Plane-form enamel hypoplasia (PFEH) is a severe dental defect in which large areas of the crown are devoid of enamel. This condition is rare in humans and even rarer in wild primates. The etiology of PFEH has been linked to exposure to severe disease, malnutrition, environmental toxins, and associated with systemic conditions. In this study, we examined the prevalence of enamel hypoplasia in several populations of wild Japanese macaques (Macaca fuscata) with the aim of providing context for severe defects observed in macaques from Yakushima Island. We found that 10 of 21 individuals (48%) from Yakushima Island displayed uniform and significant PFEH; all 10 specimens were from two adjacent locations in the south of the island. In contrast, macaques from other islands and from mainland Japan have low prevalence of the more common types of enamel hypoplasia and none exhibit PFEH. In Yakushima macaques, every tooth type was affected to varying degrees except for first molars and primary teeth, and the mineral content of the remaining enamel in teeth with PFEH was normal (i.e., no hypo- or hyper mineralization). The aetiology of PFEH might be linked to extreme weather events or high rates of environmental fluoride causing enamel breakdown. However, given that the affected individuals underwent dental development during a period of substantial human-related habitat change, an anthropogenic related etiology seems most likely. Further research on living primate populations is needed to better understand the causes of PFEH in wild primates.

Specimens studied originate from three Japanese islands and mainland (Yakushima: 21 individuals; Honshu: 10 individuals; Koshima: 19 individuals). All specimens are curated at the Primate Research Institute (PRI) (now the Center for the Evolutionary Origins of Human Behavior), Kyoto University, Japan. All 48 individuals studied lived in the wild, with those on Koshima (Kojima) Island provisioned regularly as part of a primatological study.

Enamel hypoplasia data was collected following Towle and Irish (2019). Teeth were held under a lamp and rotated allowing light to hit the surface at different angles. The smallest discernible macroscopic defect was recorded, with a hand lens used to rule out postmortem damage. Postmortem damage was distinguished from enamel hypoplasia by distinct characteristics, including sharp edges and contrasting coloration between the fractured enamel and the rest of the crown. These features seldom resemble common forms of enamel hypoplasia, such as pitting, linear, or plane form defects. Under a hand lens, postmortem damage also lacks evidence of wear during the individual's life. Methods for recording LEH follow Goodman and Rose (1990) and Miszkiewicz (2015). Localized hypoplasia was recorded following Skinner et al. (2016). PEH was recorded if there were multiple circular/oval enamel defects on a tooth crown (Towle and Irish, 2019). If pitting was present within a LEH band, then it was recorded as LEH not PEH, but the pitting was noted. Plane-form enamel hypoplasia was recorded following Towle et al. (2017). Data are presented by tooth count rather than individual, with the number of hypoplastic teeth displayed as a percentage of the total number of observable permanent teeth.

The data analyzed includes the location, year of inclusion into the PRI collection and sex of each individual studied when it was available. Sex assignment was taken from the PRI database, and in the present study we did not attempt to assign sex to remaining specimens. Further details on the samples, as well as other information on tooth wear and pathologies for these populations can be found in Towle et al. (2022) and Towle and Loch (2021).

References

Towle, I., & Irish, J. D. (2019). A probable genetic origin for pitting enamel hypoplasia on the molars of Paranthropus robustus. Journal of Human Evolution, 129, 54-61.

Goodman, A. H., & Rose, J. C. (1990). Assessment of systemic physiological perturbations from dental enamel hypoplasias and associated histological structures. American Journal of Physical Anthropology, 33(S11), 59‐110.

Miszkiewicz, J. J. (2015). Linear enamel hypoplasia and age‐at‐death at medieval (11th–16th Centuries) St. Gregory's Priory and Cemetery, Canterbury, UK. International Journal of Osteoarchaeology, 25(1), 79-87.

Skinner, M. F., Skinner, M. M., Pilbrow, V. C., & Hannibal, D. L. (2016). An enigmatic hypoplastic defect of the maxillary lateral incisor in recent and fossil orangutans from Sumatra (Pongo abelii) and Borneo (Pongo pygmaeus). International Journal of Primatology, 37(4), 548-567.

Towle, I., Dove, E. R., Irish, J. D., & De Groote, I. (2017). Severe plane-form enamel hypoplasia in a dentition from Roman Britain. Dental Anthropology, 30(1).

Towle, I., & Loch, C. (2021). Tooth chipping prevalence and patterns in extant primates. American Journal of Physical Anthropology, 175(1), 292-299.

Towle, I., MacIntosh, A. J., Hirata, K., Kubo, M. O., & Loch, C. (2022). Atypical tooth wear found in fossil hominins also present in a Japanese macaque population. American Journal of Biological Anthropology, 178(1), 171-181. https://doi.org/10.1002/ajpa.24500