Give a monkey two shapes and play the words “Bouba” or “Kiki” in the background, and it seems to make the same call most humans do.
A new experiment found macaque monkeys tended to match “Bouba” with a round shape and “Kiki” with a spiky one, echoing a well-known psychological effect first conceived in the 1920s.
Nine monkeys at the Mandai Wildlife Reserve in Singapore took part in the study: eight long-tailed macaques, Macaca fascicularis, and one southern pig-tailed macaque, Macaca nemestrina.
Researchers first trained the monkeys without playing any sound. Each monkey stood in front of two cups with food hidden below a lid. One lid was blank, while the other showed a shape combining spiky and rounded elements.
After the monkeys learned that the cup with the shape contained food, they moved to the main test. They were presented with two cups covered by lids showing either the spiky or round shapes used in the Bouba-Kiki experiment, while a recording of either “Bouba” or “Kiki” played repeatedly in the background.
“Each monkey completed 24 trials divided into short blocks of three, with the position of the round and spiky shapes, as well as Bouba and Kiki playback, counterbalanced between trials,” the authors explain.
The monkeys had not been trained that any particular response was correct. But in most cases, they chose the round shape when they heard “Bouba” and the spiky shape when they heard “Kiki”.
The researchers said statistical analysis showed those choices happened at rates above random chance.
“Consistent with these condition-specific effects, the odds of choosing the round shape were nearly six times higher in Bouba than in Kiki trials,” the authors note.
The research has not yet been peer-reviewed or published in a scientific journal, but a preprint is available on bioRxiv.
“These findings are consistent with the idea that at least some crossmodal correspondences may rest on ancient perceptual biases inherited from a distant common ancestor, rather than having evolved only within humans,” the authors write.
The evidence, they add, “shifts the evolutionary question away from whether the effect is uniquely human, and toward how broadly distributed across vertebrates these perceptual mappings are.”
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