1. School of Biology, University of St Andrews, St Andrews, KY16 9TH, UK.
2. Animal Demography Unit, Department of Biological Sciences, University of Cape Town, Cape Town, South Africa.
3. Centre for Research into Ecological and Environmental Modelling, University of St Andrews, KY16 9TH, UK.
4. The Roslin Institute and Royal (Dick) School of Veterinary Studies, University of Edinburgh, 50 George Square, Edinburgh, EH8 9LH, UK.