Chimpanzee Sign Language Study Reveals Uncanny Parallels to Human Speech

"This is another piece in the puzzle."

When a chimpanzee wants to flirt, it nibbles on a leaf. A request to be groomed is more direct: it will show exactly where it wants a pinch. Scientists have identified 66 gestures that chimps use to communicate, theorizing that they resemble human language. Now, a new study shows these gestures actually follow human linguistic rules, revealing how our own language evolved.

In a paper released Wednesday in the journal Proceedings of the Royal Society B, international scientists report that the chimpanzees of the Sonso community in Uganda’s Budongo Forest Reserve, who they have observed for a long time, use language that is mathematically similar to our own. Co-author Cat Hobaiter, Ph.D., a lecturer at the University of St. Andrews, observed these wild chimps while conducting the first systematic study of their gestures.

“It’s an incredible privilege to spend time with them — observing their lives in the forest; getting to know them as individuals,” Hobaiter tells Inverse. “When you first work with chimps there’s a lot of noise and chaos! But with more time, you get to see that they use gestures to communicate all of their day-to-day requests, like ‘Come here,’ ‘I want that,’ ‘Get lost!’”

Chimpanzee gestures adhere to human linguistic laws.

In this study, Hobaiter and her colleagues recorded the chimpanzees using gestural communication during social play. In these moments, two or more chimps might laugh, wrestle, chase, tickle, or play-bite. In the 359 video clips that resulted, they looked for gestures that met at least one of the key criteria for intentional communication: sensitivity for the receiver’s attentional state, response waiting, or goal persistence.

Doing so identified 58 different types of “play” gestures. Further examination revealed that the most commonly used gestures were short, quick actions, and longer gestures were typically broken up by multiple shorter gestures. These gestures, the team argues, adhere to two firmly established laws of human linguistics, which apply to all human languages: Zimph’s law of abbreviation, which states that the words used most frequently in speech are shorter, and Menzerath’s law, which states that all longer words consist of shorter syllables.

Because these linguistic laws apply to all human languages, their presence in chimp communication demonstrates that both communication systems are underpinned by the same mathematical principles, the team argues. This finding not only advances our understanding of how great apes communicate but also increases our understanding of how humans developed language in the first place.

“A lot of research has focused on exploring the similarities between primate vocalizations and language, but we’ve been able to show that there are several unique features in ape gestures,” Hobaiter says. “That means they probably played an important role in early hominin communication. This is another piece in the puzzle — some of the fundamental ways in which language is organized is shared with chimpanzee gesture!”

Now, the team hopes to examine chimpanzees while they communicate out of the context of play and to also watch the communication styles of bonobos — another close relative of ours. Chimpanzees, bonobos, and humans share 98.8 percent of their DNA, so it stands to reason we may all share communication techniques as well.