Leaf Surface Roughness Elicits Leaf Swallowing Behavior in Captive Chimpanzees (Pan troglodytes) and Bonobos (P. paniscus), but not in Gorillas (Gorilla gorilla) or Orangutans (Pongo abelii)
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Researchers have described apparently self-medicative behaviors for a variety of nonhuman species including birds and primates. Wild chimpanzees, bonobos, and gorillas have been observed to swallow rough leaves without chewing, a behavior proposed to be self-medicative and to aid control of intestinal parasites. Researchers have hypothesized that the presence of hairs on the leaf surface elicits the behavior. We investigated the acquisition and the underlying mechanisms of leaf swallowing. We provided 42 captive great apes (24 chimpanzees, six bonobos, six gorillas, and six orangutans) with both rough-surfaced and hairless plants. None of the subjects had previously been observed to engage in leaf swallowing behavior and were therefore assumed naïve. Two chimpanzees and one bonobo swallowed rough-surfaced leaves spontaneously without chewing them. In a social setup six more chimpanzees acquired the behavior. None of the gorillas or orangutans showed leaf swallowing. Because this behavior occurred in naïve individuals, we conclude that it is part of the behavioral repertoire of chimpanzees and bonobos. Social learning is thus not strictly required for the acquisition of leaf swallowing, but it may still facilitate its expression. The fact that apes always chewed leaves of hairless control plants before swallowing, i.e., normal feeding behavior, indicates that the surface structure of leaves is indeed a determinant for initiating leaf swallowing in apes where it occurs.
KeywordsCulture Great apes Latent solution Leaf swallowing Self-medication Social learning
We thank the Max Planck Society, the Department of Developmental and Comparative Psychology, and the Department of Primatology of the Max-Planck-Institute for Evolutionary Anthropology. We thank Joanna Setchell and three anonymous reviewers for their helpful comments. We also thank the staff of the Leipzig Zoo and Wolfgang Köhler Primate Research Center, especially the keepers, Charlotte Rahn, Daniel Hanus, Johannes Großmann, Matthias Allritz, Julia Löpelt, and Andreas Bernhard. Josefine Kalbitz, Hagen Knofe, and Julia Watzek were additional experimenters and we thank them for their support. Heike Heklau, Martin Freiberg, Ulf Schilling, and Helmut Eißner helped with selection and supply of plants. We are grateful for permission to collect plant parts in the city area of Leipzig and Halle/Saale, Germany. We thank Siobhan Loftus for reliability coding. We also thank Barbara Fruth and Gottfried Hohmann for their continued support during the study.
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