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Objectives While dental development is important to life history investigations, data from wild known-aged great apes are scarce. We report on the first radiographic examination of dental development in wild Virunga mountain gorillas, using known-age skeletal samples recovered in Rwanda. Materials and methods In 43 individuals (0.0-14.94 years), we collected radiographs of mandibular molars, and where possible, cone beam CT scans. Molar crown and root calcification status was assessed using two established staging systems, and age prediction equations generated using polynomial regression. Results were compared to available data from known-age captive and wild chimpanzees. Results Mountain gorillas generally fell within reported captive chimpanzee distributions or exceeded them, exhibiting older ages at equivalent radiographic stages of development. Differences reflect delayed initiation and/or an extended duration of second molar crown development, and extended first and second molar root development, in mountain gorillas compared to captive chimpanzees. However, differences in the duration of molar root development were less evident compared to wild chimpanzees. Discussion Despite sample limitations, our findings extend the known range of variation in radiographic estimates of molar formation timing in great apes, and provide a new age prediction technique based on wild specimens. However, mountain gorillas do not appear accelerated in radiographic assessment of molar formation compared to chimpanzees, as they are for other life history traits. Future studies should aim to resolve the influence of species differences, wild versus captive environments, and/or sampling phenomena on patterns observed here, and more generally, how they relate to variation in tooth size, eruption timing, and developmental life history. [ABSTRACT FROM AUTHOR]

The microanatomy of enamel and dentine records important information about the rate and duration of tooth development, and the timing of key developmental milestones and stress events occurring during an individual's life. While much attention has focused on relationships between dental development and life history in great apes, opportunities to examine individuals from well-documented wild populations are exceptionally rare. Further, many taxa remain poorly known. We report on a histological examination of dental development in a young juvenile female mountain gorilla from Volcanoes National Park, Rwanda. This individual died an early death; associated records document the last several months of her life. In first and second mandibular permanent molars, we used histological methods to reconstruct cuspal formation times, daily secretion rates, and age at death, providing the first such data for mountain gorillas. Following a blind protocol, we also charted the timing of stress as recorded by accentuated growth increments. Calculated age at death of this individual was 3.10 years, at which time mandibular M1s were partially erupted through the gingiva. From analyses of daily and long-period growth increments, M1 mesiobuccal cusp calcification initiated 87 days before birth and total cuspal formation time was 2.27 years. Periodicity of long period growth increments was 6 days. A prominent stress line was determined to have formed four months before death, corresponding closely in time to a documented poaching event that left this individual orphaned. Subsequent to this, accentuated increments record repeated incidence of stress during a period of documented reintroduction attempts, injury resulting from aggression by other gorillas and a follow-up surgery. Our observations are consistent with accelerated dental development reported previously for a captive juvenile western gorilla (Schwartz et al. 2006), and demonstrate the value of dental histology for revealing aspects of individual life history and stress in wild great apes. [ABSTRACT FROM AUTHOR]

Deeper or more 'severe' linear enamel hypoplasia (LEH) defects are hypothesized to reflect more severe stress during development, but it is not yet clear how depth is influenced by intrinsic enamel growth patterns. Recent work documented inter- and intraspecific differences in LEH defect depth in extant great apes, with mountain gorillas having shallower defects than other taxa, and females having deeper defects than males. Here, we assess the correspondence of inter- and intraspecific defect depth and intrinsic aspects of enamel growth: enamel extension rates, outer enamel striae of Retzius angles, and linear enamel thickness. Thin sections of great ape canines (n = 40) from Gorilla beringei beringei , Gorilla gorilla gorilla , Pan troglodytes , and Pongo spp. were analyzed. Enamel extension rates were calculated within deciles of enamel-dentine junction length. Linear enamel thickness and the angle of intersection between striae of Retzius and the outer enamel surface were measured in the imbricational enamel. Mountain gorillas have faster enamel extension rates and shallower striae angles than the other taxa examined. Mountain gorillas have thinner imbricational enamel than western lowland gorillas and orangutans, but not chimpanzees. In the combined-taxon sample, females exhibit larger striae angles and thicker imbricational enamel than males. Enamel extension rates are highly negatively correlated with striae angles and LEH defect depth. Enamel growth variation corresponds with documented inter- and intraspecific differences in LEH defect depth in great ape canines. Mountain gorillas have shallower striae angles and faster extension rates than other taxa, which might explain their shallow LEH defect morphology and the underestimation of their LEH prevalence in previous studies. These results suggest that stressors of similar magnitude and timing might produce defects of different depths in one species or sex vs. another, which has implications for interpretations of stress histories in hominins with variable enamel growth patterns. [ABSTRACT FROM AUTHOR]