Dental topography indicates ecological contraction of lemur communities

Dental topography of prosimian premolars predicts diet: A comparison in premolar and molar dietary classification accuracies

OBJECTIVES

This study tests whether (1) premolar topography of extant "prosimians" (strepsirrhines and tarsiers) successfully predicts diet and (2) whether the combination of molar and premolar topography yields higher classification accuracy than using either tooth position in isolation.

MATERIALS AND METHODS

Dental topographic metrics (ariaDNE, relief index, and orientation patch count rotated) were calculated for 118 individual matched-pairs of mandibular fourth premolars (P4) and second molars (M2). The sample represents 7 families and 22 genera. Tooth variables were analyzed in isolation (P4 only; M2 only), together (P4 and M2), and combined (PC1 scores of bivariate principal component analyses of P4 and M2 for each metric). Discriminant function analyses were conducted with and without a measure of size (two-dimensional surface area).

RESULTS

When using topography only, "prosimian" P4 shape predicts diet with a success rate that is slightly higher than that of M2 shape. When absolute size is included, premolars and molars perform comparably well. Including both premolar and molar topography (separately or combined) improves classification accuracy for every analysis beyond considering either in isolation. Classification accuracy is highest when premolar and molar topography and size are included.

DISCUSSION

Our findings indicate that molar teeth incompletely summarize the functional requirements of oral food breakdown for a given diet, and that the mechanism selecting for premolar form is more varied than what is expressed by molar teeth. Finally, our findings suggest that fossil P4s (in isolation or with the M2) can be used for meaningful dietary reconstruction of extinct primates.

Dental complexity and diet in amniotes: A meta-analysis

Tooth morphology is among the most well-studied indicators of ecology. For decades, researchers have examined the gross morphology and wear patterns of teeth as indicators of diet, and recent advances in scanning and computer analysis have allowed the development of new and more quantitative measures of tooth morphology. One of the most popular of these new methods is orientation patch count (OPC). OPC, a measure of surface complexity, was originally developed to distinguish the more complex tooth crowns of herbivores from the less complex tooth crowns of faunivores. OPC and a similar method derived from it, orientation patch count rotated (OPCR), have become commonplace in analyses of both modern and fossil amniote dietary ecology. The widespread use of these techniques makes it possible to now re-assess the utility of OPC and OPCR. Here, we undertake a comprehensive review of OPC(R) and diet and perform a meta-analysis to determine the overall difference in complexity between herbivores and faunivores. We find that the relationship between faunivore and herbivore OPC or OPCR values differs substantially across studies, and although some support the initial assessment of greater complexity in herbivores, others do not. Our meta-analysis does not support an overall pattern of greater complexity in herbivores than faunivores across terrestrial amniotes. It appears that the relationship of OPC or OPCR to diet is taxon-specific and dependent on the type of faunivory of the group in question, with insectivores often having values similar to herbivores. We suggest extreme caution in comparing OPC and OPCR values across studies and offer suggestions for how OPCR can constructively be used in future research.

Dental topography of the Oligocene anthropoids Aegyptopithecus zeuxis and Apidium phiomense: Paleodietary insights from analysis of wear series

Fossil primate dietary inference is enhanced when ascertained through multiple, distinct proxies. Dental topography can be used to assess changes in occlusal morphology with macrowear, providing insight on tooth use and function across the lifespans of individuals. We measured convex Dirichlet normal energy-a dental topography metric reflecting occlusal sharpness of features such as cusps and crests-in macrowear series of the second mandibular molars of two African anthropoid taxa from ∼30 Ma (Aegyptopithecus zeuxis and Apidium phiomense). Wear was quantified via three proxies: occlusal dentine exposure, inverse relief index, and inverse occlusal relief. The same measurements were calculated on macrowear series of four extant platyrrhine taxa (Alouatta, Ateles, Plecturocebus, and Sapajus apella) to provide an analogical framework for dietary inference in the fossil taxa. We predicted that Ae. zeuxis and Ap. phiomense would show similar patterns in topographic change with wear to one another and to extant platyrrhine frugivores like Ateles and Plecturocebus. The fossil taxa have similar distributions of convex Dirichlet normal energy to one another, and high amounts of concave Dirichlet normal energy 'noise' in unworn molars-a pattern shared with extant hominids that may distort dietary interpretations. Inverse relief index was the most useful wear proxy for comparison among the taxa in this study which possess disparate enamel thicknesses. Contrary to expectations, Ae. zeuxis and Ap. phiomense both resemble S. apella in exhibiting an initial decline in convex Dirichlet normal energy followed by an increase at the latest stages of wear as measured by inverse relief index, lending support to previous suggestions that hard-object feeding played a role in their dietary ecology. Based on these results and previous analyses of molar shearing quotients, microwear, and enamel microstructure, we suggest that Ae. zeuxis had a pitheciine-like strategy of seed predation, whereas Ap. phiomense potentially consumed berry-like compound fruits with hard seeds.

Variability in molar crown morphology and cusp wear in two Western chimpanzee populations

OBJECTIVES

Chimpanzees (Pan troglodytes) possess a relatively generalized molar morphology allowing them to access a wide range of foods. Comparisons of crown and cusp morphology among the four subspecies have suggested relatively large intraspecific variability. Here, we compare molar crown traits and cusp wear of two geographically close populations of Western chimpanzees, P. t. verus, to provide further information on intraspecific dental variability.

MATERIALS AND METHODS

Micro-CT reconstructions of high-resolution replicas of first and second molars of two Western chimpanzee populations from Ivory Coast (Taï National Park) and Liberia, respectively were used for this study. First, we analyzed projected tooth and cusp 2D areas as well as the occurrence of cusp six (C6) on lower molars. Second, we quantified the molar cusp wear three-dimensionally to infer how the individual cusps alter with advancing wear.

RESULTS

Both populations are similar in their molar crown morphology, except for a higher appearance rate of a C6 in Taï chimpanzees. In Taï chimpanzees, lingual cusps of upper molars and buccal cusps of lower molars possess an advanced wear pattern compared to the remaining cusps, while in Liberian chimpanzees this wear gradient is less pronounced.

DISCUSSION

The similar crown morphology between both populations fits with previous descriptions for Western chimpanzees and provides additional data on dental variation within this subspecies. The wear pattern of the Taï chimpanzees are in concordance with their observed tool rather than tooth use to open nuts/seeds, while the Liberian chimpanzees may have consumed hard food items crushed between their molars.

Feeding ecology of the last European colobine monkey, Dolichopithecus ruscinensis

Currently, very little is known about the ecology of extinct Eurasian cercopithecids. Dietary information is crucial in understanding the ecological adaptations and diversity of extinct cercopithecids and the evolution of this family. For example, the colobine genus Dolichopithecus is represented by multiple large-bodied species that inhabited Eurasia during the Pliocene-Early Pleistocene. The available evidence, though limited, suggests semiterrestrial locomotion, which contrasts with most extant African and Asian colobines that exhibit morphological and physiological adaptations for arboreality and folivory. These differences raise questions regarding the dietary specialization of early colobine taxa and how/if that influenced their dispersion out of Africa and into Eurasia. To further our understanding of the ecology of Plio-Pleistocene cercopithecids, we characterized the dental capabilities and potential dietary adaptations of Dolichopithecus ruscinensis through dental topographic and enamel thickness analyses on an M¹ from the locality of Serrat d'en Vacquer, Perpignan (France). We also assessed the feeding behavior of D. ruscinensis through dental microwear texture analysis on a broad sample of fossil molars from fossil sites in France, Greece, Bulgaria, and Romania. Dental topographic and enamel thickness analyses suggest that D. ruscinensis could efficiently process a wide range of foods. Results of the dental microwear texture analysis suggest that its diet ranged from folivory to the consumption of more mechanically challenging foods. Collectively, this suggests a more opportunistic feeding behavior for Dolichopithecus than characteristic of most extant colobines.

Wild versus lab house mice: Effects of age, diet, and genetics on molar geometry and topography

Molar morphology is shaped by phylogenetic history and adaptive processes related to food processing. Topographic parameters of the occlusal surface, such as sharpness and relief, can be especially informative regarding diet preferences of a species. The occlusal surface can however be deeply modified by wear throughout an animal's life, potentially obliterating other signals. Age being difficult to assess in wild populations, especially small rodents, experimental studies of wear through age in laboratory populations may constitute a powerful way to assess its impact on molar geometry and topography, and to validate descriptors of molar morphology that could mitigate this issue. Molar morphology was therefore quantified using 3D geometric morphometrics and topographic estimates in four groups of house mice: wild-trapped mice, lab-bred offspring of these wild mice, typical laboratory mice, and their hybrids. Three descriptors of the molar morphology were considered: the surface of the whole molar row, the surface of the first upper molar, and a truncated template of the first upper molar mimicking advanced wear. Increasing wear with age was demonstrated in the different groups, with a more pronounced effect in the wild-trapped population. The geometry of the molar row is not only modified by wear, but also by the relative position of the late developing molars on the jaw due to loading during mastication. As a consequence, the alignment of the molars is modified in wild mice, showing a qualitative difference between wild animals and their lab-bred offspring. Results obtained from the lab should thus be transferred with caution to the interpretation of differences in wild populations. Topographic estimates computed for the first upper molar seems to provide more stable parameters than those based on the whole molar row, because issues related to non-planar occlusal surface along the molar row are discarded. The truncated template was proven efficient in discarding the wear effect to focus on genetic differences, allowing an efficient characterization of the hybridization signature between wild and lab mice. Dominance of the wild phenotype for the first molar shape supports that the lab strain evolved in a context of relaxation of the selective pressures related to nutrition.

Molar biomechanical function in South African hominins Australopithecus africanus and Paranthropus robustus

Diet is a driving force in human evolution. Two species of Plio-Pleistocene hominins, Paranthropus robustus and Australopithecus africanus, have derived craniomandibular and dental morphologies which are often interpreted as P. robustus having a more biomechanically challenging diet. While dietary reconstructions based on dental microwear generally support this, they show extensive dietary overlap between species, and craniomandibular and dental biomechanical analyses can yield contradictory results. Using methods from anthropology and engineering (i.e. anthroengineering), we quantified the molar biomechanical performance of these hominins to investigate possible dietary differences between them. Thirty-one lower second molars were 3D printed and used to fracture gelatine blocks, and Bayesian generalized linear models were used to investigate the relationship between species and tooth wear, size and shape, and biomechanical performance. Our results demonstrate that P. robustus required more force and energy to fracture blocks but had a higher force transmission rate. Considering previous dietary reconstructions, we propose three evolutionary scenarios concerning the dietary ecologies of these hominins. These evolutionary scenarios cannot be reached by investigating morphological differences in isolation, but require combining several lines of evidence. This highlights the need for a holistic approach to reconstructing hominin dietary ecology.

Evolutionary and phylogenetic insights from a nuclear genome sequence of the extinct, giant, "subfossil" koala lemur Megaladapis edwardsi

No endemic Madagascar animal with body mass >10 kg survived a relatively recent wave of extinction on the island. From morphological and isotopic analyses of skeletal "subfossil" remains we can reconstruct some of the biology and behavioral ecology of giant lemurs (primates; up to ∼160 kg) and other extraordinary Malagasy megafauna that survived into the past millennium. Yet, much about the evolutionary biology of these now-extinct species remains unknown, along with persistent phylogenetic uncertainty in some cases. Thankfully, despite the challenges of DNA preservation in tropical and subtropical environments, technical advances have enabled the recovery of ancient DNA from some Malagasy subfossil specimens. Here, we present a nuclear genome sequence (∼2× coverage) for one of the largest extinct lemurs, the koala lemur Megaladapis edwardsi (∼85 kg). To support the testing of key phylogenetic and evolutionary hypotheses, we also generated high-coverage nuclear genomes for two extant lemurs, Eulemur rufifrons and Lepilemur mustelinus, and we aligned these sequences with previously published genomes for three other extant lemurs and 47 nonlemur vertebrates. Our phylogenetic results confirm that Megaladapis is most closely related to the extant Lemuridae (typified in our analysis by E. rufifrons) to the exclusion of L. mustelinus, which contradicts morphology-based phylogenies. Our evolutionary analyses identified significant convergent evolution between M. edwardsi and an extant folivore (a colobine monkey) and an herbivore (horse) in genes encoding proteins that function in plant toxin biodegradation and nutrient absorption. These results suggest that koala lemurs were highly adapted to a leaf-based diet, which may also explain their convergent craniodental morphology with the small-bodied folivore Lepilemur.

The landscape of tooth shape: Over 20 years of dental topography in primates

Diet plays an incontrovertible role in primate evolution, affecting anatomy, growth and development, behavior, and social structure. It should come as no surprise that a myriad of methods for reconstructing diet have developed, mostly utilizing the element that is not only most common in the fossil record but also most pertinent to diet: teeth. Twenty years ago, the union of traditional, anatomical analyses with emerging scanning and imaging technologies led to the development of a new method for quantifying tooth shape and reconstructing the diets of extinct primates. This method became known as dental topography.

Dental topographic change with macrowear and dietary inference in Homunculus patagonicus

Homunculus patagonicus is a stem platyrrhine from the late Early Miocene, high-latitude Santa Cruz Formation, Argentina. Its distribution lies farther south than any extant platyrrhine species. Prior studies on the dietary specialization of Homunculus suggest either a mixed diet of fruit and leaves or a more predominantly fruit-eating diet. To gain further insight into the diet of Homunculus, we examined how the occlusal surfaces of the first and second lower molars of Homunculus change with wear by using three homology-free dental topographic measures: Dirichlet normal energy (DNE), orientation patch count rotated (OPCR), and relief index (RFI). We compared these data with wear series of three extant platyrrhine taxa: the folivorous Alouatta, and the frugivorous Ateles and Callicebus (titi monkeys now in the genus Plecturocebus). Previous studies found Alouatta and Ateles exhibit distinctive patterns of change in occlusal morphology with macrowear, possibly related to the more folivorous diet of the former. Based on previous suggestions that Homunculus was at least partially folivorous, we predicted that changes in dental topographic metrics with wear would follow a pattern more similar to that seen in Alouatta than in Ateles or Callicebus. However, wear-induced changes in Homunculus crown sharpness (DNE) and complexity (OPCR) are more similar to the pattern observed in the frugivorous Ateles and Callicebus. Based on similar wear modalities of the lower molars between Homunculus and Callicebus, we infer that Homunculus had a primarily frugivorous diet. Leaves may have provided an alternative dietary resource to accommodate fluctuation in seasonal fruiting abundance in the high-latitude extratropical environment of late Early Miocene Patagonia.

Duplication and parallel evolution of the pancreatic ribonuclease gene (RNASE1) in folivorous non-colobine primates, the howler monkeys (Alouatta spp.)

In foregut-fermenting mammals (e.g., colobine monkeys, artiodactyl ruminants) the enzymes pancreatic ribonuclease (RNASE1) and lysozyme C (LYZ), originally involved in immune defense, have evolved new digestive functions. Howler monkeys are folivorous non-colobine primates that lack the multi-chambered stomachs of colobines and instead digest leaves using fermentation in the caeco-colic region. We present data on the RNASE1 and LYZ genes of four species of howler monkey (Alouatta spp.). We find that howler monkey LYZ is conserved and does not share the substitutions found in colobine and cow sequences, whereas RNASE1 was duplicated in the common ancestor of A. palliata, A. seniculus, A. sara, and A. pigra. While the parent gene (RNASE1) is conserved, the daughter gene (RNASE1B) has multiple amino acid substitutions that are parallel to those found in RNASE1B genes of colobines. The duplicated RNase in Alouatta has biochemical changes similar to those in colobines, suggesting a novel, possibly digestive function. These findings suggest that pancreatic ribonuclease has, in parallel, evolved a new role for digesting the products of microbial fermentation in both foregut- and hindgut-fermenting folivorous primates. This may be a vital digestive enzyme adaptation allowing howler monkeys to survive on leaves during periods of low fruit availability.

Ecometric modelling of tooth shape and precipitation gradients among lemurs on Madagascar

Ecometric modelling relates spatial environmental variables to phenotypic characters to better understand morphological adaptation and help reconstruct past environments. Here, the community means of the dental topography metrics Dirichlet normal energy (DNE) and orientation patch count (OPC) are tested against annual precipitation and precipitation seasonality among lemurs across Madagascar. Dry, seasonal environments are expected to be associated with high DNE and OPC, as lemurs living in these environments are more likely to rely on tougher foods. Ecometric models are also used to calculate ecometric loads for lemur taxa hypothesized to be experiencing evolutionary disequilibria and to reconstruct annual precipitation and precipitation seasonality at the ~500 years BP subfossil cave site of Ankilitelo. DNE was highest in highly seasonal but wet environments. Seasonal exploitation of fallback foods and the availability of new leaves during wet periods may be most important in driving community DNE. OPC was weakly predicted by annual precipitation and seasonality but its distribution appeared to be driven by a stepwise increase in its community values in rainforest environments. The lemur fauna from Ankilitelo appears to resemble communities from moister environments than occur in the spiny desert zone in which the site is situated today.

The frugivorous insectivores? Functional morphological analysis of molar topography for inferring diet in extant treeshrews (Scandentia)

The ecology, and particularly the diet, of treeshrews (order Scandentia) is poorly understood compared to that of their close relatives, the primates. This stems partially from treeshrews having fast food transit times through the gut, meaning fecal and stomach samples only represent a small portion of the foodstuffs consumed in a given day. Moreover, treeshrews are difficult to observe in the wild, leading to a lack of observational data in the literature. Although treeshrews are mixed feeders, consuming both insects and fruit, it is currently unknown how the relative importance of these food types varies across Scandentia. Previous study of functional dental morphology has provided an alternative means for understanding the diet of living euarchontans. We used dental topographic metrics to quantify aspects of functional dental morphology in a large sample of treeshrews (n = 58). We measured relief index, Dirichlet normal energy, and three-dimensional orientation patch count rotated, which quantify crown relief, occlusal curvature, and complexity, respectively. Our results suggest that treeshrews exhibit dental morphology consistent with high levels of insectivory relative to other euarchontans. They also suggest that taxa such as Dendrogale melanura and Tupaia belangeri appear to be best suited to insectivory, whereas taxa such as T. palawanensis and T. gracilis appear to be best adapted to frugivory. Our results suggest that Ptilocercus lowii is characterized by a dentition better adapted to insectivory than the early primate Purgatorius. If P. lowii represents a good modern analogue for primitive euarchontans, this contrast would support models of primate origins that include a shift to greater frugivory.

Effects of cropping, smoothing, triangle count, and mesh resolution on 6 dental topographic metrics

Dental topography is a widely used method for quantifying dental morphology and inferring dietary ecology in animals. Differences in methodology have brought into question the comparability of different studies. Using primate mandibular second molars, we investigated the effects of mesh preparation parameters smoothing, cropping, and triangle count/mesh resolution (herein, resolution) on six topographic variables (Dirichlet normal energy, DNE; orientation patch count rotated, OPCR; relief index, RFI; ambient occlusion, portion de ciel visible, PCV; enamel surface area, SA; tooth size) to determine the effects of smoothing, cropping, and triangle count/resolution on topographic values and the relationship between these values and diet. All topographic metrics are sensitive to smoothing, cropping method, and triangle count/resolution. In general, smoothing decreased DNE, OPCR, RFI, and SA, increased PCV, and had no predictable effect on tooth size. Relative to the basin cut off (BCO) cropping method, the entire enamel cap (EEC) method increased RFI, SA, and size, and had no predictable effect on DNE and OPCR. Smoothing and cropping affected DNE/OPCR and surfaces with low triangle counts more than other metrics and surfaces with high triangle counts. There was a positive correlation between DNE/OPCR and triangle count/resolution, and the rate of increase was weakly correlated to diet. PCV tended to converge or decrease with increases in triangle count/resolution, and RFI, SA, and size converged. Finally, there appears to be no optimal triangle count or resolution for predicting diet from this sample, and constant triangle count appeared to perform better than constant resolution for predicting diet.

Ambient occlusion and PCV (portion de ciel visible): A new dental topographic metric and proxy of morphological wear resistance

Recently, ambient occlusion, quantified through portion de ciel visible (PCV) was introduced as a method for quantifying dental morphological wear resistance and reconstructing diet in mammals. Despite being used to reconstruct diet and investigate the relationship between dental form and function, no rigorous analysis has investigated the correlation between PCV and diet. Using a sample of platyrrhine and prosimians M₂s, we show average PCV was significantly different between most dietary groups. In prosimian, insectivores had the lowest PCV, followed by folivores, omnivores, frugivores, and finally hard-object feeders. In platyrrhines, omnivores had the lowest average PCV, followed by folivores, frugivores, and finally hard-object feeders. PCV was correlated to two topographic variables (Dirichlet normal energy, DNE, and relief index, RFI) but uncorrelated to three others (orientation patch count rotated, OPCR, tooth surface area, and tooth size). The OPCR values here differed greatly from previously published values using the same sample, showing how differences in data acquisition (i.e., using 2.5D vs. 3D surfaces) can lead to drastic differences in results. Compared to other popular topographic variables, PCV performed as well or better at predicting diet in these groups, and when combined with a metric for size, the percent of successful dietary classifications reached 90%. Further, using an ontogenetic series of hominin (Paranthropus robustus) M₂s, we show that PCV correlates well with probability of wear, with PCV values being higher on the portions of the occlusal surface that experience more wear (e.g., cusps and crest tips, wear facets) than the portions of the tooth that experience less. This relationship is strongest once wear facets have begun to form on the occlusal surface. These results highlight the usefulness of PCV in quantifying morphological wear resistance and predicting diet in mammals.

A validation study of the Stoyanova et al. method (2017) for age-at-death estimation quantifying the 3D pubic symphyseal surface of adult males of European populations

The age-at-death estimation thresholds have recently been shifted towards a more objective assessment of the aging process. Such a non-subjective approach offers quantitative methods of age estimation; for instance, the method relating to the surfaces of pubic symphyses of males published by Stoyanova et al. (J Forensic Sci 62:1434-1444, 2017). A validation study was conducted to test the method performance in European samples. The sample consisted of 96 meshes of pubic symphyses of male individuals (known sex and age) that came from four different samples (two Portuguese collections, one Swiss, and one Crete). Stoyanova's method based on five regression models (three univariate and two multivariate models) performed worse in our sample, but only when the whole sample (without age limitation) was included. A sample limited to individuals under 40 years of age achieved better results in our study. The best results were reached through the thin plate spline algorithm (TPS/BE) with a root mean square error of 5.93 years and inaccuracy of 4.47 years. Generally, the multivariate regression models did not contribute to better age estimation. In our sample in all age categories, age was systematically underestimated. The quantitative method tested in this study works best for individuals under 40 years of age and provides a suitable basis for further research.

Feeding Ecology and Morphology Make a Bamboo Specialist Vulnerable to Climate Change

Animals with dietary specializations can be used to link climate to specific ecological drivers of endangerment. Only two mammals, the giant panda (Ailuropoda melanoleuca) in Asia and the greater bamboo lemur (Prolemur simus) in Madagascar, consume the nutritionally poor and mechanically challenging culm or trunk of woody bamboos [1-3]. Even though the greater bamboo lemur is critically endangered, paleontological evidence shows that it was once broadly distributed [4, 5]. Here, integrating morphological, paleontological, and ecological evidence, we project the effects of climate change on greater bamboo lemurs. Both the giant panda and the greater bamboo lemur are shown to share diagnostic dental features indicative of a bamboo diet, thereby providing an ecometric indicator [6, 7] of diet preserved in the fossil record. Analyses of bamboo feeding in living populations show that bamboo culm is consumed only during the dry season and that the greater bamboo lemur is currently found in regions with the shortest dry season. In contrast, paleontological localities of the greater bamboo lemurs have the longest dry seasons. Future projections show that many present-day greater bamboo lemur populations will experience prolonged dry seasons similar to those of the localities where only fossils of the greater bamboo lemur are found. Whereas abundant foods such as bamboo allow feeding specialists to thrive, even a moderate change in seasonality may outstrip the capacity of greater bamboo lemurs to persist on their mechanically demanding food source. Coupling known changes in species distribution with high-resolution ecological and historical data helps to identify extinction risks.

Investigating the dental toolkit of primates based on food mechanical properties: Feeding action does matter

Although conveying an indisputable morphological and behavioral signal, traditional dietary categories such as frugivorous or folivorous tend to group a wide range of food mechanical properties together. Because food/tooth interactions are mostly mechanical, it seems relevant to investigate the dental morphology of primates based on mechanical categories. However, existing mechanical categories classify food by its properties but cannot be used as factors to classify primate dietary habits. This comes from the fact that one primate species might be adapted to a wide range of food mechanical properties. To tackle this issue, what follows is an original framework based on action-related categories. The proposal here is to classify extant primates based on the range of food mechanical properties they can process through one given action. The resulting categories can be used as factors to investigate the dental tools available to primates. Furthermore, cracking, grinding, and shearing categories assigned depending on the hardness and the toughness of food are shown to be supported by morphological data (3D relative enamel thickness) and topographic data (relief index, occlusal complexity, and Dirichlet normal energy). Inferring food mechanical properties from dental morphology is especially relevant for the study of extinct primates, which are mainly documented by dental remains. Hence, we use action-related categories to investigate the molar morphology of an extinct colobine monkey Mesopithecus pentelicus from the Miocene of Pikermi, Greece. Action-related categories show contrasting results compared with classical categories and give us new insights into the dietary adaptations of this extinct primate. Finally, we provide some possible directions for future research aiming to test action-related categories. In particular, we suggest acquiring more data on mechanically challenging fallback foods and advocate the use of other food mechanical properties such as abrasiveness. The development of new action-related dental metrics is also crucial for primate dental studies.

The relationship between diet and tooth complexity in living dentigerous saurians

Living saurian reptiles exhibit a wide range of diets, from carnivores to strict herbivores. Previous research suggests that the tooth shape in some lizard clades correlates with diet, but this has not been tested using quantitative methods. I investigated the relationship between phenotypic tooth complexity and diet in living reptiles by examining the entire dentary tooth row in over 80 specimens comprising all major dentigerous saurian clades. I quantified dental complexity using orientation patch count rotated (OPCR), which discriminates diet in living and extinct mammals, where OPCR-values increase with the proportion of dietary plant matter. OPCR was calculated from high-resolution CT-scans, and I standardized OPCR-values by the total number of teeth to account for differences in tooth count across taxa. In contrast with extant mammals, there appears to be greater overlap in tooth complexity values across dietary groups because multicusped teeth characterize herbivores, omnivores, and insectivores, and because herbivorous skinks have relatively simple teeth. In particular, insectivorous lizards have dental complexities that are very similar to omnivores. Regardless, OPCR-values for animals that consume significant amounts of plant material are higher than those of carnivores, with herbivores having the highest average dental complexity. These results suggest reptilian tooth complexity is related to diet, similar to extinct and extant mammals, although phylogenetic history also plays a measurable role in dental complexity. This has implications for extinct amniotes that display a dramatic range of tooth morphologies, many with no modern analogs, which inhibits detailed dietary reconstructions. These data demonstrate that OPCR, when combined with additional morphological data, has the potential to be used to reconstruct the diet of extinct amniotes. J. Morphol. 278:500-522, 2017. © 2017 Wiley Periodicals, Inc.

Testing the adaptive radiation hypothesis for the lemurs of Madagascar

Lemurs, the diverse, endemic primates of Madagascar, are thought to represent a classic example of adaptive radiation. Based on the most complete phylogeny of living and extinct lemurs yet assembled, I tested predictions of adaptive radiation theory by estimating rates of speciation, extinction and adaptive phenotypic evolution. As predicted, lemur speciation rate exceeded that of their sister clade by nearly twofold, indicating the diversification dynamics of lemurs and mainland relatives may have been decoupled. Lemur diversification rates did not decline over time, however, as predicted by adaptive radiation theory. Optimal body masses diverged among dietary and activity pattern niches as lineages diversified into unique multidimensional ecospace. Based on these results, lemurs only partially fulfil the predictions of adaptive radiation theory, with phenotypic evolution corresponding to an 'early burst' of adaptive differentiation. The results must be interpreted with caution, however, because over the long evolutionary history of lemurs (approx. 50 million years), the 'early burst' signal of adaptive radiation may have been eroded by extinction.

On the Relationship Between Tooth Shape and Masticatory Efficiency: A Finite Element Study

Dental topography has successfully linked disparate tooth shapes to distinct dietary categories, but not to masticatory efficiency. Here, the relationship between four dental topographic metrics and brittle food item breakdown efficiency during compressive biting was investigated using a parametric finite element model of a bunodont molar. Food item breakdown efficiency was chosen to represent masticatory efficiency as it isolated tooth-food item interactions, where most other categories of masticatory efficiency include several aspects of the masticatory process. As relative food item size may affect the presence/absence of any relationship, four isometrically scaled, hemispherical, proxy food items were considered. Topographic metrics were uncorrelated to food item breakdown efficiency irrespective of relative food item size, and dental topographic metrics were largely uncorrelated to one another. The lack of a correlation between topographic metrics and food item breakdown efficiency is not unexpected as not all food items break down in the same manner (e.g., nuts are crushed, leaves are sheared), and only one food item shape was considered. In addition, food item breakdown efficiency describes tooth-food item interactions and requires location and shape specific information, which are absent from dental topographic metrics. This makes it unlikely any one efficiency metric will be correlated to all topographic metrics. These results emphasize the need to take into account how food items break down during biting, ingestion, and mastication when investigating the mechanical relationship between food item shape, size, mechanical properties, and breakdown, and tooth shape.

Age-related changes in molar topography and shearing crest length in a wild population of mountain Gorillas from Volcanoes National Park, Rwanda

OBJECTIVES

Great ape teeth must remain functional over long lifespans. The molars of the most folivorous apes, the mountain gorillas, must maintain shearing function for 40+ years while the animals consume large quantities of mechanically challenging foods. While other folivorous primates experience dental senescence, which compromises their occlusal surfaces and affects their reproductive success as they age, it is unknown whether dental senescence also occurs in mountain gorillas. In this article, we quantified and evaluated how mountain gorilla molars change throughout their long lifespans.

MATERIALS AND METHODS

We collected high-resolution replicas of M(1)s (n = 15), M(2)s (n = 13), and M(3)s (n = 11) from a cross-sectional sample of wild mountain gorilla skeletons from the Virunga Volcanoes, ranging in age from 4 to 43 years. We employed dental topographic analyses to track how aspects of occlusal slope, angularity, relief index, and orientation patch count rotated change with age. In addition, we measured the relative length of shearing crests in two- and three-dimensions.

RESULTS

Occlusal topography was found to decrease, while 2D relative shearing crest length increased, and 3D relative crest lengths were maintained with age.

DISCUSSION

Our findings indicate that shearing function is maintained throughout the long lifetimes of mountain gorillas. Unlike the dental senescence experienced by other folivorous primates, mountain gorillas do not appear to possess senesced molars despite their long lifetimes, mechanically challenging diets, and decreases in occlusal topography with age.

MorphoTester: An Open Source Application for Morphological Topographic Analysis

The increased prevalence and affordability of 3D scanning technology is beginning to have significant effects on the research questions and approaches available for studies of morphology. As the current trend of larger and more precise 3D datasets is unlikely to slow in the future, there is a need for efficient and capable tools for high-throughput quantitative analysis of biological shape. The promise and the challenge of implementing relatively automated methods for characterizing surface shape can be seen in the example of dental topographic analysis. Dental topographic analysis comprises a suite of techniques for quantifying tooth surfaces and component features. Topographic techniques have provided insight on mammalian molar form-function relationships and these methods could be applied to address other topics and questions. At the same time implementing multiple complementary topographic methods can have high time and labor costs, and comparability of data formats and approaches is difficult to predict. To address these challenges I present MorphoTester, an open source application for visualizing and quantifying topography from 3D triangulated polygon meshes. This application is Python-based and is free to use. MorphoTester implements three commonly used dental topographic metrics–Dirichlet normal energy, relief index, and orientation patch count rotated (OPCR). Previous OPCR algorithms have used raster-based grid data, which is not directly interchangeable with vector-based triangulated polygon meshes. A 3D-OPCR algorithm is provided here for quantifying complexity from polygon meshes. The efficacy of this metric is tested in a sample of mandibular second molars belonging to four species of cercopithecoid primates. Results suggest that 3D-OPCR is at least as effective for quantifying complexity as previous approaches, and may be more effective due to finer resolution of surface data considered here. MorphoTester represents an advancement in the automated quantification of morphology, and can be modified to adapt to future needs and priorities.

The first major primate extinction: An evaluation of paleoecological dynamics of North American stem primates using a homology free measure of tooth shape

OBJECTIVES

The disappearance of the North American plesiadapoids (stem primates, or plesiadapiforms) in the latest Paleocene has been attributed to competition with rodents over dietary resources. This study compares molar morphology of plesiadapoids and early rodents to assess whether all taxa were adapted to consuming foods of the same structural properties with similar mechanical efficacy.

MATERIALS AND METHODS

Micro-CT scans of second mandibular molars (M2 s) of plesiadapoids (n = 181) and ischyromyid (early fossil) rodents (n = 13) were evaluated using Dirichlet normal energy (DNE), a dental topographic metric that quantifies the curvature of a tooth's occlusal surface, independent of the orientation of the occlusal plane; this metric can be used to infer diet.

RESULTS

Comparisons of DNE values for plesiadapoids and rodents show that rodents shared functionally similar dental morphology with at least some plesiadapid plesiadapoids and thus were likely adapted to processing foods with similar physical properties. However, the DNE values for rodents contrast markedly with those for the other two plesiadapoid families, the Carpolestidae and Saxonellidae.

CONCLUSIONS

It is unlikely that direct competition over food resources with rodents played a major role in the extinction of carpolestids and saxonellids, as members of these families were capable of consuming a range of foods that were not accessible to rodents. Although several plesiadapid species overlap with rodents in their range of DNE values, only three overlap in time. One of these (Plesiadapis cookei) may have been too large to be in direct competition with rodents, another (Plesiadapis dubius) has DNE values substantially different (higher) than those of rodents, whereas the third, Chiromyoides, has teeth of both a similar size and DNE value to those of Clarkforkian rodents. If dietary niche overlap with rodents played a direct role in the decline of plesiadapiforms, it can only have potentially done so for Chiromyoides.

What did Hadropithecus eat, and why should paleoanthropologists care?

Over 40 years ago, Clifford Jolly noted different ways in which Hadropithecus stenognathus converged in its craniodental anatomy with basal hominins and with geladas. The Malagasy subfossil lemur Hadropithecus departs from its sister taxon, Archaeolemur, in that it displays comparatively large molars, reduced incisors and canines, a shortened rostrum, and thickened mandibular corpus. Its molars, however, look nothing like those of basal hominins; rather, they much more closely resemble molars of grazers such as Theropithecus. A number of tools have been used to interpret these traits, including dental microwear and texture analysis, molar internal and external morphology, and finite element analysis of crania. These tools, however, have failed to provide support for a simple dietary interpretation; whereas there is some consistency in the inferences they support, dietary inferences (e.g., that it was graminivorous, or that it specialized on hard objects) have been downright contradictory. Cranial shape may correlate poorly with diet. But a fundamental question remains unresolved: why do the various cranial and dental convergences exemplified by Hadropithecus, basal hominins, and Theropithecus exist? In this paper we review prior hypotheses regarding the diet of Hadropithecus. We then use stable carbon and nitrogen isotope data to elucidate this species' diet, summarizing earlier stable isotope analyses and presenting new data for lemurs from the central highlands of Madagascar, where Hadropithecus exhibits an isotopic signature strikingly different from that seen in other parts of the island. We offer a dietary explanation for these differences. Hadropithecus likely specialized neither on grasses nor hard objects; its staples were probably the succulent leaves of CAM plants. Nevertheless, aspects of prior hypotheses regarding the ecological significance of its morphology can be supported. Am. J. Primatol. 78:1098-1112, 2016. © 2015 Wiley Periodicals, Inc.

The effects of relative food item size on optimal tooth cusp sharpness during brittle food item processing

Teeth are often assumed to be optimal for their function, which allows researchers to derive dietary signatures from tooth shape. Most tooth shape analyses normalize for tooth size, potentially masking the relationship between relative food item size and tooth shape. Here, we model how relative food item size may affect optimal tooth cusp radius of curvature (RoC) during the fracture of brittle food items using a parametric finite-element (FE) model of a four-cusped molar. Morphospaces were created for four different food item sizes by altering cusp RoCs to determine whether optimal tooth shape changed as food item size changed. The morphospaces were also used to investigate whether variation in efficiency metrics (i.e. stresses, energy and optimality) changed as food item size changed. We found that optimal tooth shape changed as food item size changed, but that all optimal morphologies were similar, with one dull cusp that promoted high stresses in the food item and three cusps that acted to stabilize the food item. There were also positive relationships between food item size and the coefficients of variation for stresses in food item and optimality, and negative relationships between food item size and the coefficients of variation for stresses in the enamel and strain energy absorbed by the food item. These results suggest that relative food item size may play a role in selecting for optimal tooth shape, and the magnitude of these selective forces may change depending on food item size and which efficiency metric is being selected.

Eco-evo-devo of the lemur syndrome: did adaptive behavioral plasticity get canalized in a large primate radiation?

BACKGROUND

Comprehensive explanations of behavioral adaptations rarely invoke all levels famously admonished by Niko Tinbergen. The role of developmental processes and plasticity, in particular, has often been neglected. In this paper, we combine ecological, physiological and developmental perspectives in developing a hypothesis to account for the evolution of 'the lemur syndrome', a combination of reduced sexual dimorphism, even adult sex ratios, female dominance and mild genital masculinization characterizing group-living species in two families of Malagasy primates.

RESULTS

We review the different components of the lemur syndrome and compare it with similar adaptations reported for other mammals. We find support for the assertion that the lemur syndrome represents a unique set of integrated behavioral, demographic and morphological traits. We combine existing hypotheses about underlying adaptive function and proximate causation by adding a potential developmental mechanism linking maternal stress and filial masculinization, and outline an evolutionary scenario for its canalization.

CONCLUSIONS

We propose a new hypothesis linking ecological, physiological, developmental and evolutionary processes to adumbrate a comprehensive explanation for the evolution of the lemur syndrome, whose assumptions and predictions can guide diverse future research on lemurs. This hypothesis should also encourage students of other behavioral phenomena to consider the potential role of developmental plasticity in evolutionary innovation.

Tooth cusp sharpness as a dietary correlate in great apes

Mammalian molars have undergone heavy scrutiny to determine correlates between morphology and diet. Here, the relationship between one aspect of occlusal morphology, tooth cusp radius of curvature (RoC), and two broad dietary categories, folivory and frugivory, is analyzed in apes. The author hypothesizes that there is a relationship between tooth cusp RoC and diet, and that folivores have sharper teeth than frugivores, and further test the correlation between tooth cusp RoC and tooth cusp size. Eight measures of tooth cusp RoC (two RoCs per cusp) were taken from 53 M(2) s from four species and subspecies of frugivorous apes (Pongo pygmaeus, Pan troglodytes troglodytes, Pan troglodytes schweinfurthii, and Gorilla gorilla gorilla) and two subspecies of folivorous apes (Gorilla beringei beringei, and Gorilla beringei graueri). Phylogenetically corrected ANOVAs were run on the full dataset and several subsets of the full dataset, revealing that, when buccolingual RoCs are taken into account, tooth cusp RoCs can successfully differentiate folivores and frugivores. PCAs revealed that folivores consistently had duller teeth than frugivores. In addition, a weak, statistically significant positive correlation exists between tooth cusp size and tooth cusp RoC. The author hypothesizes differences in tooth cusp RoC are correlated with wear rates, where, per vertical unit of wear, duller cusps will have a longer length of exposed enamel ridge than sharper cusps. More data need to be gathered to determine if the correlation between tooth cusp RoC and tooth cusp size holds true when small primates are considered.

Dental topography of platyrrhines and prosimians: convergence and contrasts

Dental topographic analysis is the quantitative assessment of shape of three-dimensional models of tooth crowns and component features. Molar topographic curvature, relief, and complexity correlate with aspects of feeding behavior in certain living primates, and have been employed to investigate dietary ecology in extant and extinct primate species. This study investigates whether dental topography correlates with diet among a diverse sample of living platyrrhines, and compares platyrrhine topography with that of prosimians. We sampled 111 lower second molars of 11 platyrrhine genera and 121 of 20 prosimian genera. For each tooth we calculated Dirichlet normal energy (DNE), relief index (RFI), and orientation patch count (OPCR), quantifying surface curvature, relief, and complexity respectively. Shearing ratios and quotients were also measured. Statistical analyses partitioned effects of diet and taxon on topography in platyrrhines alone and relative to prosimians. Discriminant function analyses assessed predictive diet models. Results indicate that platyrrhine dental topography correlates to dietary preference, and platyrrhine-only predictive models yield high rates of accuracy. The same is true for prosimians. Topographic variance is broadly similar among platyrrhines and prosimians. One exception is that platyrrhines display higher average relief and lower relief variance, possibly related to lower relative molar size and functional links between relief and tooth longevity distinct from curvature or complexity. Explicitly incorporating phylogenetic distance matrices into statistical analyses of the combined platyrrhine-prosimian sample results in loss of significance of dietary effects for OPCR and SQ, while greatly increasing dietary significance of RFI.

How does tooth cusp radius of curvature affect brittle food item processing?

Tooth cusp sharpness, measured by radius of curvature (RoC), has been predicted to play a significant role in brittle/hard food item fracture. Here, we set out to test three existing hypotheses about this relationship: namely, the Blunt and Strong Cusp hypotheses, which predict that dull cusps will be most efficient at brittle food item fracture, and the Pointed Cusp hypothesis, which predicts that sharp cusps will be most efficient at brittle food item fracture using a four cusp bunodont molar. We also put forth and test the newly constructed Complex Cusp hypothesis, which predicts that a mixture of dull and sharp cusps will be most efficient at brittle food item fracture. We tested the four hypotheses using finite-element models of four cusped, bunodont molars. When testing the three existing hypotheses, we assumed all cusps had the same level of sharpness (RoC), and gained partial support for the Blunt Cusp hypotheses. We found no support for the Pointed Cusp or Strong Cusp hypotheses. We used the Taguchi sampling method to test the Complex Cusps hypothesis with a morphospace created by independently varying the radii of curvature of the four cusps in the buccolingual and mesiodistal directions. The optimal occlusal morphology for fracturing brittle food items consists of a combination of sharp and dull cusps, which creates high stress concentrations in the food item while stabilizing the food item and keeping the stress concentrations in the enamel low. This model performed better than the Blunt Cusp hypothesis, suggesting a role for optimality in the evolution of cusp form.

Adaptive dynamics under development-based genotype-phenotype maps

It is not known whether natural selection can encounter any given phenotype that can be produced by genetic variation. There has been a long-lasting debate about the processes that limit adaptation and, consequently, about how well adapted phenotypes are. Here we examine how development may affect adaptation, by decomposing the genotype-fitness map-the association between each genotype and its fitness-into two: one mapping genotype to phenotype by means of a computational model of organ development, and one mapping phenotype to fitness. In the map of phenotype and fitness, the fitness of each individual is based on the similarity between realized morphology and optimal morphology. We use three different simulations to map phenotype to fitness, and these differ in the way in which similarity is calculated: similarity is calculated for each trait (in terms of each cell position individually), for a large or a small number of phenotypic landmarks (the 'many-traits' and 'few-traits' phenotype-fitness maps), and by measuring the overall surface roughness of morphology (the 'roughness' phenotype-fitness map). Evolution is simulated by applying the genotype-phenotype map and one phenotype-fitness map to each individual in the population, as well as random mutation and drift. We show that the complexity of the genotype-phenotype map prevents substantial adaptation in some of the phenotype-fitness maps: sustained adaptation is only possible using 'roughness' or 'few-traits' phenotype-fitness maps. The results contribute developmental understanding to the long-standing question of which aspects of phenotype can be effectively optimized by natural selection.