What Bonobos Can Teach Us about Human Speech

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International team of researchers with members from Freie Universität Berlin examines the fundamentals of language development

Researchers from Europe and the United States of America conducted a study on "sound symbolism" in primates. The experiment involved experts from Freie Universität Berlin, the University of St. Andrews, the Max Planck Institute for Evolutionary Anthropology, as well as Pennsylvania State University and the Ape Initiative. The animal subject was a language-competent bonobo named "Kanzi." The ape has drawn attention due to its ability to learn English words and match spoken words with images of objects - a basic ability required for language acquisition. In the experiment, researchers would play nonsense syllables, for example "kiki," for Kanzi and show him corresponding abstract shapes that were either angular or curved. In human trials, subjects showed a tendency to pair sharp sounding pseudowords like "kiki" with angular shapes, whereas soft sounding pseudowords like "bouba" were frequently associated with curved shapes. This connection between sounds and abstract symbols is known as "sound symbolism." Kanzi, however, did not demonstrate this matching preference in the experiment, which did not come as a great surprise, given that other great apes also fail to connect sounds and shapes in this manner. Scientists have long assumed that sound symbolism indicated a kind of implicit knowledge important to the evolution of human language. The study discusses the evolution of human speech and how language ability differs between humans and other primates. The study was published in the scientific journal Proceedings of the Royal Society B: Biological Sciences.

"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.

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