Scientists crack Bonobos’s facial expression code

According to new research, Bonobos can make 28 different facial movements, from subtle nostril flares to dramatic lip leaks.

This study represents the first comprehensive analysis of Bonobo’s facial expressions using standardized scientific methods.

An international research team successfully adapted the Chimpfacs’ facial motion coding system (Chimpfacs) in collaboration with Bonobos to create the content of these endangered great apes’ facial expression dictionary. This work fills a critical gap in primate communication research and provides new tools for understanding how Bonobos navigates in its well-known peaceful societies.

Something beyond the eyes

The researchers found that Bonobos had 22 different muscle-driven facial movements, plus three other action descriptors and three ear movements. Although this represents less expression than humans can produce, it matches the chimpanzee facial repertoire and shows significant complexity in Bonobo communication.

What makes this study particularly interesting is how it reveals the anatomical basis of Bonobo expression. The team found that Bonobos can independently control specific facial muscles, resulting in subtle but meaningful differences in their expression. For example, they can lift only the inner eyebrows or independent nostrils, which requires precise muscle control.

The study also found several facial movements in BONOBOS that were not documented in the original chimpanzee system, including what the researchers called “Glabella reduction”, which is actually a frowning movement between eyebrows that is more common in Bonobos than Chimpanzes.

The Science Behind BONOBO Smile

One of the most fascinating discoveries involves how Bonobos communicates with their unique bright pink lips. The researchers found that contrasting lip colors actually make certain facial movements more pronounced and easier to detect than chimpanzees, which may give Bonobos an advantage in visual communication.

“This adaptation to Bonobos fills an important gap in our study of facial expressions in different primates,” explains Dr. Catia Correia-Caeiro, principal investigator at the University of Leipzig. “Now, we can systematically compare facial movements between humans, chimpanzees and BONOBOS to provide insights into the evolution of facial communication.”

The team analyzed more than 500 videos totaling approximately 55 hours and examined BONOBOS in various settings in the Democratic Republic of the Congo. They recorded each facial movement through the frame, linking specific muscle contractions to visible changes in the appearance of the facial.

Understand the impact of a peaceful society

The timing of this study is particularly important given the ongoing debate on BONOBO’s social behavior. While Bonobos are generally considered quieter than chimpanzees, recent research challenges this view by documenting higher male aggression than previously thought.

The new facial coding system can help resolve these ambivalent discoveries by providing objective measures of emotional expression in different social interactions. Previous studies rely on subjective descriptions of facial behavior, so it is difficult to compare results from different study groups.

This study reveals important differences in how bob bones and chimpanzees use facial expressions. Some facial movements that are difficult to detect in chimpanzees seem to be more prominent in Bonobos, potentially reflecting their different social structures and communication needs.

Technological breakthroughs and challenges

One of the most important technical achievements of the study is to establish reliable methods to distinguish similar facial movements. For example, researchers must develop precise criteria to distinguish between true eyebrow rise and simple reduction of eyebrow movement release, i.e., this finding that requires frame-by-frame video analysis.

Bonobos’ dark skin tone presents a unique challenge for the research team, making it harder to detect certain skin primates with lighter facial wrinkles and skin movements. However, the researchers developed solutions by focusing on motion patterns and using high-definition recording devices.

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The new system provides immediate practical applications for Bonobo benefits in captivity. “This tool is particularly valuable for evaluating Bonobos welfare in human care,” noted Dr. Correia-Caeiro. “By better understanding of their facial expressions, we can more accurately assess their emotional state and well-being.”

Perhaps more importantly, this study opens the door to comparative studies of all great ape species. With the standardized facial coding system now available for humans, chimpanzees, bonobos, orgutans and Gorillas, scientists can begin to draw the evolutionary history of facial communication with unprecedented details.

Researchers suggest that their findings may help explain why facial communication is so important in primate society. The ability to generate and interpret complex expressions may be crucial to managing complex social relationships in the traited Great Ape community.

This work represents not only a catalogue of facial movements, but also an understanding of how our closest relatives experience and communicate emotions, bringing new windows into the minds of some of the most endangered species on our planet.

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