While bones appear devoid of identity to the untrained eye, a forensic anthropologist can spy subtle features and differences. This pattern of clues can help them piece together the person those bones represent, an important skill in crime victim identification.
Some of those subtle differences pertain to biological sex - which, it should be pointed out, isn't the same as gender, which is a social construction. In her research, Binghamton University graduate student Mary St. John uses software techniques and quantitative measures to track differences in human cheekbones, which will lead to a new method for determining the sex of unknown individuals.
Humans are a sexually dimorphic species, meaning that there are discernible size and shape differences between males and females, St. John explained. When determining a skeleton's sex, experts normally look to the pelvis because female pelvic girdles are designed to allow for childbirth, an attribute obvious to the trained eye.
In general, biological males have larger builds - "robust," to use the terminology - with larger muscle attachment sites. Male skulls are more likely to feature prominent brow bones and jaws that are strong and square; also larger are the occipital crest, where muscles attach in the back of the head, and the mastoid process, the bony bump behind your ear. Female features are smaller, or "gracile."
Sex-linked skull traits are a bit more variable than those of the pelvic girdle, however. Differences are often population-based, and many individuals have a combination of gracile and robust traits, or intermediate traits that fall between the two poles.
"Humans are quite variable, so when we talk about structural differences, we are talking about general trends and not hard facts," said St. John, who will complete her master's in biomedical anthropology in May.
Ideally, forensic anthropologists would use both the pelvis and skull to estimate sex, but sometimes only one or the other is available. Morphological traits, especially in the skull, are ranked on a scale of one to five, with one being the most gracile and therefore most likely female, and 5 the most robust and most likely male, she said.
"This can be incredibly subjective and different people might rank traits differently," she said. "The more accurate we can be with these identification techniques, the better we are able to assist law enforcement and families that may be searching for their loved one."
St. John and her research partner, Jefry Garcia-Bernabe, are assessing the level of sexual dimorphism in the zygomatic bone using three-dimensional geometric morphometrics (3D GM), which relies on landmarks to capture shape variables.
The two researchers captured the bone structure of 10 males and 10 females. Using specialized forms of analysis, they were able to demonstrate that differences exist and where most variations were likely to occur. They ran their study blindly, so they didn't know the individuals' sexes until they interpreted the data. The resulting graph correctly split the individuals into two groups, based on the cheekbone features: one male, one female, confirming their hypothesis.
Finding her path
A native of Michigan, St. John completed her bachelor's in anthropology from Central Michigan University (CMU). During her first semester as an undergraduate, she ended up taking an introductory course in biological anthropology, although she was unfamiliar with the field. By the third class, she knew that she had found her path.
"I'm fascinated by the physical processes that define the human experience, both during life and after. As a career, it will allow me to use my knowledge to help others, which is important to me," she explained.
One of her CMU professors had graduated from Binghamton's anthropology program and recommended it. She particularly appreciates the Binghamton program's hands-on nature, which allows her to complete her MS degree through an internship rather than a written thesis.
At Binghamton, she found her professors to be both inspiring and encouraging. St. John first learned about 3D GM through a virtual anthropology class taught by her mentor, Professor Rolf Quam, who has been instrumental in her research project. Associate Professor Elizabeth DiGangi had suggested looking at sex differences in the zygomatic bone and helped solidify St. John's desire to work in the forensic field.
"It is truly gratifying to see students take their class project in a course you taught and turn it into a real research project that contributes to the field," Quam said. "Mary and her research partner Jefry have been persistent in their study, overcoming several technical obstacles, learning new software programs and forms of analysis, and interpreting the results. This study is a promising approach to estimating sex from the skeleton, a topic of central importance in forensic cases."
For their next steps, St. John and Garcia-Bernabe will expand the project, gathering a larger sample size to reproduce their results. They also will present their findings this March at the American Association of Biological Anthropologists' annual conference in Denver.
"We're excited about the research that we're doing, and grateful for the opportunities we've been given to do so!" St. John said. "It's a dream to be able to contribute to the body of 3D GM research that is out there, and we hope that what we are doing will in the future become another useful tool to help with sex estimation."
