Social contagion of affiliation in female macaques

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.