Baby chicks pass the ‘bouba-kiki’ test, challenging a theory of language evolution

“Stunning” chick study challenges theory about how humans evolved language

By Cody Cottier edited by Allison Parshall

Why does “bouba” sound round and “kiki” sound pointed? This intuition that links certain sounds to shapes is strangely reliable around the world, and for at least a century, scientists have considered it a clue to the origin of language, theorizing that our ancestors may have built their first words on these instinctive associations between sound and meaning. But now a new study adds an unexpected twist: Baby chickens make these same sound connections, suggesting the connection to human language may not be so unique.

The results, published today in Sciencechallenge a long-standing theory about the so-called bouba-kiki effect: it could explain how humans first attached meaning to sound to create language. Perhaps, it is thought, people naturally agree on certain associations between shapes and sounds because of some innate feature of our brains or our world. But if the barnyard hen also agrees with such associations, you might wonder whether we have the wrong linguistic seed.

Maria Loconsole, a comparative psychologist at the University of Padua in Italy, and her colleagues decided to study the bouba-kiki effect in chicks because the birds could be tested almost immediately after hatching, before their brains were influenced by exposure to the world. The researchers placed the chicks in front of two panels: one had a flower shape with slightly rounded curves; the other had a prickly spot reminiscent of a cartoon explosion. They then played recordings of humans saying “bouba” or “kiki” and observed the birds’ behavior. When the chicks heard “bouba”, 80% of them approached the round shape first and spent on average more than three minutes exploring it, compared to just under a minute on average exploring the spiky shape. Exploration preferences were reversed when chicks heard “kiki.”

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Because the testing took place during the chicks’ carefully supervised first hours of life outside their eggshell, this association between particular sounds and shapes could not be learned through experiment. Instead, it could be evidence of an innate perceptual bias that goes back much further in our evolutionary history than previously thought.

“We separated from birds during evolution 300 million years ago,” says Aleksandra Ćwiek, a linguist at the Nicolaus Copernicus University in Toruń, Poland, who was not involved in the study. “It’s just breathtaking.”

In a paper published in 2022, Úwiek and colleagues demonstrated that the bouba-kiki effect applied to diverse cultures and writing systems around the world. Other experiments have shown that human infants perform similarly on these tests, even before they have learned to speak. And in 2019 and 2022, researchers tested the effect in great apes and found that they failed the bouba-kiki test, further reinforcing the idea that the effect was exclusive to humans and our linguistic abilities.

Loconsole argues that the monkeys’ prior communicative training may have distorted their performance. Jared Taglialatela, director of the Ape Initiative and co-author of one of the monkey studies, agrees. The study subject, Kanzi the recently deceased bonobo, was often subjected to similar language-related tests. It is possible that when Kanzi encountered these absurd new words, he tried to guess their “meaning” rather than following his instincts.

In light of the new findings about chicks, Úwiek also took a broader view. “This actually makes the question of bouba-kiki as a solution to language evolution less interesting because it is a pre-language,” she says. “I think it shows us something deeper about cognition, about the ability to connect the senses.”

As for what makes “bouba” round and “kiki” pointy, perhaps we can rule out a long-standing theory: that these associations are inspired by the shape your mouth takes as you say each word. Although the “b” sound requires rounding your lips and the “k” sound requires an explosive blow to the roof of your mouth, chickens obviously can’t pronounce them at all. Instead, the bouba-kiki effect might come from the physical properties of the objects themselves, as some researchers have suggested. When round objects hit the ground or roll, they generally produce more continuous, low-frequency sounds than sharp sounds. An integrated understanding of these dynamics, linking sight and hearing, could help newborn animals quickly understand their environment, possibly to locate food or avoid predators.

The bouba-kiki effect may have played a role in the emergence of language, as well as many other cognitive faculties. But for chickens (and probably other animals), these same predispositions appear to serve an evolutionarily older purpose. “Even if the language [is] unique to humans,” says Loconsole, “that does not mean that it comes from a capacity it’s unique to humans.

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