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Great apes’ understanding of biomechanics: eye-tracking experiments using three-dimensional computer-generated animations

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Visual processing of the body movements of other animals is important for adaptive animal behaviors. It is widely known that animals can distinguish articulated animal movements even when they are just represented by points of light such that only information about biological motion is retained. However, the extent to which nonhuman great apes comprehend the underlying structural and physiological constraints affecting each moving body part, i.e., biomechanics, is still unclear. To address this, we examined the understanding of biomechanics in bonobos (Pan paniscus) and chimpanzees (Pan troglodytes), following a previous study on humans (Homo sapiens). Apes underwent eye tracking while viewing three-dimensional computer-generated (CG) animations of biomechanically possible or impossible elbow movements performed by a human, robot, or nonhuman ape. Overall, apes did not differentiate their gaze between possible and impossible movements of elbows. However, some apes looked at elbows for longer when viewing impossible vs. possible robot movements, which indicates that they may have had knowledge of biomechanics and that this knowledge could be extended to a novel agent. These mixed results make it difficult to draw a firm conclusion regarding the extent to which apes understand biomechanics. We discuss some methodological features that may be responsible for the results, as well as implications for future nonhuman animal studies involving the presentation of CG animations or measurement of gaze behaviors.

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Availability of data and material

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request. Note that relevant data are visualized in this published article (and in Supplementary file 1).

Code availability

The R code used during the current study is available from the corresponding author on reasonable request.


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We would like to especially thank Mr. H. Ishimoto and Mr. K. Nagamachi at the Department of Computer Science and Engineering, Toyohashi University of Technology, for making the high-quality 3D animations. Further, we are grateful to Dr. N. Morimura, Dr. F. Kano, and all the staff at KS; Ms. H. Bando, Mr. M. Sato, Dr. Y. Yamanashi, and Dr. M. Tanaka at KCZ; and Ms. E. Ichino, Dr. Y. Kawaguchi, and members of the Language & Intelligence section of the PRI, for their kind help in conducting the current study. We also thank Dr. T. Matsuzawa for financial support. We thank Sydney Koke, MFA, from Edanz (, for editing a draft of this manuscript. Finally, we are grateful to the editor and anonymous reviewers for their constructive feedback during the review process.


This study was supported financially by the Japan Society for the Promotion of Science, Ministry of Education, Culture, Sports, Science and Technology, to YS (grant no. 19J22889), MT (grant nos. 15H05709, 18H04194), SH (grant nos. 26245069, 18H05524), and TM (grant no. 16H06283, LDG-U04); the Cooperative Research Program of the PRI; and the Great Ape Information Network. These funding sources had no involvement in the study design, collection, analysis and interpretation of data, manuscript writing, or in the decision to submit the article.

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Authors and Affiliations



Conceptualization: YS, MK, SI, SH. Methodology: MK. Formal analysis, investigation, data curation, writing—original draft, and visualization: YS. Resources: MK, TM, YS, MT, SH. Writing—review and editing: MK, SI, TM, YS, MT, SH. Supervision: SI, SH. Project administration: SH. Funding acquisition: SH, YS.

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Correspondence to Yutaro Sato.

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The authors declare that they have no conflict of interest.

Ethics approval

The animal husbandry and research protocols complied with the Guide for Animal Research Ethics of the Wildlife Research Center, Kyoto University for the KS experiments (chimpanzees, no. WRC-2018-KS006A; bonobos, no. WRC-2018-KS007A), the Research Ethics Guideline of KCZ for the KCZ experiments (no. 2019-KCZ-001), and the Guidelines for the Care and Use of Laboratory Primates of the PRI for the PRI experiments (no. 2019-001).

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All authors approved the submission of the article. The funding sources had no involvement in the decision to submit this article for submission.

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Sato, Y., Kitazaki, M., Itakura, S. et al. Great apes’ understanding of biomechanics: eye-tracking experiments using three-dimensional computer-generated animations. Primates 62, 735–747 (2021).

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