Linear enamel hypoplasia (LEH) is a defect of tooth enamel that reflects periods of nutritional or disease stress experienced during the early years of life, when teeth are forming. These defects form when such physiological stresses disrupt normal enamel growth, causing the formation of linear groves on the enamel surface. There are consistent differences among both extinct and living primate species in the frequency with which they exhibit these enamel defects. Of the great apes, bonobos have the highest frequencies of LEH. Chimpanzees and orangutans exhibit higher frequencies than gorillas, and lowland gorillas exhibit higher frequencies than mountain gorillas. Among human ancestors, Australopithecus has higher frequencies of multiple defects than Paranthropus, and Neanderthals exhibit LEH frequencies higher than some modern human groups but not others.
Researchers usually assume that these LEH frequency differences among species are a direct reflection of differences in stress experience. However, species also differ in the way that their enamel grows. It is highly likely that species-specific differences in aspects of enamel growth are influencing the patterns that have been observed in the distribution of LEH in primates. Recent studies in modern humans have demonstrated that aspects of enamel growth affect how teeth record physiological stresses. Specifically, the percentage of the tooth crown composed of imbricational enamel (enamel covered by perikymata, external manifestations of internal growth increments known as striae of Retzius) affects the opportunity for defects to form, while the depth of perikymata affects defect perceptibility.
This study represents the first systematic investigation of the extent to which these enamel growth variables can account for differences in LEH prevalence across primate species. The intellectual merit of this study lies in the insight it will provide into how the structure of enamel constrains defect formation across species. By understanding these constraints, clearer interpretations of what linear enamel hypoplasias reveal about physiological stress in fossil and living primates, including humans, will be possible. To answer the question of how enamel growth variables affect LEH prevalence across species, this study examines both internal (striae of Retzius) and external (perikymata) enamel growth structures across 70 primates species.
Broader impacts include the creation of a large replica sample of primate dental specimens selected for the preservation of perikymata on their surfaces. Because perikymata provide a record of enamel growth, they can be used in additional studies of primate growth and development. This sample will be become part of the Menegaz-Bock dental cast collection housed in the Bioarchaeology Laboratory at The Ohio State University and will be made available to other researchers. Thus, this project will have a positive impact on the infrastructure of research in physical anthropology. The PI has used raw data from previous research in lab exercises in both graduate and undergraduate classes; data generated from this research will be used in this way as well. Finally, dental anthropology is a field in which female scientists have been traditionally underrepresented. The post-doctoral collaborator on this project is a female scientist, and this project will support her professional development.
