1. School of Biological Sciences University of Bristol Bristol BS8 1TQ U.K.
2. Edward Grey Institute, Department of Zoology University of Oxford Oxford OX1 3SZ U.K.
3. Department of Evolution and Ecology University of California Davis Davis CA 95616 U.S.A.
4. Centre de Recherches sur la Cognition Animale (UMR5169), Centre de Biologie Intégrative, CNRS, UPS Université de Toulouse 31062 Toulouse France