Chimps and humans share a surprising amount in common when it comes to communication. Researchers studying the facial expressions of chimpanzees at play reveal that they make the same “laugh face” when they’re laughing out loud and when they’re silently laughing -- just like us. The work, published in PLoS ONE last week, suggests that cracking up was used by our common ancestor as a positive expression. 

“Humans have the flexibility to show their smile with and without talking or laughing,” University of Portsmouth’s Marina Davila-Ross says in a news release. “This ability to flexibly use our facial expressions allows us to communicate in more explicit and versatile ways, but until now we didn’t know chimps could also flexibly produce facial expressions free from their vocalizations.”

Davila-Ross and colleagues filmed 46 chimpanzees as they played with each other at the Chimfunshi Wildlife Orphanage in Zambia. The male and female chimps ranged in age from 2 to 35 years old, and about half of them were born in the wild. The team was able to identify 1,270 “open-mouth faces” in the videos: 44 chimps produced 697 open-mouth faces with laughter, 41 produced 573 silent open-mouth faces. Little differences in these facial expressions include raising the upper lips to expose teeth, sticking out the tongue, and dropping the jaw. The researchers then used a system called ChimpFACS to measure the apes’ facial movements in the video recordings and compare them to our own facial expressions and their underlying musculature. 

They discovered that chimps produced the same 14 configurations of open-mouth faces with sounds accompanying the laugh and when laugh sounds were absent. 

Furthermore, the chimps seem to have different uses for laughing out loud and laughing silently: Laughter accompanies open-mouth faces during highly interactive social play. That includes physical contact with playmates and also when they try to match the faces of their playmates. 

The faces we make when we chuckle and cackle, the findings suggest, gradually emerged from laughing, open-mouth faces of ancestral apes. The flexibility in facial expressions was already present long before our species evolved. Although, there are still key differences between how humans and our ape ancestors laughed, Davila-Ross adds: “Chimps only rarely display crow’s feet when laughing, but this trait is often shown by laughing humans. Then, it is called Duchenne laughter, which has a particularly positive impact on human listeners.” 

Image Credit: University of Portsmouth (middle)