1. Centre for Social Learning and Cognitive Evolution, School of Psychology, St Andrews KY16 9JU, UK
2. St John's College, Cambridge CB2 1TP, UK
3. Centre for Social Learning and Cognitive Evolution, School of Biology, University of St Andrews, St Andrews KY16 9TS, UK
4. Department of Palaeontology, The Natural History Museum, London SW7 5BD, UK