Mechanical Constraints on the Hominin Pelvis and the “Obstetrical Dilemma”

Abstract

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. © 2017 Wiley Periodicals, Inc.

Human variation in the shape of the birth canal is significant and geographically structured

“…Several authors have recently criticized the notion of an 'obstetrical dilemma', first of all by showing that the larger female pelvis is not less efficient than the narrower male pelvis during walking or running [5][6][7] (but see [61]). Grabowski and colleagues [62,63] showed that the human pelvis is a less constrained structure than in other apes, with lower covariance between pelvic traits, allowing it to evolve in more independent directions.…”

Cortical bone distribution in the femoral neck of Paranthropus robustus

“…Compared to the modern human condition, among other distinctive postcranial features, the australopith proximal femur shows a small head relative to the shaft breadth; a proportionally long and anteroposteriorly compressed neck (or, more appropriately, a superoinferiorly expanded neck relative to the femoral head breadth; Ruff and Higgins, 2013); a reduced neck-shaft angle; and a less laterally-projected greater trochanter positioned below the femoral head (e.g., Lovejoy et al, 1973;Lovejoy, 1975;McHenry, 1975;Tague and Lovejoy, 1986;Ruff, 1995Ruff, , 2010Ruff et al, 1999Ruff et al, , 2016Harmon, 2009;Berge and Goularas, 2010;Kibii et al, 2011;Ruff and Higgins, 2013). Such anatomy is associated with a relatively wide biacetabular breadth which appears to have characterized early hominin pelves (Gruss and Schmitt, 2015;Ruff, 2017;Vansickle, 2017). While it is still debated whether some morphological traits of the australopith hip joint evolved in response to functional demands or are retentions from an ancestral condition (Ward, 2013;Ruff et al, 2016), it has been shown that a complex and changing pattern of natural selection drove hominin hip and femoral evolution.…”

Evolutionary Theories and Men's Preferences for Women's Waist-to-Hip Ratio: Which Hypotheses Remain? A Systematic Review

“…However, a large pelvis can be an obstacle to efficient locomotion (Leutenegger, 1974; Lovejoy, 1988; Ruff, 2017 but see Warrener et al, 2015). A woman with a lower ability to walk will have higher difficulties to secure resources for her children, which will decrease their survival or quality.…”