As the Rolling Stones, Revlon, and Angelina Jolie can attest, not many body parts are more sexualized than the lips. A new study published online today in Royal Society Open Science, suggests that we’re not the only primates that feel this way. Black-and-white snub-nosed monkeys (Rhinopithecus bieti, pictured) have a strict social hierarchy in which a few, older males mate with multiple females, while the younger males form bachelor groups and bide their time. The males’ lips naturally redden with age, but the story seems a little more complicated than that: A series of photographs taken over multiple months shows that mating males’ lips redden during the mating season, whereas the bachelor males’ become paler. Scientists still aren’t sure why the animals’ lips seem to correspond with their social rank, but one idea is that females prefer the redder shades when choosing a mate, similar to how a female peacock chooses the male with the most elaborate tail. Another explanation could be that the males are using lip color as a preemptive indicator of their status in order to minimize conflict: Paler lips could make bachelors appear less threatening, allowing the mating males to focus their aggression on other red-lipped competitors. Both mechanisms could also be acting simultaneously, the authors say.