Emotional states after grooming interactions in Japanese macaques (Macaca fuscata)

In animal societies, the effect of grooming interactions on anxiety reduction is unclear. This study examined the effects of giving and receiving grooming on anxiety reduction in free ranging female Japanese macaques (Macaca fuscata) by measuring rates of self-scratching as an index of anxiety. In this study, the authors used a focal-animal sampling method, targeting 17 females at Katsuyama, Okayama prefecture, Japan. They evaluated affiliative relationships, which were defined by standard proximity rates, and found that females' self-scratching rates were lower after grooming affiliated partners than during matched-control periods (occurring on another day, beginning at approximately the same time of day as the corresponding postgrooming period) and not after grooming unaffiliated partners. Moreover, regardless of affiliative relationships, self-scratching rates were lower after receiving grooming than during matched-control periods. These findings did not change after excluding data in which groomer and groomee were in proximity after the grooming interaction. In addition, multivariable analysis showed that affiliative relationships, but not kinship or rank distances, were related to differences in the rates of self-scratching between giving grooming and matched-control periods. In contrast, neither affiliative relationships nor kinship nor rank distances affected differences in self-scratching rates between receiving grooming and matched-control periods. Therefore, individuals' anxiety levels decreased both after giving grooming to affiliated partners and after receiving grooming, regardless of affiliative relationships. This is the first empirical study to support the notion that giving grooming to affiliated partners is self-rewarding in Japanese macaques.