A pair of red, glossy lips is a popular advertisement for sexiness, and this ideal has left its mark all over human culture: Applying a "cupid’s bow" was a key part of flapper attire in the 1920s, for example, and people’s thirst for lip color is strong enough that they have endured some strange and dangerous lipstick ingredients. But we aren’t the only species to find reddened lips alluring.
Black-and-white snub-nosed monkeys enjoy a bit of natural lipstick, reports David Shultz for Science. The smaller primate’s twist on the phenomenon is that the red color also decorates the lips of males, and only the sexiest males get to wear the brightest colors.
Researchers hailing from China, the U.K, Australia and the U.S. describe how males of the large, endangered primate species have lips that redden as they age. Unlike the flashy plummage of male birds and bright colors of male fish, the monkey lip color isn’t just a way to attract females, the researchers write in Royal Society Open Science. Since older males have redder lips and bachelor males keep their lips pale, the "lipstick" might also serve as a way to enforce social hierarchy.
"Paler lips could make bachelors appear less threatening, allowing the mating males to focus their aggression on other red-lipped competitors," Shultz writes. It’s also possible that the ladies prefer red-lipped mates, but the researchers’ data couldn’t offer a firm indication either way. Both mechanisms—sexual selection and male-male competition—could be at play. Lip color intensity was most intense during the peak of the mating season, the researchers note.
Other monkeys also redden in various part of their bodies, such as their faces, around their genitals and all over their skin. Some of these changes appear to be related to sexual availability and some in response to social order. At least it seems that for primates the world over, red is an exciting color.