Crossmodal pitch-luminance association in tortoises

If you think that high-pitched sounds match bright colours more than dark ones, you are not alone. Spontaneous associations between different sensory modalities --crossmodal associations-- are widely observed in humans. More recently, crossmodal associations have been reported in chimpanzees, monkeys, dogs, chickens, and tortoises, indicating a general cognitive strategy that reflects natural correlations in the environment or similarities in the architecture of the nervous system. Alternatively, or complementarily, crossmodal associations may arise from learning of species-specific occurrences. Recent research on primates has tentatively linked human pitch-luminance associations to the evolution of language. While humans and chimpanzees spontaneously match a high-pitched sound with a white shape and a low-pitched sound with a black shape, baboons and chickens do not exhibit any preferential association. Identifying spontaneous pitch-luminance associations in species that do not rely on vocal communication, such as the solitary Hermanns tortoises, would support the idea of a language-independent phenomenon. To test this idea, we studied tortoises for pitch-luminance associations in a spontaneous food-searching task. After hearing a high-pitched (700 Hz) vs low-pitched (450 Hz) sound, animals could choose to search for food behind either a light- or dark-coloured wall. When hearing the high-pitched sound, tortoises chose the white stimulus, whereas when hearing the low-pitched sound, they chose the black stimulus, similarly to what humans and chimpanzees do. These results demonstrate crossmodal pitch-luminance associations in tortoises. While showing this commonality in the perceptual organization of phylogenetically distant species, we shift the question on whether pitch-luminance associations reflect homology or convergent evolution.