The human pelvis is a complex trait that has to perform multiple functions, from providing skeletal stability to enabling childbirth. Not all of these demands likely align so as to require an identical morphology; instead, the hard and soft tissue of the human pelvis reflect functional and evolutionary trade offs. One such, notable, trade off exists in the dimensions of the pelvic canal (birth cana l in women). On the one hand, selection favors a spacious canal to enable birth of large brained fetuses. On the other hand, there is selection in the opposite direction favoring a smaller pelvic canal, evinced by the strong sexual dimorphism in the human pelvis. A longstanding explanation is that our bipedal mode of locomotion necessitates a narrow pelvis (Washburn 1960; Rosenberg 1992). Another hypothesis poses that a small pelvic canal benefits the supportive capacity of the muscles and connective tissue of the pelvic floor inside it (Abitbol 1988; Brown et al. 2013). The pelvic floor spans the ‘open’ space inside the pelvic canal and in upright, bipedal humans, provides critical support to the abdominopelvic organs and a heavy fetus during pregnancy. Ind eed, the high incidence of pelvic organ prolapse and other pelvic floor disorders (e.g. incontinence) in women may reflect the evolutionary conflict between a pelvis that needs to be large enough to birth large babies and small enough to provide other func tions vital to survival and reproductive functioning.
The extent to which human bony pelvic evolution may have been driven by the functions of the pelvic floor is gaining interest among researchers, but has received limited empirical attention thus far. Here, we propose to study how the dimensions and shape of the bony pelvis co vary with the size of the abdominal cavity. Specifically, we will investigate how pelvic canal size and shape scales with variation in body height and size of the abdominal cavity One can demonstrate that intra abdominal pressure increases with an increase in stature, and thus one can predict that taller individuals would benefit from relatively smaller pelvic canals in order to compensate for the increase in pressure on their pelv ic floor. Furthermore, women are known to possess absolutely wider pelvic canals than men across populations, and may thus be more susceptible to developing pelvic floor disorders as a result of this size increase alone. However, the abdominal contents are not only supported by the soft tissue of the pelvic floor; the upper part of the hipbones (the ilia) also contribute. I will therefore investigate whether the shape and orientation of women’s hipbones result in a greater surface to support abdominal conte nts, relative to the size of the abdominal cavity and body height, as a way to mitigate the increased strain on the pelvic floor. In addition to yielding valuable new insights into co evolution between the upper and lower part of the human torso, the findi ngs of this project may also have clinical relevance for understanding risks of pelvic floor disorders in women and men.