title: Distribution of a Chimpanzee Social Custom Is Explained by Matrilineal Relationship Rather Than Conformity.
abstract: High-arm grooming is a form of chimpanzee grooming in which two individuals mutually groom while each raising one arm. Palm-to-palm clasping (PPC) is a distinct style of high-arm grooming in which the grooming partners clasp each other's raised palms. In wild communities, samples of at least 100 observed dyads grooming with raised hands showed PPC frequencies varying from <5% (M group, Mahale) to >30% dyads grooming (Kanyawara, Kibale), and in a large free-ranging sanctuary group, the frequency reached >80% dyads (group 1, Chimfunshi) [1, 2]. Because between-community differences in frequency of PPC apparently result from social learning, are stable across generations, and last for at least 9 years, they are thought to be cultural, but the mechanism of transmission is unknown [2]. Here, we examine factors responsible for individual variation in PPC frequency within a single wild community. We found that in the Kanyawara community (Kibale, Uganda), adults of both sexes varied widely in their PPC frequency (from <10% to >50%) and did not converge on a central group tendency. However, frequencies of PPC were highly consistent within matrilines, indicating that individuals maintained lifelong fidelity to the grooming style of their mothers. Matrilineal inheritance of socially learned behaviors has previously been reported for tool use in chimpanzees [3] and in the vocal and feeding behavior of cetaceans [4, 5]. Our evidence indicates that matrilineal inheritance can be sufficiently strong in nonhuman primates to account for long-term differences in community traditions.
authors.affiliation: Department of Human Evolutionary Biology, Harvard University, 11 Divinity Avenue, Cambridge, MA 02138, USA. Electronic address: wrangham@fas.harvard.edu.
authors.forename: Richard W
authors.initials: RW
authors.lastname: Wrangham
authors.affiliation: Department of Human Evolutionary Biology, Harvard University, 11 Divinity Avenue, Cambridge, MA 02138, USA; Anthropological Institute and Museum, University of Zurich, Winterthurerstrasse 190, 8057 Zürich, Switzerland.
authors.forename: Kathelijne
authors.initials: K
authors.lastname: Koops
authors.affiliation: Department of Human Evolutionary Biology, Harvard University, 11 Divinity Avenue, Cambridge, MA 02138, USA; Department of Anthropology, Tufts University, 5 The Green, Medford, MA 02155, USA.
authors.forename: Zarin P
authors.initials: ZP
authors.lastname: Machanda
authors.affiliation: Institute for Quantitative Social Science, Harvard University, 1737 Cambridge Street, Cambridge, MA 02138, USA.
authors.forename: Steven
authors.initials: S
authors.lastname: Worthington
authors.affiliation: Department of Anthropology, University of Michigan, 101 West Hall, S. University Avenue, Ann Arbor, MI 48109, USA.
authors.forename: Andrew B
authors.initials: AB
authors.lastname: Bernard
authors.affiliation: Department of Epidemiology, Gillings School of Global Public Health, University of North Carolina, 1335 Dauer Drive, Chapel Hill, NC 27599, USA.
authors.forename: Nicholas F
authors.initials: NF
authors.lastname: Brazeau
authors.affiliation: National Geographic Magazine, 1144 17(th) Street NW, Washington, DC 20036, USA.
authors.forename: Ronan
authors.initials: R
authors.lastname: Donovan
authors.affiliation: Department of Human Evolutionary Biology, Harvard University, 11 Divinity Avenue, Cambridge, MA 02138, USA.
authors.forename: Jeremiah
authors.initials: J
authors.lastname: Rosen
authors.affiliation: Department of Psychology, University of York, James Way, Heslington, York YO10 5DD, UK.
authors.forename: Claudia
authors.initials: C
authors.lastname: Wilke
authors.affiliation: Kibale Chimpanzee Project, Makerere University Biological Field Station, Kamwenge Road, Fort Portal, Uganda.
authors.forename: Emily
authors.initials: E
authors.lastname: Otali
authors.affiliation: Department of Anthropology, University of New Mexico, Albuquerque, NM 87131, USA.
authors.forename: Martin N
authors.initials: MN
authors.lastname: Muller
mesh_terms: D000818:Animals; D005260:Female; D006120:Grooming; D008297:Male; D002679:Pan troglodytes; D012919:Social Behavior; D000067570:Social Learning; D014454:Uganda
keywords: behavioral diffusion; conformity; conservatism; hand-clasp grooming; high-arm grooming; palm-to-palm clasp; social tradition; transmission bias