1. School of Biological Sciences; University of East Anglia; Norwich Research Park; Norwich; Norfolk NR4 7TJ; UK
2. Department of Animal and Plant Sciences; University of Sheffield; Sheffield; S10 2TN; UK
3. Behavioural Ecology and Self-organisation; Centre for Ecological and Evolutionary Studies; University of Groningen; P.O. Box 11103, 9700 CC; Groningen; The Netherlands