Obstetric Constraints in Six Monkey Genera

Our previous work has shown that the pelvic inlet is not a point of obstetric constraint in nonhuman apes. In this study, we have extended that work to include monkey genera where research indicated the presence of significant cephalopelvic constraint, even at the pelvic inlet. We identified the true minimum dimensions of the birth canal in these monkeys and compared them to fetal head dimensions in order to determine the actual obstetric constraints in these species.

Pelves of six monkey genera (Ateles, Callithrix, Cebus, Lagothrix, Papio, and Saimiri) were digitized in order to measure the minimum dimensions within each birth canal. Fetal head dimensions from published sources were used to estimate the degree of constraint at these minimum pelvic dimensions.

The pelvic inlet was not the narrowest part of the birth canal in these monkeys. In all six genera, the minimum midsagittal dimension was a line connecting the caudal end of the sacrum with the craniodorsal surface of the pubis. The genera Callithrix, Cebus, and Saimiri exhibit a much tighter fit between the fetal head and the narrowest part of the birth canal than the other monkey genera, actually exceeding the degree of constraint seen in humans.

To allow the fetal head to pass through the birth canal, these three genera must utilize birth mechanisms not seen in humans, including face-first presentation and radical relaxation of the pelvic ligaments. Human specializations for bipedality may have necessitated the evolution of other, uniquely human obstetrical adaptations.