Testosterone has been long thought of as a “male” hormone. However, recent studies on meerkats have revealed this binary categorization is not so simple.

In a pair of studies, researchers at Duke found that female meerkats can produce more testosterone than their male counterparts, which results in weakened immune systems. This level of testosterone fuels females' aggressive behavior. 

“Female meerkats, despite being cute as the dickens, are actually mean and feisty," said Christine Drea, Earl D. McLean professor of evolutionary anthropology and a leader of the study. "That’s not a usual pattern in mammals. It’s unusual for females to be aggressive outside of protecting their young. That offers a bit of a puzzle because usually a mechanism for aggression, the one that we know best, is testosterone.” 

In the study, the researchers examined hormone levels in both sexes of meerkats, who held both high and low rank positions in their groups. They measured the levels when the animals were in their normal phase of life and when they were in periods of high reproductive activity.

Levels of estrogen, the primary female sex hormone, were higher in males than expected, and androgens, including testosterone, were higher in females. In males, there was hardly any difference in testosterone between dominant and subordinate males. However, testosterone levels were higher in males actively pursuing a mate.

Drea explained that these results are unusual for meerkats, which are cooperative breeders, meaning that one dominant male meerkat can account for 80 percent of the offspring. Researchers expected that males would have higher levels of reproductive hormones in order to monopolize reproduction, but found that was not the case.

In fact, even the subordinate female meerkats had testosterone levels equivalent to those in males. Pregnant and dominant females had androgen concentrations that were even greater. However, these differences may negatively impact the overall health of the animals.

“In some species, it has been shown that higher testosterone levels can come at a cost because so much is invested into one system, the reproductive aggressive system," Drea said. "The system in question here is the immune system."

The second study, conducted by Kendra Smyth, a graduate student in the Nicholas School of the Environment, sought to explain the effects of these high androgen levels on the immune system by measuring hormone levels and counting parasite eggs in female meerkats' droppings. The levels of parasite eggs were used as a reflection of the overall immune system health.

“Any female, regardless of rank, would have more parasites if she had a lot of androgens,” Smyth said.

Although an animal with more parasites was considered to have a weaker immune system, Smyth noted that this does not necessarily have to be the case. The next step in this field of research is to directly measure meerkat immune responses through blood tests, a paper that Smyth is currently working on, she said.

Smyth acknowledged the discrepancy in the fact that dominant female meerkats live longer, yet have weaker immune systems, explaining that most meerkats die before their first birthday due to aerial predation or separation. As a result of this early mortality rate, the distribution of life span is skewed.

Because meerkats and humans are both cooperative species, meerkat research can explain the evolution of cooperation, Drea noted. Likewise, these studies provide insight on hormone-immune interactions in humans and mammals as a whole.

“Understanding how hormones affect behavior in a mammal is universal," Drea said. "We have the same hormones that meerkats have, so studying meerkats can help us understand the role of androgens in females more broadly."