Sex classification using the human sacrum: Geometric morphometrics versus conventional approaches

Area of Science:

• Anthropology
• Paleontology
• Forensic Anthropology

Background:

• The human pelvis exhibits significant sexual dimorphism due to evolutionary pressures from bipedalism and childbirth.
• The sacrum, a key component of the pelvic girdle, is often analyzed for sex determination.
• Previous studies report highly variable sacrum-based sex classification accuracies (54%-98%).

Purpose of the Study:

• To investigate inconsistencies in sacrum-based sex classification accuracies.
• To compare 3D geometric morphometrics, linear metrics, and qualitative assessments for quantifying sacral sexual dimorphism.
• To analyze a geographically diverse sample of 164 modern humans from Africa, Europe, Asia, and America.

Main Methods:

• 3D geometric morphometrics using 44 landmarks and 56 semilandmarks.
• Linear measurements including sacral width, corpus depth and width, and derived indices.
• Qualitative assessment of macroscopic features like vertebral proportions and sacral curvature.
• Linear discriminant function analysis for sex classification accuracy.

Main Results:

• 3D geometric morphometrics showed extensive overlap between male and female sacral shapes, yielding 72% classification accuracy.
• Anteroposterior corpus depth (83%) and corpus-area index (78%) were the most effective linear discriminators.
• Qualitative inspection provided lower accuracies (64-76%).
• Classification accuracy decreased with greater geographical heterogeneity within subsamples.

Conclusions:

• Sacral sex-related variation is surprisingly limited, despite its role in the birth canal.
• Morphological variation appears influenced by factors such as body size.
• Sacrum shape is likely shaped more by biomechanical selection than obstetric selection.