Multiple behavioral and biomechanical analyses have demonstrated that capuchin monkeys (Cebus and Sapajus) are specialized for breaking down hard-object foods as compared to other cebid monkeys. In addition to a complex suite of craniodental adaptations, it has specifically been demonstrated that capuchins possess highly complex dental enamel, with extensive Hunter-Schreger banding and other decussation, that likely serve as an adaptation to resist crack propagation during hard-object feeding. Furthermore, it has been demonstrated that robust capuchins (Sapajus spp., formerly Cebus apella) demonstrate further adaptation for hard-object feeding than other capuchins, routinely breaking down extremely mechanically challenging foods. However, there has been no comparison of dental enamel complexity in robust versus gracile capuchins, to assess whether the dental enamel in Sapajus follows this same pattern of further specialization. Therefore, this study compares dental enamel complexity in images of dental thin sections from a sample of robust versus gracile capuchins using image compression ratio (ICR) analysis. ICR is a variable that correlates with enamel complexity, such that higher ICR values are indicative of increased complexity in the form of enamel decussation. We found no significant difference between robust and gracile capuchins when assessing all teeth in our sample together, however, we did find that robust capuchins have significantly higher ICR values than gracile capuchins for canine teeth, specifically.Our results support prior studies suggesting that robust capuchins are specialized to generate increased masticatory loads with their anterior dentition, specifically, as compared to gracile species.

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 pr...