1. Department of Environmental Systems Science, ETH Zurich, CHN G 73.2, Universitatstrasse 16, CH-8092, Switzerland
2. Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, Singapore 117543
3. Anthropological Institute and Museum, Universitat Zürich, CH-8057 Zürich, Switzerland
4. Sumatran Orangutan Conservation Program (PanEco-YEL), Medan 20154, Indonesia