We are used to the idea that testosterone levels in males can drive aggressive behavior and dominance. But in new research on meerkat societies in South Africa, it seems that females with high levels of testosterone run the show.
A team of researchers led by Christine Drea, professor of Evolutionary Anthropology at Duke University, has shown that testosterone-fueled aggression in female meerkats may have played a crucial part in the evolution of cooperation in meerkat societies.
Meerkats are social mammals that live in small to medium-sized groups in arid environments in southern Africa. A single dominant matriarch rules the group and is assisted in rearing her pups by the subordinate members. Meerkat parents cannot raise offspring on their own as they need the help of others to find food for the pups and protect them while the adults are off foraging.
The research team worked with 22 clans of meerkats that live in the Kuruman River Reserve, in South Africa’s Kalahari Desert. These meerkats have been the subjects of numerous scientific studies, over decades, and are habituated to the presence of humans. Researchers were able to observe the behavior of the matriarchs during their pregnancies, and to note instances of aggression. They also collected samples of blood and feces from individuals to measure testosterone levels.
Observations showed that matriarchs do not lead their groups by being kind or nice. They dominate by pushing, shoving, and biting the subordinates, and mark their territories with a pungent substance secreted from glands under their tails. If any subordinate females do become pregnant, a matriarch may expel them from the group or kill their new-born pups.
As a result, few of the adult subordinate females in a clan manage to have surviving pups in any given year. By contrast, a successful matriarch, can have as many as three or four successful litters in a good year.
Analysis of testosterone levels indicated that the matriarch has very high levels of this hormone, which is usually associated with male aggression.
“We always think of male competition being driven by testosterone, but here we’re showing that it’s driving female competition too,” she said.
“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,” said Drea.
During pregnancy, the matriarch’s levels of testosterone increases, as does her aggressiveness towards subordinate individuals. In addition, once born, her pups are also aggressive, furiously demanding attention from the subordinates, like spoiled children.
In order to test whether this aggressiveness is being driven by testosterone, the researchers dosed some matriarchs with flutamide, a testosterone-receptor blocker that prevents testosterone’s action in the body. These matriarchs soon lost their edge; they didn’t shove, bite, or growl as much, nor did they scent-mark their territories as often. Subordinates lost their respect and showed less deference.
Interestingly, any offspring born to a matriarch that had been treated with flutamide were also less aggressive and demanding towards the subordinates. Without the influence of testosterone while in the womb, these offspring were calmer and less dominant.
“The subordinate females and their pups are also aggressive, but not as much as the matriarchs and their pups” said Drea. “It’s this difference that gives matriarchs their edge, and it’s this difference that we completely erased with testosterone blockers.”
The fact that matriarchs passed on the benefits of high testosterone levels to their offspring implies that, in addition to helping the matriarch keep control and produce more pups, this hormone also helps her pups to get the best possible start in life by ensuring that the subordinates cooperate and supply food and care to them. Hormone levels may thus be driving the maintenance of a cooperative family dynasty.
“Here we have experimental results revealing a new mechanism for the evolution of cooperative breeding,” Drea said, “one that is based on testosterone-mediated aggression and competition between females.”
“Females are not primarily competing for food,” she said. “Competition is about ensuring that other individuals help raise their kids. And testosterone helps them win that reproductive battle.”
The researchers say that the matriarch’s testosterone-fueled aggression is the glue that holds the cooperative group together. If females were treated with testosterone blockers for longer, they expect that the matriarch would be overthrown, and the group’s structure would be temporarily destabilized.
“When people think about cooperation, they usually think about altruism or helping others,” Drea said. “This study is showing that cooperation can also arise through aggressive means, and quite effectively.”
The study is published in the journal Nature Communications.