Variation in kin bias over time in a group of Tibetan macaques at Huangshan, China: contest competition, time constraints or risk response?

We examine variation in grooming kin bias intensity (KBI) among wild female Tibetan macaques (Macaca thibetana hitangshanensis) in one group over time. We test three hypotheses based on socioecological theory, time constraints and risk-related responses. Only the time constraints hypothesis was supported. Grooming KBI was higher when the group was larger, but was unrelated to other indicators of within-group competition. Allies were not necessarily frequent grooming partners; thus, support did not depend on maintaining strong grooming relationships. Females groomed similar amounts, regardless of group size, but groomed smaller percentages of available partners when the group was larger, suggesting that they were unable to maintain grooming relationships with all females as the group expanded. Females with several close kin groomed each of them less than females with few. The lowest ranking females tended to groom close kin almost exclusively, as expected if they had less grooming time to spare than other females. Although grooming KBI was higher when stressful external risks (many humans) were present, the correlation was unsustained when group size was controlled. We suggest that kin-focused grooming networks are shaped at least in part by time constraints and may not be linked directly or indirectly to within-group competition.