Reversed sound–shape association in tortoises (Testudo marginata)

Abstract

Crossmodal correspondences, spontaneous links between sensory modalities, are well documented in humans and other animals. The Bouba–Kiki effect, where sharp sounds typically map onto angular shapes and soft sounds onto rounded ones, possibly constitute a peculiar case of such correspondences, as it was documented in preverbal infants and non-human species. Here we test whether a spontaneous association between “Bouba” and “Kiki” sounds, and round and spiky shapes respectively, also appears in a reptile, the Testudo marginata. In a free-exploration task, 10 tortoises were exposed to repeated “Bouba” or “Kiki” sounds while presented with a round and a spiky 3D object. Strikingly, tortoises showed a reversed mapping compared to humans and birds: they spent longer exploring the round object during “Kiki” playbacks and the spiky object during “Bouba”. This unexpected inversion raises key questions about the origins of crossmodal cognition. We argue that the reversed pattern may reflect the interaction between an underlying sound-shape correspondence and the structure of the free-exploration task. Under this interpretation, auditory stimuli may generate expectations regarding visual shapes, whereas exploratory behaviour may promote investigation of unexpected sound-shape combinations. Alternatively, tortoises may rely on fundamentally different principles of perceptual organisation, revealing that crossmodal associations are not universally aligned across species.