The lips of black-and-white snub-nosed monkeys redden over the course of mating season – but only if they’re the head of a harem. Their younger rivals literally pale in comparison, according to findings published in Royal Society Open Science this week. 

Black-and-white snub-nosed monkeys (Rhinopithecus bieti) in China are one of the most vividly colored primates. They live in bands of one-male, multi-female units (OMU) with associated bachelor groups consisting of young males kicked out of their family unit. While bachelor males stay close to OMUs, they don’t have any reproductive access to females in the harem. These bands often aggregate into larger, semi-permanent, multilevel systems of up to 500 monkeys. Surely they need a way to assess quality without relying on individual recognition.

To study how lip coloration might be correlated to status, the University of Western Australia’s Cyril Grueter and colleagues photographed and quantified the redness of the lower lip of 15 males (seven OMU holders and eight bachelors) at Xiangguqing in the Baimaxueshan Nature Reserve of Yunnan, China.

The OMUs they observed had between two and five females each; all of the group-holding males were at least eight years old, while the bachelors ranged between juveniles and adults. To the right, you can see the face and lip regions that were selected for analysis. The lip color was divided by the face color to control for different lighting and photographic set-ups between the images. The team ended up with an average of six of these standardized photos per male. 

Lip redness increases with age, they found, and there was no difference in lip redness between OMU males and bachelors during non-mating season. But during mating season – which lasts from August to October – color intensity for OMU males was much higher than for bachelor males. In fact, the lip color actually fades in males living in all-male groups during those months of the year. 

These findings indicate that lip coloration is a badge of male social status during mating season, but redder lips could still mean one of two things: strength or sexiness. In birds, the flashy colors and extravagant ornamentation of males are typically aimed at attracting females. Brightly colored skin in male mammals hasn’t been studied as much, and the researchers have yet to figure out if this reddening is a product of male–male competition or female mate choice or both.

^{Image in the text: C.C. Grueter et al., R. Soc. Open Sci. 2015}