Summary: A new study of male rhesus macaques shows that exposure to oxytocin and vasopressin reduces differences in status within groups, making dominant males more relaxed and subordinate males more confident.
Source: University of Pennsylvania
Background: The effects of oxytocin on human social behavior are mixed—some studies report positive changes while others do not. In many non-human animals, however, oxytocin reliably promotes affiliative behaviors, increases attention to others, and reduces aggression and vigilance. To better understand how these neuropeptides influence spontaneous social interactions, researchers at the University of Pennsylvania examined the behavioral consequences of oxytocin and vasopressin exposure in male rhesus macaques during naturalistic encounters.
In work published in Scientific Reports, neuroscientist Michael Platt and postdoctoral researcher Yaoguang Jiang report that both oxytocin and vasopressin tend to “flatten” social hierarchies among male macaques. Dominant animals became more relaxed, while subordinate animals showed increased confidence. Remarkably, these effects occurred even when only one member of a pair received the hormone, pointing to non-verbal signaling and rapid behavioral feedback between animals.
“This society, which is often described as despotic, hierarchical, and regulated by aggression and submission, becomes more egalitarian. Everyone is a little nicer to everyone else,” says Michael Platt, a Penn Integrates Knowledge professor with appointments in the Perelman School of Medicine, the School of Arts and Sciences, and Wharton. “They synchronize their facial expressions and their behavior more tightly in time. In other words, they’re paying more attention to each other and when you do this, you get information more quickly and you respond more quickly.”
The study is the first to examine spontaneous social behavior after both inhaled and injected administration of oxytocin and vasopressin in non-human primates. The protocol involved treating one male macaque with oxytocin, vasopressin, or saline, then pairing him seven times: six sessions with different conspecifics and one session with an empty chair. Interactions occurred at a distance that prevented physical contact but allowed sight, sound, and smell. Each encounter lasted five minutes and was recorded; two independent observers scored behaviors frame by frame. Seven animals participated in the inhalation experiments and seven in the injection experiments.
Yaoguang Jiang, who has worked in Platt’s lab for more than two years, describes the shift in social balance: “Social dominance in monkeys is a really big deal. They live and breathe for it. But here, the curve got flattened. If you were in the middle, you stayed in the middle. If you were lower-ranking and used to be timid, you became a little more assertive. 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.”
Beyond reducing status differences, the hormones boosted behavioral synchrony within pairs—animals matched facial expressions and actions more closely in time. That synchrony appeared even when only one monkey received treatment, indicating that subtle, nonverbal cues and reciprocal feedback carried the effect across partners. This observation aligns with previous findings from Platt’s lab showing that oxytocin increases the duration of gaze and attention directed at other monkeys.
Interestingly, vasopressin produced effects similar to oxytocin. Because oxytocin and vasopressin receptors are distributed differently across brain regions and can interact, the researchers tested focal injections into the anterior cingulate gyrus (ACCg), a brain area linked to empathy and social evaluation. The ACCg is rich in vasopressin receptors but lacks oxytocin receptors, yet injections of oxytocin into this area reproduced behavioral changes. This suggests that oxytocin can act, at least in part, by binding to vasopressin receptors, complicating simple models of how these peptides shape social behavior.
“Our understanding of how all of this is going to work is much more complicated than originally thought,” Platt says. “We have to consider this whole other system, the vasopressin system.”
Clinical implications are suggested but remain tentative. Deeper knowledge of the mechanisms by which oxytocin and vasopressin influence social attention and synchrony could help guide treatments for social impairments in neurodevelopmental and psychiatric disorders, such as autism, schizophrenia, and bipolar disorder. The hormones also regulate feeding and social behavior, so they may be relevant for children who develop hypothalamic damage after pituitary surgery and then show severe overeating. Platt and colleagues are exploring some of these possibilities in clinical trials at the Children’s Hospital of Philadelphia.
This research builds on more than two decades of work with non-human primates in Platt’s lab. Rhesus macaques serve as valuable models because their social systems and interpersonal behaviors resemble those of humans in many respects: they live in large groups, form long-term bonds, and display clear dominance hierarchies. The parallel responses to oxytocin and vasopressin in macaques and humans underscore both the promise and the complexity of translating peptide-based interventions across species.
While these findings advance understanding of how peptide hormones reshape social interactions in real time, Platt emphasizes that much remains unknown. “We have a lot more to learn about how, when, and in what manner we use these peptide hormones to treat various problems,” he says.
Funding: Supported by the National Institute for Mental Health and the Simons Foundation for Autism Research.
Authors and Affiliations: Michael L. Platt is James S. Riepe University Professor with appointments in psychology, neuroscience, and marketing at the University of Pennsylvania. Yaoguang Jiang is a postdoctoral researcher in the Platt Labs.
Source and Publication: University of Pennsylvania. Results reported in Scientific Reports under the title “Oxytocin and vasopressin flatten dominance hierarchy and enhance behavioral synchrony in part via anterior cingulate cortex.” Published May 29, 2018. doi: 10.1038/s41598-018-25607-1
Oxytocin and vasopressin flatten dominance hierarchy and enhance behavioral synchrony in part via anterior cingulate cortex
The neuropeptides oxytocin (OT) and arginine vasopressin (AVP) influence social functions across mammals. In humans and rhesus macaques, intranasal OT can promote prosocial behavior in certain contexts, but mechanisms remain unclear. Treating male macaques with aerosolized OT relaxed spontaneous social interactions and reduced behavioral differences between dominant and subordinate individuals, flattening status hierarchies and increasing behavioral synchrony. Aerosolized AVP produced similar effects with greater efficacy. Focal injections of OT or AVP into the anterior cingulate gyrus (ACCg) replicated these outcomes; ACCg is rich in AVP receptors but lacks OT receptors, suggesting exogenous OT may act in part via nonspecific binding to AVP receptors. Both treated animals and their untreated partners showed altered behavior, consistent with enhanced reciprocal social feedback. These findings have important implications for basic research and potential therapeutic uses of OT and AVP in social impairments.