Molar topography covaries with insectivory in primates

Researchers studying molar topography have traditionally categorized primate taxa based on their purported primary dietary guild, typically as insectivores, folivores, frugivores, or omnivores. Categorizing primates by diet can be subjective and reductionist as different researchers may define a given dietary category differently and primate diet tends to be quite complex, varying seasonally, for example. As the earliest members of the order Primates were said to experience a shift from insectivory to frugivory, it is important to develop methods to study that dietary shift. The following work treats diet as a continuous variable as the percentage of foraging time spent feeding on insects (%insects) instead of categorizing primates by broad dietary categories. Extant taxa were chosen for this analysis where both relevant dietary data were available in the literature, as well as previously published dental topographic data; in this case Dirichlet normal energy (DNE), which is a measure of surface curvature or sharpness. This study considered 16 non-folivorous primate taxa that vary in %insects, including tarsiers, lorises, galagos, lemurs, platyrrhines, and a cercopithecoid. Results suggest that as taxa spend more of their foraging time feeding on insects, their molar curvature increases. A phylogenetic generalized least squares regression suggests that %insects is significantly correlated ( P < 0.001) and a moderate predictor ( R 2 = 0.579) of molar curvature. This method of treating diet as a continuous variable served as a more nuanced approach to characterize variation in molar topography compared to previous methods of categorizing taxa by diet. Overall, these results provide further evidence that DNE is a useful method for capturing molar shape relevant to important dietary transitions throughout primate evolution, and that treating an aspect of diet as a continuous variable rather than a categorical variable provides insight into important differences in molar form among primates and mammals more generally.