1. Department of Comparative Cognition, University of Neuchâtel, 2000 Neuchâtel, Switzerland
2. Budongo Conservation Field Station, Masindi, Uganda
3. School of Psychology and Neuroscience, University of St Andrews, St Andrews, KY16 9JP, UK
4. Swiss Center for Affective Sciences, University of Geneva, 1211 Geneva, Switzerland
5. Department of Zoology, University of Oxford, Oxford OX1 3PS, UK