HMN 2026: How Great apes mirror facial expressions with surprising precision,

New research from the University of Portsmouth has found that great apes exhibit exactness in mimicking one another’s facial expressions in social contexts. The study, published in Scientific Reports, explored how orangutans and chimpanzees mirror expressions during social interactions, particularly laugh faces, drawing comparisons with human behaviors such as the Duchenne smile—a genuine smile that engages both the mouth and eyes.

The research reviewed the behaviors of 96 great apes and investigated whether the animals mirrored each other’s laugh faces within three seconds, examining both with and without their upper teeth exposed. It analyzed the behaviors of 39 orangutans and 57 chimpanzees living in eight separate groups, comprising a mix of ages and genders.

The study showed that the animals were most likely to mirror each other’s expressions when the upper teeth were not exposed, as this is a gentler expression. These accurate ways of matching can help people better understand others’ emotions and predict how they might behave.

Measuring this mimicry provides scientists with unique insights into face-to-face exchanges, allowing them to test how exact the responses of primates really are. While humans are known for having a precise and versatile language, this study suggests that great apes 10–16 million years ago already matched the level of facial communication exactness seen in humans—an ability that gave them important advantages.

The research also explored play duration and play intensity, analyzing whether the length of a play session was linked to how likely the animals were to mirror their playmate’s expression.

When orangutans engaged in longer play sessions, they showed a greater precision in mirroring their playmate’s expression.

Lead author Diane Austry, who completed her Masters at the University of Portsmouth, said, “What we found was remarkable—these animals don’t just replicate a general expression, they copy the exact same facial movement pattern their social partner uses, much like when a human mirrors another person’s natural Duchenne smile.

“The apes showed the same pattern for both automatic rapid responses as well as more delayed responses.”

Dr. Marina Davila Ross, co-author and Associate Professor in Comparative Psychology at the University of Portsmouth, said, “This level of explicit replication wasn’t previously known across the great apes. What’s more, these expressions are shared with humans—previous research measuring muscle activity shows that chimpanzees and humans use the same muscles to produce laugh faces. This points to a real complexity in positive communication across great apes.”

While both orangutans and chimpanzees showed such levels of exactness in their facial responses, they seemed to differ in how this was done. The chimpanzees appeared to avoid mimicking the upper tooth rows of their social partners in order not to signal risky play, which might be disadvantageous to match, at least when interacting with older partners or males.

Dr. Davila-Ross from the University of Portsmouth’s School of Psychology, Sport and Health Sciences and Center for Comparative and Evolutionary Psychology added, “In the future, we are interested in exploring how these expressions might be used beyond play.

“The concept of evolutionary continuity is fascinating—in humans, smiles serve many purposes, from expressing happiness to conveying mockery, so it would be interesting to examine these expressions outside of a play context.”

The orangutans involved in this study lived at the Sepilok Orangutan Rehabilitation Center (SORC) near to Kabili-Sepilok Forest Reserve in Malaysia. The chimpanzees lived in large outdoor enclosures in the Chimfunshi Wildlife Orphanage (CWO) in Zambia.

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