1Department of Oral Anatomy, School of Dentistry, Aichi Gakuin University, 2Department of Human Evolutionary Biology, Harvard University, 3Department of Biological Sciences, Wellesley College, 4Department of Anthropology, Wake Forest University, 5Graduate School of Science, Kyoto University, 6Primate Research Institute, Kyoto University, 7Japan Monkey Center
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Molar enamel thickness has an important role in studies of primate taxonomy, phylogeny, and functional morphology, although its variation among hominins is poorly understood. Macaque species parallel hominins in their widespread geographic distribution, relative range of body sizes, and radiation during the last 5.5 million years. In order to explore enamel thickness variation, we examined average (AET) and relative (RET) enamel thickness in 279 molars from 79 individuals of four species (Macaca arctoides, Macaca fascicularis, Macaca fuscata and Macaca mulatta). Virtual sections were generated from micro-CT images sectioned bucco-lingually across mesial cusps. Enamel cap area, dentine area, and enamel-dentine junction length were measured on unworn or lightly worn sections, yielding AET and RET indices. Sex differences were not found in tooth-specific AET or RET comparisons within species, which differs from great ape and human trends. Comparisons of macaque species reveal that M. fuscata has the highest AET and RET, M. fascicularis shows the lowest AET, and M. arctoides has the lowest RET. Patterns of AET, an absolute measure of enamel thickness, are consistent with average annual fruit consumption, with the most frugivorous species showing the lowest AET. Moreover, M. fuscata consumes the highest percentage of nuts, seeds, and pods, which has been suggested as a correlate of thick enamel in hominins. Relative enamel thickness, a size-corrected metric, follows phylogenetic groupings from a recent nuclear genome study. Additional data on fallback foods and material properties are needed to assess current hypotheses about the relationship between primate dietary ecology and enamel thickness.
Funded by Harvard University, Wake Forest University, Primate Research Institute Kyoto University, Aichi Gakuin University, and the Nikon Corporation (Japan).