Our attempts to communicate with our ape cousins with sign language have long captured our imagination, but surprising new research shows that chimpanzees and bonobos share common gestures and meanings. That's fascinating in itself, but it raises intriguing questions as to how sign language is inherited, and whether humans share any gestures and meanings with other apes.

Scientists already knew that chimps and bonobos share around 90 percent of their gestures (compared to about 80 percent for chimps and orangutans and 60 percent for chimps and gorillas). However, this statistic doesn't take meaning into account — a chimp may mean one thing by a particular gesture while a bonobo may mean something else entirely.

But in a new analysis of the meanings of gestures used by the two species, the researchers found that the overlap in meaning was "much greater" than would be expected by chance.

It's increasingly apparent that when great apes (bonobos, gorillas, chimps and orangutans) communicate, they're able to identify a recipient, gauge how much attention they're paying, choose gestures accordingly and wait for a response. If none is forthcoming, they can repeat the gestures, even elaborating with other signals if necessary. By gauging the recipient's response, and the signalling ape's satisfaction with the result, the researchers were able to gauge the meaning of gestures when used by several individuals.

Assessing the species overlap isn't straightforward. Bonobos and chimps use over 70 distinct gestures, and capturing robust data is harder in the wild than in captivity, where gestures tend to be focused on play.

The team analysed 2,321 instances of gesture use in bonobos with a successful outcome for the communicating ape. These gestures covered meanings as diverse as "give me food" (or an object), "climb on me," "follow me," "come closer," "stop what you're doing" to several unfit for reading in polite society.

Some gestures could result in two or more possible outcomes, implying the gestures have multiple meanings, just as words can. The researchers posit that context, facial expressions or the sequence of gestures may help to put across the specific intended meaning.

By performing statistical analyses on gestures where at least three individuals had used a gesture at least three times, the team compared the outcomes with what one would expect to see if those outcomes had been random. The analysis showed that bonobos gestures clearly have distinct sets of meanings.

Further, by comparing the data from bonobos with similar data for chimpanzees, the team could say with confidence that bonobos and chimps share more gestures with common meanings than would be expected by chance. In other words, bonobos and chimps share common gestures which sometimes have the same meanings.

More research is needed to establish how the species came to share gestures, but it's possible that the gestures were shared by a common ancestor. This raises the intriguing possibility that this ancestor could be the common ancestor both species share with humans. The researchers actually think this is more likely than the gestures emerging after the Pan genus (which includes bonobos and chimpanzees) split from the Homo genus which has led to human beings.

"In future, we hope to learn more about how gestures develop through the apes' lifetimes," says Kirsty Graham, lead author of the research. "We are also starting to examine whether humans share any of these great ape gestures and understand the gesture meanings, so watch this space."

The team notes that great apes also communicate vocally, but this was not the focus of the research.

The team's paper, Bonobo and chimpanzee gestures overlap extensively in meaning, was published in PLOS Biology and can be viewed online in full. If you'd like to see videos of some of the ape gestures in question, just visit The Great Ape Dictionary.

Source: University of York

View gallery - 6 images