Draft:Skeletal sex estimation

This article states "determining sex from human skeletal remains is not impossible."

Human skeletal sex estimation is a process by which the biological sex of skeletonized human remains is determined. Estimation of the sex of a skeleton forms an important part of the individual's biological profile, which generally consists of information such as their sex, age, height and ancestry. Biological profiling is useful in multiple fields where skeletal remains may be the only surviving identifier of a person, such as in archaeology and forensic anthropology. When using sex estimation to develop a biological profile it is also important to note the distinction between sex and gender in this context. Sex estimation can only provide an estimate for the biological sex of the individual, not their gender, which is a social identifier rather than a defined set of biological characteristics. In an archaeological context, studying other aspects of a burial such as burial type, location and the presence of burial goods may help to provide context to the gender identity of an individual alongside sex estimation. As well as gender identity not being a corollary of biological sex, it is also important to recognise that sex determination from skeletal remains, especially when incomplete or poorly preserved, is not an exact method.

Advances in DNA analysis that can be applied to bone mean that sex can be determined with high levels of accuracy due to the different chromosomes present in male and female DNA. However, in many cases DNA analysis is not possible, due to high expenses and the destructive nature of sampling methods. When DNA analysis is not an available option during the process of sex determination, methods that assess the physical characteristics of the skeleton will be employed instead. These include morphological identifiers and anthropometric measurements. Morphological osteology focuses on identifying aspects of the skeleton which can reveal identifying information such as estimated age and sex, whereas anthropometrics measures skeletal remains for a similar purpose. These methods are not as accurate as DNA analysis, but experienced osteological assessment of skeletal remains can provide significant amounts of information, including an estimation of sex, especially if there are multiple surviving aspects of a skeleton which are the most sexually dimorphic. In comparison to other great ape species who often show large levels of sexual dimorphism between males and females, humans have considerably less noticeable sex characteristics in their skeletal morphology, but determining sex from human skeletal remains is not impossible.

Sex estimation via osteological investigation can only obtain high levels of precision when the skeletal remains are of a mature adult, as skeletal remains of infants, children and adolescents may not have acquired the skeletal identifiers associated with their biological sex which often accompany puberty and later development. As well as the age of an individual affecting the morphological characteristics of their skeleton, the assessment of population differences and how these may affect skeletal presentation is also vital. For example, male bones can generally be assumed to be more robust and larger than female bones. However, across different populations these traits will differ in what is expected, and therefore there cannot be a universal standard measurement to estimate sex and skeletons must be assessed on a smaller population-based scale.

Methodology

Osteology and anthropometrics

There are many parts of the skeleton which can be used to determine the sex of an individual, but the most commonly used identifiers are located on the skull and the pelvis, both of which consist of a set of smaller bones. Some other areas include the vertebral column and the long bones, although these tend to have much more morphological overlap between males and females in humans. Generally, male skeletons are larger, more robust, and have more pronounced muscular attachments due to a greater muscle mass, whereas female bones will be smaller, lighter and more gracile. However, there will always be exceptions to this.

The skull

Representation of a male skull.

Especially in an archaeological context, skulls are rarely non-fragmentary, and therefore can be difficult to sex. However in situations where the skull is complete or partial enough to provide useful information in this area, the key points of reference for sex identification include the supraorbital ridge above the eye sockets, the eye sockets or orbita themselves, sinuses, key muscular ridges such as the nuchal crest and the external occipital protuberance on the base and back of the skull, the teeth and the palate at the roof of the mouth, and the mandible, or lower jaw.

As well as assessing size and weight of the skull, these details allow for a more refined estimation of sex. For example, in a male skull, muscular ridges, the occipital protuberance and the supraorbital ridge will likely all be more defined and pronounced. Sinuses will be larger, the upper edge of the eye socket will be more rounded, the jawline will be squared and robust, teeth will be larger and the palate broader. In contrast, female skulls will typically be more rounded and smaller in size.

Representation of a female skull.

Identifying the sex of immature skeletons is much more difficult than adults, however it has been theorised that where permanent teeth are present, these could be measured and cross-compared with adult teeth to help estimate sex.

The pelvis

The pelvis in particular is perhaps the most sexually dimorphic skeletal structure, considering its direct relationship to gestation and parturition. In particular, the female pelvis tends to be significantly wider than the male pelvic region, in order to allow for childbirth, where the infant must pass through the birth canal in the pelvic opening. It has been claimed that the pelvis is the most reliable region of the skeleton to use when estimating sex, with an accuracy of between 90% and 95%.

Similarly to the skull, the male pelvis tends to have more pronounced muscular attachments and be larger in size, especially the acetabulum and pubic symphysis. However, the female pelvis has some uniquely prominent features, such as the sciatic notch, which tends to be considerably wider and shallower in female pelvises than in males.

Anthropometrics can be particularly useful when assessing the pelvic region, and several measurements can be taken, such as the sub-pubic angle, the ischio-pubic index and the angle of the sciatic notch. A wider angle in the sciatic notch and a sub-pubic angle of greater than 90% is associated with the female sex, and a larger ischio-pubic index which is measured from the lengths of the ischial and pubic bones is also associated with the female sex.

DNA Testing

DNA testing of skeletal remains to identify sex consists of sampling the bone, and then studying the genes present in the sample, such as the amelogenin gene, which presents differently in males and females. This allows for a highly accurate determination of the biological sex of an individual, however it faces many problems such as high cost, destructive sampling methods and possible contamination during analysis.

Sex estimation in case studies

The determination of the sex of skeletal remains is very important, and can alter the way that contextual information is interpreted, especially in archaeological burial contexts. This is particularly true in cases where the sex of a buried individual has originally been misidentified, and therefore other conclusions made about them may be incorrect. Such was the case for the Birka warrior, buried in the Swedish Viking-agearchaeological site of Birka in grave Bj 581. When the grave was first excavated in 1941 it was assumed that the warrior was male, due to the presence of warrior equipment such as blades, arrows and two horses, and at that time it was not known that there were female Viking warriors. However, when a more detailed osteological analysis of the skeleton was undertaken, it turned out that the burial was more likely of a female. As well as later osteological analysis, DNA sequencing was also undertaken which further confirmed the probability that the warrior was female. This demonstrates the importance of both osteological assessment as well as DNA analysis, which have not only aided archaeologists in better understanding this specific individual, but have also helped in developing understanding of the relationships of Vikings to sex and gender. The fact that a female skeleton was found amongst prestige burial goods typically associated with male warriors could indicate that women were also able to build status and renown as warriors, something which was not previously well understood until studies of skeletal remains and their burial contexts were undertaken.

As well as helping resolve mistaken cases of identity, methods already used in sex estimation can be used to develop new and increasingly detailed processes for sexing. For example, it was theorised that measuring the humeri of skeletons could help determine their sex, focusing on populations from the medieval Adriatic Coast. Several measurements were taken from the humeri of these skeletons and then cross-compared with DNA analysis from the same individuals to assess how accurate the humeral anthropometrics had been. This revealed that measuring the humerus was a relatively accurate method of determining the sex of an individual, although it was more precise for males than females. This shows how when osteological methods and DNA sequencing are used in combination, new and unique methods for sex determination can be developed, which may be particularly useful in situations where the humerus is the only suitable bone for assessment. It is also a useful case for displaying the importance of population wide studies rather than focusing on individual skeletal remains alone, as this paper focused on medieval skeletons of the Adriatic Coast. The measurements found may not be applicable to other populations, but the method could be explored in other contexts, and would be especially useful in situations where long bones are the only consistently surviving part of skeletal remains or where further evidence for sex identification is needed.