Variation and adaptation in the mechanical properties of primate molar tooth enamel

Tooth enamel is a durable tissue with mechanical properties (stiffness and hardness) that vary throughout the teeth of primates with implications for dietary adaptation. Typically, estimations of tooth strength rely on averaged values of mechanical properties, which relegates variation within the crown that may be important for testing functional hypotheses. Here, we measured the mechanical properties of enamel in a taxonomically broad sample of primates and used generalized additive models to describe variation as gradients from the enamel-dentine juncture (EDJ) to the tooth surface. We found a sharp reduction in enamel stiffness at the EDJ and overall lower stiffness in the crown cervical margin across the sample. These regions are predicted to be vulnerable to fracture propagation during mastication, which reduced stiffness would help to mitigate. High hardness was identified at the cusp tips, consistent with increased wear resistance at the tooth surface. Large-bodied and hard-object feeding primates exemplified these patterns in our dataset. Our results suggest that mechanical properties in enamel vary to accommodate the maintenance of high strength throughout the cusp while simultaneously protecting vulnerable regions within from failure. This variation likely constitutes a key dietary adaptation with important implications for understanding dental function in medical and evolutionary contexts.