The Calls of Bonobos Show Compositionality in Their Calls

After listening to hundreds of hours of ape calls, Dr. Townsend and his team of scientists say they have detected something distinctive in the human language: the ability to put together strings of sounds to create new meanings.

To convey specific meanings, numerous animals use a wide range of sounds and acoustic signals for communication. When an animal makes a sound, it usually means just one thing. For example, monkeys make one warning call if a leopard appears, and a different one if there is an incoming eagle.

In contrast, humans string words together to combine their individual meanings into something new. Linguists call this compositionality and have long considered it to be an essential part of language, for it allows us to understand complex expressions, such as “She is a bad dancer,” based on our understanding of the individual words and their arrangement. “It’s the force behind language’s creativity and productivity,” Simon Townsend, a comparative psychologist at the University of Zurich, said to The New York Times. “Theoretically, you can come up with any phrase that has never been uttered before.” Scientists have found no clear sign of compositionality in other species for decades. However, Dr. Townsend and his colleagues discovered a hint of it in chimpanzees a few years ago.

In a Ugandan forest, Dr. Townsend and his team recorded more than 330 hours of chimpanzee life and identified 15 distinct calls. They noticed that certain calls followed others more than would be expected. The scientists pondered if a pair of calls meant something different than two calls on their own.

To test this hypothesis, they spent two years studying one pair in particular: a call known as “waa-bark”, followed by another known as “alarm-huu”. To bring other chimpanzees to them, chimpanzees make the waa-bark call. For example, they could use it to bring allies during a fight. They make the alarm-huu call when frightened or surprised.

Simon Townsend and his colleagues wondered if “alarm-huu” was followed by “waa-bark”. They noticed that the chimpanzee made the calls when it encountered a snake while other chimpanzees were within earshot.

To test this, experiments followed. One time, the researchers put a fake snake on a trail as chimpanzees passed by. As predicted, they often responded with “alarm-huu” followed by “waa-bark”. Then, the researchers played the calls through loudspeakers and watched how the chimpanzees reacted. When both calls were played, the apes tended to look at the loudspeaker for almost a minute.

In contrast, if it only played “alarm-huu” or “waa-bark” individually, the animals only glanced over for a few seconds. When some chimpanzees heard the paired calls, they jumped onto a tree, a common response among apes when snakes are around. This suggested that the two calls combine to form a snake alarm.

To speed the research along, Dr. Townsend started working with Martin Surbeck, a behavioral ecologist at Harvard who studies bonobos, a species of ape that split off from chimpanzees two million years ago. Dr. Surbeck’s team has spent many years following apes in the Koklopori Bonobo Reserve in the Democratic Republic of Congo.

In 2022, Melissa Berthet, a postdoctoral researcher in Dr. Townsend’s lab joined to closely study the apes in their natural habitat. She made 400 hours of recordings, capturing 567 single calls and 425 pairs. She also made a note of what had happened just before the bonobos made their calls by filling out a 336-item checklist for every call.

In Zurich, Dr. Berthet listened to the recordings and classified the calls into a dozen different types. She analyzed her checklist to know what the meaning of the calls is. Then, she and her colleagues used some of the mathematical techniques that artificial intelligence systems use to learn how words are related to each other. This analysis allowed the researchers to see the bonobo calls visually using a map. The closer the calls are to each other on the map, the more similar the meanings are.

Four pairs of calls stood out. These landed on the map far from the placement of their two individual calls; they carried a meaning unlike either call alone.

Together, researchers argue that the studies on bonobos and chimpanzees suggest that human’s common ancestor with these apes also possessed compositionality.

The peer reviews were mixed. When the finding was published on Thursday, April 3rd, some scholars praised their work, while others were skeptical.

For example, Federica, who is a primatologist at the University of Leipzig in Germany, stated that the study helped place the roots of language further back in time, millions of years before the evolution of humans. “Differences between humans and other primates, including in communication, are far less distinct and well-defined than we have long assumed,” Dr. Amici said to The New York Times.

Other researchers said the study, which was on bonobos, close relatives of chimpanzees, did not reveal much about how we use words. “The present findings don’t tell us anything about the evolution of language,” said Johan Bolhuis, a neurobiologist at Utrecht University in the Netherlands. Many researchers share the same view as Bolhuis; they are skeptical of this data and do not think it is linked to the evolution of language