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All races and ethnic groups have some degree of dental anatomic variations. Asian populations present one of the widest variations in coronal shape, external root form and internal canal space morphology.

Teeth were cleaned from all debris, attached tissue and calculus using an Ultrasonic Scaler and were preserved in 10% of formalin solution. Teeth were measured for length, using Electronic Vernier Caliper (MEKA Electronic Vernier Caliper 150 mm, Model no. 06912, Hangzhou Meka Tools Co. Ltd., Zhejiang, China), from the tip of the crown to the apex of the root. For curved roots, tangents were drawn to the curved portion of the tooth and end length was measured by connecting the points of tangency.

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All the mandibular first premolar teeth examined in this study were found to have 2 cusps which is in keeping with the established external morphology of this tooth.17,18 Evagination manifested as an extra cusp or enamel pearl, although commonly reported in mandibular premolars in people of Asian descent was not found.

Craniolateral view onto the incisors and canines of a lowland tapir (T. terrestris) in (A) closed and (B) opened position. View of the cheek tooth row of the same skull in occlusion (C) viewed from caudal to rostral along the lingual side of the left cheek tooth row, indicating little overlap on the lingual side, and (D) viewed from rostral to caudal along the buccal side of the left cheek tooth row, showing overlap of the maxillary teeth on the buccal side.

Mean ( SD) (A) length of individual teeth of the maxillary and mandibular cheek tooth row in relation to the total length of both rows combined (100% = length of maxillary + mandibular cheek tooth row). T. terrestris (n = 13), T. bairdii (n = 4), T. indicus (n = 2); (B) relative width of the anterior and posterior lophs of individual teeth in relation to the width of the anterior loph of the M2 (which is set at 100%). The maxillary tooth row is illustrated with a darker shade, compared to the mandibular, overlying tooth row. There is no P1. P1 only has one loph. Note that in all cases, maxillary teeth are wider than mandibular ones. T. terrestris (n = 17), T. bairdii (n = 6), T. indicus (n = 3).

When relating tooth length to loph width in the maxillary row, the P1 was longer than wide, whereas all other teeth were generally wider than long, with the P2 being most extreme and the M2 closest to a quadratic shape (Fig 7).

The shape change was mainly related to the posterior loph: whereas the relationship of the anterior loph width and tooth length was more or less constant (Fig 8), the relationship of the posterior loph width and tooth length indicated that absolute posterior loph width was constant across tooth lengths from P2 to M3 (Fig 9).

Comparing the width of the anterior and posterior loph, the molars are broadening towards their anterior part. By contrast, the premolar teeth are rather broadening towards the posterior loph or have a constant width, except P4, which more resembles the shape of the molar teeth. The lower cheek teeth show similar features; the molar teeth are broader on the anterior loph and the premolars are broader on the posterior loph, including P4. Comparing the anterior loph width with tooth length, there is a correlation along all teeth, meaning the anterior loph has a constant width in every cheek tooth compared to its length (Fig 8). In comparison, the posterior loph width does not change proportionately with tooth length, so that the posterior loph of upper premolars is proportionally wider than the one of upper molars (Fig 9). This might be due to the fact that the molar teeth erupt consecutively while the individual is still growing, and that at the point of eruption, less space is available in the tooth socket on the posterior end compared to the anterior end (Fig 22). And as the M1 is the first permanent cheek tooth [30], the permanent premolars, which erupt later, have more space on the posterior end compared to the molar teeth.

Example of a tapir in which the M2 (in the photo, the tooth farthest to the right) is in the process of eruption. Note that the space available for the tooth becomes smaller towards the posterior (here, right) part of the jaw. The shape of the M1 (second from right) probably indicates the conditions of a shorter jaw at the time of its own eruption.

Maffei [32] and Gibson [33] developed keys to determine tapirs ages by tooth wear based on hunted free-ranging individuals. As we mostly had no information about age from our specimens, we cannot compare our material to theirs. The only skull with identified age in our study was labelled as 10 years old and had non-occluding third molars, which is contradiction to the age keys of Maffei [32] and Gibson [33] (wear on M3 present at 8 years). We hope that the more detailed anatomical description of macrowear stages provided in Fig 17, and the possibility of quantifying wear using objective measurements (RLD, RLL, RLW), will facilitate more detailed future studies on relationships of tapir tooth wear and age. In other herbivores, tooth wear is often quantified as a loss of dental substance in mm over time [48]. Even if individual ages of animals were known, this would be hardly applicable to tapirs, as the loss of tooth substance is uneven along the width of maxillary cheek teeth.

Heavy tooth wear can have a serious impact on animal health. For example in giraffes (Giraffa camelopardialis), mesowear classified captive individuals as grazing herbivores, which even lead to the assumption that this intensive tooth wear might have a negative impact on their longevity [27, 53]. In zoo tapirs, tooth and apical abscesses and teeth abnormalities have been reported [54, 55], but to our knowledge, no reports about unnatural excessively worn teeth exist. Furthermore, in captive Malayan Tapirs, a prevalence for resorptive tooth root lesions of 52% is described, which was much higher than in their free-ranging conspecifics (6.5%) [56]. Therefore, regarding dental health in zoo tapirs, tooth wear seems not to be an issue, while other possible dental problems require more attention.

Reviewer #2: Overall, I enjoyed reading the manuscript and found it to generally be well done and clearly written. The authors have provided a detailed examination of chewing, tooth anatomy, and tooth wear in tapirs. The findings of the study highlight the orthal movements of the jaws when chewing, which is distinct from other perissodactyls. As a consequence of those movements, the teeth of tapirs have characteristic wear patterns as well. The authors have also compared wear patterns in samples of both wild caught and zoo specimens, revealing generally higher wear in wild populations.

Also, in Figure 4 there are two distinct gradients used to indicate tooth position in A and B. I find the direct transition of tones in Fig. 4B to be more easily interpreted, and suggest changing A to be a similar sort of transition in grayscale tones.

Page 28, line 508 to page 29, line 510 discusses the lophodont pattern and inferred orthal movement of the tapirs. Many mammals with some form of lophodont occlusal morphology (including loxodont, ptychodont, and selenolophodont teeth) have primary orientation of those folds/ridges perpendicular to the primary direction of jaw movement (Ungar 2010). Proboscideans (loxodont), rodents (many lophodont or ptychodont lineages), and some marsupials (like kangaroos and wombats) show mediolaterally oriented lophs on their teeth and a primary orthal (propalinal) movement of the jaws when chewing. Mentioning those sorts of common patterns helps support the interpretations here.

Page 29, line 531 to page 30, 537 discusses the size of the temporalis muscle and sagittal crest. Outside of ungulates, there are many other herbivorous mammals with large and prominent temporalis and sagittal crest, including many rodents like beavers and chisel tooth digging burrowers (Samuels 2009) and primates like Gorillas.

Feeding practice in herbivorous mammals can impact their dental wear, due to excessive or irregular abrasion. Previous studies indicated that browsing species display more wear when kept in zoos compared to natural habitats. Comparable analyses in tapirs do not exist, as their dental anatomy and chewing kinematics are assumed to prevent the use of macroscopic wear proxies such as mesowear. We aimed at describing tapir chewing, dental anatomy and wear, to develop a system allowing comparison of free-ranging and captive specimens even in the absence of known age. Video analyses suggest that in contrast to other perissodactyls, tapirs have an orthal (and no lateral) chewing movement. Analysing cheek teeth from 74 museum specimens, we quantified dental anatomy, determined the sequence of dental wear along the tooth row, and established several morphometric measures of wear. In doing so, we showcase that tapir maxillary teeth distinctively change their morphology during wear, developing a height differential between less worn buccal and more worn lingual cusps, and that quantitative wear corresponds to the eruption sequence. We demonstrate that mesowear scoring shows a stable signal during initial wear stages but results in a rather high mesowear score compared to other browsing herbivores. Zoo specimens had lesser or equal mesowear scores as specimens from the wild; additionally, for the same level of third molar wear, premolars and other molars of zoo specimens showed similar or less wear compared specimens from the wild. While this might be due to the traditional use of non-roughage diet items in zoo tapirs, these results indicate that in contrast to the situation in other browsers, excessive tooth wear appears to be no relevant concern in ex situ tapir management.

Citation: Hohl CJM, Codron D, Kaiser TM, Martin LF, Mller DWH, Hatt J-M, et al. (2020) Chewing, dental morphology and wear in tapirs (Tapirus spp.) and a comparison of free-ranging and captive specimens. PLoS ONE 15(6): e0234826. 2351a5e196