Sexual dimorphism using elliptical Fourier analysis: shape differences in the craniofacial complex

Abstract

The assessment of sexual dimorphism plays an essential role in numerous disciplines such as: (1) forensics, with its concern with skeletal identification, including sexing; (2) archeology, where the sexing of skeletal materials is an essential aspect; (3) primatology, where sex differences are diverse; (4) paleoanthropology, where the identification of sex can influence taxonomic and phylogenetic decisions, and (5) growth and development, where one strives to identify differences before and after puberty. Other disciplines where sexual dimorphism plays an important function include orthodontics, gerontology, nutrition, and medicine. Quantitative studies of morphological forms in general, and sexual dimorphism in particular, is not a trivial endeavor in that at least two aspects are involved, namely size and shape. Focusing on the human skeletal system, sexual dimorphism affects all bones, including the cranium, pelvis, long bones, and vertebral column. However, the overwhelming numbers of studies of sexual dimorphism are based solely on size differences. These studies have repeatedly demonstrated that males are larger for most dimensions. The question of shape differences, if indeed present, cannot be readily inferred from these studies. Consequently, a number of studies were initiated, over some 20 years duration, that specifically focused on the presence of sexually dimorphic shape changes in the skeletal craniofacial complex. The following anatomical structures were examined: (1) the human nasal bones, (2) the primate cranial base, (3) the human cranial base, (4) the human dental arch, (5) the human mandibular arch, and (6) the human cranial vault. These six datasets involved 16 samples for a total of 1110 specimens. Every dataset generated statistically significant sexually dimorphic differences in shape. Thus, it is apparent that the craniofacial complex not only exhibits size differences but also specific shape differences.

Technical note: Quantification of neurocranial shape variation using the shortest paths connecting pairs of anatomical landmarks

“…Although low degrees of sexual dimorphism in human cranial shape have been reported, this study suggested that sexually dimorphic differences in the 3D contour of neurocranial shape exist in the modern Japanese population. Although direct comparison between the results of this study and those of previous studies was made difficult by the differences in the landmarks chosen to represent cranial morphology, the results of this study were found to be generally consistent with those of previous morphometric studies (Rosas and Bastir, ; Franklin et al, ; Bigoni et al, ; Lestrel et al, ). Although it is well known that sexual dimorphism exists in forehead contour (in female crania the forehead rises more vertically, whereas in male crania it slopes back), this morphological tendency was not observed in this study.…”

“…They also observed that the occipital region of the male cranium was posteriorly projected and acutely angled at the inion, whereas the corresponding region in the female cranium was more rounded. Using elliptical Fourier analysis, Lestrel et al () reported that 2D outlines of the female cranial vault tended to be more globular and superoinferiorly higher, whereas those of males tended to be anteroposteriorly elongated, as the glabella and occipital regions in males were projected relatively more anteriorly and posteriorly, respectively. Using a three‐dimensional (3D) geometric morphometric technique, Franklin et al () demonstrated that, in female crania, the forehead was more vertical and rounded; the glabella region was smaller and less convexly protrudent; the cranial vault was lower; the parietal eminences were larger and more laterally projected; the mastoid process was smaller in length and height and less antero‐inferiorly projected; and the occipital bone was less posteriorly projected.…”

The utility of elliptical Fourier analysis for estimating ancestry and sex from lateral skull photographs

“…The average outlines exhibit all the main features of traditionally described sexually dimorphic traits [2,16,26,31,54,55,58,62]. As anticipated, the major difference is overall size, with female skull outlines being smaller than their male counterparts for all ancestry groups (Fig.…”

“…It is possible to supplement basic landmarks with pseudo-and/or semi-landmark procedures [5,6], but alternative outline-based GMM methods such as elliptical Fourier analysis (EFA) are inherently geared toward higher density sampling of continuous margins, and can also be applied with few constraints on feature limited boundaries (e.g., not possessing type I landmarks) [7,8]. The suitability of EFA to the quantification of skeletal morphology has been discussed at length elsewhere [7,9], so suffice it to briefly summarize here that these methods have been used for individuation [10][11][12][13][14], as well as to study sex and ancestry patterning of the skull [15][16][17][18][19][20] and infracranial elements, e.g., proximal humerus [21] and greater sciatic notch [22]. Accuracy of sex and ancestry classification using EFA is similar to (and sometimes better than) methods that employ linear measurements (see Table 1 in [7] for a summary).…”

Loss of molar occlusion and mandibular morphology in adults in an ancient human population consuming a coarse diet

“…Mandibular dimensions are routinely recorded during the preparation of osteological reports on skeletal remains from archaeological sites, and measurement definitions feature in standard laboratory manuals (e.g., Martin, ; Brothwell, ; Bass, ; Buikstra and Ubelaker, ; Schwartz, ; Brothwell and Zakrzewski, ). Mandibular morphology is an important focus in work on skeletal sexual dimorphism (Franklin et al, ; Lestrel et al, ), in studies of fossil hominins (Trinkaus, ; Lague et al, ), in the investigation of temporal (Jonke et al, ; Martin and Danforth, ) and ecogeographic (Humphrey et al, ; Nicholson and Harvati, ) variability in skeletal morphology, and in the study of the consistency of the diets of earlier peoples (Pinhasi et al, ; Paschetta et al, ).…”