Social contagion of non-interactive behavior is widespread among animals including humans. It is thought to facilitate behavioral synchronization and consequently group cohesion, coordination, and opportunities for social learning. Contagion of interactive behavior - particularly affiliation - has received much less attention. Here, we investigated in female rhesus macaques (Macaca mulatta) the effect of observing group members groom on a subject’s subsequent grooming behavior and the potential modulation of contagion by relationship quality and social status. We recorded behavior after subjects witnessed a grooming event and compared it to behavior in a control condition with the same individuals in proximity but in the absence of a stimulus grooming event. Compared to the control condition, after observing others groom, females engaged in a grooming interaction sooner, and were more likely to be the initiator and to take on the active groomer role. Dominance rank of the focal individual and more weakly also of the stimulus individuals affected the latency to the next grooming interaction of the focal subject. Latency to the next grooming interaction decreased with increasing rank of the subject potentially reflecting lower social constraints faced by high ranking individuals in this highly despotic species. Relationship quality between the subject and the stimulus individuals had no effect on latency to grooming. Collectively, our findings provide evidence for visual contagion of affiliation in rhesus macaques. Future studies should explore the systematic variation in contagion of interactive behavior in relation to a gradient of social tolerance.

uploaded data files:

Data file_GLM_dominance rank - difference in latency (in seconds) to the next grooming interaction the focal was involved in between matched control and post-grooming  observations predicted by standardized focal rank and stimulus rank. Data from all three study groups.

Data file_GLM_relationship Quality -difference in latency (in seconds) to the next grooming interaction the focal was involved in between matched control and post-grooming observations predicted by relationship quality between focal animal and stimulus individual. Data from 2 study groups with relationship information.

Data file_Wilcoxon tests - Data for Wilcoxon tests (predictions 1, 5, 6) from 19 focal females.