1Human Evolutionary Biology, Harvard University, 2Freelance Nature Photographer, Merchantville, NJ, 3Freelance Nature Photographer, Bozeman, MT, 4School of Human Evolution and Social Change, Arizona State University, 5Anthropology, University of New Mexico
March 26, 2015 11:15, Grand Ballroom E/F/G
Hand-clasp-grooming (HCG) is a distinct and visually obvious style of chimpanzee mutual grooming that is restricted to certain communities. We analyzed inter-individual variation in the practice of HCG so as to assess whether HCG signals a special quality of social relationship. Using > 200 focal observations of ≥10 hours duration from the Kanyawara community of Kibale chimpanzees from 2009-2013, we found that adult males averaged 2.1 minutes of HCG per day, compared to 0.7 min/day for adult females. In single-sex grooming dyads, males used HCG at twice the rate of females (HCG as % of grooming minutes: male-male 4.4%, male-female 4.1% female-female 2.2%). Despite these sex differences there was no overall relationship between the frequency of HCG and the strength of affiliative relationship (determined by high rates of temporal and spatial association). By contrast, unlike HCG frequency, HCG style varied in meaningful ways. Photographs of 542 HCG dyads showed that among adults, individuals had higher rates of palm-to-palm contact with maternal kin (mean 36.2%) than non-kin (16.9%, N = 11, P < .02). Kanyawara chimpanzees also exhibited a higher frequency of palm-to-palm contact (median male 19.8%, female 22.2%) than in Mahale M-group chimpanzees (median 5.3%), the only wild chimpanzee population with sufficient data available for comparison. We suggest that the style, but not the frequency, of HCG is a socially learned trait that reflects the proximity of social relationship.
We thank the Leakey Foundation and NSF grant BCS 1355014.