Abstract
Mothers provide indispensable care for infants in many mammalian species. In some long-lived species, the maternal-offspring bond persists after infancy with mothers continuing to provide resources and social support to their adult progeny. Maternal presence is associated with fitness benefits through adolescence for male chimpanzees despite the fact that mature males dominate females and form their strongest bonds with other males. How mothers support grown sons is unknown, because few studies have examined developmental shifts in mother-son relationships during adolescence and adulthood. We investigated social interactions between 29 adolescent (9–15 years) and young adult male (16–20 years) chimpanzees and their mothers at Ngogo in Kibale National Park, Uganda, over 3 years. All males under 12 years old had their mother as their top grooming and proximity partner, as did one-third of the young adult males. As males grew older, the amount of time they associated with, maintained proximity to, groomed with, and kept track of their mothers while traveling decreased. When males were together in the same party as their mothers, however, young adult males affiliated with their mothers as frequently as did adolescent males, with sons initiating the majority of these interactions. In contrast to adult sons, however, adolescent sons became distressed when separated from mothers and relied on their mothers for agonistic support and reassurance after conflicts. These findings indicate that the chimpanzee maternal-offspring bond continues but changes through adolescence and adulthood, with mothers remaining occasional social companions for most adult sons and frequent companions for some.
Significance statement
Mammalian mothers protect and provision their infants, and in some species, mothers provide social support for their adult offspring. The importance of mothers in the lives of adult sons is clear in humans and in one of our closest relatives, bonobos. Here we show mothers are also important social partners for sons throughout adolescence and into young adulthood in chimpanzees. Despite prevailing knowledge that adult males are key alliance partners for male chimpanzees, we demonstrate that male chimpanzees maintain social bonds with their mothers into adulthood. We also suggest that social bonds reflect not only the total time spent together, but what animals do during those times, as many young adults rarely groomed their mothers, but at times when they were in the same subgroup with her, they did so frequently. Future research will assess the physiological, psychological, and reproductive benefits of such mother-son bonds.
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All data generated or analyzed during this study are included in this published article and its supplementary information files.
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Acknowledgments
We thank the Uganda Wildlife Authority, the Uganda National Council on Science and Technology, and Makerere University Biological Field Station for permitting us to conduct this research.
We thank John Mitani for his guidance, feedback and critique at all stages and on all aspects of this project. For additional mentorship in the field, we are grateful to Kevin Langergraber, Jeremiah Lwanga, and David Watts. We thank many members of the Ngogo Chimpanzee Project, for the help and company in the field, including Samuel Angedakin, Chris Aliganyira, Charles Birungi, Charles Businge, Jeremy Clift, Nathan Chesterman, Rebecca Davenport, Brian Kamugyisha, Sarah Dunphy-Leili, Godfrey Mbabazi, Braise Mugyisha, Lawrence Ndangizi, Jacob Negrey, Carolyn Rowney, Thomas Struhsaker, William Sunday, Ambrose Twineomujuni, Alfred Tumusiime, and James Tibisimwa. For feedback on our findings, we also thank Jacinta Beehner, Kevin Langergraber, Andrew Marshall, Alexandra Rosati, Barbara Smuts, David Watts, and Henry Wellman. Statistical advice was provided by Chris Andrews and others at the Center for Statistics, Computing, and Analytics Research and the University of Michigan. We thank two anonymous reviewers for their thoughtful feedback on this manuscript. We dedicate this article to our mothers, Kiku Adatto, and Sushma Reddy.
Funding
This project was supported by grants from the National Geographic Society (9742-15), the National Science Foundation (BCS-1613392, DGE-1256260, 1540259, F031543), the Nacey-Maggioncalda Foundation, the L.S.B. Leakey Foundation, and the African Studies Center, International Institute, Rackham Graduate School and Department of Anthropology at the University of Michigan.
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All applicable international, national, and institutional guidelines for the use of animals were followed. All research protocols reported in this manuscript were granted exemption by the Institutional Animal Care and Use Committee at the University of Michigan. Work conducted in Uganda was approved by the Uganda National Council for Science and Technology (NS488) and the Uganda Wildlife Authority (EDO-35-01).
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Reddy, R.B., Sandel, A.A. Social relationships between chimpanzee sons and mothers endure but change during adolescence and adulthood. Behav Ecol Sociobiol 74, 150 (2020). https://doi.org/10.1007/s00265-020-02937-7
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DOI: https://doi.org/10.1007/s00265-020-02937-7