An experimental study of leaf swallowing in captive chimpanzees: insights into the origin of a self-medicative behavior and the role of social learning


Chimpanzees in the wild swallow the rough hispid leaves of certain plant species as a means of physically expelling intestinal parasites. A plant with such a leaf texture was introduced in 36 trial sessions to a captive group of 11 healthy adult chimpanzees to investigate the possible origin and acquisition of leaf swallowing behavior. One male (housed separately from the group during testing) and one female, both captive born, spontaneously exhibited the behavior on their first trial without prior opportunity to observe others with this plant. Six other chimpanzees on their first trial displayed a phobic response to these leaves and rejected them entirely, while another two chewed and swallowed the leaves in a normal way. Four individuals eventually exhibited the behavior, after having approached and closely observed the leaf swallowing of the first female to exhibit the behavior in the group. Four of the six individuals that initially avoided the leaves never overcame their phobia toward this plant and were not in proximity to a chimpanzee performing leaf swallowing during test sessions. Individuals born to wild chimpanzee mothers were no more likely to perform the behavior than captive-reared group mates. These results suggest that the acquisition of this behavior is based in part on a propensity to fold and swallow rough, hispid leaves, but that the acquisition and spread of leaf swallowing within a group is likely to be socially influenced. This study provides support for the hypothesis that leaf swallowing originated in the wild from opportunistic feeding behavior and was later passed down in the form of a self-medicative behavioral tradition.

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We wish to give our sincere thanks to Testuro Matsuzawa, Professor of the Section of Language and Intelligence (PRI) and Professor Iver Iversen of Northern Florida State University, for their encouragement and support in conducting this study. We are indebted to the primate health care staff of the Center for Human Evolution Modeling Research for their assistance and day-to-day care of the chimpanzees. The present research was partially financed by Grants 07102010, 12301006, and 10CE2005 from the Ministry of Education, Science, Sports, and Culture, Japan. We thank our anonymous referees and Jean-Baptiste Leca for their insightful comments and criticisms of the manuscript in its final stages. Last but not least, we thank the chimpanzees for their patience and cooperation, without which this study would not have been possible.

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Correspondence to Michael A. Huffman.

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Huffman, M.A., Hirata, S. An experimental study of leaf swallowing in captive chimpanzees: insights into the origin of a self-medicative behavior and the role of social learning. Primates 45, 113–118 (2004).

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  • Feeding
  • Social tolerance
  • Behavioral tradition
  • Self-medication