Many mammals use olfaction to recognize important social cues, including territory markers, group status and individual familiarity. It is generally thought that most primates use olfaction to a lesser degree, relying chiefly on sight and sound as their primary senses; however, recent research has shown that even primate species that have reduced olfactory physiology and behaviors can produce and perceive complex distinct odor profiles.

Rhesus macaques (Macaca mulatta) are one such primate species that lacks scent glands and does not exhibit scent-marking behaviors, though they have been observed using smell to inspect food and body parts of other macaques. Rhesus macaques also participate in structured social groups and behave very differently toward familiar and foreign monkeys depending on their discernible group affiliation. Recently, a team of researchers led by Stefanie Henkel of University of Leipzig (Germany) investigated how macaques respond to body odors from individuals within and outside their social groups (Behav. Ecol. Sociobiol. 69, 2019–2034; 2015). Henkel's team introduced both male and female macaques to odor samples swabbed from the genital region of female macaques; they presented these samples in metal tea eggs, and then recorded behavioral responses. This study took place on the research island Cayo Santiago, Puerto Rico, so Henkel's team was able to control for kinship and familiarity using contextual pedigree data, and the study took place during a non-mating season.

In this scenario both male and female macaques showed more interest toward the odors of unfamiliar females (from other social groups) than those of familiar females (from their own groups), placing their noses near unfamiliar odors for longer periods of time. Additionally, males and older macaques licked the tea egg more often than females and younger macaques, respectively, and individuals lingered at the odor longer when it came from a social group with a higher social rank.

Henkel et al. note that these findings are not surprising, as they comport with similar behaviors seen in other mammals that use olfaction similarly. However, these behaviors are still poorly understood in macaques, and these findings demonstrate the complex sensory signals that inform social behaviors in this common biomedical model. In a press release Henkel acknowledged that olfaction is only part of the story, noting that “the recognition of conspecifics might be a more multimodal process also including visual cues or a combination of olfactory and auditory signals. Further research is necessary to fully understand the underlying mechanisms of recognition processes”.