Aggression and cooperation. Meerkats teach us that one without the other may not be possible.

The matriarch is the evident leader of Meerkat societies. She governs over a group of subservient females and males of all ages with her lucky mate. Her dominion, according to these new findings, is almost totally dependent on her extremely high testosterone levels.

Subordinates assist in the raising of the matriarch’s pups. They are cooperative breeders who are unable to rear their progeny alone. While they are busy gathering food for themselves, parents need the help of their group to find food and protect their children.

The matriarchs, on the other hand, aren’t exactly benevolent rulers. She will sometimes assault pregnant subordinates, remove them from the community, or kill their newborn pups in order to ensure that her cubs receive undivided attention.

As a result, only a small percentage of a clan’s adult subordinate females have surviving pups in any given year. In a good year, a successful matriarch can have three or four successful litters.

In addition to restricting the reproduction of the inferior females, matriarchs rule by pushing and shoving, biting and growling, and marking their territory by rubbing their behinds against rocks and plants, spreading a pungent scent-marking substance produced in glands buried under their tail.

Now, researchers have discovered that the matriarch’s bossiness, and thus her success, is caused by extremely high testosterone levels.

“We always think of male competition being driven by testosterone, but here we’re showing that it’s driving female competition too,” says professor Christine Drea, lead author.

To explore the relationship between testosterone levels and matriarchal success, the researchers worked with 22 meerkat tribes at the Kuruman River Reserve in South Africa’s Kalahari Desert.

These meerkats have been researched for decades and have developed a tolerance for human contact. This enabled researchers to observe the matriarchs’ behavior throughout their pregnancy – noting any instances of aggression – and to collect blood and excrement samples to determine their testosterone levels over time.

“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,” adds Drea.

As their pregnancy advanced, both the matriarchs’ aggression and testosterone levels grew simultaneously. When their pups were born, they were likewise hostile, feverishly seeking care and feeding from their subordinates, much like spoiled little brats.

However, is testosterone the true source of all of this aggression? To solve this, researchers administered flutamide to some matriarchs, a testosterone receptor blocker that inhibits testosterone’s activity in the body.

Flutamide-treated matriarchs were less likely to shove, bite, or snarl. Additionally, they did not mark their area as frequently. Subordinates recognized this and ceased to be so submissive. Their boss had succumbed to insanity.

Additionally, the boss’s children lost their edge. Their behavior shifted in the absence of the testosterone boost they would have received in their mother’s womb. Puppies of flutamide-treated matriarchs were calmer and less aggressive toward their subordinates.

“The subordinate females and their pups are also aggressive, but not as much as the matriarchs and their pups” says Drea.

“It’s this difference that gives matriarchs their edge, and it’s this difference that we completely erased with testosterone blockers.”

As hormones have a cross-generational influence, testosterone does not only aid the matriarch in producing more pups. Additionally, it assists her offspring in getting a good start in life by bullying the subordinates.

Because inhibiting the matriarch’s testosterone alters the behavior of the pups, hormones may be responsible for the maintenance of a cooperative family dynasty.

“Here we have experimental results revealing a new mechanism for the evolution of cooperative breeding,” Drea adds, “one that is based on testosterone-mediated aggression and competition between females.”

“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.”

According to the researchers, the matriarch’s testosterone-fueled hostility serves as the binding agent that keeps the cooperative group together. Females anticipate that if they are treated with testosterone blockers for an extended period of time, the matriarch will be deposed and the group’s structure will be temporarily destabilized.

“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.”

Source: 10.1038/s41467-021-27496-x

Image Credit: Charli Davies

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