Mechanical Constraints on the Hominin Pelvis and the "Obstetrical Dilemma".

The "obstetrical dilemma" posits that increases in birth canal dimensions during hominin evolution were constrained by mechanical factors associated with bipedal gait. This model has recently been challenged, in part on the basis of experimental data showing little association between pelvic and proximal femoral dimensions and locomotor costs among human experimental subjects. However, complete rejection of the model is premature, for two reasons: (1) it is difficult to extrapolate experimental results to naturalistic conditions and the much more varied body form and possibly locomotor style of early hominins. Relative pelvic breadth and femoral neck length in both australopiths and early Homo are at or beyond the limits observed among modern humans, and there is evidence that australopiths may have used an altered form of bipedal gait. (2) Other mechanical factors in addition to locomotor cost are important when assessing pelvic and proximal femoral morphology. Increasing biacetabular breadth increases the joint reaction force (JRF) on the femoral head. Increasing femoral neck length may reduce hip JRF, but also increases stress and fracture risk in the femoral neck. Increasing both biacetabular breadth and femoral neck length also increases mediolateral bending stress in the proximal femoral shaft, necessitating substantial buttressing of the diaphysis. Thus, there are mechanical consequences beyond simply locomotor costs that must be considered when evaluating variation in pelvic and femoral morphology among past and extant hominins. Anat Rec, 300:946-955, 2017.