Triadic grooming among adult females in a free-ranging group of Japanese macaques.

This study aimed to quantitatively describe triadic grooming bouts in a free-ranging group of Japanese macaques (Macaca fuscata). Two types of triadic grooming bouts occurred less frequently and for a shorter duration than dyadic grooming bouts: straight-line type (SL), where individual A grooms B, who then grooms C; and two-to-one type (TO), where individuals X1 and X2 groom Y. TO-type grooming was recorded more often than SL-type grooming. As in the dyadic grooming bouts, two females who had direct grooming interactions in both SL-type (i.e., between A and B or between B and C) and TO-type groomings (i.e., between X1 and Y and between X2 and Y) and two groomers who did not have direct grooming interactions in TO-type grooming were closely related to each other in more than the half of the pairs recorded. Groomers were more likely to be subordinate to groomees in triadic grooming. Almost all of the observed pairs in SL- and TO-type groomings were also recorded in dyadic grooming bouts. These findings indicate that like dyadic grooming, triadic grooming bouts are largely influenced by blood relatedness through maternal lines and dominance relationships between participants. Based on affiliative relationships maintained through dyadic grooming, triadic grooming could be influenced by such affiliative relationships and then function to strengthen bonds between participants. Possible relationships between triadic grooming and the level of social tolerance among individuals are discussed from the viewpoints of interspecies differences among macaque species and regional differences in Japanese macaques.