Electronic Thesis/Dissertation


From the inside out: Understanding stress-related enamel defects in great ape canines Open Access

Developmental defects of enamel are commonly used to reconstruct aspects of the health, growth patterns, and life history of modern humans, hominins, and nonhuman primates. This dissertation focuses on hypoplastic defects that appear as grooves on the outer enamel surface and are often termed linear enamel hypoplasia, or LEH. LEH defects have been linked to stressors such as malnutrition, injury, and illness in clinical and experimental settings. LEH is common in great apes, but previous work reported that one species, Virunga mountain gorillas, have far fewer defects than other taxa. This dissertation characterizes LEH defect morphology on the outer enamel surface of great ape canines (Chapter 2). The results suggest that mountain gorilla defects are shallow compared to those in other taxa, which may have led to their underestimation in previous studies. Females in the pooled sample were found to have deeper defects than males. The deepest defect in the sample belongs to a western lowland gorilla that was captured as an infant. Based on the location and approximate developmental timing of this defect, it might correspond to her capture. In Chapter 3, I incorporate histological data to assess whether enamel growth variables, namely linear enamel thickness, enamel extension rates, and striae of Retzius angles, correspond to the documented variation in LEH defect depth described in Chapter 2. Inter- and intraspecific variation in enamel extension rates and striae of Retzius angles, and to a lesser extent linear enamel thickness, tracked the sex- and species-differences in defect depth. This suggests that enamel growth variation influences LEH defect morphology on the outer enamel surface. Enamel growth patterns should therefore be carefully considered when reconstructing stress severity based on the appearance of defects on the outer enamel surface alone. In Chapter 4, I conduct a detailed histological analysis of four mountain gorilla individuals, three of which are of known age and sex. I found that all LEH defects from Chapter 2 co-occur with underlying disruptions to enamel matrix secretion in the form of accentuated lines. However, there are many more accentuated lines than there are LEH defects in this sample, and because accentuated lines occur throughout the height of the tooth crowns, they provide a more complete history of growth disruptions. One specimen, GP.075, demonstrates a major plane-form defect in the permanent third molar that corresponds to a relatively minor accentuated line in the concurrently forming canine. Three poaching-related snare removals by veterinarians were recorded as LEH defects with co-occurring accentuated lines in the canines of two individuals, providing rare data on defect etiology in wild primates. Future work will incorporate more diverse hominoid samples into these analyses, including more frugivorous Bwindi mountain gorillas and eastern lowland gorillas, to better understand the links between behavioral ecology and enamel growth in closely related taxa. Taken together, these results increase our understanding of inter- and intraspecific canine enamel growth variation in great apes. This work characterizes “normal” growth variation as well as growth disruptions in the form of LEH defects and accentuated lines. A key finding is that enamel geometry plays an important role in determining defect severity or depth; it is important to characterize defect morphology within and among species and sexes to understand whether a given defect represents an outlier at the population level. Particularly deep LEH defects might correspond to severe stressors, as in the case of the wild-captured apes in Chapter 2. Detailed growth histories are best understood via histologic analyses, as Chapter 4 demonstrates. This work has relevance for researchers interested in reconstructing the growth histories of hominins and past populations of modern humans on the basis of enamel defects alone as they too exhibit variable enamel growth patterns, which might influence the morphology and interpretation of enamel defects.

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