1. Ecology and Epidemiology Group; Department of Biological Sciences; University of Warwick; Coventry CV4 7AL
2. Department of Clinical Veterinary Science; University of Bristol; Langford House Langford Bristol BS40 5DU
3. Welfare and Behaviour Group; Scottish Agricultural College; Bush Estate Penicuick Midlothian EH26 0PH
4. Livestock Behaviour Research Unit; Purdue University; West Lafayette IN 47907 USA