The teeth of most mammals are covered with a very hard substance called enamel. Several aspects of this enamel cap, such as its shape and thickness, vary from species to species. Many scientists have long thought that this variation reflects adaptation to diet, but nobody has been able to come up with a convincing mechanism as to what the precise relationship is. Borrowing from the field of materials science, this study uses principles of fracture and deformation of solids to provide quantitative predictions as to how enamel may be adapted to diet. The importance of using this body of theory lies in its unification of many aspects of tooth form. It connects cusp form (in terms of sharpness-bluntness), enamel thickness, enamel mechanical properties (i.e., hardness, toughness), and enamel microstructure (i.e., how the enamel is formed), quantifying their combined effect on protecting the tooth against fracture. Thus, data on these various aspects of tooth form in different species can tell how these species differ in their ability to process different food types. In addition, the outer casings of many hard foods, such as seeds and mollusk shells, appear often to have evolved structures similar to those of enamel, making it possible to predict the forces at which they break as well. To test the theory, fracture experiments are being conducted on extracted teeth from various mammal species, including humans and other primates, to identify forces at which the enamel yields or cracks. After fracturing the specimens, they are sectioned and examined under a light microscope to measure enamel thickness, and the mechanical properties of the enamel and dentin are determined by nanoindentation. The combination of data on yield/fracture forces, enamel thickness, and enamel and dentinal properties allows accurate tests of theoretical predictions to be made for both living and fossil species. The thickness of the enamel can also provide an indirect measure of the maximum bite force of an animal, which could be a useful dietary indicator for both extant and extinct taxa. The broader impact of this study, including interdisciplinary student training, is the integration of material science engineering and physical anthropology to address the relationship of tooth enamel fracture to bite force and food hardness.
