Sex differences in hip bone and sacrum morphology: a geometric morphometric analysis

The pelvis plays a crucial role in sex estimation when reconstructing an individual's biological profile from skeletal remains. The present study aims to evaluate the sex differences in size and shape of the hip bones and sacrum using geometric morphometrics and to compare the degree of sexual dimorphism captured by size and shape variables within and between these skeletal elements. The study sample included the abdominal computed tomography scans of 255 adult Bulgarians (125 males and 130 females). Surface models of the pelvis were generated and used for landmark digitization. The three-dimensional coordinates of 35 landmarks were collected from the left and right hip bones, and 34 landmarks were recorded from the sacrum. The landmark data of each bone were subjected to Generalized Procrustes Analysis, Principal Component Analysis, Procrustes ANOVA, and classification methods (linear discriminant analysis of centroid size and support vector machine analysis of shape variables). In the present study, classification accuracies were interpreted as indicators of sexual dimorphism within the studied sample rather than as estimates of predictive model performance. The results indicated that the size and shape of all three bones differed significantly between males and females. The ability of centroid size to distinguish between the sexes was low, with classification accuracies of 69% for the right and left hip bones and 66% for the sacrum. In contrast, shape variables exhibited substantially greater sex differentiation, with values of 99% for the right and left hip bones and 91% for the sacrum, indicating pronounced sexual dimorphism in the shape of these bones within the studied sample.