The study was led by Martina Francesconi and Elisabetta Palagi from the University of Pisa, in collaboration with researchers from Belgium, Italy, and beyond. The study is based on detailed video recordings of bonobos in zoos, analyzed frame by frame. The focus lay on the tempo of repetitive movements during sex and the role of facial expressions, such as the "silent bared-teeth display" (grin).
"The concept of 'rapid facial mimicry' proved essential here: bonobos often imitate each other's expressions almost instantaneously. However, this effect is only powerful enough to observe when the imitation happens immediately. The fact that this response is so lightning-fast and synchronized raises the fundamental question of whether we are looking at conscious communication or an involuntary reflex," says Martina Francesconi of the University of Pisa.
The analysis shows that the tempo of movements is remarkably high, averaging around seven movements per second. Notably, this tempo does not necessarily increase when one of the animals begins to smile or when that smile is imitated by the partner. What researchers did establish, however, is that the tempo drops significantly shortly after the cessation of rapid facial mimicry.
"This suggests that these facial expressions likely do not play a role in coordinating or accelerating the behavior," says Yannick Jadoul (VUB). "But the sharp decrease in tempo afterward could potentially be linked to physiological processes, such as orgasm. We cannot say that with certainty based purely on observational data, but the pattern is striking."
"Our study is part of a broader field investigating how rhythm, tempo, and vocalization evolved in humans and animals," concludes Professor Palagi of the University of Pisa. "Bonobos, along with chimpanzees, are our closest living relatives. Unlike chimps, they live in a matriarchal society where sex plays a vital social role. By comparing their behavior with humans and other species, we hope to better understand which elements of communication are uniquely human and which building blocks are much older in our evolutionary history."
Within this project, Yannick Jadoul primarily provided expertise in data analysis and the quantification of rhythm. "This is not a study involving artificial intelligence in the sense of large language models or neural networks," he emphasizes. "It is about rigorous data analysis and pattern recognition. But it is precisely these kinds of methods that are crucial for objectively measuring complex behaviors."
This research aligns with a long-standing tradition at the VUB regarding language, communication, and learning systems. By systematically comparing rhythm and tempo across species, researchers aim to unravel, step by step, how traits we consider 'typically human'—such as music, speech, and social communication—first emerged.
Reference
Martina Francesconi, Alice Galotti, Yannick Jadoul, Federico Giovannini, Andrea Ravignani, Elisabetta Palagi 2026 SEX in bonobos: The intensity of sexual stimulation sharply drops after facial mimicry Evolution and Human Behavior , Volume 47, Issue 1. https://doi.org/10.1016/j.evolhumbehav.2025.106786
Evolution and Human Behavior