1. Agricultural and Food Research Council, Institute of Animal Physiology and Genetics Research, Babraham, Cambridge CB2 4AT, United Kingdom.
2. Laboratoire de Comportement Animal, Institut National de Ia Recherche Agronomique, Nouzilly, 37380 France.
3. Subdepartment of Animal Behavior, University of Cambridge, Madingley, Cambridge CB3 8AA, United Kingdom