The 88th Annual Meeting of the American Association of Physical Anthropologists (2019)

Obstetric constraints in large-brained cebids and modern humans: a comparison of coping mechanisms


1Anthropology, Boston University, 2Evolutionary Anthropology, Duke University

March 29, 2019 , CC Ballroom BC Add to calendar

Compared to other primates, humans are claimed to have extreme maternal-fetal cephalopelvic disproportion. This claim assumes that other primates, like humans, have a birth canal that is constrained at the pelvic inlet and a vertex presentation of the fetal head as it enters the birth canal. But these are false assumptions; in non-human primates the inlet is not an area of obstetric constraint, and presentation of the fetal head is generally face-first. Yet even when more realistic assumptions are adopted, the ratio of the effective (coronal) area of the fetal head to that of the bony birth canal at its narrowest point approaches or exceeds the human ratio in some New World monkeys, rising as high as 141% in Cebus albifrons and 151% in Cebus apella (coronal presentation), compared to a mean of 108% for a vertex presentation in our human sample (n≥10 for all species). In humans, the disproportion is accommodated through cranial remodeling and relaxation of the pelvic ligaments. But platyrrhine neonates lack large fontanels in the skull roof, and little or no cranial remodeling can occur during delivery. The necessary expansion of the birth canal during labor is produced by extreme pelvic relaxation. Combining published radiographs of Saimiri births with 3D animation modeling, we reconstruct the way these primates give birth, compare their birth mechanisms to those known or inferred for humans and apes, and propose functional and evolutionary explanations for the differences.

Wenner-Gren Foundation Dissertation Fieldwork Grant and Boston University Summer Graduate Research Abroad Fellowship