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Lethal aggression in Pan is better explained by adaptive strategies than human impacts

Nature volume 513, pages 414–417 (2014)Cite this article

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Abstract

Observations of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) provide valuable comparative data for understanding the significance of conspecific killing. Two kinds of hypothesis have been proposed. Lethal violence is sometimes concluded to be the result of adaptive strategies, such that killers ultimately gain fitness benefits by increasing their access to resources such as food or mates1,2,3,4,5. Alternatively, it could be a non-adaptive result of human impacts, such as habitat change or food provisioning6,7,8,9. To discriminate between these hypotheses we compiled information from 18 chimpanzee communities and 4 bonobo communities studied over five decades. Our data include 152 killings (n = 58 observed, 41 inferred, and 53 suspected killings) by chimpanzees in 15 communities and one suspected killing by bonobos. We found that males were the most frequent attackers (92% of participants) and victims (73%); most killings (66%) involved intercommunity attacks; and attackers greatly outnumbered their victims (median 8:1 ratio). Variation in killing rates was unrelated to measures of human impacts. Our results are compatible with previously proposed adaptive explanations for killing by chimpanzees, whereas the human impact hypothesis is not supported.

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Figure 1: Number of victims killed per year by members of study communities.
Figure 2: Number of killings per year for each community versus number of males and population density.

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Acknowledgements

This study was funded by National Science Foundation grants BCS-0648481 and LTREB-1052693 and National Institutes of Health grant R01 AI 058715. Numerous additional sources of funding have supported the long-term studies that contributed data to this study. We thank J. H. Jones for statistical advice; L. Pintea for preparing the map for Extended Data Fig. 1b; I. Lipende and R. Lawrence for providing details on recent cases at Gombe and Kanyantale; S. Amsler for helping to calculate the range of the Kanyantale community, and the many field assistants who collected data.

Author information

Authors and Affiliations

  1. Department of Anthropology, University of Minnesota, 395 Humphrey Center, 301 19th Avenue South, Minneapolis, Minnesota 55455, USA,

    Michael L. Wilson

  2. Department of Ecology, Evolution and Behavior, University of Minnesota, 1987 Upper Buford Circle, St Paul, Minnesota 55108, USA,

    Michael L. Wilson

  3. Department of Primatology, Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, 04103 Leipzig, Germany,

    Christophe Boesch, Gottfried Hohmann, Julia Riedel & Roman M. Wittig

  4. Division of Neurobiology, Ludwig-Maximilians Universitaet Muenchen, Germany

    Barbara Fruth

  5. Centre for Research and Conservation, Royal Zoological Society of Antwerp, Belgium

    Barbara Fruth

  6. Primate Research Institute, Kyoto University, 41-2 Kanrin, Inuyama, Aichi 484-8506, Japan,

    Takeshi Furuichi, Chie Hashimoto & Tetsuro Matsuzawa

  7. Department of Evolutionary Anthropology, Duke University, 104 Biological Sciences Building, Box 90383, Durham, North Carolina 27708-0680, USA,

    Ian C. Gilby & Anne E. Pusey

  8. School of Human Evolution and Social Change, Arizona State University, PO Box 872402, Tempe, Arizona 85287-2402, USA,

    Ian C. Gilby

  9. School of Psychology and Neuroscience, University of St Andrews, Westburn Lane, St Andrews, Fife KY16 9JP, UK,

    Catherine L. Hobaiter & Klaus Zuberbühler

  10. Wildlife Research Center, Kyoto University, 2-24 Tanaka-Sekiden-Cho, Sakyo, Kyoto, Japan,

    Noriko Itoh & Michio Nakamura

  11. Division of Biological Anthropology, Department of Archaeology & Anthropology, University of Cambridge, Henry Wellcome Building, Fitzwilliam Street, Cambridge CB2 1QH, UK,

    Kathelijne Koops

  12. Zoology Department, Makerere University, P.O. Box 7062, Kampala, Uganda,

    Julia N. Lloyd & Nicole Simmons

  13. Japan Monkey Center, 26 Kanrin, Inuyama, Aichi 484-0081, Japan,

    Tetsuro Matsuzawa

  14. Department of Anthropology, University of Michigan, 101 West Hall, 1085 S. University Avenue, Ann Arbor, 48109, Michigan, USA

    John C. Mitani

  15. Gombe Stream Research Centre, The Jane Goodall Institute – Tanzania, P.O. Box 1182, Kigoma, Tanzania,

    Deus C. Mjungu

  16. The Lester E. Fisher Center for the Study and Conservation of Apes, Lincoln Park Zoo, Chicago, 60614, Illinois, USA

    David Morgan

  17. Department of Anthropology, MSC01-1040, Anthropology 1, University of New Mexico, Albuquerque, 87131, New Mexico, USA

    Martin N. Muller

  18. Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, 04103 Leipzig, Germany,

    Roger Mundry

  19. Department of Anthropology, Iowa State University, 324 Curtiss, Ames, Iowa 50011, USA,

    Jill Pruetz

  20. Department of Anthropology, Washington University in St Louis, Campus Mailbox 1114, One Brookings Drive, St Louis, Missouri 63130, USA,

    Crickette Sanz

  21. Department of Psychology, University of York, Heslington, York, YO10 5DD, UK,

    Anne M. Schel

  22. Department of Anthropology, University of Oregon, Eugene, 97403, Oregon, USA

    Michel Waller & Frances White

  23. Department of Anthropology, Yale University, 10 Sachem Street, New Haven, Connecticut 06511, USA,

    David P. Watts

  24. Université de Neuchâtel, Institut de Biologie, Rue Emile-Argand 11, 2000 Neuchâtel, Switzerland,

    Klaus Zuberbühler

  25. Department of Human Evolutionary Biology, Harvard University, 11 Divinity Avenue Cambridge, Massachusetts 02138, USA,

    Richard W. Wrangham

Authors
  1. Michael L. Wilson
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  2. Christophe Boesch
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Contributions

All authors contributed to the acquisition, analysis and interpretation of the data; M.L.W., R.W.W., and J.C.M. initiated and conceived the study; M.L.W. and R.M. performed statistical analyses; C.B., B.F., T.F., C.H., C.L.H., G.H., N.I., K.K., J.N.L., T.M., J.C.M., D.C.M., D.M., M.N.M., M.N., J.P., A.E.P., C.S., N.S., D.P.W., F.W., K.Z., M.L.W., R.M.W., and R.W.W. conducted and supervised fieldwork; C.B., T.F., I.C.G., C.H., C.L.H., G.H., J.N.L., T.M., J.C.M., D.C.M., D.M., M.N.M., M.N., J.P., J.R., C.S., A.M.S., N.S., M.L.W., M.W., D.P.W., F.W., R.W.W. and K.Z. provided demographic and ranging data; C.B., T.F., C.H., G.H., J.N.L., T.M., J.C.M., M.N., J.P., A.E.P., N.S., F.W., M.L.W., R.W.W., and K.Z. provided data on site characteristics and human disturbance ratings; M.L.W. coordinated the contributions of all authors; M.L.W. wrote the paper with J.C.M., D.P.W., R.W.W. and input from all authors.

Corresponding author

Correspondence to Michael L. Wilson.

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Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Summary data and location of study sites.

a, Summary data for each community. Clade: bonobos (B), eastern chimpanzees (E), western chimpanzees (W); community: mean total size of the community; males: mean number of males ≥ 12 years old; females: mean number of females ≥ 12 years old; home range: mean size of the community’s home range (km2); density = (community)/(home range); area: size of protected area inhabited by the community; provisioned: whether community was regularly provisioned with food; disturbance: sum of the disturbance rating scores. b, Location of chimpanzee (circles; n = 10) and bonobo (squares; n = 3) study sites in Africa.

Extended Data Figure 2 Disturbance ratings.

a, Disturbance ratings for each site: disturbance to habitat (black bars); harassment of study animals by people (vertical lines); amount of hunting of study animals (grey); degree of habituation to people at start of study (diagonal hatching); and whether major predators have been eliminated (white). Clade is indicated by letters following community name: bonobos (B), eastern chimpanzees (E), and western chimpanzees (W). b, Number of killings per year vs. disturbance. Rates for each community are indicated by black diamonds (chimpanzees; n = 18) and open squares (bonobos; n = 4).

Extended Data Table 1 Intercommunity killings of weaned victims
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Extended Data Table 2 Intercommunity infanticides
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Extended Data Table 3 Intracommunity killings of weaned victims
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Extended Data Table 4 Intracommunity infanticides
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Extended Data Table 5 Summary of model averaged parameters using different subsets of the data
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Extended Data Table 6 Summary of parameter estimates for test of the effect of age and sex on probability of being killed
Full size table
Extended Data Table 7 Number of attackers and defenders on each side for intercommunity killings
Full size table

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Wilson, M., Boesch, C., Fruth, B. et al. Lethal aggression in Pan is better explained by adaptive strategies than human impacts. Nature 513, 414–417 (2014). https://doi.org/10.1038/nature13727

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