Visual categorization of surface qualities of materials by capuchin monkeys and humans

- Capuchin monkeys and humans behaved similarly in visual matching of materials.
- Both species showed short choice reaction time for glossy materials like metal.
- Capuchin monkeys showed an excellent categorization ability for fur.

Visually identifying and categorizing the material composition of objects before actually interacting with them is an important skill for operating smoothly and safely in the world. This ability is assumed to have been shaped by evolution; therefore, non-human animals should share similar categorization abilities. Little is known, however, about how non-human animals do this. We tested whether tufted capuchin monkeys (Cebus apella) were able to visually categorize images that represented nine different materials (metal, ceramic, glass, stone, bark, wood, leather, fabric, and fur), and we compared their performance with that of humans. Capuchins showed excellent categorization abilities for images of fur, which is a familiar material to captive monkeys. Humans showed a tendency to confuse material categories that resembled each other visually and/or semantically. Correlation analyses on reaction time showed that both species made correct choices rapidly in selecting glossy categories like metal and ceramic compared with matte categories like fabric and stone, which contain minute patterns. Overall, our results suggest that monkeys share similar perceptual tendencies with humans in visual categorization of material images to some extent and the potential to categorize materials frequently encountered in their daily lives by visual observation.