1. Department of Evolutionary Anthropology, Duke University, Durham, NC 27708, USA.
2. Kumamoto Sanctuary, Wildlife Research Center, Kyoto University, Kumamoto, Japan.
3. Primate Research Institute, Kyoto University, Inuyama, Japan.
4. School of Psychology and Neuroscience, University of St Andrews, St Andrews, UK.
5. Department of Developmental and Comparative Psychology, Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany.
6. Department of Psychology and Neuroscience, Duke University, Durham, NC 27708, USA.