REPUBLIKA.CO.ID, WASHINGTON -- Just like humans, chimpanzees can smile without making a laughing sound, a new study revealed Wednesday.

The new findings, published in the U.S. journal PLOS ONE, suggested that chimpanzees' communication is more similar to humans than was previously known.

"Humans have the flexibility to show their smile with and without talking or laughing," lead author Marina Davila-Ross from Britain's University of Portsmouth said in a statement.

"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."

In the new study, Davila-Ross and her colleagues filmed 46 chimpanzees at the Chimfunshi Wildlife Orphanage in Zambia and used a facial action coding system called ChimpFACS to measure their subtle facial movements.

The study showed that chimpanzees produced the same 14 types of "laugh faces" when laugh sounds were present as when they weren't.

Furthermore, the study indicated that the facial expression plus vocalization, as well as the facial expression alone, were used differently in social play, for instance, when in physical contact with playmates and when matching playmates' open-mouthed faces.

The findings led the researchers to suggest "laugh faces of humans must have gradually emerged from laughing open-mouth faces of ancestral apes."

There are still key differences between humans and our ape ancestors, Davila-Ross noted.

"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," she said.