Geometric morphometric analysis of the use of mandibular gonial eversion in sex determination

Physical anthropologists are continually looking for new ways to determine sex from skeletal remains. Determination of sex is done either metrically or descriptively (morphologically). Many characteristics cannot be metrically assessed and descriptive characteristics cannot always be objectively compared. Geometric morphometrics is a relatively new method that provides a mechanism to quantify descriptive morphological characteristics and provides statistics to interpret findings. In this study gonial eversion as a sexually dimorphic trait was re-evaluated, with the use of geometric morphometrics. Twenty-eight adult black female mandibles and 46 adult black male mandibles from South Africa were used. Photographs were taken of the posterior aspect of each mandible with a digital camera in a fixed position. The mandibles were positioned so as to optimise the demonstration of the gonial eversion.The captured electronic images (in jpg format) were then analysed with the tps series of programs (thin-plate spline analysis of digitized landmarks). Statistical analysis revealed that 71.4% of females and 73.9% of males could be accurately sexed by using the shape of the posterior aspect of the mandible. The males had a more variable shape, while the females were more similar to each other. A smaller gonial eversion involving the lower part of the mandibular ramus was seen in the females while the males tend to have a uniform and much wider curve involving the whole ramus. There was, however, considerable overlap between the sexes. Although different tendencies exist between the gonial eversion of the mandible of males and females, the extent of these differences is not adequate to predict the sex of a single individual.