Sexual Dimorphism of the First Rib: A Comparative Approach Using Metric and Geometric Morphometric Analyses

Validation of discriminant functions from the rib necks in two Portuguese adult identified populations

Sexual dimorphism is the basis for sex estimation research in skeletal remains, helping the positive identification of individuals in forensic backgrounds. In this regard, it has been proved that the metrical analysis of the costal neck of the first four ribs in Spanish contemporary adult skeletons shows high sexual dimorphism, especially in the first rib. However, the validation of any method developed in identified skeletal collections must be compulsorily reproduced in other skeletonized sets to verify either its potential application in individuals from other locations, or the existence of possible biases associated with inter-population variation. Due to the geographical and socioeconomic proximity between Spain and Portugal, this paper aims to check the utility of the discriminant functions designed by Partido-Navadijo et al. (2021) in the two skeletal collections of the University of Coimbra (Portugal). Results show the utility of these discriminant functions in Portuguese population, with frequencies of cases correctly assigned reaching up to 93.6% in the Coimbra Identified Skeletal Collection (CISC); and up to 92.6% in the 21^st Century Identified Skeletal Collection (CEI/XXI). However, two particularities between the Spanish and Portuguese osteological assemblages can be highlighted: the possible existence of secular changes in relation to the CISC collection (1904-1938); and a reversal in the degree of costal dimorphism, being the third and fourth ribs of the CEI/XXI collection more dimorphic than the first ones.

Sexual dimorphism in the first rib of Homo sapiens

This work aimed to study sexual dimorphism in the first rib of modern humans, with a special focus on whether differences in shape are due to divergent allometric growth in males and females. Also, we compare the accuracy of sex classification using different approaches based on two methodologies, traditional morphometry based on linear measurements and geometric morphometric analysis based on 2D landmark coordinates. The sample studied here comprised 121 right and left first ribs from 65 female and male adult recent Euro-American Homo sapiens individuals. For traditional morphometrics, 12 metric variables were collected from each rib using a digital caliper, and for geometric morphometrics, six landmarks and 31 semilandmarks were captured from photographs using digital software. Both geometric morphometric and metric data were analyzed to calculate the index of sexual dimorphism, variation related to lateral asymmetry, variation in size and shape, and allometric trends between males and females. Finally, a linear discriminant analysis (LDA) was performed comparing both methodologies to test the best approach for sex classification. Results indicated that there are significant sex differences in the size and shape of the first ribs of recent Euro-American Homo sapiens. Regression analysis revealed different allometric patterns for males and females, and this could partially explain shape differences between sexes. Additionally, traditional morphometrics showed that all characteristics analyzed are significantly dimorphic, with the midshaft minimum craniocaudal diameter, the sternal end minimum diameter, and the neck minimum craniocaudal diameter displaying the most dimorphic scores. Similarly, geometric morphometrics results indicated that males have more curved and interno-exteriorly wider first ribs. Finally, analysis of sex classification using LDA yielded slightly better accuracy for traditional morphometry (83.8%) than the geometric morphometrics approach (81.3%) based on form Procrustes coordinates. This study demonstrates the usefulness of applying two different morphometric approaches to obtain more comprehensive results.

Discriminant functions for sex estimation using the rib necks in a Spanish population

One of the first steps in the identification of human remains is sex estimation. Discriminant functions are very useful for this purpose, as they are based on the fundamental premise of the sexual dimorphism in osseous remains. However, the absence of studies of this kind in the thoracic cage is visible, and adequate cause to justify this work. To this purpose, we analysed and measured a set of ribs, R1 to R4, belonging to skeletons of contemporary chronology (1970-2010), in order to obtain discriminant functions enabling sex estimation through equations applied to the rib neck. The sample comes from the Cemetery of San José, which is located at the Laboratory of Anthropology at the University of Granada. All data from this sample was known: ancestry, sex, age-at-death and cause of death. Results showed a percentage of success of approximately 80-88% in ribs R2 to R4, and approximately 85-90% in R1, reaching up to 93.2%. Results from intra-/inter-observer error tests show excellent reproducibility of the method. The data confirms that it is possible to estimate sex from the rib neck.

New methods for sex estimation using sternum and rib morphology

Sex estimation of skeletal remains is of great importance in the fields of physical and forensic anthropology. Since skeletons are often incomplete, it is essential to estimate sex from as many skeletal remains as possible. The aim of this study was to establish new methods for estimating sex using the morphology of the sternum and the fifth to ninth ribs. We considered two conditions of skeletal preservation: All skeletal elements measured are available, or only a single element is available. Traditional or virtual measurements were carried out on three samples: (1) A skeletal sample from the Hamann-Todd Human osteological collection, Cleveland Museum of Natural History, USA (N = 413), was used to create prediction equations for sex estimation. (2) A recent, CT-based sample from Israel (N = 33) was used to cross-validate the accuracy of the prediction equations. (3) A skeletal sample from the Anthropological Collection at Tel Aviv University (N = 15) was used to test the validity of the virtual measurements. Reliability and validity analyses were carried out via intraclass correlation coefficient analysis. Prediction equations for sex were created using logistic regression. The measurements were found to be highly reliable and valid. Success rates for sex estimation were high (> 80%) and correspond well between the skeletal and recent samples, especially for the left sixth, left eigth, and left ninth ribs. To conclude, measurements of the sternum and ribs are valuable for estimating sex and can be carried out using either traditional or virtual tools. Of all the skeletal elements examined in this study, the sternum, left sixth, left eighth, and left ninth ribs were found to be the most reliable skeletal elements for estimating the sex of an individual.

An Automated Two-dimensional Pairwise form Registration Method for Pair-matching of Fragmented Skeletal Remains

This study introduces an automated pairwise method for osteological pair-matching of fragmented skeletal remains using two-dimensional fragmented outlines extracted from photographs. The form data are used in pairwise iterative closest point registrations with rigid transformations. A modified version of the average Hausdorff distance is calculated to remove any coordinate correspondences with outline fracture margins, which allow the distance analysis of fragmented outlines. A dilation modification to the Hausdorff distance is proposed creating a greater separation between true- and false-pairs. The sample consists of 122 calcanei (61 pairs) from the UI-Stanford collection. Performance statistics are provided for simulated fragmented and complete assemblages. Results indicate up to 98% accuracy for fragmented and complete assemblages. The dilated Hausdorff distance performed similarly across assemblages, but showed a slight decrease in performance for the complete assemblage. This approach provides a useful short listing tool to reduce the number of visual comparisons required in large commingled assemblages.