1. Cognitive & Cultural Ecology Research Group, Max Planck Institute of Animal Behavior , Am Obstberg 1, 78315 Radolfzell , Germany
2. Department of Biology, University of Konstanz , Universitätsstraße 10, 78464 Konstanz , Germany
3. Department of Human Behavior, Ecology and Culture, Max Planck Institute for Evolutionary Anthropology , Deutscher Pl. 6, 04103 Leipzig , Germany
4. Ecoscience, Aarhus University , Nordre Ringgade 1, 8000 Aarhus C , Denmark
5. Division of Ecology and Evolution, Research School of Biology, The Australian National University , 134 Linnaeus Way, Acton ACT 2601 , Australia
6. Department of Evolutionary Biology and Environmental Science, University of Zurich , Winterthurerstrasse 190, 8057 Zürich , Switzerland