Severe enamel defects in wild Japanese macaques

Abstract

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 rarer in wild primates. The etiology of PFEH has been linked to exposure to severe disease, malnutrition, environmental toxins, and other systemic conditions. Similar defects have also been associated with genetic conditions such as amelogenesis imperfecta. In this study, we examined the prevalence of all types of enamel hypoplasia in several populations of wild Japanese macaques (Macaca fuscata) with the aim of providing context for the severe defects found in macaques from Yakushima Island. We found that macaques from other islands and from the mainland of Japan have low prevalence of the more common types of enamel hypoplasia and none exhibit PFEH. In contrast, 10 of the 21 individuals (48%) from Yakushima Island displayed uniform and significant PFEH, with all 10 living in two adjacent locations in the south of the island. All permanent teeth were affected to varying degrees except for first molars, and the mineral content of the remaining enamel in teeth with PFEH is normal (i.e., no hypo- or hyper mineralization). Given that the affected individuals have smaller first molars compared to non-affected macaques, and that they all underwent dental development during a period of substantial human-related habitat change, we conclude that the PFEH likely resulted from environmental stress. Extreme weather events on the island may also have influenced the formation of these defects. Additionally, it is plausible that a documented recent population bottleneck could have heightened the susceptibility of these macaques to PFEH. Further research on living primate populations is needed to better understand the causes of PFEH in wild primates and to evaluate whether such features can be used to assess the impact of human-related disturbance.