Mandibular ramus flexure: A new morphologic indicator of sexual dimorphism in the human skeleton

In the skeleton, male and female characteristics lie along a continuum of morphologic configurations and metric values. Size alone is not the best indicator of sex. In contrast, morphologic differences that arise from genetically sex-linked growth and development allow better separation of the sexes. This study presents a new morphologic indicator of sexual dimorphism in the human mandible. A sample of 300 mandibles from adults of known sex primarily from the Dart collection was analyzed. Of these, 100 were found to have obvious bony pathologies and/or excessive tooth loss (“pathologic” sample). Thus, the normative sample consisted of 200 individuals (116 males, 84 females). Examination of morphologic features led to the discovery of a distinct angulation of the posterior border of the mandibular ramus at the level of the occlusal surface of the molars in adult males. Flexure appears to be a male developmental trait because it is only manifest consistently after adolescence. In most females, the posterior border of the ramus retained the straight juvenile shape. If flexure was noted, it was found to occur either at a higher point near the neck of the condyle or lower in association with gonial prominence or eversion. In the normative sample, overall prediction accuracy from ramus shape was 99%. When the “pathologic” sample was analyzed separately, 91.0% were correctly diagnosed. Because the African samples were overwhelmingly black, this trait was also tested on American samples (N = 247) of whites (N = 85), Amerinds (N = 66), and blacks (N = 96) that included a mix of healthy individuals and those with extensive tooth loss and evidence of pathology. The results were nearly identical to those of the “pathologic” African sample, with accuracies ranging from about 91% in whites and blacks to over 92% in Amerinds. Total accuracy for all African and American samples combined (N = 547) is 94.2%. In conclusion, at 99%, sexing from the shape of the ramus of a healthy mandible is on a par with accuracy attainable from a complete pelvis. Moreover, there is no record that any other single morphologic or metric indicator of sex (that has been quantified from the adult skeleton) surpasses the overall accuracy attained from the more representative mixed sample produced by combining all groups assessed in this study. The usefulness of this trait is enhanced by the survivability of the mandible and the fact that preliminary investigations show that the trait is clearly evident in fossil hominids. © 1996 Wiley-Liss, Inc.     

Technical note: evaluating mandibular ramus flexure as a morphological indicator of sex

Described as a highly reliable method of sex identification, mandibular ramus flexure is a morphological trait expressed on the posterior border of the ramus at the occlusal plane (Loth and Henneberg [1996] Am. J. Phys. Anthropol. 99:473-485). In a blind test, 158 mandibles were examined for the presence of flexure as defined by Loth and Henneberg, resulting in 79.1% accuracy, which is well below the reported 91-99% accuracy. Twenty-five of these mandibles were assigned the ambiguous score of 0, an outcome of a +1 score for one side, and a -1 score for the other. Seventeen mandibles were examined twice to measure intraobserver error. Only 64.7% of the scores were duplicated in the second session, suggesting difficulty in consistent identification of flexure. Low overall accuracy, an invalid scoring system, and high intraobserver error indicate that mandibular ramus flexure is an unreliable technique for estimation of sex.     

Technical note: A blind test of mandibular ramus flexure as a morphologic indicator of sexual dimorphism in the human skeleton

Loth and Henneberg (1996, Am. J. Phys. Anthropol. 99:473–487) identified a single morphological feature of the mandible, the presence or absence of a distinct flexure or angulation of the posterior margin of the mandibular ramus at the level of the occlusal plane, which appears to be an extraordinarily accurate predictor of sex. Using only this feature, Loth and Henneberg were able to predict sex with 94% accuracy in a large sample of mandibles. In this article, we report the results of a blind test of mandibular ramus flexure as a predictor of sex. In our blind test, only 62.5% of the mandibles were correctly sexed, and virtually identical results were obtained when the same sample of mandibles was examined by a second observer. Overall, our results demonstrate that: 1) the association between ramus flexure and sex is weak; 2) the predictive accuracy of Loth and Henneberg's method is better than chance for only one sex, males; and 3) the method is based on a trait that cannot be reliably or consistently identified. Am J Phys Anthropol 107:363–366, 1998 © 1998 Wiley-Liss, Inc.     

Investigation into the usability of geometric morphometric analysis in assessment of sexual dimorphism

Understanding sexual dimorphism is very important in studies of human evolution and skeletal biology. Sexual dimorphic characteristics can be studied morphologically and metrically, although morphologic studies pose several problems such as difficulties with quantification and interobserver error. Geometric morphometrics is a relatively new method that allows better assessment of morphologic characteristics. This paper aims to investigate the usability of this method by assessing three different morphologic characteristics in a sample of South African blacks: shape of the greater sciatic notch, mandibular ramus flexure, and shape of the orbits. Relative warps, thin-plate splines, and canonical variates analysis (CVA) analyses were performed. As expected, the shape of the greater sciatic notch provided the best separation between the sexes. Surprisingly, however, the shape of the orbits performed better that ramus flexure. Several possible explanations for this result are possible, which include the possibility that orbit shape is more sexually dimorphic than previously expected, or that biological reality is not reflected by this technique. More research is, however, needed.     

Mandibular ramus flexure and gonial eversion as morphologic indicators of sex

Recently, two mandibular traits--ramus flexure and gonial eversion--have come under close scrutiny (Loth & Henneberg 1996, 2000). The present study investigates the reliability of these two traits when each is applied as a single and independent indicator of sex, including the question of repeatability. The investigation was designed to give insights into possible confounding factors such as age and remodeling after tooth loss. Two samples, one of forensic (N = 153) and one of archaeological provenance (N = 80), were examined. The forensic sample was evaluated by a single observer while the archaeological sample was independently scored by three different observers. The results document that age and localized tooth loss seriously reduce the accuracy of these traits. For ramus flexure, male accuracy was only 66%, while female accuracy was even lower (32%). Overall accuracy was 59%. It is believed that the original scoring system devised by Loth and Henneberg (1996) creates an inherent bias in favor of males. For gonial eversion, a similar picture emerged (75.4% for males, 45.2% for females and 69.3% overall accuracy). Furthermore, both indicators are prone to intra- as well as inter-observer bias. While both possess some merit as sex indicators, they show marked functional and adaptive responses and may not be suitable for all samples.     

Sexual dimorphism in the Atapuerca-SH hominids: the evidence from the mandibles

The pattern of sexual dimorphism in 15 mandibles from the Atapuerca-SH Middle Pleistocene site, attributed to Homo heidelbergensis, is explored. Two modern human samples of known sex are used as a baseline for establishing sexing criteria. The mandible was divided for analysis into seven study regions and differential expression of sexual dimorphism in these regions is analysed. A total of 40 continuous and 32 discrete variables were scored on the mandibles. The means method given in Regh & Leigh (Am. J. phys. Anthrop.110, 95-104, 1999) was followed for evaluating the potential of correct sex attribution for each variable.On average, the mandibles from the Atapuerca-SH site present a degree of sexual dimorphism about eight points higher than in H. sapiens samples. However, mandibular anatomy of the European Middle Pleistocene hominid records sexual dimorphism differentially. Different areas of the Atapuerca-SH mandibles exhibit quite distinct degrees of sexual dimorphism. For instance, variables of the alveolar arcade present very low or practically no sexual dimorphism. Variables related to overall size of the mandible and symphysis region present a medium degree of sex differences. Finally, ramus height, and gonion and coronoid process present a high degree of sexual dimorphism (indexes of sexual dimorphism are all above 130%). Whether this marked sexual dimorphism in specific anatomical systems affects sexual differences in body size is not completely clear and further studies are needed.Sexual differences detected in the mandible of modern humans have at least two components: differences related to musculo-skeletal development and differences related to a different growth trajectory in males and females (relative development of some of the basal border features). The Atapuerca-SH mandibles display little variation in the basal border, however. The limited variation of this mandibular region may indicate that the pattern of sexual variation in H. heidelbergensis is different enough to that of H. sapiens to caution against simple extrapolation of criteria from one pattern to the other.     

Cephalometric study of mandibular dimorphism in living population in South-West France

The role of the mandible in sex determination is still poorly defined because of the plasticity of this bone. We carried out a teleradiographic study of the mandibles of 63 living white subjects, measuring bigonial breadth, bicondylar breadth (anterior radiographs), mandibular symphysis height, mandibular length, articulare-gonion height, mandibular ramus height and mandibular body length (lateral radiographs). All these paramters showed significant sexual dimorphism (p<0.001 in all cases). Mandibular symphysis height had the highest percentage of dimorphism (19.44%). Based on this study, two discriminant functions were calculated, one of which used all the measurement and the other, two. These functions made it possible to determine the sex of 87.3% of the sample studied, using the mandible alone.     

Sexually dimorphic pelvic morphology in South African whites and blacks

It is well known that there is metric and morphologic variation in the expression of sexual dimorphism between racial phenotypes and populations. Therefore, the purpose of this research is to assess morphologic sex differences in the pelves of South African whites and blacks. Results will be used to improve the identification of human skeletal remains by producing group specific standards. Morphologic features of both left and right os coxae were studied in a sample of 400 known sex/race individuals from the Pretoria and Dart collections. Bones that were clearly pathologic or visibly deformed were excluded from the study. Data were subjected to SPSS analysis.

Results indicated that overall, pubic bone shape was the easiest to assess and was the most consistently reliable morphological indicator of sex in both sexes and population groups. At 88% average accuracy, the most discriminating traits in whites were pubic bone shape and subpubic concavity form. In blacks, greater sciatic notch form allowed the highest separation, averaging 87.5%, followed by pubic shape at 84.5%. Important findings included the fact that there were significant differences in the accuracy of sex determination from pelvic morphology between both males and females and whites and blacks.

In conclusion, this study provides quantification of individual morphological traits in the os coxae of white and black South Africans that will be of value in forensic and archaeological analyses, especially when dealing with fragmentary remains. It also demonstrates that population differences affect the expression of sexual dimorphism and must be accounted for to develop the most effective methods of analysis.     

Methodological considerations on the study of sexual dimorphism in past human populations

The degree of sexual dimorphism in human populations is influenced by stress, social role and by labour division. However, studies on ethnographic populations provided contradictory results. Unfortunately, most of these studies were based on stature only, which, as we could observe in a survey on pre-protohistoric circum-Medirerranean samples, is a poor indicator of functionally related dimorphism. A number of skeletal measurements were examined: skull, stature, transverse trunk diameters, long bones length, circumference and section, in order to assess their usefulness as indicators of functionally related dimorphism. The best indicators were represented by section and circumference of the long bones of the limbs, followed by cross-shoulder breadth (biclavicular length), stature and limb bone length, facial measurements, cranial measurements and sacral breadth. From the methodological point of view, it was found that:

1. It is better to calculate the index of dimorphism for each trait or set of traits within each sample. Then a weighted average of all the available samples is taken. The index derived from pooling a number of samples does not make biological sense. In pooled samples the distinction of between versus within sample differences is obscured;
2. It is better to combine an index which is based on the difference between averages and one which takes variability into account, because variability can also be an aspect of sexual dimorphism;
3. It is better to apply some allometric correction to the measurements used. For instance, the log transformation produces clearer trends of differential dimorphism among the various traits.

     

Disproportionate sexual dimorphism in the human face

Analysis of facial dimensions of 86 young adults and their 76 parents indicates that a disproportionate sexual dimorphism exists in the ramus of the mandible, demonstrating a regional difference in growth response. The male ramus is on the average 14% longer than the female ramus, whereas other facial dimensions approximate an 8% sex difference. The findings have relevance to the analysis of skeletal remains and suggest the desirability of age specific discriminant function analysis for the sexing of adult mandibles.     

Sexual dimorphism of the human mandible and its association with dental development

The present study investigates whether the human mandible is sexually dimorphic during early postnatal development and whether early dimorphic features persist during subsequent ontogeny. We also examine whether mandibular dimorphism is linked to dimorphism of dental development. Dense CT-derived mandibular meshes of 84 females and 75 males, ranging from birth to adulthood, were analyzed using geometric morphometric methods. On the basis of the specimen's chronological ages and mineralization stages of the deciduous and permanent teeth, we compute dental age as proxy for dental development by the additive conjoint measurement method. By birth, males have, on average, more advanced age-specific shapes than females. However, sex differences decrease quickly as females catch up via a different association between shape and size. This leads to an almost complete reduction of sexual dimorphism between the ages of 4 and 14. From puberty to adulthood, males are characterized by allometric shape changes while the shape of the female mandible continues to change even after size has ceased to increase. Dimorphism of dental maturation becomes visible only at puberty. Sexual dimorphism, concentrated at the ramus and the mental region during the earliest ontogenetic stages and again at adulthood, is not associated with the development of the teeth. At puberty there is a simultaneous peak in size increase, shape development, and dental maturation likely controlled by the surge of sex hormones with a dimorphic onset age. We argue that the infant and adult dimorphism of the mental region may be associated with the development of supralaryngeal structures.     

Changes in the sexual dimorphism of the human mandible during the last 1200 years in Central Europe

According to many investigations, changes in mandibular morphology can occur synchronously with changes in the environment, and sexual dimorphism of the mandible can be influenced by the environment. Sexual dimorphism during the last 1200 years was evaluated using geometric morphometric analysis of virtual cranial models. The method of geometric morphometrics allows differences in size and shape to be assessed separately. We analyzed groups of adult individuals dating to Early Middle Ages, High Middle Ages, Early Modern Ages and from a modern Czech population (21st century). Significant sexual dimorphism in mandibular size was found in all populations. A trend in the sexual dimorphism of size was seen, with differences between the sexes increasing gradually over time. Size changes in female mandibles were a better reflection of environmental conditions and climate than size changes in male mandibles. Regarding changes in the sexual dimorphism of shape, significant dimorphism was found in all four samples. However, the pattern of mandibular shape dimorphism was different and varied considerably between samples. There was only one stable shape trait showing sexual dimorphism across all four samples in our study: the gonion lies more laterally in male than in female mandibles and male mandibles are relatively wider than female mandibles. Sexual dimorphism of shape is not influenced by the climate; instead sexual selection might play a role. This research supports earlier studies that have found that the degree and pattern of sexual dimorphism is population-specific and the factors regulating sexual dimorphism today may not be the same as those in the past.     

Three-dimensional geometric morphometric analysis of cranio-facial sexual dimorphism in a Central European sample of known sex

This article presents an approach for estimating the sexual dimorphism of adult crania using three-dimensional geometric morphometric methods. The study sample consisted of 139 crania of known sex (73 males and 66 females) belonging to persons who lived during the first half of the 20th century in Bohemia. The three-dimensional co-ordinates of 82 ecto-cranial landmarks and 39 semi-landmarks covering the midsagittal curve of the cranial vault were digitised using a MicroScribe G2X contact digitiser. The purposes of the investigation were to define the regions of the cranium where sexual dimorphism is most pronounced and to investigate the effectiveness of this method for determining sex from the shape of the cranium. The results demonstrate that it is better to analyse apportionable parts of the cranium rather than the cranium as a whole. Significant sexual differences (significance was determined using multivariate analysis of variance) were noted in the shape of the midsagittal curve of the vault, upper face, the region of the nose, orbits, and palate. No differences were recorded either in the shape of the cranium as a whole or in the regions of the base and the neurocranium. The greatest accuracy in determining sex was found in the region of the upper face (100% of study subjects correctly classified) and the midsagittal curve of the vault (99% of study subjects correctly classified).     

Bioanthropological analysis of human occipital condyles using geometric morphometric method

Sex differences are present in all parts of the body, including the skeletal system. Several methods are used to analyze the sex differences of skeleton, while more recently, a new method called geometric morphometry has been used. The aim of this study was to examine the sexual dimorphism of occipital condyles on human skulls originating from the population of Bosnia and Herzegovina using the geometric morphometric method.Material and methodsThe study was conducted on 214 human skulls of known gender from Bosnian population. For analysis of sexual dimorphism of occipital condyles, we used geometric morphometry, where all the skulls were scanned to obtain three-dimensional skull models. On the obtained models, we marked anthropometric points on occipital condyles in a Landmark Editor program from which we exported data in the form NTSYS file and analyzed it in MorphoJ program.ResultsFirst principal component PC1 describes 26.917% of total variability, the second principal component PC2 describes 20.992% of total variability, while the first eight principal components together describe 100% of total variability. The greatest variability between the male skulls and female skulls was present in the anterior-posterior diameter (length of occipital condyles). Discriminant functional analysis of the shape and size of the occipital condyles was possible with 69.50% accuracy for male skulls and with 60.27% accuracy for female skulls. The size of the occipital condyles showed a statistically significant effect on sexual determination. Discriminant functional analysis of the shape of the occipital condyles without affecting size enabled the determination of gender with with 65.96% accuracy for male skulls and with 63.01% accuracy for female skulls.ConclusionAnalysis of sexual dimorphism of occipital condyles using geometric morphometry showed statistically significant differences in the shape and size of occipital condyles between the sexes. The accuracy of sex determination based on occipital condyles was higher for male gender.     

The estimation of sex on the basis of the talus and calcaneus

The present report records and describes sexual dimorphism of the talus and calcaneus in American Blacks and Whites from the Terry Collection housed in the Smithsonian Institution, Washington, D.C. The greater amount of sexual dimorphism was observed in the talus, where 81 percent of the study sample could be accurately sexed. Four discriminant functions based on measurements from the talus and/or the calcaneus allowed sexing 79 to 89% of the study sample accurately. The techniques developed were then applied to two North American Indian samples, and sex of the individuals in these samples was assessed with the same degree of accuracy.     

Sexual dimorphism of root length on a Greek population sample

Sexual dimorphism in teeth has been an area of research for osteoarchaeologists and forensic anthropologists studying human skeletal remains. As most studies have been based on the mesiodistal and buccolingual crown measurements, sexual dimorphism from root length dimensions remains “neglected” by comparison to crown dimensions. The aim of the present study was to test the existence of sexual dimorphism in the root length of single-rooted teeth with the purpose of investigating whether maximum root length can be reliably used to determine sex. A total of 774 permanent teeth in 102 individuals (58 males and 44 females) from the Athens Collection were examined. The maximum root length of each tooth was measured on the mesial, distal, buccal, and lingual side. Almost all teeth presented a high degree of sexual dimorphism with males showing numerically higher values in root length than females. The most dimorphic teeth were the maxillary second incisors followed by maxillary canines. The percentage of sexual dimorphism reached 16.56%, with maxillary teeth showing the highest degree of dimorphism. The classification results show that the overall correctly specified group percentage ranged from 58.6% to 90.0%. The data generated from this study suggest that root length measurements offer a reliable method for determining sex and are therefore useful in osteoarchaeological studies, particularly in cases of fragmented or cremated material, but also in forensic contexts. Moreover, root length can be used to separate the remains of female and male subadult individuals with a high level of accuracy thus addressing one of the most problematic issues in human osteoarchaeology and anthropology as immature skeletons are the most difficult to sex.     

Brief communication: Sexual dimorphism of the juvenile basicranium

The purpose of this article is to examine the level of sexual dimorphism exhibited in the foramen magnum and occipital condyles of juveniles, and to test the utility of this sexual dimorphism for estimating sex. Using five basicranial measurements taken from 36 juveniles of known sex and age from the Lisbon documented collection (Portugal), we evaluated sexual dimorphism in the juvenile cranial base. Our application of a method previously applied solely to adults indicated that the basicranium is sexually dimorphic in juveniles, with larger foramen magnum and occipital condyle dimensions observed in males. Significant univariate differences between males and females were found for length and breadth of the foramen magnum, and breadth of the left occipital condyle. Using these measurements, multivariate discriminant analysis indicated that sex was correctly assigned 75.8% of time. Obtained accuracy, however, was lower than reported by previous studies of adult samples. We suggest that this discrepancy is a result of population variability rather than age. Am J Phys Anthropol 2010. © 2009 Wiley‐Liss, Inc.