Leaf swallowing behavior in chimpanzees (Pan troglodytes): biased learning and the emergence of group level cultural differences

Abstract

Demonstrating the ability to ‘copy’ the behavior of others is an important aspect in determining whether social learning occurs and whether group level differences in a given behavior represent cultural differences or not. Understanding the occurrence of this process in its natural context is essential, but can be a daunting task in the wild. In order to test the social learning hypothesis for the acquisition of leaf swallowing (LS), a self-medicative behavior associated with the expulsion of parasites, we conducted semi-naturalistic experiments on two captive groups of parasite-free, naïve chimpanzees (Pan troglodytes). Individuals in the group were systematically provided appropriate stimuli (rough hispid leaves) identical to those used by chimpanzees in the wild. Individuals initially responded in a variety of ways, ranging from total aversion to normal chewing and swallowing. Over time, however, the two groups adopted different variants for inserting and folding the leaves in the mouth prior to swallowing them (complete and partial LS), following the specific method spontaneously displayed by the first and primary LS models in their respective groups. These variants were similar to LS displayed by chimpanzees in the wild. Using the option-bias method, we found evidence for social learning leading to group-level biased transmission and group-level stabilization of these two variants. This is the first report on two distinct cultural variants innovated in response to the introduction of natural stimuli that emerged and spread spontaneously and concurrently within two adjacent groups of socially housed primates. These observations support the assertion that LS may reflect a generalized propensity for ingesting rough hispid leaves, which can be socially induced and transmitted within a group. Ingesting an adequate number of these leaves induces increased gut motility, which is responsible for the subsequent expulsion of particular parasite species in the wild. Cultural transmission and maintenance of LS within a group and associative learning by the individual of the positive consequences of this otherwise non-nutritive mode of ingestion is proposed to be the pivotal link between this feeding propensity and its maintenance as a self-medicative behavior by great apes in the wild.

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Acknowledgments

We are grateful to the director Dr. Cesare Avesani Zaborra, Ms Donata Grassi, and other staff of Parco Natura Viva for their tireless support of this research and for their dedication to the well-being of the chimpanzees. Special thanks to Prof. Emanuela Prato Previde and Prof. Giorgio Vallortigara for their advice and encouragement. During the later stages of the preparation of this manuscript, JBL was supported by a JSPS (Japan Society for the Promotion of Science) postdoctoral fellowship (No. 07421). We are also grateful to the reviewers of this manuscript, who both challenged and encouraged the development of our ideas. This study complies with the current laws of the country in which it was performed, and was approved in advance by the Parco Natura Viva administration.

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Correspondence to Michael A. Huffman.

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Huffman, M.A., Spiezio, C., Sgaravatti, A. et al. Leaf swallowing behavior in chimpanzees (Pan troglodytes): biased learning and the emergence of group level cultural differences. Anim Cogn 13, 871–880 (2010). https://doi.org/10.1007/s10071-010-0335-8

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Keywords

  • Captive chimpanzees
  • Leaf swallowing
  • Pan troglodytes
  • Social learning
  • Self-medication