1. Department of Ecology, Faculty of Science, Charles University , Vinicna 7, 128 44 Prague 2 , Czech Republic
2. Czech Academy of Sciences, Institute of Vertebrate Biology , Kvetna 8, 603 65 Brno , Czech Republic
3. Department of Botany and Zoology, Faculty of Sciences, Masaryk university , Kotlarska 2, 611 37 Brno , Czech Republic
4. Department of Psychology, University of Essex , Wivenhoe Park, CO4 3SQ Colchester , UK