Visual discrimination of primate species based on faces in chimpanzees
Many primate studies have investigated discrimination of individual faces within the same species. However, few studies have looked at discrimination between primate species faces at the categorical level. This study systematically examined the factors important for visual discrimination between primate species faces in chimpanzees, including: colour, orientation, familiarity, and perceptual similarity. Five adult female chimpanzees were tested on their ability to discriminate identical and categorical (non-identical) images of different primate species faces in a series of touchscreen matching-to-sample experiments. Discrimination performance for chimpanzee, gorilla, and orangutan faces was better in colour than in greyscale. An inversion effect was also found, with higher accuracy for upright than inverted faces. Discrimination performance for unfamiliar (baboon and capuchin monkey) and highly familiar (chimpanzee and human) but perceptually different species was equally high. After excluding effects of colour and familiarity, difficulty in discriminating between different species faces can be best explained by their perceptual similarity to each other. Categorical discrimination performance for unfamiliar, perceptually similar faces (gorilla and orangutan) was significantly worse than unfamiliar, perceptually different faces (baboon and capuchin monkey). Moreover, multidimensional scaling analysis of the image similarity data based on local feature matching revealed greater similarity between chimpanzee, gorilla and orangutan faces than between human, baboon and capuchin monkey faces. We conclude our chimpanzees appear to perceive similarity in primate faces in a similar way to humans. Information about perceptual similarity is likely prioritized over the potential influence of previous experience or a conceptual representation of species for categorical discrimination between species faces.
KeywordsSpecies discrimination Colour Familiarity Perceptual similarity Chimpanzees
We sincerely thank Prof. Tetsuro Matsuzawa and Drs. Ikuma Adachi, Yuko Hattori and Misato Hayashi of the Language and Intelligence Section, Primate Research Institute, Kyoto University, for their helpful comments and generous support of this research. We also thank Dr. Fumito Kawakami, Mr. Yoshiki Kurosawa and Ms. Etsuko Ichino of the Language and Intelligence Section, Primate Research Institute, Kyoto University, for assistance with conducting the experiments and staff at the Center for Human Evolution Modelling Research, Primate Research Institute, Kyoto University, for their daily care of the chimpanzees. In addition, we thank Drs. Yumi Yamanashi, Yoko Sakuraba and Renata Mendonça for permission to use their photographs of chimpanzees, gorillas and orangutans. This research was financially supported by the MEXT (#150528), JSPS-MEXT KAKENHI (#15H05709, #16H06283, #20002001, #23220006, #24000001), JSPS-CCSN and the JSPS Leading Graduate Program in Primatology and Wildlife Science (U04) at Kyoto University.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflicts of interest.
This study was approved by the Animal Welfare and Care Committee of the Primate Research Institute, Kyoto University and the Animal Research Committee of Kyoto University, and followed the Guidelines for the Care and Use of Laboratory Primates of the Primate Research Institute, Kyoto University (Version 3, 2010).
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