Obstetric adaptation of the human pelvic inlet based on a sample of females and males matched for body size, and comparison with three species of primates that are sexually monomorphic in body size.

Human males are larger than females in body size; females are larger than males in pelvic size. Percentage pelvic sexual size dimorphism (sex with larger mean/sex with lower mean*100%) is used to infer selection for obstetric adaptation. As pelvic size is associated with body size, interpretation of percentage sexual size dimorphism necessitates control of body size. This study uses a larger sample of 221 females and 204 males to identify a subsample of females and males matched for femoral length, femoral head diameter, and clavicular length (25 females, 46 males). For five measures of the pelvic inlet, the human sample with control of body size is compared with three primate species that are sexually monomorphic in body size and give birth to newborns with higher relative body masses than those of humans: Aotus azarae (15 females, 19 males), Hylobates lar (29 females, 29 males), and Saguinus geoffroyi (28 females, 30 males). Results show that humans (112.0%) have the highest percentage sexual size dimorphism of pelvic inlet circumference: A. azarae (108.2%), H. lar (105.4%), and S. geoffroyi (103.3%). Humans have higher sexual size dimorphism of inlet circumference despite lower relative newborn body mass compared to the other species because human newborns are of larger body mass than predicted for an anthropoid with our adult female body mass. In humans, birthing of bigger babies than predicted for our body mass increases selection pressure on females for enlargement of the pelvic inlet.