Do exaggerated sexual swellings function in female mating competition in primates? A comparative test of the reliable indicator hypothesis

The reliable indicator hypothesis proposes that exaggerated sexual swelling s in female primates serve as honest signals of female quality that functio n in female-female competition over mates. We examined a version of this hy pothesis using interspecific data to test whether exaggerated sexual swelli ngs are associated with female mating competition, as measured using the ad ult sex ratio, female canine size, and expected female mating synchrony. Th e ratio of females to males and relative canine size declined over evolutio nary transitions in swelling state, thus providing no support for the relia ble indicator hypothesis. Expected female mating synchrony increased over e volutionary transitions in swelling state, but this pattern did not approac h significance, and the patterns were opposite to predictions when controll ing for the number of males in the group. In addition to these comparative tests, we reviewed evidence concerning individual attributes of females rel ative to characteristics of their swellings. Contrary to the reliable indic ator hypothesis, the least fertile females, or those least likely to raise surviving offspring, often have larger swellings. We consider the statistic al power of our tests, discuss the theoretical and empirical bases for our comparative predictions, and consider other lines of evidence needed to tes t the reliable indicator hypothesis. We also discuss an alternative hypothe sis, the graded signal hypothesis, which combines the benefits of biasing a nd confusing paternity through a novel mechanism, and is testable in the fi eld and the laboratory.