not universal among lemurs, and the key difference may lie in how their brains deploy a single hormone: oxytocin.

Gender equality and oxytocin receptors

A new study led by Duke University takes a close look at seven species in the genus Eulemur. Some of these species, such as the blue‑eyed black lemur, operate under rigid female dominance, while others, like the collared lemur, live in more egalitarian groups.

By mapping oxytocin‑binding sites in the animals’ brains, the researchers discovered an intriguing pattern: lemur societies that are more equitable between the sexes possess a denser network of oxytocin receptors. This is especially true in a brain region that governs fear, anxiety, and aggression.

“It’s more of an even playing field,” said first author Allie Schrock, who completed her PhD in evolutionary anthropology at Duke. Her remark contrasts with blue-eyed black lemurs, where dominant females control males through intimidation and strength.

Lemur queens rule the canopy

Dominant females in species like the blue‑eyed black lemur do not confine their aggression to maternal defense or resource scarcity. They wield authority even when circumstances are calm, reinforcing status through spontaneous acts of aggression.

According to senior author Christine Drea, a professor of evolutionary anthropology at Duke, aggression in these females is often entirely unprovoked, simply to remind others who’s in charge.

“Males let females have priority access to whatever they want,” Drea said. This arrangement places males perpetually on the defensive, ceding first rights to fruiting trees, coveted shade, or safe resting platforms.

Though unusual to humans, these social structures are common in lemurs – except for notable deviations in the Eulemur genus.

Oxytocin rewires lemur behavior

To understand how some Eulemur species shed female domination for gender parity, the team turned to preserved brain tissue housed at the Duke Lemur Center. These samples came from naturally deceased, flash-frozen lemurs – a vital resource given Madagascar’s endangered primates.

Using autoradiography, a high‑resolution imaging method, the researchers traced where oxytocin molecules could bind throughout the brain. Oxytocin, popularly dubbed the “love hormone,” is well known for its role in mammalian bonding, trust, and social recognition.

The surprising discovery was that egalitarian lemurs had significantly more oxytocin receptors than their female‑dominated cousins. The density spike was most pronounced in the amygdala, a neural hub for emotion regulation.

This pattern suggests that animals with more oxytocin “targets” in emotional centers may experience a dampening of aggression across both sexes, creating conditions favorable to equal status. Rather than males meeting female hostility blow‑for‑blow, the entire group becomes somewhat less confrontational.

For Drea, the data indicate that egalitarian species achieved gender parity by becoming less aggressive towards others overall.

Implications for human neuroscience

The study slots neatly into a broader conversation about oxytocin’s influence on lemur social behavior.

In humans, variations in oxytocin signaling have been tied to spectrum disorders such as autism, as well as to patterns of aggression and anxiety. The Duke study shows tiny chemical shifts in receptors can drive major social changes over evolutionary time.

Scientists have already documented an “oxytocin paradox” in which the hormone can foster in‑group bonding but also fuel suspicion or aggression toward outsiders.

The lemur findings offer a new perspective. When oxytocin receptors increase in brain areas that reduce fear or promote calm, the hormone’s bonding effects may spread more widely. This could help ease social hierarchies rather than intensify them.

A million‑year experiment

Genetic evidence indicates that egalitarian Eulemur species diverged from their more dictatorial relatives within the past million years. That is lightning‑fast in evolutionary terms – and hints moderate adjustments in oxytocin pathways may be sufficient to trigger a social overhaul.

“We have a natural experiment right here,” Schrock said. He added that closely related species provide a powerful test case for linking neurobiology and behavior.

Researchers will study how oxytocin receptors relate to group size, social habits, and the length of pair bonds. “There’s a lot more that we can learn from lemurs about how the brain regulates behavior,” Schrock emphasized.

The story of lemur society

Though lemurs occupy only a sliver of the world’s biodiversity, their social quirks resonate widely.

Understanding how neurochemical circuitry shapes aggression and cooperation could inform conservation strategies. It can also improve welfare in captive primates, and even shed light on human behavioral disorders.

For endangered species, understanding social harmony can aid reintroduction efforts by reducing conflict and improving survival.

In the end, lemurs remind us that social structures – whether matriarchal dominance or egalitarian negotiation – do not exist in a vacuum. They are built on biochemical foundations that evolution can remodel.

As scientists continue to uncover those hidden mechanisms, the story of lemur society becomes a window into the delicate interplay between brain chemistry, behavior, and the bonds that hold communities together.

The study is published in the journal Biology Letters.

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