Testosterone promotes aggression, “cuddling”: study on gerbils

A recent rodent study found that testosterone, while often associated with aggression, can also promote friendly behaviors in males.

For the study, published in the peer-reviewed journal The Proceedings of the Royal Society B, neuroscientists from Emory University in Atlanta, Georgia, looked at how the hormone affects Mongolian gerbils, with findings showing that testosterone has the ability to promote aggression, as well as “cuddling” behavior.

“For what we believe to be the first time, we have demonstrated that testosterone can directly promote nonsexual and prosocial behavior, in addition to aggression, in the same individual,” Aubrey Kelly, assistant professor of psychology at the Emory University and first author of the study, said in a press release.

“This is surprising because normally we think testosterone increases sexual behaviors and aggression. But we have shown that it can have more nuanced effects, depending on social context.”

Scientists chose to study Mongolian gerbils because of their tendency to form long-lasting pair bonds and raise their young together.

While males can be aggressive during mating, researchers say they will cuddle more and more after a female becomes pregnant, while protecting their pups.

For the study, the scientists waited for a male and female gerbil to pair up and the female to become pregnant, after which the males exhibited their usual cuddling behaviors. The researchers then gave the men an injection of testosterone.

While they expected testosterone to reduce cuddly behavior in males, the researchers say the male gerbils became even more cuddly and prosocial with their partners — becoming more of a “super mate” in this context, a said Kelly.

As a follow-up, the researchers removed the females and introduced an unknown male.

Similarly, men who had been injected with testosterone were friendlier to their intruders.

However, this changed when the male subjects received another injection of testosterone. Once this happened, they exhibited the typical chase and avoidance behaviors you would see if an intruder was present.

“It appears that testosterone enhances context-appropriate behavior,” Kelly said. “It seems to play a role in amplifying the tendency to be cuddly and protective or aggressive.”

The researchers say that testosterone could prompt male gerbils to be more social in the future, even with other males, and potentially allow them to quickly switch between pro and anti-social behaviors depending on social context.

The study also looked at the influence of testosterone on oxytocin cells, which the researchers described as the so-called “love hormone” associated with social connection.

Their research found that male subjects who received testosterone injections produced more oxytocin activity in their brains when they were with their partners, compared to men who did not receive the injection.

“We know that the oxytocin and testosterone systems overlap in the brain, but we don’t really understand why,” Kelly said. “Taken together, our findings suggest that one reason for this overlap could be that they may work together to promote pro-social behavior.”

While the researchers say human behavior is much more complex than that of Mongolian gerbils, they hope the study will complement research on other species, including humans.

“Our hormones are the same, and the parts of the brain they act on are the same,” said Richmond Thompson, a neuroscientist at Emory University’s Oxford College, study co-author and Kelly’s husband.

“Thus, learning how hormones like testosterone help other animals adapt to rapidly changing social contexts will help us not only to understand the biological inner workings that affect their behavior, but also to predict and ultimately to understand how the same molecules in the human brain help shape our responses to the social world around us.”