New study reveals that humans can understand the sign language of apes: 'It's really incredible'

Despite not utilizing gestures, humans can comprehend the signs made by other great apes. This understanding is either inherited or a component of more general cognition, per a study published on January 24, 2023, in the journal PLOS Biology. One of these signals is the “big loud scratch,” which means “groom me” and is used by apes to brush dirt or insects from each other’s hair. The "directed push" signals "climb on my back" for bonobos or "move to a different position" for chimpanzees, while the "object shaking" signs "intercourse," "groom me," or "move away." The study reversed a video playback technique traditionally used to assess language comprehension in non-human primates, as reported by CNN. 

The study has found that Chimpanzees and bonobos are the closest living relatives of humans, sharing more than 90% of their gestures. Primatologist Catherine Hobaiter, a principal investigator at the university's Wild Minds Lab and research scholar Kirsty E. Graham of the University of St. Andrews in Scotland say that these gestures may have played a significant role in the framework of the evolution of human language. More than 50 gestures from the ape repertoire have been found to use by infants between the age of 1 to 2. Therefore, it was assumed that humans may have retained their knowledge of ape gestures.

About 5,656 participants watched 20 short videos of ape movements and then responded to multiple-choice questions to select the correct meaning of these gestures. The participants effectively understood the chimpanzee and bonobo gestures with over 50% accuracy, which is double what was predicted, Graham wrote to CNN via email on Thursday. The deciphering of gestures such as the “mouth stroke” (meaning “give me that food”) and “big loud scratch,” had over 80% success. “That our participants were able to interpret primate signals complements recent findings that suggest humans may be able to perceive affective cues in primate vocalizations,” the study authors explained. “These gestures are shared by all other great ape species,” Graham told CNN, “and if humans understand them, then it seems like a great ape gesture ability that would have been used by our last common ancestors.”

However, researchers emphasized that the underlying mechanism through which humans can understand apes is still a mystery. Some possibilities for this conundrum include the biological inheritance of the great ape repertoire, the similarity in general intelligence to interpret signals and the fact that humans and apes share body plans and social goals. “We need to examine how participants understood the gestures — do humans inherit a vocabulary or a capacity, or are we reasoning our way through it? It’s a big question that will require a variety of approaches to tackle,” Graham said. “But this experiment is an important proof of concept and from here, we can play around with the information that participants receive and ask more about how they are interpreting these gestures. We are also studying different communities of great apes in the wild to get a fuller picture of their gestures.”

Also, the researchers highlighted that although the meanings of the ape species' gestures have not yet been interpreted by humans, the movements and gestures of gorillas and orangutans gestures could be well understood by humans. “Dogs are interesting too because we aren’t that closely related, but we have domesticated and co-evolved with them over tens of thousands of years.” Graham added, “so how we communicate with them can be very instructive for researchers too.” Dr. Catherine Hobaiter, the co-author of the study, said: "On one hand, it's really incredible that we're able to do this - Kirsty and I have spent years living in the forest with chimpanzees and bonobos and working hard to study their communication."