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Sexual dimorphism of the human pelvis is linked intimately with its adaptive functions. The peculiarly shaped hominid pelvis represents the total response to the diverse forces that have moulded its structure. These diverse forces are requirements for efficient bipedalism and parturition. In some respects, the structural demands of these unrelated functions have been in conflict. The morphological response to the dominant requirement, bipedalism, is clearly discernible, while the changes serving the needs of parturition are seen as compensatory modifications as reflected with greater emphasis for pelvic sexual dimorphism in the female. In addition, sexual selection has made sexual dimorphism even more pronounced. The female buttocks have undergone sexual elaboration through mate choice by males. Thus, total pelvic architecture is a mosaic constituted of the aggregate of differential responses to different functional goals.  There are complications during parturition that have been repeatedly prevented or interceded by medical technology, close monitoring, surgical practices such as caesarian sections, and other strategies. With these complications, one could hypothesize that these natural operators, which have exerted their influence since the beginning of mankind, might be becoming increasingly destabilized, attenuated or stochastic.  The Male and Female Pelvic Blueprint and Anatomic Variations In general, the structure of the male pelvis is significantly heavier and thicker than that of the female. The male pelvic bones are also adapted to fit a more massive and sturdy body architecture. For example, the male acetabulum has been designed to fit a bigger femur. Though a large amount of the sexual dimorphism of the pelvis is accounted for by size differences, sex-linked shape variation is also very conspicuous and cannot be considered an allometric consequence of differences in body size between the sexes (1). These variations in shape are demonstrated by the more rounded frame of the female pelvis. The sciatic notches are broader, the greater pelvis is shallower, the lesser pelvis is wider and the pelvic inlet and outlet are larger (longer pubic bones and a greater degree of curvature of the pectineal line). The female hipbones are also different in traits associated in the position of the sacroiliac joint in the iliac bone (2). As a result of this ‘flattened’ appearance, the female obturator foramen is more elliptical. The pubic arch is formed by the conjoined rami of the pubis and ischium of the two sides. These rami meet at the pubic symphysis to form the subpubic angle. The subpubic angle is nearly a right angle in females and is considerably less in males; approximately 30° narrower (3).  Superficial to the skeleton and musculature of the pelvis, sexual dimorphism in pelvic morphology is most apparent in body fat distribution as measured by waist hip ratio (WHR). The WHR has been shown to be independent of overall body weight and an accurate predictor of risk for various diseases, premature mortality, degree of estrogenicity and fecundity of women (4). Undoubtedly, healthy women have a greater propensity to possess rounder hips and a lower WHR compared to most men (4).  Nonetheless, the human pelvis is not always distinctly dimorphic. It has been well established that nature has allowed individual anatomical variation and departures from set norms within each sex. Hence, one could infer that it is possible to find any of the previously defined archetypal features in the ‘wrong’ sex. In addition, there is metric and morphologic variation in the expression of sexual dimorphism between racial phenotypes and populations (5). In other words, the final shape of the female pelvis is affected by multiple etiological factors - cultural, environmental and genetic.  The gynecoid pelvis (rounded shape) is said to be the normal female type while the android pelvis (heart-shaped) is often designated a male variant. Women with the android pelvis do not typically present with hyperandrogenism, and signs of hyperandrogenism have been similarly encountered in patients with gynecoid and other types of pelvis. This android pelvis was also found more frequently in women exposed to strenuous physical activity during adolescence and observed more often when the acquisition of erect posture was delayed beyond the usual age of 14 months, while a platypelloid pelvis was more frequent when erect posture was acquired before 14 months (6).  In addition to being developmentally discriminating, the distribution of the anthropoid pelvis between both sexes and frequency is ethnically discriminating as well. It was observed that there were significant differences in the accuracy of sex determination from pelvic morphology between both males and females, and whites and blacks. Pubic bone shape was the easiest to assess and was the most consistently reliable morphological indicator of sex in both sexes and population groups. However, in blacks, the greater sciatic notch form allowed the highest separation. This demonstrates that racial differences significantly affect the expression of sexual dimorphism (5).

Sexual dimorphism of the pelvic architecture: A struggling response to destructive and parsimonious forces by natural & mate selection