The Biomechanical and Energetic Advantages of a Mediolaterally Wide Pelvis in Women

Abstract

Here, we argue that two key shifts in thinking are required to more clearly understand the selection pressures shaping pelvis evolution in female hominins: (1) the primary locomotor mode of female hominins was loaded walking in the company of others, and (2) the periodic gait of human walking is most effectively explained as a biomechanically controlled process related to heel-strike collisions that is tuned for economy and stability by properly-timed motor inputs (a model called dynamic walking). In the light of these two frameworks, the evidence supports differences between female and male upper-pelvic morphology being the result of the unique reproductive role of female hominins, which involved moderately paced, loaded walking in groups. Anat Rec, 300:764-775, 2017. V C 2017 Wiley Periodicals, Inc.

Expanding the evolutionary explanations for sex differences in the human skeleton

“…But as Plavcan 40,41 has cautioned, there is not a straight‐forward relationship between sexual selection and primate male body size, largely because the sorts of data that are required to investigate this relationship are difficult to obtain. It is also difficult to tease selection on male body size apart from selection on female body size through the generations, which is sometimes understood within the framework of females as the “ecological sex.” 42–44 Given the nutritional, energetic, metabolic, and locomotor costs of pregnancy, lactation, and mothering, 45,46 there are (context‐specific) limits to female body size, 47 perhaps leading to biology that favors reproduction over growth 48,49 …”

Anatomy, Development, and Function of the Human Pelvis

“…To start the issue, Cara Lewis (Boston University) and colleagues provide a broad overview of basic pelvic anatomy and function in living humans. Lewis (, this issue) presents evidence that there are significant differences in pelvic function between males and females—a finding that is supported in detail by other scholars (Gruss et al, , this issue; Wall‐Scheffler and Myers, , this issue; Whitcome et al , this issue) later in the issue. Finally, Lewis (, this issue) discusses the etiology of over and under coverage of the acetabulum and the resulting complications—femoroacetabular impingement, which causes pain and limits hip mobility—in the context of hip evolution and pelvic dimorphism, suggesting that the higher prevalence in females than in males may be a result of the evolutionary challenge of bipedalism and obstetrical adequacy in human females.…”

“…Therefore, the energetics of walking are similar in men and women not because of identical anatomies, but because of slightly different walking kinematics. However, here and in her previous work (Wall‐Scheffler, ; Wall‐Scheffler and Myers, ), Cara Wall‐Scheffler of Seattle Pacific University and Marcie Myers of St Catherine University (Wall‐Scheffler and Myers, , this issue) completely reframe the original question. Perhaps we should not be asking about (and refuting) the negative consequences of a wide pelvis, but examining instead the potential adaptive benefits of such a morphology.…”

“…However, up until this point, this hypothesis has not been formally tested. Three studies published in this issue address this very question (Gruss et al, , this issue; Wall‐Scheffler and Myers, , this issue; Whitcome et al, , this issue) and are stunning in how concordant the results are. Gruss (Radford University) and colleagues present their findings (Gruss et al, , this issue) as a formal test of Rak's hypothesis.…”

Variation in obstetric dimensions of the human bony pelvis in relation to age‐at‐death and latitude

“…Under the argument of the obstetrical dilemma, the form of the pelvis has evolved through tradeoffs between locomotor and obstetric selective pressures (Ruff, ; Wells, DeSilva, & Stock, ), although recent studies have questioned whether such an evolutionary tradeoff exists (Rosenberg & Trevathan, ; Dunsworth, Warrener, Deacon, Ellison, & Pontzer, ; Betti, von Cramon‐Taubadel, Manica, & Lycett, ; Grabowski & Roseman, ; Warrener, Lewton, Pontzer, & Lieberman, ,; Dunsworth, ; Stone, ; Wall‐Scheffler & Myers, ). Successful childbirth, for example, requires that the birth canal accommodate the large heads and wide shoulders of human fetuses (Rosenberg, ; Wells, ; Wells et al, ).…”