1. 1Dept. of Animal Behaviour, Zoological Institute, Tagensvej 16, DK-2200 Copenhagen N, Denmark
2. 2Dept. of Animal Behaviour, Zoological Institute, Tagensvej 16, DK-2200 Copenhagen N, Denmark; CEFE/CNRS, 1919 route de Mende, F-34293 Montpellier Cedex 5, France; Fitzpatrick Institute, University of Cape Town, Rondebosch 7701, South Africa)