Dietary signals in the premolar dentition of primates

Systematic review and meta-analysis of root morphology and canal configuration of permanent premolars using cone-beam computed tomography

INTRODUCTION

The efficacy of root canal treatment is greatly impacted by a thorough understanding of root canal anatomy. This systematic review and meta-analysis aim to thoroughly investigate the root morphology and canal configuration (RMCC) of permanent premolars (PMs).

METHODOLOGY

A comprehensive analysis was conducted following the PRISMA guidelines. Literature exploration was carried out across four electronic databases (PubMed, Embase, Cochrane, and Web of Science). The risk of bias assessment was conducted for the included studies utilizing the Anatomical Quality Assessment (AQUA) tool. Data analysis was performed utilizing SPSS and RevMAN5.3.3. The meta-analysis was applied with a 95% confidence interval to calculate odds ratios (OR).

RESULTS

Among the 82 selected studies, 59 studies exhibited potential bias in domain one (objective(s) and subject characteristics), followed by domain three (methodology characterization). The majority of maxillary PM1s had either single root (46.7%) or double roots (51.9%), while three-rooted variants were uncommon (1.4%). Conversely, most other PMs exhibited a single root. In terms of canal configuration, maxillary PM1s predominantly featured double distinct canals (87.2%), with the majority of maxillary PM2s displaying either a single canal (51.4%) or double canals (48.3%). Mandibular PMs were primarily characterized by single canals, accounting for 78.3% of mandibular PM1s and 90.3% of mandibular PM2s. Subgroup analyses revealed higher incidences of single-rooted and single-canalled PMs among Asians compared to Caucasians. Additionally, women exhibited a higher incidence of single-rooted PMs, while men showed a greater frequency of double-rooted PMs.

CONCLUSIONS

The comprehensive analysis indicated that maxillary PM1s predominantly possess double roots and double canals, whereas maxillary PM2s and mandibular PMs were primarily characterized by single-rooted with a single canal. Notably, single root and single canal were more prevalent among women and Asian samples.

Zygomatic arch root position in relation to dietary type in haplorhine primates

The zygomatic root, along with other key craniofacial features, is hypothesized to play a crucial role in strengthening the face in response to stresses and strains related to feeding. As such, it has been cited as indicative of dietary specialization among fossil taxa, although it remains unknown how variable zygomatic arch root position is among living primates, and whether its positioning predicts differences in diet. We test whether primates that consume more mechanically challenging foods possess more anteriorly positioned zygomatic roots compared to those consuming less challenging foods. Zygomatic root position, as defined by the zygomaxillare landmark, was identified and recorded from digital images and physical specimens of adult primate crania. Data were collected from 33 haplorhine species (n = 722). Published data were used to assign species to a dietary type based on patterns of overall consumption along with reliance on especially challenging foods. Pairwise comparisons between mechanically challenging (hard and/or tough) and less mechanically challenging (soft) consumers found significant differences (p < 0.05) in the position of the zygomatic root in 17 of 20 pairs, 11 of which supported the prediction that a more mechanically challenging diet is associated with a more anteriorly placed zygomatic root. PGLS analysis found no significant effect of phylogeny on root position. This suggests that a more anteriorly positioned zygomatic root is useful for identifying dietary specialization in some taxa but is not required for consuming a mechanically challenging diet given that other craniofacial and behavioral factors can facilitate the consumption of such foods.

Enamel chipping and its ecological correlates in African papionins: Implications for hominin feeding behavior

African papionins are classic paleoecological referents for fossil hominins. Enamel chips on the teeth of baboons and hominins are argued to represent responses to similar dietary habits; however, a comprehensive analysis of modern papionin chipping is lacking, leaving open the question of analog suitability. Here, we investigate patterns of antemortem enamel chipping across a diverse set of African papionin species occupying a range of ecological niches. We compare papionin chipping frequencies to estimates for Plio-Pleistocene hominins to address hypotheses of habitat and/or dietary similarities. Antemortem chips in seven African papionin species were scored on intact postcanine teeth (P3-M3) using established protocols. Chip size was scored on a tripartite scale. Papio hamadryas and Papio ursinus-two common paleoecological referents-display higher levels of chipping than Plio-Pleistocene hominin taxa (Australopithecus and Paranthropus) posited to have similar dietary habits. Papio populations occupying dry or highly seasonal habitats accumulate more large chips than Papio taxa occupying more mesic habitats, and terrestrial papionins chip their teeth more often than closely related taxa occupying arboreal niches. Chipping is present on the teeth of all Plio-Pleistocene hominins; however, chipping in baboons (P. ursinus and P. hamadryas) consistently exceeds most hominin taxa. Chipping frequencies on their own do not reliably sort taxa into major dietary groupings. We conclude that the large differences in chipping frequency may instead reflect habitat use and food processing idiosyncrasies. Less chipping in Plio-Pleistocene hominin teeth compared to modern Papio is more likely attributable to differences in dental morphology rather than diet.

Estimates of absolute crown strength and bite force in the lower postcanine dentition of Gigantopithecus blacki

Gigantopithecus blacki is hypothesized to have been capable of processing mechanically challenging foods, which likely required this species to have high dental resistance to fracture and/or large bite force. To test this hypothesis, we used two recently developed approaches to estimate absolute crown strength and bite force of the lower postcanine dentition. Sixteen Gigantopithecus mandibular permanent cheek teeth were scanned by micro-computed tomography. From virtual mesial cross-sections, we measured average enamel thickness and bi-cervical diameter to estimate absolute crown strength, and cuspal enamel thickness and dentine horn angle to estimate bite force. We compared G. blacki with a sample of extant great apes (Pan, Pongo, and Gorilla) and australopiths (Australopithecus anamensis, Australopithecus afarensis, Australopithecus africanus, Paranthropus robustus, and Paranthropus boisei). We also evaluated statistical differences in absolute crown strength and bite force between the premolars and molars for G. blacki. Results reveal that molar crown strength is absolutely greater, and molar bite force absolutely higher, in G. blacki than all other taxa except P. boisei, suggesting that G. blacki molars have exceptionally high resistance to fracture and the ability to generate exceptionally high bite force. In addition, G. blacki premolars have comparable absolute crown strength and larger bite force capabilities compared with its molars, implying possible functional specializations in premolars. The dental specialization of G. blacki could thus represent an adaptation to further facilitate the processing of mechanically challenging foods. While it is currently not possible to determine which types of foods were actually consumed by G. blacki through this study, direct evidence (e.g. dental chipping and microwear) left by the foods eaten by G. blacki could potentially lead to greater insights into its dietary ecology.

Functional and phylogenetic variation in anthropoid incisor size

OBJECTIVES

Previous studies have shown that frugivorous anthropoids have wider incisors than folivores relative to body mass, and that catarrhines have relatively wider incisors than platyrrhines. This study reexamines these contrasts using mandibular length as a biomechanical standard to quantify relative incisor width.

MATERIALS AND METHODS

Dental, mandibular, and body-mass data for 86 anthropoid species were taken from the literature. Incisor width was size-adjusted using shape ratios, with mandibular length and body mass as the denominators. Dietary and phylogenetic effects were examined using phylogenetic generalized least squares.

RESULTS

Mandible-based ratios provide a signal that is very similar to the one derived from ratios computed using body mass. Frugivores have relatively wider incisors than folivores, as expected. There is limited support for a stronger dietary effect in platyrrhines when mandible-based ratios are used, but neither type of ratio indicates an overall difference between platyrrhines and catarrhines.

DISCUSSION

Although both ratios support a link between incisor size and diet, there is some evidence indicating that mandible-based ratios are more sensitive to dietary variation at smaller phylogenetic scales. Understanding why the signals from the two ratios diverge at such scales may help clarify the functional significance of variation in incisor width. The results of this study undermine the view that platyrrhines, as a group, tend to have narrower incisors than catarrhines, regardless of diet. Instead, the difference between the two clades noted in previous studies can be explained by greater incisor functional diversity in platyrrhines.

The effect of high wear diets on the relative pulp volume of the lower molars

OBJECTIVES

One role of dental pulp is in the upkeep and maintenance of dentine. Under wear, odontoblasts in the pulp deposit tertiary dentine to ensure the sensitive internal dental tissues are not exposed and vulnerable to infection. It follows that there may be an adaptive advantage for increasing molar pulp volume in anthropoid primate taxa that are prone to high levels of wear. The relative volume of dental pulp is therefore predicted to covary with dietary abrasiveness (in the sense of including foods that cause high degrees of wear).

MATERIALS AND METHODS

We examined relatively unworn lower second molars in pairs of species of extant hominoids, cebids, and pitheciids that vary in the abrasiveness of their diet (n = 36). Using micro-CT scans, we measured the percent of tooth that is pulp (PTP) as the ratio of pulp volume to that of the total volume of the tooth.

RESULTS

We found that in each pair of species, the taxa that consume a more abrasive diet had a significantly higher PTP than the closely related taxa that consume a softer diet.

CONCLUSIONS

Our results point to an adaptive mechanism in the molars of taxa that consume abrasive diets and are thus subject to higher levels of wear. Our results provide additional understanding of the relationship between dental pulp and diet and may offer insight into the diet of extinct taxa such as Paranthropus boisei or into the adaptive context of the taurodont molars of Neanderthals.

Tooth chipping prevalence and patterns in extant primates

OBJECTIVES

A tooth chip occurs when a hard object forcefully contacts the surface of the tooth, typically removing enamel from the occlusal edge. In this study, chipping patterns in extant primates were compared, and hard-object-feeding assessed alongside other factors (e.g., grit mastication and dental properties), to elucidate dietary and behavioral inferences in archeological and paleontological samples.

MATERIALS AND METHODS

Thirteen species of extant primates were studied, including eight species within the Cercopithecidae, two within the Ceboidea, and three within the Hominoidea. Four additional species were also incorporated from the literature for some of the analyses. The severity (Grade 1-3), position (buccal, lingual, mesial, and distal) and number of tooth fractures were recorded for each specimen.

RESULTS

Species considered hard-object-feeding specialists presented higher rates of chipping, with sakis, mandrills, sooty mangabeys and Raffles' banded langurs having high chipping rates (28.3%, 36.7%, 48.4%, and 34.7% of teeth, respectively). Species that seasonally eat harder foods had intermediate chipping frequencies (e.g., brown woolly monkeys: 18.5%), and those that less commonly consume hard food items had the lowest chipping frequencies (e.g., Kloss gibbon: 7.3%; chimpanzees: 4.4%).

DISCUSSION

The results suggest hard food mastication influences differences in chipping prevalence among the species studied. Although Homo fossil samples show high rates of chipping comparable to hard-object-feeding extant primates, they display a different pattern of chipping, supporting the hypothesis that these fractures are mostly non-food related (e.g., grit mastication in Homo naledi; non-masticatory tooth use in Neanderthals).

Enamel chipping in Taï Forest cercopithecids: Implications for diet reconstruction in paleoanthropological contexts

Antemortem enamel chipping in living and fossil primates is often interpreted as evidence of hard-object feeding (i.e., 'durophagy'). Laboratory analyses of tooth fracture have modeled the theoretical diets and loading conditions that may produce such chips. Previous chipping studies of nonhuman primates tend to combine populations into species samples, despite the fact that species can vary significantly in diet across their ranges. Chipping is yet to be analyzed across population-specific species samples for which long-term dietary data are available. Here, we test the association between enamel chipping and diet in a community of cercopithecid primates inhabiting the Taï Forest, Ivory Coast. We examined fourth premolars and first molars (n = 867) from naturally deceased specimens of Cercocebus atys, Colobus polykomos, Piliocolobus badius,Procolobus verus, and three species of Cercopithecus. We found little support for a predictive relationship between enamel chipping and diet across the entire Taï monkey community. Cercocebus atys, a dedicated hard-object feeder, exhibited the highest frequencies of (1) chipped teeth and (2) chips of large size; however, the other monkey with a significant degree of granivory, Co. polykomos, exhibited the lowest chip frequency. In addition, primates with little evidence of mechanically challenging or hard-food diets-such as Cercopithecus spp., Pi. badius, and Pr. verus-evinced higher chipping frequencies than expected. The equivocal and stochastic nature of enamel chipping in the Taï monkeys suggests nondietary factors contribute significantly to chipping. A negative association between canopy preference and chipping suggests a role of exogenous particles in chip formation, whereby taxa foraging closer to the forest floor encounter more errant particulates during feeding than species foraging in higher strata. We conclude that current enamel chipping models may provide insight into the diets of fossil primates, but only in cases of extreme durophagy. Given the role of nondietary factors in chip formation, our ability to reliably reconstruct a range of diets from a gradient of chipping in fossil taxa is likely weak.

Evidence of strong stabilizing effects on the evolution of boreoeutherian (Mammalia) dental proportions

The dentition is an extremely important organ in mammals with variation in timing and sequence of eruption, crown morphology, and tooth size enabling a range of behavioral, dietary, and functional adaptations across the class. Within this suite of variable mammalian dental phenotypes, relative sizes of teeth reflect variation in the underlying genetic and developmental mechanisms. Two ratios of postcanine tooth lengths capture the relative size of premolars to molars (premolar-molar module, PMM), and among the three molars (molar module component, MMC), and are known to be heritable, independent of body size, and to vary significantly across primates. Here, we explore how these dental traits vary across mammals more broadly, focusing on terrestrial taxa in the clade of Boreoeutheria (Euarchontoglires and Laurasiatheria). We measured the postcanine teeth of N = 1,523 boreoeutherian mammals spanning six orders, 14 families, 36 genera, and 49 species to test hypotheses about associations between dental proportions and phylogenetic relatedness, diet, and life history in mammals. Boreoeutherian postcanine dental proportions sampled in this study carry conserved phylogenetic signal and are not associated with variation in diet. The incorporation of paleontological data provides further evidence that dental proportions may be slower to change than is dietary specialization. These results have implications for our understanding of dental variation and dietary adaptation in mammals.