Developmental evidence for obstetric adaptation of the human female pelvis

Abstract

The bony pelvis of adult humans exhibits marked sexual dimorphism, which is traditionally interpreted in the framework of the "obstetrical dilemma" hypothesis: Giving birth to large-brained/ large-bodied babies requires a wide pelvis, whereas efficient bipedal locomotion requires a narrow pelvis. This hypothesis has been challenged recently on biomechanical, metabolic, and biocultural grounds, so that it remains unclear which factors are responsible for sex-specific differences in adult pelvic morphology. Here we address this issue from a developmental perspective. We use methods of biomedical imaging and geometric morphometrics to analyze changes in pelvic morphology from late fetal stages to adulthood in a knownage/known-sex forensic/clinical sample. Results show that, until puberty, female and male pelves exhibit only moderate sexual dimorphism and follow largely similar developmental trajectories. With the onset of puberty, however, the female trajectory diverges substantially from the common course, resulting in rapid expansion of obstetrically relevant pelvic dimensions up to the age of 25-30 y. From 40 y onward females resume a mode of pelvic development similar to males, resulting in significant reduction of obstetric dimensions. This complex developmental trajectory is likely linked to the pubertal rise and premenopausal fall of estradiol levels and results in the obstetrically most adequate pelvic morphology during the time of maximum female fertility. The evidence that hormones mediate female pelvic development and morphology supports the view that solutions of the obstetrical dilemma depend not only on selection and adaptation but also on developmental plasticity as a response to ecological/nutritional factors during a female's lifetime.pelvis | development | evolution | obstetrical dilemma | sex steroids

Is the Pelvis Sexually Dimorphic in Turtles?

“…Pelvic aperture area increased with body size in both female and male S. odoratus , which is unsurprising. This is comparable to allometric scaling trends described in primates (Fischer and Mitteroecker, ; Huseynov et al, ; Fischer and Mitteroecker, ), as well as in small‐bodied turtles hypothesized to experience pelvic constraints on egg size (Long and Rose, ). My analyses indicate that this scaling relationship might differ slightly between sexes, such that females of comparable size to males attain a larger pelvic aperture.…”

“…The pelvis is a fascinating example of how sex-specific functional demands influence skeletal variation in tetrapods. Nonetheless, sexual pelvic dimorphism has been primarily evaluated in live-bearing species (Tague, 2000;Fischer and Mitteroecker, 2015;Huseynov et al, 2016;Rosenberg and DeSilva, 2017). Using turtle anatomy as a model, I tested the hypothesis that pelvic morphology is also sexually dimorphic in egg-laying tetrapods.…”

“…Variation in the architecture of the pelvic girdle is intrinsically linked to locomotor performance, but also to reproduction (Congdon and Gibbons, 1987;Sinervo and Licht, 1991;Fischer and Mitteroecker, 2015;Huseynov et al, 2016). Sexual dimorphism in the pelvis of primates is a classic example of this important form-tofunction relationship (Moffett, 2017;Rosenberg and DeSilva, 2017).…”

“…This transformation, however, constrained cranial size, leading to female-biased evolutionary changes in the size and shape of the pelvic inlet (the obstetric dilemma; Fischer and Mitteroecker, 2015). A major challenge in understanding mechanisms underlying this dimorphic trend concerns the disentangling of allometric (correlated with body size) or modular (uncorrelated with body size) trait effects (Fischer and Mitteroecker, 2015;Huseynov et al, 2016;Fischer and Mitteroecker, 2017). Another, and equally important issue, is the lack of comparative phylogenetic outgroups, particularly that of egg-laying tetrapods.…”

Transwomen in elite sport: scientific and ethical considerations

“…For example, women have a wider pelvis changing the hip structure significantly between the sexes. Pelvis shape is established during puberty and is driven by oestrogen 33. The different angles resulting from the female pelvis leads to decreased joint rotation and muscle recruitment34–36 ultimately making them slower 37 38.…”