Sexual swellings in mandrills (Mandrillus sphinx): a test of the reliable indicator hypothesis

Abstract

Various hypotheses have been proposed to account for the function of sexual swellings in female primates, but few empirical data exist to test predictions arising from these hypotheses. Controversy has recently arisen over a field study that appeared to support the predictions of the reliable indicator hypothesis. This hypothesis proposes that females compete for males or matings, that differences in swelling size between females reliably advertise female quality, and that males use swelling characteristics to differentially allocate mating effort to females with certain swelling characteristics, hence to females of higher quality. To provide an independent test of this hypothesis, we collected data concerning the size and coloration of 40 sexual swellings for 29 semi-free-ranging female mandrills, varying in age and parity, along with data concerning the behavior of males toward the females, and compared these with the long-term reproductive history of the females. We examined the following predictions: (1) swelling characteristics are consistent across subsequent cycles for individual females, (2) swelling characteristics indicate aspects of female reproductive quality, and (3) males prefer to mate with females that show particular swelling characteristics. Our results support prediction 1; we found little change in swelling characteristics across swellings for individual females. However, we found no significant relationships between female reproductive history and swelling characteristics and, thus, no support for prediction 2. Finally, we found only limited support for prediction 3; females with larger (wider) sexual swellings were more likely to have a sperm plug when maximally swollen. However, male mate-guarding was not significantly related to female swelling characteristics. Furthermore, in situations in which more than one female was maximally swollen, the alpha male (who has "free" choice) did not show the most interest in the female with the largest swelling. We conclude that the reliable indicator hypothesis does not explain variation in sexual swellings in female mandrills.