“In human language, the connection between a word, a written signifier, and that which it signifies is usually arbitrary. The meaning of a word usually has nothing to do with the sound of the word – but there are exceptions,” says Friedemann Pulvermüller, a professor of neuroscience at Freie Universität Berlin, who works on the neuroscience of language and pragmatics. Humans tend to associate the sound of pseudowords with certain shapes, explains Pulvermüller, who co-authored the study. Sounds like “bouba” or “maluma” tend to be matched with curved shapes, while “kiki” or “takete” are often matched with pointy or angular shapes.

Previous studies have shown that speakers of different languages from across cultures share this tendency to map sounds and shapes. The theory is that sound symbolism was key to the early development of spoken language in humans, and this is known as the “bouba-kiki effect.”

The team of researchers from Europe and the United States wanted to examine whether this phenomenon is only prevalent among humans or whether animals are also capable of mapping sounds and shapes in a similar manner.

Dr. Konstantina Margiotoudi, a neuroscientist, led the team in the USA that conducted the experiment. The animal subject, Kanzi the bonobo, has a strong understanding of language. He was able to learn how to communicate with humans using hundreds of different symbols, called lexigrams. The great ape can differentiate between English words and identify objects that correspond to the words on a touchscreen.

For the experiment, scientists played Kanzi “bouba-kiki” sounds and other pseudowords that were new to him. Unlike humans, the bonobo did not demonstrate a tendency to match sharp sounds (like “kiki”) with angular shapes or round sounds (like “bouba”) with curved shapes.

The findings indicate that a highly developed, intuitive understanding of sound-symbol associations is a unique quality among humans. This ability likely played a significant role in the emergence of protowords – an important step in the evolutionary history of speech. However, the experiment did not establish any substantial conclusions about apes’ language abilities. “Bonobos might indeed be able to perceive other sound-symbol associations, for example between low frequency sounds and larger objects,” Professor Pulvermüller notes. The study did advance scientists’ knowledge of fundamental speech phenomena, such as the bouba-kiki effect, and gave insights into the evolution of human language.