1. Institute of Zoology, Zoological Society of London, Regent's Park, London NW1 4RY, UK
2. Conservation Programmes, Zoological Society of London, Regent's Park, London NW1 4RY, UK
3. Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC, Canada V5A 1S6
4. Centre for Ecology and Hydrology, Maclean Building, Benson Lane, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB, UK