Affiliation: | a Department of Psychology, University of Pennsylvania, Philadelphia, PA, U.S.A. b Department of Biology, University of Pennsylvania, Philadelphia, PA, U.S.A. c Department of Psychology, University of Michigan, Ann Arbor, MI, U.S.A. d Department of Ecology & Evolutionary Biology, University of Michigan, Ann Arbor, MI, U.S.A. e Cognitive Ethology Laboratory, German Primate Center, University of Gottingen, Gottingen, Germany f Wissenschaftskolleg, Institute for Advanced Study, Berlin, Germany |