The study was conducted by an international team of scientists from institutions in Germany, Switzerland, France, the United States, and the United Kingdom, led by Dr Catia Correia-Caeiro from Leipzig University’s Institute of Biology and researchers from the Max Planck Institute for Evolutionary Anthropology.
One of their findings is that although bonobos display fewer facial movements than humans, they share a similar repertoire with another closely related species – chimpanzees. According to the researchers, this similarity points to the importance of facial communication in the social interactions of both species. To enable this comparison, the team extended the existing Chimpanzee Facial Action Coding System (ChimpFACS) to include bonobos. “This adaptation of ChimpFACS for bonobos fills an important gap in our ability to study facial expressions across different primate species,” says Correia-Caeiro. “We can now systematically compare facial movements in humans, chimpanzees and bonobos – providing new insights into the evolution of facial communication.”
The new coding system, which was originally developed for humans and previously adapted for nine other animal species, is not limited to academic research, according to Correia-Caeiro. “This tool will be especially valuable for assessing the welfare of bonobos in human care. By better understanding their facial expressions, we can more accurately evaluate their emotional states and overall well-being,” says the biologist. Facial Action Coding Systems (FACS) are tools used to measure animal facial behaviour in a detailed and objective way by coding independent muscle movements known as action units (AUs). The use of FACS has already led to several new findings in the field of animal communication – for example, that orangutans and gibbons use their faces flexibly and intentionally during play with others in order to influence the course of the interaction.
Bonobos and chimpanzees have similar facial muscles but differ in their facial morphology – for example, in species-specific features such as lip and eyelid colouring or the size of the brow ridge. These differences can affect how individual muscle movements are identified and measured. “They must be taken into account when studying facial expressions in both species – something that is possible with FACS,” explains Correia-Caeiro.
The human FACS has been used for over 40 years in research and in the creation of realistic facial expressions for animated films, as well as in clinical settings to help diagnose psychiatric and psychological conditions.
The researchers also recorded short video clips of individual muscle movements in bonobos to document the morphological differences in facial expressions between the two species. Future users of the tool can now be trained to recognise these muscle movements in bonobos and study their facial behaviour. “This tool will be available online and free of charge to anyone interested in learning more about bonobo facial expressions – even those with no prior experience with the species or the tool,” says Dr Correia-Caeiro. She adds that it can be used not only by researchers to explore empirical questions about the evolution of human communication and emotions, but also by veterinarians and animal care staff to help assess animal welfare.
Publication in PeerJ Journal:
"Adapting the facial action coding system for chimpanzees (Pan troglodytes) to bonobos (Pan paniscus): the ChimpFACS extension for bonobos", doi.org/10.7717/peerj.19484
