We can be very good sisters and brothers with unknown, not-related-to-us guys if our family co-members are away: Grooming behavior of female Japanese macaques (Macaca fuscata yakui)

Behavioral responses to changes in group size and composition: a case study on grooming behavior of female Japanese macaques (Macaca fuscata yakui). Yosuke Kurihara, Mari Nishikawa, Koji Mochida. Behavioural Processes, Volume 162, May 2019, Pages 142-146. https://doi.org/10.1016/j.beproc.2019.03.005

•    Changes from multi-female to one-female groups were observed in Japanese macaques.
•    Adult females increased their grooming efforts in one-female periods.
•    They continued kin-biased grooming in one-female periods.
•    Unrelated juveniles were alternative grooming partners for them in one-female periods.
•    Rare observations of social changes are useful for understanding animal societies.

Abstract: Primates flexibly change their grooming behavior depending on group size and composition to maintain social relationships among group members. However, how drastic social changes influence their grooming behavior remains unclear. We observed the grooming behavior of adult female Japanese macaques in two groups temporarily formed as one-female groups from multi-female groups and compared their behaviors between the multi-female and one-female periods. Adult females more frequently performed grooming with both their relatives and unrelated juveniles during the one-female period when other adult females were unavailable as alternatives to their absent familiar partners. The increased grooming time and diversity of grooming partners might alleviate the short-term stress caused by the loss of grooming partners and reduce social instability or mitigate the long-term stress due to disadvantages in intergroup conflicts. Our study provides rare evidence on the flexibility in grooming behavior of primates and encourages accumulating case reports for understanding behavioral responses of primates to drastic social changes.