Sexually selected lip colour indicates male group-holding status in the mating season in a multi-level primate society

Abstract

Sexual selection typically produces ornaments in response to mate choice, and armaments in response to male–male competition. Unusually among mammals, many primates exhibit colour signals that may be related to one or both processes. Here, we document for the first time correlates of facial coloration in one of the more brightly coloured primates, the black-and-white snub-nosed monkey (Rhinopithecus bieti). Snub-nosed monkeys have a one-male unit (OMU) based social organization, but these units aggregate semi-permanently into larger bands. This form of mating system causes many males to become associated with bachelor groups. We quantified redness of the prominent lower lip in 15 males (eight bachelors, seven OMU holders) in a group at Xiangguqing, China. Using mixed models, our results show that lip redness increases with age. More interestingly, there is a significant effect of the interaction of group-holding status and mating season on redness; that is, lip colour of OMU males undergoes reddening in the mating season, whereas the lips of subadult and juvenile bachelor males become paler at that time of year. These results indicate that lip coloration is a badge of (group-holding) status during the mating season, with non-adults undergoing facial differentiation, perhaps to avoid the costs of reproductive competition. Future research should investigate whether lip coloration is a product of male–male competition, and/or female mate choice.

Platyrrhine color signals: New horizons to pursue

“…Catarrhine primates are well known to possess large patches of exposed skin on their faces and hindquarters, and to possess skin color that varies in response to fertility, pregnancy, and other reproductive phases, as well as social status . In addition to chroma (the hue and saturation components of color), patterns of luminance variation is a frequent but often underemphasized component of color signaling in catarrhines . Chromatic and achromatic components of signals seem to have different mechanisms of inheritance but often covary .…”

“…31 In addition to chroma (the hue and saturation components of color), patterns of luminance variation is a frequent but often underemphasized component of color signaling in catarrhines. 32,33 Chromatic and achromatic components of signals seem to have different mechanisms of inheritance 34 but often covary. 35 For example, red male rhesus macaques (Macaca mulatta) receive more sexual solicitations by more females than pale pink males, suggesting the potential importance of both chroma and luminance.…”

Male characteristics as predictors of genital color and display variation in vervet monkeys

“…In geladas (Theropithecus gelada), for instance, the skin color of leader males holding larger groups was redder than that of males holding smaller groups (Bergman et al 2009). Similarly, a study of black-and-white snub-nosed monkeys (Rhinopithecus bieti) showed that males' lip color reddened with both increasing age and when holding a group compared to subadult and non-group holding males (Grueter et al 2015), indicating that color does covary with social status.…”

“…However, it is not clear what drives color variation within these species (Setchell 2005;Dubuc et al 2014a). Correlational studies on primates suggest that male coloration is more likely to represent social status (Setchell et al 2008;Bergman et al 2009;Marty et al 2009;Grueter et al 2015). In geladas (Theropithecus gelada), for instance, the skin color of leader males holding larger groups was redder than that of males holding smaller groups (Bergman et al 2009).…”

“…Previous studies on signals of male social status in primates have addressed the covariates of single color patches (e.g., Setchell and Wickings 2005;Marty et al 2009). However, multi-component color signals may be more accurate and informative for communication than single colors (Renoult et al 2011;Grueter et al 2015;reviewed in Setchell 2015).…”