Bonobos use a kind of syntax once thought to be unique to humans

Bonobos combine their calls in a complex way that forms distinct phrases, a sign that this type of syntax is more evolutionarily ancient than previously thought.

Human language, often described as the hallmark of our species, is made up of many different building blocks. One core block is syntax, where meaningful units are combined into longer sequences, like words into sentences. This is made possible through compositionality, where the meaning of the whole is derived from the meaning of the parts.

Compositionality in itself isn’t unique to humans. For example, chimpanzees combine calls to warn others of snakes. But, so far, only “trivial compositionality” has been identified in non-human animals, whereby each unit adds independently to the meaning of the whole. For example, the phrase “blonde dancer” has two independent units: a blonde person who is also a dancer. Humans were thought to be unique in also having “non-trivial compositionality”, where the words in a combination means something different to what they mean individually. For example, the phrase “bad dancer” doesn’t mean a bad person who also dances.

The issue was that biologists didn’t have the tools to assign a clear meaning to animal vocalisations, says Mélissa Berthet at the University of Zurich in Switzerland, so they couldn’t be certain if a combination was trivial or non-trivial.

Berthet and her colleagues spent years learning and tweaking methods from linguistics to try to find unambiguous evidence of non-trivial compositionality in our closest living relatives. This first involved spending five months following 30 adult bonobos in the Kokolopori Bonobo Reserve in the Democratic Republic of the Congo, recording almost 1000 instances when a bonobo called out. Of these utterances, roughly half were combinations where at least two different call types were paired together in quick succession.

In a new step, the researchers noted everything that was happening at the time of the call and in the minutes after. They recorded over 300 of these observations, including what the caller was doing at the time, what was happening in the environment and the behaviour of the caller and audience after the vocalisation.

To reveal the meaning of each call, they used a technique from linguistics to create a cloud of utterance types, placing vocalisations that occurred in similar circumstances closer together. “We kind of established this dictionary,” says Berthlet. “We have one vocalisation and one meaning.”

Once they had this semantic cloud, they could see whether the individual calls in a combination had distinct meanings, and found that the combinations were close to the units that they were made of, which would suggest compositionality. Using this approach, they identified four compositional calls, of which three were clearly non-trivial, with their meanings not directly overlapping with their constituent parts. For example, “high-hoot + low-hoot” combines the calls that seem to mean “pay attention to me” and “I am excited” to say “pay attention to me because I am in distress”, which bonobos often used to call for support when another individual was intimidating them.

Almost all of the bonobos’ chatter was about coordinating the group, says Berthlet. Team member Martin Surbeck at Harvard University thinks this is because bonobos have a fission-fusion group dynamic, where smaller breakaway groups can do their own thing.

“It’s the first time in any animal species that there is an unambiguous evidence for non-trivial syntax, non-trivial compositionality, and so that changes the game,” says Maël Leroux at the University of Rennes in France. “It’s revolutionary. It’s the cornerstone for the next decade of comparative linguistics, basically, and evolutionary linguistics.”

This finding doesn’t mean that bonobos have language, though, because language is the human communication system, says Berthet. “But we’re showing that they have a very complex communication system that shares parallels with human language.”

Now we have evidence that both chimps and bonobos have syntax, it is inevitable that this capacity for compositionality was inherited from our last common ancestor, says Leroux. “They just showed, unambiguously, that this core building block is evolutionary ancient and at least 7 million years old, and maybe even older.”