Sacral‐Coxal Covariation in the Developing Human Pelvis

Abstract

The adult human bony pelvis is sexually dimorphic with females typically exhibiting wider pelvic canals than males. Previous research suggests that pelvic dimorphism results from sex-specific developmental trajectories that diverge at puberty (2016 PNAS 113(19): 5227-5232). Other research demonstrates that developmental pelvic shape changes are associated with increased sacral slope, or the degree of sacral anterior tilt (2013 J Hu Ev 65: 209-222). The current study tests for sex differences in sacral-coxal covariation across ontogeny. I analyzed decedent computed tomography data for 44 females and 48 males aged 0-21 years from the Center for Forensic Imaging at the University of New Mexico. I used SlicerMorph to collect three-dimensional landmark coordinate data representing pelvic shape along with R Geomorph and Morpho to complete geometric morphometric. After a single Generalized Procrustes Analysis on the entire pelvis, I analyzed sacral-coxal covariation with a two-block partial least squares analysis and assessed sex-specific covariation patterns through comparison of slopes. I tested effect sizes of sex-specific two-block partial least squares to compare covariation magnitudes. Results from the two-block partial least squares analysis indicate significant covariation between the sacrum and coxal bones (p=0.001). The first latent variable explains 65.5% of the total covariation and appears to relate to ontogeny. As the sacral body becomes more concavely curved, the pelvic inlet anteroposteriorly elongates and the outlet widens. Slope comparison between sexes indicates similar patterns of sacral-coxal covariation. The sex-specific two-block partial least squares analyses exhibit similar magnitudes of sacral-coxal covariation. The study results suggest that the observed pelvic dimorphism in adults does not result from sex differences in sacral-coxal covariation during development. Although sacral-coxal covariation patterns are similar between sexes, pelvic dimorphism may relate to mean sex differences in sacral slope. This research aims to provide an understanding of the processes that produce pelvic sex dimorphism, which will aid in the refinement of sex estimation techniques in forensic and bioarchaeological contexts.