Adolescent male chimpanzees do not form a dominance hierarchy with their peers
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Dominance hierarchies are a prominent feature of the lives of many primate species. These hierarchies have important fitness consequences, as high rank is often positively correlated with reproduction. Although adult male chimpanzees strive for status to gain fitness benefits, the development of dominance relationships is not well understood. While two prior studies found that adolescent males do not display dominance relationships with peers, additional research at Ngogo in Kibale National Park, Uganda, indicates that adolescents there form a linear dominance hierarchy. These conflicting findings could reflect different patterns of rank acquisition across sites. An alternate possibility arises from a recent re-evaluation of age estimates at Ngogo and suggests that the report describing decided dominance relationships between adolescent males may have been due to the accidental inclusion of young adult males in the sample. To investigate these issues, we conducted a study of 23 adolescent male chimpanzees of known age during 12 months at Ngogo. Adolescent male chimpanzees exchanged pant grunts, a formal signal of submission, only 21 times. Recipients of pant grunts were late adolescent males, ranging between 14 and 16 years old. In contrast, younger adolescent males never received pant grunts from other males. Aggression between adolescent males was also rare. Analysis of pant grunts and aggressive interactions did not produce a linear dominance hierarchy among adolescent males. These data indicate that adolescent male chimpanzees do not form decided dominance relationships with their peers and are consistent with the hypothesis that the hierarchy described previously at Ngogo resulted from inaccurate age estimates of male chimpanzees. Because dominance relationships develop before adulthood in other primates, our finding that adolescent male chimpanzees do not do so is surprising. We offer possible explanations for why this is the case and suggest future studies that may help clarify the matter.
KeywordsRank Adolescence Aggression Development Pan troglodytes
Our fieldwork was sponsored by the Uganda Wildlife Authority, the Uganda National Council for Science and Technology, and the Makerere University Biological Field Station. We thank Nathan Chesterman for providing invaluable assistance in the field. For additional support in the field, we are grateful to David Watts, Kevin Langergraber, Sam Angedakin, Alfred Tumusiime, Ambrose Twineomujuni, Godfrey Mbabazi, and Laurence Ndangizi. We thank Michio Nakamura, Nicholas Newton-Fisher, and one anonymous reviewer for comments on the manuscript. Thore Bergman and biological anthropology students and faculty in the Department of Anthropology at the University of Michigan furnished additional advice. A.A.S. was supported by The Leakey Foundation, the University of Michigan, the Nacey-Maggioncalda Foundation, and the National Science Foundation Graduate Research Fellowship under Grant No. F031543. R.B.R. was supported by a National Geographic Young Explorer grant and the National Science Foundation Graduate Research Fellowship under Grant No. DGE 1256260. J.C.M. is currently supported by NIH RO1AG049395.
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The authors declare that they have no conflict of interest.
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