Dimorphism and integration in the pelves of anthropoid primates

Abstract

The link between obstetric demand (natural selection for birth) and pelvic morphology in anthropoids has remained ambiguous for the past century. Though it is assumed that the dramatic sexual dimorphism seen in modern human pelves is due to obstetric demand, this does not explain why other primates that give birth to relatively small neonates also exhibit pelvic dimorphism. This thesis explores the correlation between obstetric demand and dimorphism and morphological integration in anthropoid pelves. Dimorphism in birth canal and non-obstetric pelvic morphology is present across most primates regardless of obstetric demand, but those primates that give birth to relatively large babies have the most pelvic sexual dimorphism. Sexual dimorphism in sacral breadth is likewise most apparent in species that give birth to large neonates relative to the maternal birth canal. In species that give birth to large neonates, females also have higher magnitudes of integration in birth canal morphology compared to males, but this is not true in species that give birth to small neonates. This study demonstrates a clear link between obstetric demand, pelvic dimorphism, and the magnitudes of pelvic integration in anthropoid primates.