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.
Highlights
Selected lip colour indicates male group-holding status in the mating season in a multi-level primate society
Plotting shows that during the non-mating season, AMU and one-male unit (OMU) males do not differ in lip redness
Males in OMUs who possess near universal monopolization of sexually mature females scored higher on lip redness than bachelor males
Summary
Selected lip colour indicates male group-holding status in the mating season in a multi-level primate society. A number of studies of primate species have shown that they indicate mate social status and are used in male–male agonistic interactions, suggesting that like many mammalian sexually selected traits, they have been selected primarily through male–male competition (e.g. mandrills [6,7], geladas [8], drills [9], vervet monkeys [10]) Such signals may provide information on the social status of the rival, and the potential risks associated with engaging in a contest with a particular competitor, allowing imminent conflicts to be resolved without the need for escalation. A study of drills only found evidence that male coloration is involved in indicating social status, not in attracting females beyond the effects of dominance rank [9]
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