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 known-age/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.

Keywords: development; evolution; obstetrical dilemma; pelvis; sex steroids.

Fig. 1.

Developmental changes in human pelvic morphology from late fetal stages to late adulthood. (A and B) Bivariate plots of shape variation along PC1 (45% of total sample variation) and PC2 (11%) (A), and along PC1 and PC3 (10%) (B). Red symbols represent females; dots indicate immature or unknown parity status; filled and open circles indicate parous and nonparous status, respectively. Blue symbols represent males. Points A–S denote moving-average positions calculated at the ages indicated in Fig. 2. (C) Anterior and superior views of sex-specific pelvic mean shapes at birth and around 2, 6, 13, 25, and 80 y. (Scale bar, 5 cm.)

Fig. 2.

Age-related change in human pelvic size (A), shape (B–D), and shape dimorphism (E). Colors and symbols are as in Fig. 1; note that the age axis is scaled logarithmically in postconception years.

Fig. 3.

Anterior, superior, and lateral views showing male and female patterns of pelvic shape change from ∼15 y (transparent) to ∼25 y (solid) (A), and from ∼40 y (solid) to ∼80 y (transparent) (B). For additional visualizations of the same patterns of shape change, see Movies S1–S6.

Fig. S1.

Anterior and superior views showing male (A) and female (B) pelvic mean shapes at age ∼15 y (Left), ∼25 y (Center), and ∼80 y (Right). For additional visualizations of the same patterns of shape change, see Fig. 3 and Movies S1–S6.

Fig. 4.

Sex-specific changes in angular (A) and size-normalized linear (B and C) pelvic dimensions and in pelvic proportions (D). Colors and symbols are as in Figs. 1 and 2.

Fig. S2.

Anterior (A), lateral (B), and posterior (C) views of an immature pelvis showing pelvic LMs and SLMs. Fixed LMs are shown in red, curve SLMs in blue, and surface SLMs in green. (Scale bar, 2 cm.)