Dimorphism and integration in the pelves of anthropoid primates

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.

Pelvic sexual dimorphism among species monomorphic in body size: relationship to relative newborn body mass

Females have larger pelves than males among eutherians to mitigate obstetrical difficulty. This study addresses 3 issues concerning pelvic sexual dimorphism using 8 species that are sexually monomorphic in nonpelvic size: Aotus azarae , Castor canadensis , Dasypus novemcinctus , Hylobates lar , Saguinus geoffroyi , Sciurus carolinensis , Sylvilagus floridanus , and Urocyon cinereoargenteus . Using published data to compute the index of relative newborn body mass (RNBM = [newborn body mass/adult female body mass]100%) for 266 eutherian species, A. azarae , H. lar , and S. geoffroyi are characterized as giving birth to relatively large newborns and the other 5 species as giving birth to relatively small newborns. The 3 issues are, compared to species giving birth to relatively small newborns, whether species that give birth to relatively large newborns have 1) higher magnitude of pelvic sexual size dimorphism (SSD), 2) lower prevalence of pelvic joint fusion, and 3) dissociation between pelvic and nonpelvic sizes. Nine measures of the pelvis were taken, and fusion of interpubic and sacroiliac joints was observed. Species grouped by high and low RNBM do not differ significantly in magnitude of SSD of pelvic inlet circumference. Species with high RNBM have significantly lower prevalence of interpubic joint fusion than those with low RNBM. Sexes do not differ in their multiple correlation coefficients between inlet circumference and nonpelvic body size in 7 of 8 species. Results suggest that 1) there are multiple anatomical pathways for pelvic obstetrical sufficiency, 2) an unfused interpubic joint is obstetrically advantageous, and 3) relative newborn size does not change the association between pelvic and nonpelvic size in females and males.