[1] Univ London Imperial Coll Sci Technol & Med, Dept Life Sci, Ascot SL5 7PY, Berks, England
[2] Univ London Imperial Coll Sci Technol & Med, Ctr Environm Policy, London SW7 1NA, England
[3] Univ Cambridge, Dept Zool, Cambridge CB2 3EJ, England
[4] Wildlife Conservat Soc, Bronx, NY 10460 USA