Sexual Dimorphism in The Birth Canal and Non‐Obstetric Pelvis among Anthropoids: Effects of Cephalopelvic Proportions

Abstract

As important as birth is hypothesized to be in shaping the primate pelvis, we actually know very little about how birth affects pelvic form across anthropoids. This study tests whether large cephalopelvic proportions are related to dimorphism in the obstetric and non‐obstetric pelvis. Landmark data for 118 anthropoid specimens were analyzed [Large cephalopelvic proportions: Ateles geoffroyi (n=4), Homo sapiens (n=40), Hylobates lar (n=22), Macaca fasicularis (n=10), Nasalis larvatus (n=13), Papio cynocephalus (n=10), Saimiri sciureus (n=38); Small cephalopelvic proportions: Alouatta palliata (n=5), Gorilla gorilla (n=21), Pan troglodytes (n=20), Pongo pygmaeus (n=12)]. Birth canal dimensions were significantly larger in females compared to males (p<0.05) within species having large cephalopelvic proportions, but not in species with small proportions. Birth canal shape, quantified using principal components analysis, was also dimorphic in species with large cephalopelvic proportions (all p‐values <0.05), but not for species with small proportions. Patterns of dimorphism, quantified using Euclidean Distance Matrix Analysis, were shared among phylogenetic groups (New World monkeys, Old World monkeys, apes). Finally, dimorphism in the non‐obstetric pelvis did not correspond predictably to the presence of birth canal dimorphism or to large cephalopelvic proportions.