Covariation Between Body Size, Sacral Slope, and Pelvic Shape in Humans: Implications for Sexual Dimorphism

Abstract

This study tests for a significant relationship between bony pelvic shape, body size, and sacral slope (SS), or the angled position of the sacrum within the pelvis. Typically, human males exhibit narrow pelves and narrow greater sciatic notches (GSNs) with shorter posterior aspects and posteriorly located apexes. Females typically exhibit wide pelves and wide GSNs with expanded posterior aspects that appear to contribute to spacious birth canals. Although these morphological differences are often attributed to obstetric selection, the proximate mechanisms responsible for pelvic dimorphism are unclear.I analyzed previously recorded pelvic shape data for 46 male and 52 female American white individuals aged 19–70 years from the Hamann‐Todd collection. Three‐dimensional landmark coordinate data for the left pelvis is stored in the Dryad repository (2015 PNAS 112:5655–5660). I began geometric morphometric analyses by estimating missing landmarks using thin‐plate spline interpolation in R Geomorph. After reflecting paired landmarks across the midplane to yield 125 total landmarks, I completed a Generalized Procrustes Analysis (GPA) of the entire symmetrized pelvis. SS is defined by the angle between a transverse plane and the first sacral cranial endplate. Body size is represented by recorded stature and body breadth of the first sacral vertebra (S1) as a proxy for body mass. Using both R Geomporph and Morpho, two‐block partial least squares analyses in the total, female, and male samples tested the relationship between a block comprised of stature, S1 body breadth, and SS, with each variable standardized by z‐score transformation, and a second block of 85 GPA‐aligned coordinates representing right and left coxal shape.In the total sample, coxal shape significantly covaries with SS and the body size variables (SA1 r‐pls: 0.718, p=0.001), with body size variables showing greater loading, or influence in the relationship compared to SS. As body size increases and SS decreases, the GSN exhibits a shorter posterior aspect, and the pelvis shows greater superoinferior expansion. Female coxal shape significantly covaries with SS and the body size variables (SA1 r‐pls: 0.579, p=0.036), with SS being the greatest loading on the first singular axis. Male coxal shape also significantly covaries with SS and the body size variables (SA1 r‐pls: 0.626, p=0.014 (Morpho)), but with S1 body breadth being the greatest loading on the first singular axis. In both the female and male analyses, as SS increases and body size decreases, the GSN exhibits a relatively expanded posterior aspect and the breadth between the left and right anterior superior iliac spines appears relatively greater. Among females, the pelvic inlet also appears relatively mediolaterally narrow with increased SS. Overall, results indicate that body size and SS relate to GSN shape and orientation of the coxal bones. SS affects how upper body mass is transmitted to the pelvis. This study therefore suggests that trunk biomechanical factors influence pelvic shape variation, including patterns of dimorphism.