Dyadic affiliative relationships influence the selection of vulnerable grooming sites in Tibetan macaques

Abstract Social grooming is distributed heterogeneously both among group members and across individual’s body surfaces. While previous studies primarily link grooming site preferences to hygienic functions, the selection of specific body locations may also be modulated by the quality of social relationships. Vulnerable body locations offer an opportunity to test the hypothesis that dyadic affiliative bonds influence preferences for these locations (the Affiliative Effect Hypothesis). To test this hypothesis, we conducted behavioral observations on a group of Tibetan macaques (Macaca thibetana), utilizing social bridging (a behavioral index of dyadic affiliative relationships) to test immediate effects. We found that vulnerable body locations were groomed predominantly during social grooming rather than self-grooming. As expected, the durations of grooming on vulnerable body locations were positively related to the strength of affiliative bonds between groomers and groomees. Furthermore, social grooming directed to vulnerable body locations was longer during post-bridging sessions than during daily interactions. These findings demonstrate that in Tibetan macaques, dyadic affiliative relationships influence the direction of allo-grooming toward vulnerable body locations in both long-term and immediate contexts. Our findings support the Affiliative Effects Hypothesis and offer new insights into the various social functions of grooming across different body locations in non-human primates.