1. Centre for Social Learning and Cognitive Evolution, and Scottish Primate Research Group, School of Psychology, University of St Andrews, St Andrews KY16 9JP, UK
2. Department of Psychology, Heriot Watt University, Edinburgh EH14 4AS, UK
3. Department of Biomedical Science and Technology, University of Milan, Milan, Italy
4. Department of Psychology, University of Durham, Durham DH1 3LE, UK