Assessing male gelada chest patches: color measurement and physiological mechanisms

Selective pressures have favored conspicuous coloration across a wide variety of taxa. A particularly striking example of conspicuous coloration is the brilliant red chest patch of male geladas (Theropithecus gelada), a species of cercopithecine monkey found in the high-altitude regions of Ethiopia. Previous research found that gelada chest patch redness increases with age (adult vs subadult), social status (“leader” vs non-leader), and mating opportunities (number of adult females), but the mechanism mediating changes in redness has not yet been examined. First, we validated and compared multiple color measurement methods (Adobe Photoshop, micaToolbox designed for use with ImageJ, and a subjective measure using the human eye). Second, we demonstrated that chest patch redness is positively associated with high-intensity physical activity, the application of a heat pack directly to the chest skin, and higher chest skin surface temperatures. Together, these results suggest that increases in chest redness are mediated by increased blood flow to this area with a concomitant increase in surface temperature. Further research is needed to understand both the energetic costs associated with redness and how other males respond to variation in the signal.