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 swellings in female primates serve as honest signals of female quality that function in female-female competition over mates. We examined a version of this hypothesis using interspecific data to test whether exaggerated sexual swellings are associated with female mating competition, as measured using the adult sex ratio, female canine size, and expected female mating synchrony. The ratio of females to males and relative canine size declined over evolutionary transitions in swelling state, thus providing no support for the reliable indicator hypothesis. Expected female mating synchrony increased over evolutionary transitions in swelling state, but this pattern did not approach significance, and the patterns were opposite to predictions when controlling for the number of males in the group. In addition to these comparative tests, we reviewed evidence concerning individual attributes of females relative to characteristics of their swellings. Contrary to the reliable indicator hypothesis, the least fertile females, or those least likely to raise surviving offspring, often have larger swellings. We consider the statistical power of our tests, discuss the theoretical and empirical bases for our comparative predictions, and consider other lines of evidence needed to test the reliable indicator hypothesis. We also discuss an alternative hypothesis, the graded signal hypothesis, which combines the benefits of biasing and confusing paternity through a novel mechanism, and is testable in the field and the laboratory.