Human communication

Some monkeys might make a face

image: Marmosets have fewer facial movements than humans, but more than expected.
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Credit: KyotoU/Catia Correia-Caeiro

Kyoto, Japan — How could a human mother say her child is upset? Humans can distinguish the meaning of facial expressions from our fellow human beings either explicitly by speech or implicitly by context.

When decoding animal expressions, however, we face the challenge of interpreting familiar features in an appropriate context. For example, when we see smiles exposing the teeth in other humans, the correct emotion it is usually associated with is joy, while for other primates it is most likely an expression out of fear or submission.

A research team from Kyoto University has reoriented the human facial action coding system, or FACS, as a tool for systematic cross-species comparisons of facial muscles to help interpret the resulting expressions. Contractions of facial muscles move parts of the skin, producing a pattern of visible changes in appearance on the face.

In turn, these changes act as cues for specific movements that FACS helps identify. FACS analyzes and classifies the visible movements performed by the facial muscles into so-called share unitsor to.

“Marmosets exhibit socio-ecological characteristics and primitive facial musculature that would suggest lower facial mobility than other primates. But surprisingly, when we developed an adapted version of FACS, there was actually little difference,” explains lead author Catia Correia-Caeiro.

The CalliFACS tool, named after the common marmosets, Callithrix jacchus, detected 33 facial movements, fifteen of which were AU. This means that the facial mobility of marmosets is lower than that of humans but similar to that of other primates studied, namely chimpanzees, orangutans, rhesus macaques and gibbons.

It is surprising that the common marmoset possesses a surprisingly high number of facial movements. For the team, this suggests that facial expressions are older and more prevalent in social animals than previously thought.

CalliFACS has clearly shown us how complex animal communication can be,” the author presumes.

According to Correia-Caeiro, the development of a FACS for the common marmoset as the first New World primate is significant. “This has furthered the expansion of research into the evolution of human communication and emotions by comparing us to a more distant primate in studies of facial expression,” concludes the author.


The article “CalliFACS: The Common Marmoset Facial Action Coding System” appeared on April 19, 2022 in PLOS Onewith doi: 10.1371/journal.pone.0266442

About Kyoto University

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