SEX in bonobos: The intensity of sexual stimulation sharply drops after facial mimicry

Nonverbal communication plays a central role in human sexual interactions, enhancing sexual satisfaction and perceived partner connectedness. Here, we investigate the potential evolutionary roots of visual communication by examining the role of facial displays in sexual interactions in bonobos (Pan paniscus), a species in which - unlike many others - sexuality serves both reproductive and social functions. Using high-resolution temporal analyses, we tested whether variations in the rate of the oscillatory movements during sex (a proxy for sexual stimulation intensity) are associated with the exchange of facial expressions, specifically silent bared-teeth (SBT) displays and rapid facial mimicry (RFM). Analyses of sexual sessions free from interference by third-parties revealed that the offset of unilateral SBTs was not followed by a significant decrease in the oscillatory move- ment rates, whereas the termination of RFM consistently triggered a decline in sexual stimulation. Restricted temporal windows surrounding RFM cessation further confirmed that disruption of facial mimicry by even one partner suffices to decrease stimulation intensity. These results indicate that RFM, rather than passive obser- vation of a partner’s expression, marks the peak of sexual stimulation and reflects fine-grained socio-emotional coordination between partners. Our findings suggest that rapid facial exchanges during sexual interactions in bonobos may represent an evolutionarily conserved mechanism linking nonverbal communication and coordi- nation in social actions. This work provides comparative insights into the communicative functions of facial expressions, highlighting parallels between humans and one of our closest phylogenetic relatives.