Meerkat societies are structured by testosterone-driven aggression

Meerkat groups have a clear, undeniable leader: the matriarch. Together with a lucky mate that she chooses, the matriarch rules over a group of subordinate males and females of all ages.

According to a new study led by Duke University, the matriarch’s dominion depends almost entirely on her extremely high levels of testosterone, particularly during her frequent pregnancies. These findings suggest that, paradoxically, testosterone-fueled aggression might be a crucial part in the evolution of cooperation in meerkat social groups. 

Meerkat matriarchs are far from being benevolent leaders. Since subordinate meerkats usually help raise the matriarch’s pups, she needs to make sure that these pups receive undivided attention. To ensure this, she often attacks pregnant subordinates, expelling them from the group, or killing their newborn pups.

Thus, while very few of a matriarch’s subordinate females manage to have surviving pups in a given year, a matriarch can have as many as three or four successful litters in a good year. 

Besides preventing female subordinates from reproducing, matriarchs shows their dominion by pushing, shoving, biting, and growling, and mark their territory by rubbing their behinds against shrubs and rocks, spreading a pungent substance produced by their anal glands.

By studying 22 clans of meerkats at the Kuruman River Reserve in South Africa’s Kalahari Desert, the scientists found that the matriarchs’ domination and success are caused by their very high levels of testosterone.

“We always think of male competition being driven by testosterone, but here we’re showing that it’s driving female competition too,” said study lead author Christine Drea, a professor of Evolutionary Anthropology at Duke University.

“In non-pregnant matriarchs, testosterone values are equivalent to the males’, and just a little bit lower in subordinate females. But when matriarchs get pregnant, they ramp up.”

Moreover, once born, their puppies were also highly assertive and aggressive, furiously demanding food and care from their mother’s subordinates. This shows that their parent’s testosterone was transmitted to them inside the womb. If pregnant mothers were given flutamide, a testosterone-blocker, both theirs and the pups’ aggression levels dropped significantly. 

“Here we have experimental results revealing a new mechanism for the evolution of cooperative breeding, one that is based on testosterone-mediated aggression and competition between females,” Drea said. “Females are not primarily competing for food. Competition is about ensuring that other individuals help raise their kids. And testosterone helps them win that reproductive battle.”

“When people think about cooperation, they usually think about altruism or helping others. This study is showing that cooperation can also arise through aggressive means, and quite effectively,” she concluded.

The study is published in the journal Nature Communications.

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By Andrei Ionescu, Earth.com Staff Writer