Empirical Test of the Relationship Between Pelvic Organs and Pelvic Cavity Dimensions as an Explanation for Female-Biased Pelvic Sex Differences

Objectives: Growing evidence obfuscates the role obstetrics is thought to have played in the evolution of female-biased pelvic dimorphism. An alternative explanation is offered by the “Virile, Active Gonads and Genitalia in Nether Area (VAGGINA) hypothesis,” which posits that females' larger pelvic organs drive female-biased pelvic dimorphism. The present study tests this hypothesis by evaluating whether females have larger pelvic organs than males and whether dimensions of pelvic organs influence those of the bony pelvic cavity. Materials and Methods: Non-pathological pelvic MRIs were compiled from de-identified patients evaluated at Stony Brook University Hospital. Organ and pelvic cavity volumes were determined from segmented structures. Mediolateral and anteroposterior organ and pelvic cavity dimensions were derived from landmark data. T-tests and ordinary least squares regression were employed to test specific predictions of the “VAGGINA hypothesis.”. Results: Comparisons of non-reproductive pelvic organ dimensions varyingly demonstrate both female- and male-biased dimorphism. Reproductive organs, however, demonstrate female-biased dimorphism of such magnitude that female-biased dimorphism is retained in analyses of summed pelvic organs. Despite this sexual dimorphism in organ dimensions, organ dimensions do not have a predictive relationship with corresponding bony pelvic cavity dimensions. Discussion: The central argument of the “VAGGINA hypothesis,” that large pelvic organs produce large bony pelves, is not supported, indicating more work is needed to understand what forces cause female-biased pelvic dimorphism. Future research may benefit from broader comparative and evolutionary contexts by exploring phylogenetic signals in female pelvic morphology.