Development of Pelvic Sexual Dimorphism in Hylobatids: Testing the Obstetric Constraints Hypothesis

Pelvic sexual dimorphism in primates is typically seen as the result of female-specific adaptations to obstetric constraints, which arise from the tight fit between the neonate head and the maternal pelvis. However, it remains debated to which extent pelvic dimorphism is a correlate of obstetric constraints, of body size dimorphism, and/or of other factors. Also, little is known on how pelvic dimorphism develops. Here we use methods of biomedical imaging and geometric morphometrics to address these questions in two hylobatid species, Hylobates lar and Symphalangus syndactylus. These species differ markedly in body size, but within each species, there is only little body size dimorphism. Results show that the neonates of H. lar are large compared to the maternal pelvis, resulting in high cephalopelvic proportions and substantial obstetric constraints. Pelvic sexual dimorphism is moderate but significant: During puberty, females develop a more capacious pelvic inlet than males, while overall pelvic size is similar in both sexes. S. syndactylus has substantially larger pelves than H. lar, but neonates are similar in size to those of H. lar. Cephalopelvic proportions are thus low, and there are no obstetric constraints. Pelvic sexual dimorphism is absent. Overall, these data indicate that pelvic sexual dimorphism in hylobatids reflects obstetric constraints. Anat Rec, 300:859-869, 2017. © 2017 Wiley Periodicals, Inc.