The Hadropithecus conundrum reconsidered, with implications for interpreting diet in fossil hominins

Tooth chipping patterns in Archaeolemur provide insight into diet and behavior

OBJECTIVES

Archaeolemur is a recently extinct genus of lemur that is often compared to some Cercopithecidae, especially baboons. This is due in part to their derived dentition, with large anterior teeth and reduced bilophodont molars. Research involving comparative morphology, analysis of coprolites, isotopes, and enamel structure, have suggested Archaeolemur had an omnivorous diet involving mechanically challenging items. Yet, microwear analysis of posterior teeth does not necessarily support this conclusion.

MATERIALS AND METHODS

In this macroscopic study, dental chipping was recorded on permanent teeth of Archaeolemur from different localities (53 individuals; 447 permanent teeth; including both A. edwardsi and A. majori specimens). This study aimed to compare chipping patterns across the dentition of Archaeolemur with chipping in other primates.

RESULTS

The results show enamel chipping was prevalent on the anterior teeth of Archaeolemur (38.9% of anterior teeth showed at least one fracture) yet rare in posterior teeth (9%). There was a decrease in chipping frequency across the dentition, moving distally from incisors (50%; 20/40), through caniniform teeth (30%; 15/50), premolars (9.5%; 16/169), and molars (8.5%; 16/188).

DISCUSSION

The results support previous research suggesting Archaeolemur had a varied omnivorous diet in which the anterior dentition was used for extensive food processing. This likely included mechanically challenging items such as tough/hard large fruits, small vertebrates, and crustaceans. Such a high rate of chipping in the anterior dentition is uncommon in other primates, with exception of hominins.

Does the model reflect the system? When two-dimensional biomechanics is not 'good enough'

Models are mathematical representations of systems, processes or phenomena. In biomechanics, finite-element modelling (FEM) can be a powerful tool, allowing biologists to test form-function relationships in silico, replacing or extending results of in vivo experimentation. Although modelling simplifications and assumptions are necessary, as a minimum modelling requirement the results of the simplified model must reflect the biomechanics of the modelled system. In cases where the three-dimensional mechanics of a structure are important determinants of its performance, simplified two-dimensional modelling approaches are likely to produce inaccurate results. The vertebrate mandible is one among many three-dimensional anatomical structures routinely modelled using two-dimensional FE analysis. We thus compare the stress regimes of our published three-dimensional model of the chimpanzee mandible with a published two-dimensional model of the chimpanzee mandible and identify several fundamental differences. We then present a series of two-dimensional and three-dimensional FE modelling experiments that demonstrate how three key modelling parameters, (i) dimensionality, (ii) symmetric geometry, and (iii) constraints, affect deformation and strain regimes of the models. Our results confirm that, in the case of the primate mandible (at least), two-dimensional FEM fails to meet this minimum modelling requirement and should not be used to draw functional, ecological or evolutionary conclusions.

Mechanical compensation in the evolution of the early hominin feeding apparatus

Australopiths, a group of hominins from the Plio-Pleistocene of Africa, are characterized by derived traits in their crania hypothesized to strengthen the facial skeleton against feeding loads and increase the efficiency of bite force production. The crania of robust australopiths are further thought to be stronger and more efficient than those of gracile australopiths. Results of prior mechanical analyses have been broadly consistent with this hypothesis, but here we show that the predictions of the hypothesis with respect to mechanical strength are not met: some gracile australopith crania are as strong as that of a robust australopith, and the strength of gracile australopith crania overlaps substantially with that of chimpanzee crania. We hypothesize that the evolution of cranial traits that increased the efficiency of bite force production in australopiths may have simultaneously weakened the face, leading to the compensatory evolution of additional traits that reinforced the facial skeleton. The evolution of facial form in early hominins can therefore be thought of as an interplay between the need to increase the efficiency of bite force production and the need to maintain the structural integrity of the face.

Seed choice differs by sex in sooty mangabeys (Cercocebus atys)

The sooty mangabey (Cercocebus atys) practices year-round durophagy. A large part of the C. atys diet consists of the oily nut of Sacoglottis gabonensis, which is accessed by post-canine crushing of the hard, protective seed coat. During a typical foraging bout, some seeds are discarded after initial crushing attempts using isometric biting, but the reason mangabeys reject some seeds and break into others is unclear. Although C. atys is sexually dimorphic, little is known about whether differences between males and females affect the selectivity of mechanically protected foods. We studied C. atys feeding on S. gabonensis in the Taï National Park,  Côte d'Ivoire, in July and August 2016. Nuts discarded after an initial crushing attempt were collected and their hardness measured using a Shore D durometer. Measurements were taken in the region of the nut where monkeys attempted to crush it. Hardness values of nuts rejected by adult male (n = 79) and adult female (n = 104) C. atys were compared to those of a control assemblage of nuts collected randomly on the forest floor (n = 69). Nuts rejected by either sex do not differ statistically from the random sample; however, they do differ from each other, with females rejecting harder nuts. This suggests that males are more effective at broaching harder seed husks, and discard seeds based on other factors.

Maximum Bony Gape in Primates

Maximum jaw gape has important functional implications for behavior and feeding habits in primates. It has been suggested that gape is correlated to canine height and ingested food size. Extending these correlations to the fossil record would provide insights about the diets and/or social behavior of extinct primates. However, this can be problematic due to uncertainty about size and location of musculature, and it depends on reliability and repeatability of maximum gape estimation using only skeletal elements. In this study, maximum bony gape (MBG) was estimated using reliable landmarks and repeatable methods. The cranium was fixed in position and then the mandible was rotated and translated to the point immediately prior to loss of condyle-glenoid contact. Then it was photographed in a steady position using an adjustable wooden frame. This protocol allowed for photographs and linear measurements to be obtained for many museum specimens in a short time. The sample included 203 individuals, representing 42 species of primates. When scaled for body size, linear MBG correlates with maximum anesthetized gape (Hylander: Am J Phys Anthropol 150 (2013) 247-259), ingested food size (Perry and Hartstone-Rose: Am J Phys Anthropol 142 (2010) 625-635), and canine length but not condylar height. Anat Rec, 302:215-225, 2019. © 2018 Wiley Periodicals, Inc.

Biting mechanics and niche separation in a specialized clade of primate seed predators

We analyzed feeding biomechanics in pitheciine monkeys (Pithecia, Chiropotes, Cacajao), a clade that specializes on hard-husked unripe fruit (sclerocarpy) and resistant seeds (seed predation). We tested the hypothesis that pitheciine crania are well-suited to generate and withstand forceful canine and molar biting, with the prediction that they generate bite forces more efficiently and better resist masticatory strains than the closely-related Callicebus, which does not specialize on unripe fruits and/or seeds. We also tested the hypothesis that Callicebus-Pithecia-Chiropotes-Cacajao represent a morphocline of increasing sclerocarpic specialization with respect to biting leverage and craniofacial strength, consistent with anterior dental morphology. We found that pitheciines have higher biting leverage than Callicebus and are generally more resistant to masticatory strain. However, Cacajao was found to experience high strain magnitudes in some facial regions. We therefore found limited support for the morphocline hypothesis, at least with respect to the mechanical performance metrics examined here. Biting leverage in Cacajao was nearly identical (or slightly less than) in Chiropotes and strain magnitudes during canine biting were more likely to follow a Cacajao-Chiropotes-Pithecia trend of increasing strength, in contrast to the proposed morphocline. These results could indicate that bite force efficiency and derived anterior teeth were selected for in pitheciines at the expense of increased strain magnitudes. However, our results for Cacajao potentially reflect reduced feeding competition offered by allopatry with other pitheciines, which allows Cacajao species to choose from a wider variety of fruits at various stages of ripeness, leading to reduction in the selection for robust facial features. We also found that feeding biomechanics in sympatric Pithecia and Chiropotes are consistent with data on food structural properties and observations of dietary niche separation, with the former being well-suited for the regular molar crushing of hard seeds and the latter better adapted for breaching hard fruits.

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.

Dietary proclivities of Paranthropus robustus from Swartkrans, South Africa

Pleistocene Paranthropus robustus fossils from Swartkrans have yielded stable isotope values suggesting some foraging on C4 plants possibly including underground storage organs. Dental microwear texture analysis on P. robustus (SK 6, SK 34 and SK 47) from Swartkrans Member 1 is performed to examine whether tooth surface damage from mastication agrees with prior dietary inferences from carbon isotopes. There is considerable variation in textural characteristics among the P. robustus specimens. Specifically, adult SK 34 stands apart from the two subadult specimens, SK 6 and SK 47, suggesting life history could be reflected in patterns of dental microwear texture characteristics, although seasonality and availability of fallback foods may also explain the variation observed in P. robustus. The fossils all exhibit elevated surface texture complexity, resembling the values for Lophocebus albigena and Cebus apella, and to a lesser extent, Pan troglodytes. Paranthropus robustus is dissimilar to primary folivores, such as Trachypithecus cristatus or folivore- frugivores such as Alouatta palliata suggesting leaves comprised very little of its diet. The textural fill volume of P. robustus differs from that observed in extant primates from tropical forests indicating extreme durophagy, perhaps a function of differences in habitat. Ingestion of extraneous grit on the underground parts of plants and from terrestrial resources, perhaps as fallback foods or as dietary staples, may account for these enamel textural properties and may help explain the mixed C3/C4 isotopic signal in P. robustus.

The feeding biomechanics and dietary ecology of Paranthropus boisei

The African Plio-Pleistocene hominins known as australopiths evolved derived craniodental features frequently interpreted as adaptations for feeding on either hard, or compliant/tough foods. Among australopiths, Paranthropus boisei is the most robust form, exhibiting traits traditionally hypothesized to produce high bite forces efficiently and strengthen the face against feeding stresses. However, recent mechanical analyses imply that P. boisei may not have been an efficient producer of bite force and that robust morphology in primates is not necessarily strong. Here we use an engineering method, finite element analysis, to show that the facial skeleton of P. boisei is structurally strong, exhibits a strain pattern different from that in chimpanzees (Pan troglodytes) and Australopithecus africanus, and efficiently produces high bite force. It has been suggested that P. boisei consumed a diet of compliant/tough foods like grass blades and sedge pith. However, the blunt occlusal topography of this and other species suggests that australopiths are adapted to consume hard foods, perhaps including grass and sedge seeds. A consideration of evolutionary trends in morphology relating to feeding mechanics suggests that food processing behaviors in gracile australopiths evidently were disrupted by environmental change, perhaps contributing to the eventual evolution of Homo and Paranthropus.

Biomechanical implications of intraspecific shape variation in chimpanzee crania: moving toward an integration of geometric morphometrics and finite element analysis

In a broad range of evolutionary studies, an understanding of intraspecific variation is needed in order to contextualize and interpret the meaning of variation between species. However, mechanical analyses of primate crania using experimental or modeling methods typically encounter logistical constraints that force them to rely on data gathered from only one or a few individuals. This results in a lack of knowledge concerning the mechanical significance of intraspecific shape variation that limits our ability to infer the significance of interspecific differences. This study uses geometric morphometric methods (GM) and finite element analysis (FEA) to examine the biomechanical implications of shape variation in chimpanzee crania, thereby providing a comparative context in which to interpret shape-related mechanical variation between hominin species. Six finite element models (FEMs) of chimpanzee crania were constructed from CT scans following shape-space Principal Component Analysis (PCA) of a matrix of 709 Procrustes coordinates (digitized onto 21 specimens) to identify the individuals at the extremes of the first three principal components. The FEMs were assigned the material properties of bone and were loaded and constrained to simulate maximal bites on the P(3) and M(2) . Resulting strains indicate that intraspecific cranial variation in morphology is associated with quantitatively high levels of variation in strain magnitudes, but qualitatively little variation in the distribution of strain concentrations. Thus, interspecific comparisons should include considerations of the spatial patterning of strains rather than focus only on their magnitudes.

Food mechanical properties, feeding ecology, and the mandibular morphology of wild orangutans

Bornean orangutan mandibular morphology has been functionally linked to the exploitation of hard and tough foods, based on evidence that Pongo pygmaeus wurmbii spends a greater percentage of time feeding on bark, seeds and vegetation compared with Pongo abelii (Sumatran orangutans) and the assumption that these tissues are more challenging to process than fruit pulp. We measured and compared toughness (R) and Young's modulus (E) of ripe and unripe foods exploited by P. abelii and P. p. wurmbii. Additionally, we recorded and compared the percentage of time these orangutans fed on plants/plant parts of varying degrees of R and E. Compared with P. abelii, P. p. wurmbii consumed significantly tougher and more displacement limited (R/E)(0.5) fruit parts, leaves and inner bark, and spent a significantly greater percentage of time feeding on immature leaves, unripe fruit and other vegetation. Modulus did not vary as expected between species, likely because we failed to capture the high-end range of modulus values for tissues consumed by P. p. wurmbii. Notably, P. p. wurmbii spent ∼40% of its feeding time on the toughest foods consumed (between 1000 and 4000 J m(-2)). Thus, the hypothesis that mandibular robusticity in P. p. wurmbii is functionally linked to feeding on tough foods is supported and is likely related to countering relatively larger external forces and/or repetitive loads required to process the toughest tissues. The importance of elastic modulus on morphological divergence awaits future studies capturing the full range of this material property for P. p. wurmbii. Finally, phenophase and fruit availability influence orangutan species differences in food material properties and percentage of time spent feeding on various foods, emphasizing the importance of incorporating these variables in future studies of feeding ecology and craniodental morphology in extant taxa.

Dietary reconstruction of Pliocene Parapapio whitei from Makapansgat, South Africa, using dental microwear texture analysis

Pliocene Parapapio whitei fossils from Makapansgat have yielded stable isotope values suggesting some foraging on C4 plants and possibly underground storage organs (USOs). Dental microwear texture analysis on Pp.whitei (M 3147, MP 62, MP 223 and MP 239) from Makapansgat Members 3-4 is performed to examine whether tooth surface damage from mastication agrees with prior dietary inferences from isotopes. The enamel surface texture of Pp. whitei is relatively complex, resembling Lophocebus albigena and Cebusapella, and lacks the anisotropy of Trachypithecuscristatus and Alouattapalliata. The textural fill volume of Pp. whitei is distinct from extant forest primates suggesting extremes in hard-object consumption. Grit adhering to USOs is offered as an explanation for these enamel textural properties, corroborating the inference that Pp. whitei supplemented its diet with terrestrial resources. © 2013 S. Karger AG, Basel.

Probabilistic finite element analysis of a craniofacial finite element model

We employed a probabilistic finite element analysis (FEA) method to determine how variability in material property values affects stress and strain values in a finite model of a Macaca fascicularis cranium. The material behavior of cortical bone varied in three ways: isotropic homogeneous, isotropic non-homogeneous, and orthotropic non-homogeneous. The material behavior of the trabecular bone and teeth was always treated as isotropic and homogeneous. All material property values for the cranium were randomized with a Gaussian distribution with either coefficients of variation (CVs) of 0.2 or with CVs calculated from empirical data. Latin hypercube sampling was used to determine the values of the material properties used in the finite element models. In total, four hundred and twenty six separate deterministic FE simulations were executed. We tested four hypotheses in this study: (1) uncertainty in material property values will have an insignificant effect on high stresses and a significant effect on high strains for homogeneous isotropic models; (2) the effect of variability in material property values on the stress state will increase as non-homogeneity and anisotropy increase; (3) variation in the in vivo shear strain values reported by Strait et al. (2005) and Ross et al. (2011) is not only due to variations in muscle forces and cranial morphology, but also due to variation in material property values; (4) the assumption of a uniform coefficient of variation for the material property values will result in the same trend in how moderate-to-high stresses and moderate-to-high strains vary with respect to the degree of non-homogeneity and anisotropy as the trend found when the coefficients of variation for material property values are calculated from empirical data. Our results supported the first three hypotheses and falsified the fourth. When material properties were varied with a constant CV, as non-homogeneity and anisotropy increased the level of variability in the moderate-to-high strains decreased while the level of variability in the moderate-to-high stresses increased. However, this is not the pattern observed when CVs calculated from empirical data were applied to the material properties where the lowest level of variability in both stresses and strains occurred when the cranium was modeled with a low level of non-homogeneity and anisotropy. Therefore, when constant material property variability is assumed, inaccurate trends in the level of variability present in modest-to-high magnitude stresses and strains are produced. When the cranium is modeled with the highest level of accuracy (high non-homogeneity and anisotropy) and when randomness in the material properties is calculated from empirical data, there is a large level of variability in the significant strains (CV=0.369) and a low level of variability in the modest-to-high magnitude stresses (CV=0.150). This result may have important implications with regard to the mechanical signals driving bone remodeling and adaptation through natural selection.

Abrasive, silica phytoliths and the evolution of thick molar enamel in primates, with implications for the diet of Paranthropus boisei

BACKGROUND

Primates--including fossil species of apes and hominins--show variation in their degree of molar enamel thickness, a trait long thought to reflect a diet of hard or tough foods. The early hominins demonstrated molar enamel thickness of moderate to extreme degrees, which suggested to most researchers that they ate hard foods obtained on or near the ground, such as nuts, seeds, tubers, and roots. We propose an alternative hypothesis--that the amount of phytoliths in foods correlates with the evolution of thick molar enamel in primates, although this effect is constrained by a species' degree of folivory.

METHODOLOGY/PRINCIPAL FINDINGS

From a combination of dietary data and evidence for the levels of phytoliths in plant families in the literature, we calculated the percentage of plant foods rich in phytoliths in the diets of twelve extant primates with wide variation in their molar enamel thickness. Additional dietary data from the literature provided the percentage of each primate's diet made up of plants and of leaves. A statistical analysis of these variables showed that the amount of abrasive silica phytoliths in the diets of our sample primates correlated positively with the thickness of their molar enamel, constrained by the amount of leaves in their diet (R(2) = 0.875; p<.0006).

CONCLUSIONS/SIGNIFICANCE

The need to resist abrasion from phytoliths appears to be a key selective force behind the evolution of thick molar enamel in primates. The extreme molar enamel thickness of the teeth of the East African hominin Paranthropus boisei, long thought to suggest a diet comprising predominantly hard objects, instead appears to indicate a diet with plants high in abrasive silica phytoliths.