When Faces Fall Out of Sync: What Bonobo Sex Tells Us About the Evolution of Nonverbal Communication
– by Martina Francesconi & Elisabetta Palagi
Sex is often thought of as a purely physical activity, driven by movements, touch, and physiological arousal. Yet in humans, sexual interactions are also deeply communicative. Eye contact, facial expressions, and subtle emotional cues shape how partners experience intimacy, influencing feelings of pleasure, connection, and satisfaction. Nonverbal communication, in particular, appears to play a central role: during sex, faces and bodies often “speak” more loudly than words.
But are these communicative dimensions of sexuality uniquely human? Or do they have deeper evolutionary roots?
To explore this question, we turned to one of our closest living relatives: the bonobo (Pan paniscus). Bonobos are famous for their rich sexual repertoire. Unlike most other primates, they engage in sexual interactions across ages and sexes, and not only for reproduction. Sex in bonobos also serves important social functions, helping to reduce tension, repair relationships, and strengthen social bonds. This makes them an ideal model species for investigating the evolutionary origins of sexual communication.
In humans, facial expressions during sex are often interpreted as spontaneous manifestations of pleasure. Yet growing evidence suggests that they may also play an active communicative role, helping partners coordinate emotionally and behaviorally. Testing this idea directly in humans is extremely difficult for obvious ethical and practical reasons. In non-human primates, observing spontaneous sexual interactions with fine-grained temporal precision is simply not feasible. Bonobos offer a rare opportunity to overcome this limitation. Their sexual interactions are frequent, often face-to-face, and occur in a social context where detailed behavioral observation is possible. Importantly, bonobos regularly display a facial expression known as the silent bared-teeth display (SBT) during sex. This expression can be produced by one partner alone, or reciprocated by both partners in a rapid, automatic exchange known as Rapid Facial Mimicry (RFM), where one individual mirrors the other’s facial expression within less than a second.
This distinction allowed us to ask a key question: is it simply seeing a partner’s facial expression that matters during sex, or is it the mutual exchange, the moment when both partners’ faces fall into synchrony?
To address this question, we analyzed sexual interactions in a captive bonobo colony using high-resolution video recordings. Rather than focusing on outcomes such as mating success, we examined the moment-to-moment dynamics of sexual interactions. Specifically, we used the rate of rhythmic pelvic or genital movements as a proxy for the intensity of sexual stimulation. Faster, more frequent movements indicate higher levels of stimulation, while slower rates suggest a decline. We then examined how these movement patterns changed before, during, and after different facial expression conditions: no facial expressions, unilateral SBTs, and reciprocal facial mimicry (RFM). Crucially, our analyses focused not just on whether these behaviors co-occurred, but on timing. We asked what happens immediately when facial mimicry begins, and what happens when it ends.
Our results revealed a striking pattern. Sexual interactions involving rapid facial mimicry were characterized by the highest levels of stimulation. When both partners mirrored each other’s facial expressions, the rate of rhythmic movements reached a peak. Even more telling was what happened when this mimicry stopped. As soon as facial synchrony was disrupted, when even one partner ceased to mirror the other, the intensity of sexual stimulation sharply dropped. This decline occurred rapidly, within seconds, and persisted even when we excluded interactions interrupted by third parties or external disturbances.
By contrast, unilateral facial expressions told a different story. When only one partner displayed the silent bared-teeth expression, without being mimicked, stimulation levels did not show a consistent or robust change. In other words, expressing a facial signal was not enough. What mattered was sharing it.
These findings suggest that rapid facial mimicry does not merely reflect pleasure as a byproduct of sexual stimulation. Instead, it appears to mark moments of fine-grained socio-emotional coordination between partners.
One possible interpretation was that facial mimicry might serve as an anticipatory signal, communicating motivation to increase stimulation. However, our data did not support this idea. Stimulation did not increase after facial mimicry occurred. Rather, it was highest during mimicry and declined immediately once it ended. This pattern indicates that RFM marks the peak of sexual coordination rather than signaling a future escalation. In short, when partners are emotionally and behaviorally aligned, stimulation is highest; when facial synchrony breaks down, so does intensity. Whether these peaks in stimulation correspond to orgasm or other reward states remains an open question. Nonetheless, our findings have broader implications. Rapid facial mimicry is observed in bonobos across multiple social contexts, including social play, and is thought to reflect shared emotional states and automatic emotional resonance.
Our study suggests that such facial coordination mechanisms may also play a role in regulating sexual interactions. Given the importance of sex in bonobo social life, not only for reproduction but also for maintaining tolerance, cooperation, and social bonds, selection may have favored individuals who could finely tune their behavior to their partners’ emotional signals.
From an evolutionary perspective, this points to a deep-rooted link between nonverbal communication, emotional synchrony, and coordinated social action. The communicative role of facial expressions during human sexual interactions may therefore not be a recent cultural invention, but part of an ancient primate toolkit for social connection.
In both bonobos and humans, it seems that during intimate moments, being in sync matters as much as, if not more than, the movements themselves.
