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We are grateful to the Royal Burgers’ Zoo for allowing this study to take place and to the chimpanzee caretakers for their dedication to the animals and assistance throughout the observations.
We also thank Tom Roth, Han de Vries, and Roger Mundry for their assistance with the analyses.
The authors declare that they have no conflicts of interest.
EJCvL is supported in the capacity of postdoctoral fellow by the Research Foundation Flanders (FWO).
CEW received a fellowship from the Living Links Center at Emory University which, together with Utrecht University, supported the research.
© 2019, The Author(s).