Research Training Program

Smithsonian Institution
National Museum of Natural History

PROJECT SUMMARY
1996

S. Elizabeth Alter
Yale University
New Haven, Connecticut

David Hunt, Ph.D.
Douglas Owsley, Ph.D.
Supervising Scientists
Department of Anthropology
"Being given the opportunity to work in the Smithsonian with some of the most important scientists and collections in the world taught me the true meaning of excellence in science. "

A Reexamination of Flexure in the Human Mandibular Ramus as an Indicator of Sexual Dimorphism

ABSTRACT
Quantifying sexually dimorphic features of the human skeleton is a process crucial to the understanding of human development and evolution. Insofar as sexual dimorphism may reflect the lifeways of organisms, these features can additionally be a source of information about early hominid sociality. Many of these features that aid in the determination of a skeleton as male or female, however, are not simply present or absent but exist in a gradient of values between the sexes, making assessment more difficult. Therefore, the knowledge of an indicator of sexual dimorphism that was present in one sex but not in the other would be extremely valuable to physical anthropologists. Recently, a new morphologic indicator of sexual dimorphism was proposed in the American Journal of Physical Anthropology by Susan Loth and Maciej Henneberg: the degree of flexure in the mandibular ramus. In this study the proposal that flexure is a hormonally-linked trait found only in males was tested on the Terry Collection housed in the Smithsonian Institution in Washington, D.C. using the methods utilized by Loth and Henneberg and, in addition, measuring flexure and other relevant mandibular features metrically. It was found that flexure is not a discrete trait that is either present or absent but is morphologically variable. It was found in both males and females, though more commonly in males. T-tests for visual assessment values showed that there was a significant difference between flexure in males and females. However, factors such as tooth loss, genial angle, tooth wear, and robusticity influence ramus shape to a degree that genetically determined shape may be masked. Finally, it was noted that a high degree of correlation existed between eversion or flaring of bone at the gonial angle and flexure. It is possible, therefore, that flexure is not a genetic effect but the product of biomechanical forces acting on the jaw.

This research was supported by a grant from the National Science Foundation Research Experiences for Undergraduates program, Award Number DBI-9531331.

Letter of Gratitude