1. School of Biology, University of St Andrews, Harold Mitchell Building, St Andrews, Fife, KY16 9TH, UK
2. School of Psychology & Neuroscience, University of St Andrews, Westburn Lane, St Andrews, Fife, KY16 9JP, Scotland
3. Department of Biological Sciences, University of Alberta, Edmonton, AB T6G 2R3, Canada
4. The Roslin Institute, The Royal (Dick) School of Veterinary Studies, The University of Edinburgh, Easter Bush, EH25 9RG, UK