1. Division of Environmental and Evolutionary Biology, Institute of Biomedical and Life Sciences, Graham Kerr Building, University of Glasgow, Glasgow G12 8QQ, UK
2. School of Biological Sciences, Center for Reproductive Biology, Washington State University, Pullman, WA 99164-4236, USA
3. Department of Veterinary Physiology and Pharmacology, University of Glasgow, Bearsden Road, Glasgow G61 1QH, UK