Abstract
Accurate sex estimation from the skeleton is important when working with human skeletal remains in biological anthropology. Forensic anthropology and bioarchaeology, for example, rely on sex estimation to help identify unknown individuals and understand past populations. The field of biological anthropology has numerous methods for estimating sex from the skeleton using the pelvis, cranium, and various other postcranial elements. These methods are based on human sexual dimorphism – or physical differences between males and females – related to shape or size differences, and frequently focus on the concept that males typically exhibit more robust size characteristics than females. This is particularly relevant to, for example, skeletal sex estimation methods using morphological observations of the cranium, as they often rely on assumed relationships between soft tissue (muscle/ligament), skeletal robusticity, and sexual dimorphism. Based on this concept, it is thought that larger muscles lead to larger attachment sites, or entheses, on the cranium, and these robust cranial characteristics are indicative of typical male morphology. However, these relationships are poorly understood and have never been studied. Despite this, many sex estimation methods are based on these concepts and, although many are reported to be accurate, it may be possible to further improve sex estimation in anthropology by basing new methods on data that incorporate the anatomy of both soft and skeletal tissues.
This study tested the relationship between skeletal sexual dimorphism and soft tissue/enthesis robusticity (size, in this context) with the aim of not only understanding their relationships, but also developing an accurate anthropological sex estimation method based on anatomical data. These aims were achieved by examining three skeletal landmarks that are used for sex estimation in anthropology and are also soft tissue attachment sites. These included the nuchal crest and mastoid process of the cranium and the rhomboid fossa of the clavicle and their associated soft tissues. This research was completed through dissection, and three-dimensional and skeletal analyses using two cadaveric samples from New Zealand and Thailand (five males and five females each). Data collected included muscle size, ligament mass, enthesis surface area, and skeletal measurements, which were used to create osteometrics to generate univariate and multivariate equations, through logistic regression analysis, to estimate sex in each sample. This resulted in the development of a novel metric sex estimation method using logistic regression equations. This method was then tested for accuracy by calculating correct classification rates for estimating sex in modern skeletal samples from New Zealand and Thailand and archaeological samples from New Zealand representing individuals from European and Chinese populations.
Results from this research challenge assumptions that skeletal sexual dimorphism is associated with muscular and entheseal robusticity. Findings also indicate that the newly developed metric sex estimation method performed well in these populations. The overall results demonstrate that the Thai sample had more significant sex differences in muscle size than the New Zealand sample, both samples exhibited some sexual dimorphism for enthesis area, and some correlations existed between muscle and enthesis size. However, positive muscle and enthesis correlations did not often correspond with observed sexual dimorphism. Further, morphological methods for sex estimation from the cranium and clavicle were found to be accurate for estimating sex in the cadaveric samples, but morphological scores had no significant relationship with muscle or enthesis size. Finally, some aspects of the novel metric sex estimation method, developed based on these data, exhibited variability in accuracy. However, overall, the method demonstrated high classification rates for modern New Zealand and Thai skeletal samples (>80%) when using results from multiple logistic regression equations to estimate sex.
This research emphasises the importance of multidisciplinary research not only for understanding the anatomical basis of anthropological sex estimation, but also for developing new metric sex estimation methods based on these foundations. A limitation of this research was the use of cadaveric samples from older-aged body donors, which could impact the applicability of this research to younger age groups. Future research is needed to further validate the metric sex estimation method created in this study, although preliminary findings suggest that it could be a useful addition to existing methods for estimating sex in biological anthropology.