Quantification of enamel decussation in capuchin monkeys

Abstract

Introduction & Objective – An intricate suite of highly complex enamel has led robust capuchin monkeys to be uniquely capable in breaking down hard-object foods as compared to other cebid monkeys, and in routinely breaking down extremely mechanically challenging foods as compared to other capuchins. The goal of this study is to compare dental enamel complexity in images of dental thin sections from a sample of robust vs. gracile capuchins using Image Compression Ratio (ICR) analysis. We hypothesize that: 1) robust capuchins will exhibit an overall pattern of higher ICR, and therefore more complex enamel, compared to gracile capuchins, and 2) the difference in ICR between the two groups will be greater for anterior as compared to posterior teeth. Materials and Methods – 113 images of thin sections from three gracile capuchin species and from unidentified species of robust capuchins, Sapajus spp were captured using a Leica-Leitz DMRX/E Universal Microscope configured with a Marzhauser motorized stage, phase contrast, and circularly polarizing filters. The images were then digitally processed to delete non-enamel components from the images and any other factor that can affect the compression ratio. After various compressions, the ICR values were compared across our images using standard non-parametric Kruskal-Wallis and Mann Whitney tests. Results – As for the first hypothesis, we found no significant difference in ICR between robust and gracile capuchins across all teeth (p = 0.35) using Mann-Whitney U tests. Likewise, we found no significant difference in ICR for all tooth categories when all gracile capuchin species were examined separately rather than assessed as one category (Kruskal Wallis, p = 0.48), nor was there a significant difference among tooth types (canines, premolars, and molars) in the total dataset (p = 0.36). As for the second hypothesis, we separately compared anterior (canines) and posterior teeth (premolars and molars) across the same taxonomic groupings. We once again found no difference in ICR between robust and gracile capuchins for the posterior dentition (p = 0.28). However, the anterior tooth sample did display a significant difference between the two groups (p = 0.04). Conclusion – Our ICR results indicate that premolars and molars among gracile and robust capuchins exhibit similar degrees of enamel complexity, but that canines in robust capuchins display increased complexity reflecting high specialization in this group for hard object feeding in the anterior masticatory apparatus. Significance/Implication – These findings provide further evidence to the hypothesis that the need to break apart obdurate foodstuffs with the anterior – as opposed to posterior – teeth has driven much of the specialized craniodental anatomy seen in robust capuchins. Future work including further sampling may help assess the degree in which different species of robust capuchins differ from one another. The findings of this research may also lead us to assess differences among the robust species, considering previous studies suggesting that tool use in some species may lessen the need for generation of high anterior bite forces compare capuchins to other primate hard object feeders such as the pitheciines and Paranthropus, the latter of which would be particularly useful for assessing patterns of hard object feeding in hominin evolution.