Technical note: converting durometer data into elastic modulus in biological materials

Researchers have increasingly recognized the need to quantify the material properties of primate food items, particularly hardness (H) and stiffness (E), which is measured as elastic modulus. Assessing E in the field is particularly difficult because the typical equipment needed to perform the requisite analyses is expensive and cumbersome. Alternatively, researchers can use hand-held, relatively inexpensive, portable durometers that measure H on Shore scales. Shore-D durometers show a reliable ability to characterize H in harder-stiffer materials, and Shore-D measures in these materials can be reliably converted to E. Shore-A durometers-employed in past field studies of food properties-do not accurately characterize the properties of harder-stiffer materials, which are likely to be those materials exerting the greatest mechanical demands on primate masticatory morphology. We offer recommendations for Shore-D durometer usage in the field.