Social relationships and caregiving behavior between recently orphaned chimpanzee siblings
When their mothers die, chimpanzees often adopt younger vulnerable siblings who survive with their care. This phenomenon has been widely reported, but few studies provide details regarding how sibling relationships change immediately following the deaths of their mothers. A disease outbreak that killed several females at Ngogo in Kibale National Park, Uganda, furnished an opportunity to document how maternal death influenced the social relationships of siblings. We describe social interactions between four adolescent and young adult males and their younger immature maternal siblings 9 months before and 8 months after their mothers died. We also show how the behavior of individuals in the four recently orphaned sibling pairs contrasts to the behavior displayed by chimpanzees in 30 sibling pairs whose mothers were alive. Following the death of their mothers, siblings increased the amount of time they associated, maintained spatial proximity, groomed, reassured, and consoled each other. During travel, younger orphans followed their older siblings, who frequently looked back and waited for them. Both siblings showed distress when separated, and older siblings demonstrated heightened vigilance in dangerous situations. Chimpanzees who were recently orphaned interacted in the preceding ways considerably more than did siblings whose mothers were alive. These findings suggest that siblings provide each other support after maternal loss. Further research is needed to determine whether this support buffers grief and trauma in the immediate aftermath of maternal loss and whether sibling support decreases the probability that orphans will suffer long-term consequences of losing a mother if they survive.
KeywordsChimpanzee Orphans Kin relationships Adoption Altruism Alloparenting
Permission to carry out this research was granted by the Uganda Wildlife Authority, the Uganda National Council for Science and Technology, and Makerere University Biological Field Station. This project would have been impossible without the work, help, company, and guidance from many people who have worked and been part of the Ngogo Chimpanzee Project, including Samuel Angedakin, Chris Aliganyira, Charles Birungi, Charles Businge, Jeremy Clift, Rebecca Davenport, Bethany Hansen, Brian Kamugyisha, Sarah Dunphy-Leilii, Kevin Langergraber, Jeremiah Lwanga, Godfrey Mbabazi, Braise Mugisha, Lawrence Ndangizi, Jacob Negrey, Carolyn Rowney, Aaron Sandel, Tom Struhsaker, William Sunday, Ambrose Twineomujuni, Alfred Tumusiime, James Tibisimwa, and David Watts. For their perspectives on the findings reported here, we thank our colleagues at the University of Michigan, especially Albert Cain, Elizabeth Tinsley Johnson, Alexandra Rosati, Barbara Smuts, and Henry Wellman. We also thank two anonymous reviewers for their thoughtful feedback. Statistical advice was provided by Chris Andrews, Josh Errickson, and others at the Center for Statistics, Computing, and Analytics Research and the University of Michigan. This project was supported by Grants to R.R. from the National Geographic Society, the National Science Foundation (BCS-1540259; DGE-1256260), 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. Additional support was provided by a Grant from the National Institutes of Health (R01AG049395) to J.M. This study involved non-invasive behavioral observations of animals and was granted an exemption from review by the Institutional Animal Care and Use Committee at the University of Michigan.
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