Abstract
Socioecological environments that promote frequent interaction and social tolerance have favored the evolution of strong and equitable social bonds, which facilitate cooperation and confer fitness benefits. In most species, kinship is the primary predictor of bond strength and quality, but it does not adequately explain partner choice among adult male chimpanzees. Instead, most bonds and cooperative interactions occur among unrelated individuals, likely because of a lack of brothers among available partners. To identify what factors drive partner choice when kinship does not, we investigated the strength and quality of social bonds among adult male chimpanzees (N = 26) at Gombe National Park, Tanzania. Maternal brothers, when present, formed stronger bonds than other dyads, and maternal brothers who were close in age or rank formed the strongest bonds. Among dyads that were not maternal brothers, however, the strength of social bonds was not associated with either age difference or rank difference. Additionally, dyads with stronger bonds groomed more equitably than other dyads, as did maternal brothers and dyads close in rank. Bonds were stable, community-wide, for 2 years on average, while lasting up to 13 years. Overall, there remains no clear-cut explanation for partner choice among male chimpanzees. Demographic constraints limit the impact of kinship, and the effects of age and rank difference are small or nonexistent, suggesting that bond strength results from a more complex process than a simple accounting of basic characteristics. Instead, dyads may possess some distinct quality that engenders strength and stability, such as compatible personalities.
Significance statement
Strong social ties are associated with increased fitness in several species, including humans, baboons, feral horses, and dolphins. Although close kin typically form the strongest bonds, kinship explains only a limited number of bonds (and associated cooperation) among adult male chimpanzees. To identify what factors influence partner choice when kinship does not, we examined social bonds among adult male chimpanzees at Gombe National Park, Tanzania. Males formed strong, stable bonds and groomed more equitably with strong bond partners. Nevertheless, kinship, age difference, and rank difference did not fully explain male partner choice. These results contrast with findings in other species, where stronger and more equitable bonds are typically formed by kin as well as those that are close in age and rank. Instead, male chimpanzee dyads may possess some distinct quality that promotes strength and stability, such as compatible personalities.
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Data availability
The datasets generated and analyzed during the current study are available from the corresponding author upon reasonable request.
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Acknowledgments
We thank the Tanzania Wildlife Research Institute, Tanzania National Parks, and the Tanzania Commission for Science and Technology for research permissions. We are deeply grateful to Jane Goodall and the Gombe Stream Research Center staff for their efforts over six decades to collect these data. We thank Anne Pusey for initiating digitization of the data and construction of the Gombe database, and Joseph Feldblum for creating a parallel version of the Gombe database in R. We thank the many research assistants at Duke University, the University of Minnesota, and Arizona State University for data entry. We thank Brendan Barrett and Tom Morgan for statistical assistance, and Joan Silk, Kevin Langergraber, Maggie Stanton, Sam Patterson, Joseph Feldblum, David Watts, and four anonymous reviewers for feedback on prior versions of the manuscript.
Funding
Fieldwork was funded primarily by the Jane Goodall Institute. Additional support came from the National Science Foundation (DBS-9021946, SBR-9319909, BCS-0452315, BCS-0648481, IIS-0431141, IOS-1052693, IOS-1457260), the National Institutes of Health (R01-AI058715), Harris Steel, the McKnight Foundation, the Windibrow Foundation, the Leakey Foundation, the Carnegie Corporation, the University of Minnesota, and Duke University. JB was supported by the National Science Foundation Graduate Research Fellowship under grant no. 026257-001.
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Conceptualization: JB and ICG; methodology: JB; formal analysis and investigation: JB; writing—original draft preparation: JB; writing—review and editing: JB and ICG.
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The authors declare that they have no conflict of interest.
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Chimpanzees have been observed at Gombe National Park since 1960. The study followed U.S. and Tanzanian laws and was approved by the Tanzania Wildlife Research Institute, Tanzania National Parks, and the Tanzania Commission for Science and Technology. All animal protocols followed the guidelines for the treatment of animals recommended by the ASAB/ABS (2019).
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Model code is available online (https://github.com/joelbray/form-male-bonds).
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Communicated by D. P. Watts
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Bray, J., Gilby, I.C. Social relationships among adult male chimpanzees (Pan troglodytes schweinfurthii): variation in the strength and quality of social bonds. Behav Ecol Sociobiol 74, 112 (2020). https://doi.org/10.1007/s00265-020-02892-3
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DOI: https://doi.org/10.1007/s00265-020-02892-3