Rhesus macaques (Macaca mulatta) recognize group membership via olfactory cues alone

Abstract

The ability to distinguish group members from conspecifics living in other groups is crucial for gregarious species. Olfaction is known to play a major role in group recognition and territorial defense in a wide range of mammalian taxa. Although primates have been typically regarded as microsmatic (having a poor sense of smell), increasing evidence suggests that olfaction may play a greater role in primates’ social life than previously assumed. In this study, we carried out behavioral bioassays using a signaler-receiver paradigm to investigate whether rhesus macaques (Macaca mulatta) can discriminate between body odors of female group members and females from different social groups. We conducted the study on the research island Cayo Santiago, Puerto Rico, in the non-mating season and controlled for kinship and familiarity using extensive pedigree and demographic data. Our results indicate that both males and females inspect out-group odors significantly longer than in-group odors. Males licked odors more often than females, and older animals licked more often than younger ones. Furthermore, individuals tended to place their nose longer towards odors when the odor donor’s group rank was higher than the rank of their own group. Reuse of odor samples decreased odor intensity (rated by human experimenters) during the course of a given test day and with longer exposure to ambient air; however, the reuse of odor samples did not significantly influence the response behaviors. Our findings uncover key roles of olfactory communication in a species not possessing distinct scent glands and thus shed light into the evolution of primate olfactory communication.