Is Brightest Best? Testing the Hamilton-Zuk Hypothesis in Mandrills

Although many primates exhibit striking coloration, including brightly colored pelage and bare areas of skin, our understanding of the function and evolution of these traits pales in the face of knowledge about color in other taxa. However, recent years have seen an increase in the number of studies of individual variation in primate color and evidence is accumulating that these traits can act as important signals to conspecifics. Mandrills are arguably the most colorful of all primates. Here, we review what we have discovered about the signal function of coloration in male and female mandrills from our long-term studies of a semi-free-ranging colony in Franceville, Gabon and test the predictions of the Hamilton-Zuk hypothesis-that bright coloration is condition dependent, and that only individuals of superior quality will be able to express color fully-in this species. We compare measures of facial coloration in both sexes with parasite load (using fecal analysis over 1 annual cycle), immune status (hematological parameters), neutral genetic diversity (microsatellite heterozygosity), and major histocompatability (MHC) genotype to examine whether red coloration acts as an honest signal of individual quality in mandrills. We found that red coloration was unrelated to parasitism and hematological parameters. Red was also unrelated to genome-wide heterozygosity and MHC diversity, although specific MHC genotypes were significantly related to red. The healthy, provisioned nature of the colony and problems associated with observational, correlational studies restrict interpretation of our data, and it would be premature to draw conclusions as to whether color signals individual quality in mandrills. We conclude with some suggestions for future studies on the signal content of color in mandrills and other primates.