Chimpanzees are indifferent to the welfare of unrelated group members

Abstract

Humans are an unusually prosocial species—we vote, give blood, recycle, give tithes and punish violators of social norms. Experimental evidence indicates that people willingly incur costs to help strangers in anonymous one-shot interactions1,2, and that altruistic behaviour is motivated, at least in part, by empathy and concern for the welfare of others (hereafter referred to as other-regarding preferences)1,2,3. In contrast, cooperative behaviour in non-human primates is mainly limited to kin and reciprocating partners, and is virtually never extended to unfamiliar individuals4. Here we present experimental tests of the existence of other-regarding preferences in non-human primates, and show that chimpanzees (Pan troglodytes) do not take advantage of opportunities to deliver benefits to familiar individuals at no material cost to themselves, suggesting that chimpanzee behaviour is not motivated by other-regarding preferences. Chimpanzees are among the primates most likely to demonstrate prosocial behaviours. They participate in a variety of collective activities, including territorial patrols, coalitionary aggression, cooperative hunting, food sharing and joint mate guarding5,6,7,8,9,10,11,12. Consolation of victims of aggression13 and anecdotal accounts of solicitous treatment of injured individuals suggest that chimpanzees may feel empathy14,15. Chimpanzees sometimes reject exchanges in which they receive less valuable rewards than others, which may be one element of a ‘sense of fairness’, but there is no evidence that they are averse to interactions in which they benefit more than others16,17,18.

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Figure 1: Proportion of 1/1 choices made by Louisiana subjects.
Figure 2: Proportion of 1/1 choices made by Texas subjects.

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Acknowledgements

This work was supported by a grant from the MacArthur Foundation Preferences Network to J.B.S., J.H. and D.J.P., and a grant from the James S. McDonnell Foundation to D.J.P. A James S. McDonnell Foundation Centennial Fellowship was awarded to D.J.P. S.F.B. was supported by an NIH/NIGMS IRACDA grant awarded to Emory University. The Texas colony was supported by NIH/NCRR. We are grateful to R. McElreath for advice about statistical procedures. Author Contributions The original idea for the experiments was conceived by J.B.S. The experimental protocol used in Louisiana was developed by D.J.P., J.V. and J.B.S. J.V. supervised experiments, coding and data tabulation in Louisiana. The experimental protocol used in Texas was developed by S.F.B., J.H., J.V., D.J.P. and J.B.S. S.F.B. supervised experiments, coding and data tabulation in Texas; A.S.R. and J.M. assisted in data collection in Texas. S.P.L. and S.J.S. provided essential support for research at the Texas colony. J.H. conducted the statistical analyses. J.B.S. drafted the manuscript.

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Correspondence to Joan B. Silk.

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Supplementary information

Supplementary Methods

This file provides a complete description of the experimental protocols used at the two study sites, complete results from multivariate logistical regression analyses presented in the main text and additional statistical analyses from multinomial logistic regression analyses and generalized estimating equations analyses. (DOC 56 kb)

Supplementary Figures

This file contains two figures depicting the experimental apparatuses used in Louisiana and Texas. (DOC 2021 kb)

Supplementary Tables

Supplementary Tables S1–S11. One table describes the test configurations used in pre-testing and testing. Two tables provide the full results from the experiments. The remaining tables provide the complete statistical results from the analyses described in the Supplementary Methods. (DOC 286 kb)

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Silk, J., Brosnan, S., Vonk, J. et al. Chimpanzees are indifferent to the welfare of unrelated group members. Nature 437, 1357–1359 (2005). https://doi.org/10.1038/nature04243

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