Variation in pelvic size between males and females in nonhuman anthropoids

Whether there is a sexual difference in phenotypic variance has been the subject of theoretical and empirical studies. The presence of such a difference is integral to some models on the evolution of sexual dimorphism. Several studies report that males are more variable than females for nonpelvic measures. This study tests for a sexual difference in variability of the pelvis. Phenotypic variance is a correlate of both the intensity and mode of selection. In both sexes, the pelvis is subject primarily to stabilizing selection. However, selection intensity is greater among females than among males because only among females does the pelvis function as a birth canal and, thereby, serve as a proximate cause of death. As selection intensity and phenotypic variability are inversely related under stabilizing selection, the implication is that females should be less variable than males in pelvic size and shape. However, the results of previous studies on this issue are equivocal or contradictory. This study compares the sexes for differences in pelvic variability by analyzing nine species (and ten samples) of nonhuman anthropoids. Data were collected on 16 measures of the pelvis; only adults were used. Levene's univariate and multivariate tests for relative variation were used in the analysis. The results show that the sexes do not differ significantly in pelvic variability either within or among the nine samples of noncaptive anthropoids. Only in the one sample of captive specimens (Saimiri sciureus) do the sexes differ significantly in pelvic variability, with males being more variable than females. Two interpretations are derived from these results. First, the sexual difference in variability in captive Saimiri may be due either to a bias in the selection of Saimiri specimens by the captors/experimentors or to a sexual difference in growth associated with the stress of captivity. Second, the sexes do not differ in pelvic variability among noncaptive anthropoids because the anlage of the pelvis is bipotential in development. The intrinsic (i.e., agonadal) pattern of growth and development is that characteristic of a female. Testicular androgens are requisite to redirect the pattern of growth and development to that characteristic of a male. However, the redirected pattern of growth in males simply shifts en masse the intrinsic (i.e., female) distribution curve. The consequence among adults is sexual dimorphism in pelvic size, but sexual equivalency in relative pelvic variability.