In a new study, investigators at the University of Pennsylvania in Philadelphia turned to rhesus macaques to investigate the effects of two hormones — oxytocin and vasopressin — on social behavior.
These primates are known for their aggressive, competitive behavior in groups that are highly hierarchized — which typically split members between dominant and submissive individuals.
But rhesus macaques were also chosen for another reason; according to the researchers involved in the recent study, these primates — like humans — live in large social groups, form long-term connections, and present some similar social behaviors.
Study authors Michael Platt and Yaoguang Jiang first turned their attention to the possible effects of oxytocin on social behavior as this hormone has previously been tied to bonding between mother and infant and pair-bonding.
As they note in the study paper they recently published in the journal Scientific Reports, "A single intranasal dose of [oxytocin] in healthy humans [has also been seen to enhance] trust, generosity, and empathy."
Certain studies have suggested that vasopressin may be implicated in the regulation of social behavior in humans, too.
Better synchronized behaviors
The study involved giving one rhesus macaque a dose of oxytocin, vasopressin, or saline solution — applied as a control method — through injection or inhalation. Seven monkeys had injections while seven were chosen to receive the oxytocin, vasopressin, or saline through inhalation.
After this process, the monkey was paired up six times with different rhesus macaques, and once with an empty chair (again, as a control measure).
While the monkeys were not able to touch each other — in order to prevent possible injury — they were able to interact in other ways; they could see, smell, and hear each other.
The team noticed that the monkeys that had received oxytocin or vasopressin tended to equalize their behavior in such a way that the other macaque would be able to pick up on cues.
In other words, aggression was taken down a notch in normally dominant monkeys, wheras the usually submissive animals became more daring, so that the interaction between the pairs of monkeys involved became more equal. This is known as "behavioral synchrony."
"[The monkeys] synchronize their facial expressions and their behavior more tightly in time," Platt explains, adding, "[T]hey're paying more attention to each other and when you do this, you get information more quickly and you respond more quickly."
That monkeys that usually thrive on being competitive become so much more amenable to each other thanks to oxytocin and vasopressin is certainly significant.
"Social dominance in monkeys is a really big deal," says Jiang, adding, "But here, the curve got flattened. If you were in the middle, you stayed in the middle."
"But if you were lower-ranking and you used to be timid, you got a little more assertive, and if you were super dominant, you still knew you were the boss but you were a little more chill about it. You weren't always trying to pick a fight."
The role of non-verbal communication
Even more interestingly, however, the effects persisted even when just one of the two monkeys in a given pair had received a dose of one of the hormones, while the other one had skipped this treatment.
This, the authors suggest, indicate that an amount of non-verbal communication — based on body language — takes place between the two monkeys, allowing the behavioral synchrony to still happen.
"Somehow they were conveying this information to each other," says Jiang. "Communication was obviously not verbal, but little gestures."
But the fact that both vasopressin and oxytocin had the same effect on the rhesus macaques makes things a lot more complicated. Though the receptors for each of these hormones are found in different parts of the brain, they can bind to both vasopressin and oxytocin.
And, when the researchers injected small amounts of both hormones into a brain region where vasopressin receptors were found, they saw that oxytocin ended up binding to those receptors instead.
"Our understanding of how all of this is going to work is much more complicated than originally thought," notes Platt.
This is because, he adds, "We have to consider this whole other system, the vasopressin system."
'A lot more to learn'
A better understanding of the mechanisms accessed by oxytocin and vasopressin may, in the future, lead to much more effective treatments for neuropsychiatric disorders, such as schizophrenia and autism.
Platt and Jiang are especially interested in seeing whether or not these hormones could be harnessed to improve social behavior impairments in these and similar conditions.
Rhesus macaques offer a good starting point for such studies; the researchers explain that the effect of oxytocin and vasopressin appear to be very similar in these monkeys and in humans. However, how these hormones actually work in people is, as yet, rather poorly understood.
"We have a lot more to learn about how, when, and in what manner we use these peptide hormones to treat various problems," concludes Platt.