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

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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.

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Authors and Affiliations

Authors

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

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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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