A model dilemma: how assumptions about locomotion and birth have shaped the obstetrical dilemma (335.1)

The obstetrical dilemma posits that the human female pelvis is under antagonistic selection pressures: narrow enough for efficient locomotion, but large enough to pass a large‐brained infant through the birth canal. This adaptive trade‐off is thought to explain the apparent difficulty of human childbirth, the helplessness of human neonates, and sex differences in human locomotor performance. However, core assumptions of the obstetrical dilemma model have gone untested. One assumption, that wider pelves in women increase locomotor cost, is based on a static biomechanical model of hip abductor function. Wider biacetabular width is thought to increase the external torque about the hip by moving the body center of mass farther from the hip joint, thereby increasing hip abductor muscle force and energetic cost to stabilize the pelvis. However, skeletal dimensions are actually poor proxies for the mechanical determinants of hip abductor function measured during locomotion and do not correlate significantly with locomotor cost. While this first assumption of the obstetrical dilemma can be empirically rejected, it is harder to directly assess the bases for and evolutionary history of cephalopelvic disproportion (CPD) that uniquely imperils human birth. A novel hypothesis to be tested suggests that recent environmental conditions have made CPD more common today than through most of human evolutionary history. Grant Funding Source: National Science Foundation BCS#0850841, The Leakey Foundation, The Wenner‐Gren Foundation