Although variation in the degree and pattern of pelvic sexual dimorphism among primates has been a subject of extensive investigation, researchers disagree on the relative contribution of various factors, including obstetrics, locomotion, body size, and phylogeny, to the observed variation in sexual dimorphism of adult pelves. The ontogenetic bases of pelvic sexual dimorphism and evolutionary significance of variability in pelvic ontogeny remain largely uncharacterized in nonhuman primates.

To better understand selective mechanisms affecting pelvic morphology, this study addresses postnatal growth patterns and trajectories underlying various degrees and patterns of pelvic sexual dimorphism in four anthropoid primates, Saimiri sciureus, Alouatta seniculus, Hylobates lar, and Pan t. troglodytes. Morphometric techniques (EDMA) were employed to characterize static patterns of pelvic sexual dimorphism at each developmental age and sexual dimorphism in pelvic growth patterns. A non-parametric regression analysis (loess) was used to determine growth trajectories of individual pelvic variables with respect to body size during postnatal growth periods.

The results of this study reveal distinct postnatal growth patterns and trajectories that are responsible for the development of pelvic sexual dimorphism. In species with high cephalopelvic ratios, sexual dimorphism in the obstetric pelvis can be derived from a higher female growth rate occurring consistently throughout growth periods or during adolescence. Sexual dimorphism in the sagittal plane of the pelvis has different ontogenetic bases than transverse dimorphism, and is typically a result of sexual differences in growth cessation among large species. The degree of inlet dimorphism is closely related to the cephalopelvic ratio, and is largely determined by sexual differences in pubic growth. The surprising result of a larger female bispinous diameter in A. seniculus may be a product of bimaturism, a process characterized by earlier female maturation or completion of maximum body weight. Hominoid transverse diameters of the birth canal share a similar degree and pattern of sexual dimorphism regardless of variation in body size and cephalopelvic ratios. Future research on important life history correlates of pelvic ontogeny may help reveal evolutionary mechanisms of pelvic sexual dimorphism in relation to the age of females’ first reproduction, infant mortality, and reproductive strategies of the organisms.