Signaling context modulates social function of silent bared-teeth displays in rhesus macaques (Macaca mulatta)

The signaling context has been found to change the meaning of the silent bared-teeth display (SBT) in pigtail macaques (Macaca nemestrina) such that the SBT in apparently peaceful contexts communicates subordination, a long-term pattern of behavior, whereas in conflict contexts it communicates immediate submission (PNAS, 104: 1581-1586). However, the context dependent nature of the SBT has not yet been explored in other species. We investigated SBT usage with respect to grooming, severe aggression, and signaler-receiver sex, rank difference, and body size in seven captive groups of rhesus macaques. Peaceful SBTs were given most often to male receivers by male and female signalers whereas conflict SBTs were given to both male and female receivers primarily by female signalers. Male signalers rarely gave SBTs (peaceful or conflict) to female receivers. Unlike pigtail macaques, peaceful SBTs in rhesus were often accompanied by withdrawal behavior (referred to as peaceful SBT-leave), which influenced grooming, but not aggression, at the dyadic level. Severe aggression was less frequent among dyads using peaceful SBTs (regardless of withdrawal behavior) than those using conflict SBTs. In contrast, grooming was more frequent among dyads using peaceful SBT-stay signals than those using peaceful SBT-leave signals or conflict SBTs. In total, our results indicate that peaceful SBTs are a functionally different signal from conflict SBTs in rhesus macaques.