Nicole Dieneke Sybille Grunstra

NICOLE GRUNSTRA, University of Vienna, Vienna, Austria, was awared funding in April 2019 to aid research on “Of Mice and Women: Disentangling the Human Obstetric Conundrum Using a Comparative Mammalian Approach.” This project was aimed at better understanding the “obstetrical dilemma.” The “obstetrical dilemma” refers to the idea that the human pelvis is under opposing selection pressures related to different functions, which simultaneously favor a larger and a smaller pelvic canal. This evolutionary “dilemma” is proposed to be the reason that the human birth canal is small relative to the size of human neonates. In turn, it proposes that this at least partly explains the difficulty of human childbirth and the incidence of obstructed labor. Classically, this antagonistic selection regime has been assumed to arise from the “competing” functions of childbirth and bipedal locomotion. That a sufficiently large pelvic (birth) canal benefits childbirth is supported, among other things, by the strong sexual dimorphism in the human pelvis, whereby women on average have quite more spacious pelvic canals in all the birth-relevant dimensions and shape features. However, subtle pelvic sexual dimorphism in mammals with small offspring — and hence the presumed absence of obstetric selection — has cast doubt on this. Furthermore, the role of bipedal locomotion in favoring a smaller pelvic canal is unclear and hotly debated. Recently, alternative — or at least additional — factors have been proposed that may also favor a smaller bony pelvis, such as pelvic floor support and thermoregulation. The importance of pelvic floor support derives from the fact that in upright bipeds the abdominopelvic organs and the fetus are carried to a large extent by the pelvic floor muscles. These are anchored inside the pelvic canal and the smaller the area across which they span (or are suspended), the better their supportive capacity. According to the pelvic floor hypothesis, the pivotal factor is thus our upright posture rather than our two-legged locomotion per se.