1. School of Biology, University of Nottingham, Nottingham NG7 2RD, UK.
2. International Livestock Research Institute (ILRI), Post Office Box 5689, Addis Ababa, Ethiopia.
3. University of Liverpool, School of Biological Sciences, Biosciences Building, Liverpool L69 7ZB, UK and ILRI, Post Office Box 30709, Nairobi, Kenya.