Crossmodal Pitch-Luminance Association in Tortoises

Crossmodal associations-spontaneous links between sensory modalities-are widely observed in humans. Similar associations have also been found in chimpanzees, monkeys, dogs, chickens, and tortoises, suggesting a shared cognitive strategy based on environmental correlations or a similar nervous system architecture. Alternatively, or complementarily, these associations may arise from learned, species-specific occurrences. Humans and chimpanzees, for instance, pair higher-pitched sounds with white shapes and lower-pitched sounds with black shapes, while baboons and chickens show no preference. Here, we provide novel evidence of pitch-luminance association in a nonvocal reptile, the Hermann's tortoise (Testudo hermanni). We studied tortoises in a spontaneous food-searching task. After hearing a relative higher-pitched (700 Hz) versus lower-pitched (450 Hz) sound, animals could choose to search for food behind either a light- or dark-colored wall. Tortoises consistently preferred the white wall with higher-pitch sounds and the black wall with lower-pitch sounds, resembling spontaneous associations seen in humans and chimpanzees. Evidence of pitch-luminance association in tortoises suggests that phylogenetically distant species may share similar perceptual organization or internalize similar statistical regularities, shifting the question toward whether such associations reflect homology or convergent evolution.