When we think of testosterone, we probably think about stereotypically masculine traits: strength, beards and aggression.

But research in rodents shows the hormone is far more complex than it might seen.

In the right context, testosterone doesn’t just promote aggression. It can also promote social behaviours and even cuddling.

Study author and assistant professor at Emory University in the US Aubrey Kelly said she believed this was the first time researchers had demonstrated the hormone could lead to non-sexual, pro-social and agressive behaviour, all in the same individual.

‘It’s surprising because normally we think of testosterone as increasing sexual behaviors and aggression,’ he said. ‘But we’ve shown that it can have more nuanced effects, depending on the social context.’

Kelly and her husband, Emory University neuroscientist Richmond Thompson, wanted to find out more about how hormones like testosterone affect the brain, quickly influencing behaviour.

They wondered if testosterone would always promote aggression and dampen friendly behaviour ⁠— or if it might play a more nuanced, context-specific role.

To find out, they first gave the hormone to male gerbils who’d bonded with females that then became pregnant.

Male Mongolian gerbils like those in the study tend to be very affectionate to their pregnant partners, forming tight social bonds before raising their pups together.

‘Super partners’

The researchers then gave these males testosterone to see if it would make them less affectionate.

‘Instead, we were surprised that a male gerbil became even more cuddly and prosocial with his partner,’ Kelly said. ‘He became like ‘super partner.’’

A week later, the researchers introduced another male to each setting to see if the lingering effects of testosterone would again bring out aggression.

Instead, the males initially acted relatively friendly towards their intruders.

After another dose of testosterone, however, the residents acted aggressively to the newbies, chasing them out or avoiding them.

‘It was like they suddenly woke up and realised they weren’t supposed to be friendly in that context,’ Kelly says.

Because of these nuanced responses, Kelly and Thompson think the testosterone enhanced regular context-specific behaviours.

The testosterone injections may have a lingering effect that reflects behaviours you’d see in nature, the researchers suggested. In the wild, males continue to bond with their pregnant partners after a testerone boost from mating has tailed off.

An invading male may then cause a surge in the hormone, helping the resident male to fend off his enemy.

The researchers think the surprising cuddle effect might also be linked to testosterone interacting with another hormone: oxytocin. The gerbils’ brains seemed to show higher levels of oxytocin in those mice who had recieved testosterone and went on to act more affectionately.

‘We know that systems of oxytocin and testosterone overlap in the brain but we don’t really understand why,’ Kelly says. ‘Taken together, our results suggest that one of the reasons for this overlap may be so they can work together to promote prosocial behaviour.’

Testerone, she added, doesn’t just affect gerbils. Humans are influenced by the same hormones. They even interact with the same parts of the brain – albeit on a much larger scale.

That doesn’t mean their effects will be the same. But they may help us better understand their impacts on humans.

‘Learning how hormones like testosterone help other animals adjust to rapidly changing social contexts will… help us predict and ultimately understand how the same molecules in human brains help shape our own responses to the social world around us,’ Kelly said.

The study was published in Proceedings of the Royal Society B Biological Sciences.

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