Grooming-related feeding motivates macaques to groom and affects grooming reciprocity and episode duration in Japanese macaques (Macaca fuscata). 
Allogrooming is considered as an altruistic behavior wherein primates exchange grooming as a tradable commodity for reciprocal grooming or other commodities such as support during aggression and tolerance during co-feeding.  First, we report a case of the grooming relationships of the lowest-ranking adult female in a group of Japanese macaques (Macaca fuscata).  The female (Lp) had lost a portion of the fur and was groomed by higher-ranking individuals without providing reciprocal grooming or other commodities.  The groomers probably fed on lice eggs from the fur of Lp more frequently than from that of other adult groomees.  This suggests that grooming-related feeding (GRF) motivated many individuals to groom Lp and influenced grooming reciprocity in dyads.  Second, we investigated quantitative grooming data for adult females.  A high GRF rate was found to lengthen the duration of grooming, suggesting that GRF motivates groomers to groom.  From these results, we proposed 2 possible reasons for groomers' sensitivity to GRF rate: (1) the nutritional benefit from GRF compensates for part of the cost of giving grooming and facilitates giving grooming and (2) groomer's sensitivity to the GRF rate maintains the efficiency of removing lice eggs and ensures the groomee's hygienic benefit in receiving grooming.