Grunts, barks, screams and pants ring through Taï National Park in Cȏte d’Ivoire. Chimpanzees there combine these different calls like linguistic Legos to relay complex meanings when communicating, researchers report May 9 in Science Advances.
Chimps can combine and flexibly rearrange pairs of sounds to convey different ideas or meanings, an ability that investigators have not documented in other nonhuman animals. This system may represent a key evolutionary transition between vocal communication strategies of other animals and the syntax rules that structure human languages.
“The difference between human language and how other animals communicate is really about how we combine sounds to form words, and how we combine words to form sentences,” says Cédric Girard-Buttoz, an evolutionary biologist at CNRS in Lyon, France.
Chimpanzees (Pan troglodytes) were known to have a particularly complicated vocal repertoire, with about a dozen single sounds that they can combine into hundreds of sequences. But it was unclear if the apes used multiple approaches when combining sounds to make new meanings, like in human language.
In 2019 and 2020, Girard-Buttoz and his colleagues recorded 53 different adult chimpanzees living in the Taï forest. In all, the team analyzed over 4,300 sounds and described 16 different “bigrams” — short sequences of two sounds, like a grunt followed by a bark, or a panted hoo followed by a scream. The team then used statistical analyses to map those bigrams to behaviors to reveal some of the bigrams’ meanings.
The result? Chimpanzees don’t combine sounds in a single, consistent way. They have at least four different methods — a first seen outside of humans. For instance, they can combine sounds A and B to get a new meaning, C. Adding a sound can also modify the meaning of another sound, a bit like adding a suffix or a prefix. The order of sounds can also make a difference. A “hoo + grunt” is mostly associated with feeding or rest. A “grunt + hoo” is made mostly during travel or the merger of chimp groups.
Previous work delving into the evolutionary origins of language in nonhuman animals had shown that species typically have a limited ability to combine sounds to expand options for communication: They rely on only one strategy. And their combinations of sounds are mostly used in relation to a particular event, like a predator encounter. Such situations are very dangerous, so the alarm signal to others in the group has to be quite specific.
“In the chimpanzee, it looks as if they use [combination] much more broadly, across a vast diversity of daily life situations,” Girard-Buttoz says. “These combinations are potentially also to communicate about more than one thing at the same time, exactly what we do in a sentence.”
The findings suggest that chimpanzees can go beyond a limited alphabet of sorts to communicate richer, more detailed messages. For example, the bigram “hoo + pant” has a very precise meaning, and seems to signal making a nest in a tree away from predators, rather than resting on the ground.
“It’s a super exciting advance of the field,” says Simon Townsend, an evolutionary anthropologist at the University of Zurich not involved with the new research. “The chimp combinatorial vocal system is more complex than we previously thought.”
Townsend and his colleagues published a study in April this year in Science that took a similar approach to understanding communication in chimpanzees’ close relatives, bonobos (P. paniscus). Bonobos combine calls, with one call modifying the meaning of the other call it’s paired with. What chimpanzees are doing is even more complex, Townsend says, with a bigger bag of linguistic tricks.
There’s been more research on chimpanzee vocal communication than on bonobos, Girard-Buttoz says, so there are more data and a greater understanding of the meaning behind the sounds they make.
“I wouldn’t be surprised if the bonobos have the same systems,” Girard-Buttoz says.
These apes “have the preliminary building blocks” of complex language, Girard-Buttoz says. In humans, such combination of sounds “exploded,” and we use it to create a veritable deluge of potential meanings.
Girard-Buttoz says he and his team are now trying to see if the chimpanzees organize calls in a sentence-like structure, with for instance a subject first, then a verb. They’re also interested if the apes are embedding bigrams in longer sequences three or four sounds long.