Social tension after grooming in wild Japanese macaques (Macaca fuscata yakui) is sex specific and sensitive to social relationships

Changes in the rate of self‐directed behavior—a proxy for social tension—offervaluable insights into individuals' experiences of social interactions. Many studies havetested the tension‐reduction hypothesis, whereby grooming is expected to reducesocial tension in the grooming partners. However, it is still unclear whether responsesto grooming are affected by social factors such as an individual's role or socialrelationship with their partner, and whether responses are similar for females andmales. Focussing on same‐sex grooming in Yakushima Japanese macaques (Macacafuscata yakui), we analyzed the effects of two social factors on the change in the rateof self‐scratching after receiving grooming: sitting in contact with their partner outsideof grooming contexts (as a measure of relationship strength) and rank distance. Wecollected 10‐min postgrooming and matched‐control focal samples on recipients ofgrooming. After controlling for postinhibitory rebound effects, grooming reducedscratching in females (suggesting that grooming was relaxing for the receiver), but notin males. In females, this reduction was associated with increased sitting in contact andincreased rank distance: being groomed was more tension‐reducing if the groomingpartners spent time together outside of grooming contexts, or if their difference inrank was greater and therefore more conspicuous. The reduction effect was mediatedby sitting in contact only when females had a higher‐ranking—that is, more dominantand potentially aggressive—groomer. Our findings suggest that not all groominginteractions are perceived as equal, which has implications for its use as an index ofrelationship quality. To our knowledge, this represents the first study of postgroomingsocial tension in male Japanese macaques and our results highlight the need forcaution when generalizing findings from only one sex to the species level.