Chewed out: an experimental link between food material properties and repetitive loading of the masticatory apparatus in mammals

Variation in Craniodental Pathologies Among Cercopithecoid Primates

Pathologies of the skull and teeth are well documented for many human populations, but there are fewer studies of other primates. We contrast lesion prevalence and patterning among cercopithecoid primates and map variation onto socioecological variables. We compare craniodental lesions in six species: Nasalis larvatus (n = 54), Colobus polykomos (n = 64), Cercopithecus mitis (n = 65), Macaca fascicularis (n = 109), Theropithecus gelada (n = 13), and Papio anubis (n = 76). One of us (C.A.K.) evaluated each adult skull for multiple lesion types using standard criteria. We also tested for a relationship between lesion prevalence and cranial suture fusion (age proxy). We used nonparametric tests for sex and species differences as well as pathology co-occurrence in SPSS. Socioecological data come from previous studies. Sex differences in lesion prevalence were only detected in P. anubis. Within taxa, some lesion types co-occurred. In Macaca, the presence of caries was associated with several other lesion types. Pulp cavity exposure co-occurred with TMJ osteoarthritis in multiple taxa. Among taxa, male P. anubis had higher lesion prevalences, particularly related to the anterior dentition and facial trauma. Because we did not detect a relationship between suture fusion and lesion prevalence, we propose that craniodental lesions may also be influenced by socioecological variables such as group composition and ratio of fruit to leaves in the diet. Our findings suggest that pain from pulp cavity exposure and related dental infections may alter chewing biomechanics and contribute to onset of TMJ osteoarthritis in nonhuman primates, as seen in humans. Further, we suggest that higher lesion prevalence in male baboons is likely related to male-male competition. Skeletal lesion analysis provides useful insight into primate socioecology, particularly for rare or difficult-to-observe phenomena, and provides additional biological context for our own species.

Ontogenetic biomechanics of tufted (Sapajus) and untufted (Cebus) capuchin mandibles

OBJECTIVES

Cortical bone geometry is commonly used to investigate biomechanical properties of primate mandibles. However, the ontogeny of these properties is less understood. Here we investigate changes in cortical bone cross-sectional properties throughout capuchin ontogeny and compare captive versus wild, semi-provisioned groups. Tufted capuchins (Sapajus spp.) are known to consume relatively hard/tough foods, while untufted capuchins (Cebus spp.) exploit less mechanically challenging foods. Previous research indicates dietary differences are present early in development and adult Sapajus mandibles can resist higher bending/shear/torsional loads.

MATERIALS AND METHODS

This study utilized microCT scans of 22 Cebus and 45 Sapajus from early infancy to adulthood from three sample populations: one captive Cebus, one captive Sapajus, and one semi-provisioned, free-ranging Sapajus. Mandibular cross-sectional properties were calculated at the symphysis, P3, and M1. If the tooth had not erupted, its position within the crypt was used. A series of one-way ANOVAs were performed to assess differences between and within the sample populations.

RESULTS

Mandible robusticity increases across ontogeny for all three sample populations. Sapajus were better able to withstand bending and torsional loading even early in ontogeny, but no difference in shear resistance was found. Semi-provisioned, free-ranging Sapajus tend to show increased abilities to resist bending and torsional loading but not shear loading compared to captive Sapajus.

DISCUSSION

This study helps advance our understanding of the primate masticatory system development and opens the door for further studies into adaptive plasticity in shaping the masticatory apparatus of capuchins and differences in captive versus free-ranging sample populations.

The effects of feeding frequency on jaw loading in two lemur species

OBJECTIVES

Studies on oral processing are often snapshots of behaviors that examine feeding through individual bouts. In this study, we expand on our previous work comparing bite/chew variables per feeding bout to summed daily biting, chewing, and food intake to interpret loading that could have potential morphological effects.

MATERIALS AND METHODS

We observed sympatric Lemur catta and Propithecus verreauxi over two field seasons in the dry forest of Bezà Mahafaly Special Reserve in southwestern Madagascar. Bite and chew rates determined from videos filmed during observations were multiplied with time spent feeding on specific foods during focal follows to calculate daily values for each feeding bout. Food mechanical properties (FMPs) were tested on dietary items with a portable tester. We contrasted daily bite/chew numbers and intake with FMPs, species, season, and food shape.

RESULTS

Daily bite and chew numbers increased with maximum, but not average, food toughness. Daily intake decreased with average and maximum toughness. Season had a strong effect on daily bites and chews, but not on intake. Food shape influenced intake and total bite and chew numbers. The lemur species did not differ in our models.

DISCUSSION

Maximum food toughness impacted feeding behaviors and intake, which is consistent with higher loads having a greater effect on morphology. In contrast to feeding per bout, cumulative biting and chewing did not differ between species; taking feeding frequency into consideration affects interpretation of jaw loading. Finally, biting, as much as chewing, may generate strains that impact morphology.

Food texture and vitamin D influence mouse mandible form and molar roots

Industrialization influenced several facets of lifestyle, including softer nutrient-poor diets that contributed to vitamin D deficiency in post-industrzialized populations, with concomitantly increased dental problems. Here we simulated a post-industrialized diet in a mouse model to test the effects of diet texture and vitamin D level on mandible and third molar (M3) forms. Mice were raised on a soft diet with vitamin D (VitD) or without it (NoD), or on a hard diet with vitamin D. We hypothesized that a VitD/hard diet is optimal for normal mandible and tooth root form, as well as for timely M3 initiation. Subsets of adult NoD/soft and VitD/soft groups were bred to produce embryos that were micro-computed tomography (μCT) scanned to stage M3 development. M3 stage did not differ between embryos from mothers fed VitD and NoD diets, indicating that vitamin D does not affect timing of M3 onset. Sacrificed adult mice were μCT-scanned, their mandibles 3D-landmarked and M3 roots were measured. Principal component (PC) analysis described the largest proportion of mandible shape variance (PC1, 30.1%) related to diet texture, and nominal shape variance (PC2, 13.8%) related to vitamin D. Mice fed a soft diet had shorter, relatively narrower, and somewhat differently shaped mandibles that recapitulated findings in human populations. ANOVA and other multivariate tests found significantly wider M3 roots and larger root canals in mice fed a soft diet, with vitamin D having little effect. Altogether our experiments using a mouse model contribute new insights about how a post-industrial diet may influence human craniodental variation.

Introduction: food processing and nutritional assimilation in animals

How animals process and absorb nutrients from their food is a fundamental question in biology. Despite the continuity and interaction between intraoral food processing and post-oesophageal nutritional extraction, these topics have largely been studied separately. At present, we lack a synthesis of how pre- and post-oesophageal mechanisms of food processing shape the ability of various taxa to effectively assimilate nutrients from their diet. The aim of this special issue is to catalyse a unification of these distinct approaches as a functional continuum. We highlight questions that derive from this synthesis, as well as technical advances to address these questions. At present, there is also a skew toward vertebrates in studies of feeding form-function mechanics; by including perspectives from researchers working on both vertebrates and invertebrates, we hope to stimulate integrative and comparative research on food processing and nutritional assimilation. Below, we discuss how the papers in this issue contribute to these goals in three areas: championing a functional-comparative approach, quantifying performance and emphasizing the effects of life history, and food substrate and extrinsic factors in current and future studies of oral food processing and nutritional assimilation. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Comparing effects of food mechanical properties on oral processing behaviors in two sympatric lemur species

OBJECTIVES

The link between diet and the masticatory apparatus in primates is complex. We investigated how food mechanical properties (FMPs) and food geometry affect feeding behaviors and subsequent jaw loading. We compared oral processing in two sympatric lemur species with distinct diets and mandibular morphologies.

MATERIALS AND METHODS

All-day focal follows of Lemur catta (Lc) and Propithecus verreauxi (Pv) were conducted in both the dry and wet seasons at Beza Mahafaly Special Reserve. We collected activity budget data, filmed feeding bouts, and collected food items to measure their mechanical properties with an FLS-1 portable tester. Feeding videos for the top food items they spent the most time consuming were analyzed frame-by-frame to assess bite and chew numbers and rates.

RESULTS

Lc bite more and at a slower rate on tougher (maximum) foods, chew more for tougher (average) foods, and chew less for stiffer leaves. Pv initially increase chew number for tougher (average) foods, but their behavior is less affected as food toughness increases. Pv chew less and more slowly but spend more of the day feeding than Lc. Additionally, they have a tougher (maximum) diet than Lc.

DISCUSSION

Lc adjust their feeding behaviors depending on the FMPs of their top food items, while Pv feed more consistently. The more robust masticatory apparatus of Pv may not require them to adjust their feeding behaviors for more mechanically challenging foods. Furthermore, the two species show distinct differences in chewing. Exploring chewing on a daily scale could aid in understanding its impact on the loading of the masticatory apparatus.

Experimental assessment of the relationship between diet and mandibular morphology using a pig model: new insights for paleodietary reconstructions

Dietary habits exert significant selective pressures on anatomical structures in animals, leading to substantial morphological adaptations. Yet, the relationships between the mandible and diet are still unclear, raising issues for paleodietary reconstructions notably. To assess the impact of food hardness and size on morphological structures, we used an experimental baseline using a model based on the domestic pig, an omnivorous mammal with bunodont, thick-enameled dentition, and chewing movements similar to hominids. We hypothesized that the consumption of different types of seeds would result in substantial differences in the morphology of the mandible despite similar overall diets. The experiment was conducted on four groups of juvenile pigs fed with mixed cereal and soy flours. The control group received only flours. We supplemented the four others with either 10 hazelnuts, 30 hazelnuts, 30% barley seeds or 20% corn kernels per day. We investigated the shape differences between the controlled-fed groups using three-dimensional geometric morphometrics. Our results provide strong evidence that the supplemental consumption of a significant amount of seeds for a short period (95 days) substantially modify the mandibular morphology of pigs. Our analyses suggest that this shape differentiation is due to the size of the seeds, requiring high and repeated bite force, rather than their hardness. These results provide new perspectives for the use of mandibular morphology as a proxy in paleodietary reconstructions complementing dental microwear textures analyses. This article is protected by copyright. All rights reserved.

Assessment of masticatory function in older individuals with bimaxillary implant-supported fixed prostheses or with a natural dentition: A case-control study

STATEMENT OF PROBLEM

A clinical assessment of the masticatory performance of edentulous people treated with bimaxillary implant-supported fixed prostheses is lacking.

PURPOSE

The purpose of this case-control study was to compare the masticatory performance of edentulous people treated with bimaxillary implant-supported fixed prostheses to that of those with a natural dentition by using a simplified comminution test with hard viscoelastic test food and a mixing ability test.

MATERIAL AND METHODS

Thirty-six participants were recruited and divided into 2 equal groups (n=18) based on their dental status. The experimental group (7 women, mean ±standard deviation age=69.8 ±7.5 years) had bimaxillary implant-supported fixed prostheses, and the control group (9 women, mean ±standard deviation age=68.3 ±5.6 years) had a natural dentition. A hard viscoelastic food comminution test and a mixing ability test with 2-colored chewing gum were performed in both groups. The number and total area of the particles during the comminution test and the variance of hue during the mixing ability test were measured. The data were analyzed with the parametric (paired t test) and nonparametric (Mann-Whitney U) tests to assess differences between the groups (α=.05).

RESULTS

The experimental group exhibited significantly fewer pieces in the food comminution test than the control group (P=.001). The experimental group also showed significantly higher variance of hue values than the control group (P<.001).

CONCLUSIONS

Patients treated with bimaxillary implant-supported prostheses had reduced masticatory performance in comparison to those with a natural dentition, despite having been provided with satisfactory and well-functioning prostheses.

Mechanical properties of food and masticatory behavior in llamas, Llama glama

Mammals typically process food items more extensively in their oral cavities than do other vertebrates. Dental morphology, jaw-muscle activity patterns, mandibular movements, and tongue manipulation work to facilitate oral fragmentation of dietary items. While processing mechanically challenging foods, mammals modulate mandibular movements and bite forces via recruitment of greater jaw-adductor muscle forces and protracted biting or chewing. Because jaw-loading patterns are influenced by magnitude; frequency; and duration of muscular, bite, and reaction forces during routine feeding behaviors, relatively larger jaws are thought to be more characteristic of mammals that experience higher masticatory loads due to the processing of mechanically challenging foods. The ease of food fracture during post-canine biting and chewing is mainly determined by food stiffness and toughness. Such foods have been associated with increased loading magnitude and/or greater amounts of cyclical loading (i.e., chewing duration). Dietary properties are thought to modulate cyclical loading through changes in chewing frequency and chewing investment. On the other hand, chewing frequency has been found to be independent of dietary properties in rabbits and primates; however, little evidence exists regarding the influence of dietary properties on these parameters in a broader range of mammals. Here, we assessed chewing behavior in seven adult llamas (Llama glama) processing foods with a wide range of mechanical properties (grain, hay, carrots, and dried corn). Each subject was filmed at 60 frames/s, with video slowed for frame-by-frame computer analysis to obtain length of feeding bout and number of chewing cycles for each food type. These parameters were used to calculate chewing frequency (chews/s), chewing investment (chews/g), and chewing duration (s/g). Chewing frequency was not significantly related to mechanical properties of food, but chewing investment and chewing duration were significantly related to dietary stiffness and toughness. Therefore, cyclical loading is positively influenced by stiff and tough foods. This suggests that variation in jaw morphology in extinct and extant mammals is positively related to dietary stiffness and toughness, which requires greater chewing investment and increased chewing duration.

Lagomorpha as a Model Morphological System

Due to their global distribution, invasive history, and unique characteristics, European rabbits are recognizable almost anywhere on our planet. Although they are members of a much larger group of living and extinct mammals [Mammalia, Lagomorpha (rabbits, hares, and pikas)], the group is often characterized by several well-known genera (e.g., Oryctolagus, Sylvilagus, Lepus, and Ochotona). This representation does not capture the extraordinary diversity of behavior and form found throughout the order. Model organisms are commonly used as exemplars for biological research, but there are a limited number of model clades or lineages that have been used to study evolutionary morphology in a more explicitly comparative way. We present this review paper to show that lagomorphs are a strong system in which to study macro- and micro-scale patterns of morphological change within a clade that offers underappreciated levels of diversity. To this end, we offer a summary of the status of relevant aspects of lagomorph biology.

Computational biomechanical modelling of the rabbit cranium during mastication

Although a functional relationship between bone structure and mastication has been shown in some regions of the rabbit skull, the biomechanics of the whole cranium during mastication have yet to be fully explored. In terms of cranial biomechanics, the rabbit is a particularly interesting species due to its uniquely fenestrated rostrum, the mechanical function of which is debated. In addition, the rabbit processes food through incisor and molar biting within a single bite cycle, and the potential influence of these bite modes on skull biomechanics remains unknown. This study combined the in silico methods of multi-body dynamics and finite element analysis to compute musculoskeletal forces associated with a range of incisor and molar biting, and to predict the associated strains. The results show that the majority of the cranium, including the fenestrated rostrum, transmits masticatory strains. The peak strains generated over all bites were found to be attributed to both incisor and molar biting. This could be a consequence of a skull shape adapted to promote an even strain distribution for a combination of infrequent incisor bites and cyclic molar bites. However, some regions, such as the supraorbital process, experienced low peak strain for all masticatory loads considered, suggesting such regions are not designed to resist masticatory forces.

Masticatory system integration in a commensal canid: interrelationships between bones, muscles and bite force in the red fox

The jaw system in canids is essential for defence and prey acquisition. However, how it varies in wild species in comparison with domestic species remains poorly understood, yet is of interest in terms of understanding the impact of artificial selection. Here, we explored the variability and interrelationships between the upper and lower jaws, muscle architecture and bite force in the red fox (Vulpes vulpes). We performed dissections and used 3D geometric morphometric approaches to quantify jaw shape in 68 foxes. We used a static lever model and bite force estimates were compared with in vivo measurements of 10 silver foxes. Our results show strong relationships exist between cranial and mandible shape, and between cranial or mandible shape on the one hand and muscles or estimated bite force on the other hand, confirming the strong integration of the bony and muscular components of the jaw system. These strong relationships are strongly driven by size. The functional links between shape and estimated bite force are stronger for the mandible, which probably reflects its greater specialisation towards biting. We then compared our results with data previously obtained for dogs (Canis lupus familiaris) to investigate the effect of domestication. Foxes and dogs differ in skull shape and muscle physiological cross-sectional area (PCSA). They show a similar amount of morphological variation in muscle PCSA, but foxes show lower variation in cranial and mandible shape. Interestingly, the patterns of covariation are not stronger in foxes than in dogs, suggesting that domestication did not lead to a disruption of the functional links of the jaw system.

Bone remodeling and cyclical loading in maxillae of New Zealand white rabbits (Oryctolagus cuniculus)

Mammalian feeding behaviors are altered when mechanically challenging (e.g., tough, stiff) foods require large bite forces or prolonged mastication. Bony responses to high bite forces are well-documented for the mammalian skull, but osteogenesis due to cyclical loading, caused by repetitive chewing, is more poorly understood. Previous studies demonstrate that cyclical loading results in greater bone formation in the rabbit masticatory apparatus and in substantial Haversian remodeling in primate postcrania. Here we assess the relationship between cyclical loading and remodeling in the rabbit maxilla. Twenty male New Zealand white rabbits (Oryctolagus cuniculus) were raised on either an overuse or control diet (10 per group) for 48 weeks, beginning at weaning onset. The control group was raised on a diet of rabbit pellets (E = 29 MPa, R = 1031 J/m² ), whereas the overuse group ate rabbit pellets and hay, which has high stiffness (E = 3336 MPa) and toughness (R = 2760 J/m² ) properties. Hay requires greater chewing investment (475 chews/g) and longer chewing durations (568 s/g) than pellets (161 chews/g and 173 s/g), therefore causing cyclical loading of the jaws. Remodeling was measured as osteon population density (OPD), percent Haversian bone (%HAV), and osteon cross-sectional area (On.Ar). The only significant difference found was greater On.Ar in the alveolar region of the maxilla (p < 0.001) in the overuse group. The hypothesis that cyclical loading engenders Haversian remodeling in the developing maxilla is not supported. The continuation of modeling throughout the experimental duration may negate the need for remodeling as newly laid bone tends to be more compliant and resistant to crack propagation.

Internal architecture of the mandibular condyle of rabbits is related to dietary resistance during growth

Although there is considerable evidence that bone responds to the loading environment in which it develops, few analyses have examined phenotypic plasticity or bone functional adaptation in the masticatory apparatus. Prior work suggests that masticatory morphology is sensitive to differences in food mechanical properties during development; however, the importance of the timing/duration of loading and variation in naturalistic diets is less clear. Here, we examined microstructural and macrostructural differences in the mandibular condyle in four groups of white rabbits (Oryctolagus cuniculus) raised for a year on diets that varied in mechanical properties and timing of the introduction of mechanically challenging foods, simulating seasonal variation in diet. We employed sliding semilandmarks to locate multiple volumes of interest deep to the mandibular condyle articular surface, and compared bone volume fraction, trabecular thickness and spacing, and condylar size/shape among experimental groups. The results reveal a shared pattern of bony architecture across the articular surface of all treatment groups, while also demonstrating significant among-group differences. Rabbits raised on mechanically challenging diets have significantly increased bone volume fraction relative to controls fed a less challenging diet. The post-weaning timing of the introduction of mechanically challenging foods also influences architectural properties, suggesting that bone plasticity can extend well into adulthood and that bony responses to changes in loading may be rapid. These findings demonstrate that bony architecture of the mandibular condyle in rabbits responds to variation in mechanical loading during an organism's lifetime and has the potential to track dietary variation within and among species.

Nanofibrous yarn reinforced HA-gelatin composite scaffolds promote bone formation in critical sized alveolar defects in rabbit model

Alveolar ridge resorption and crestal bone loss necessitate the use of bone graft substitutes for dental rehabilitation. The aim of this study was to compare the bone regenerative property of nanofibre incorporated two composite matrices (nanofibrous sheet layered matrix (CS-S) and nanofibrous yarn reinforced matrix (CS-Y)) in critical sized mandibular defect in a rabbit model (under load bearing scenario). Histological evaluation revealed continuous bone formation in the defect implanted with fibre reinforced scaffolds than those without fibres as well as commercial nanoHA-collagen graft. Interestingly, the mineralisation and the mineral density were significantly higher with nanoyarn reinforced scaffolds. Moreover, the compressive strength of new bone formed from CS-Y scaffolds was almost similar to that of native rabbit mandible. It can be concluded that the mechanical strength provided by three-dimensionally reinforced nanoyarns in the matrix could promote bone formation in load bearing mandibular region, and these can be proposed as a scaffold of choice for alveolar bone augmentation and dental rehabilitation.

Dietary variation and mechanical properties of articular cartilage in the temporomandibular joint: implications for the role of plasticity in mechanobiology and pathobiology

Due to their nature as tissue composites, skeletal joints pose an additional challenge in terms of evaluating the functional significance of morphological variation in their bony and cartilaginous components in response to altered loading conditions. Arguably, this complexity requires more direct means of investigating joint plasticity and performance than typically employed to analyze macro- and micro-anatomical phenomena. To address a significant gap in our understanding of the plasticity of the mammalian temporomandibular joint (TMJ), we investigated the histology and mechanical properties of condylar articular cartilage in rabbits subjected to long-term variation in diet-induced masticatory stresses, specifically cyclical loading. Three cohorts of male weanlings were raised for six months on different diets until adulthood. Following euthanasia, the TMJ condyles on one side were dissected away, fixed, decalcified, dehydrated, embedded and sectioned. Safranin O staining was employed to identify variation in proteoglycan content, which in turn was used to predict patterns of articular cartilage stiffness in contralateral condylar specimens for each treatment group. Hematoxylin and eosin staining was used to quantify diet-induced changes in chondrocyte hypertrophy and cellularity. Mechanical tests document significant decreases in articular cartilage stiffness corresponding to patterns of extracellular matrix relative protein abundance in rabbits subjected to greater cyclical loading. This indicates that TMJs routinely subjected to higher masticatory stresses due to a challenging diet eventually develop postnatal decreases in the ability to counter compressive loads during postcanine biting and chewing. These findings provide novel information regarding TMJ performance, with broader implications about the costs and benefits of phenotypic plasticity as well as implications for how such biological processes affect connective tissue mechanobiology and pathobiology.

Betwixt and Between: Intracranial Perspective on Zygomatic Arch Plasticity and Function in Mammals

The zygomatic arch is morphologically complex, providing a key interface between the viscerocranium and neurocranium. It also serves as an attachment site for masticatory muscles, thereby linking it to the feeding apparatus. Though morphological variation related to differential loading is well known for many craniomandibular elements, the adaptive osteogenic response of the zygomatic arch remains to be investigated. Here, experimental data are presented that address the naturalistic influence of masticatory loading on the postweaning development of the zygoma and other cranial elements. Given the similarity of bone-strain levels among the zygoma and maxillomandibular elements, a rabbit and pig model were used to test the hypothesis that variation in cortical bone formation and biomineralization along the zygomatic arch and masticatory structures are linked to increased stresses. It was also hypothesized that neurocranial structures would be minimally affected by varying loads. Rabbits and pigs were raised for 48 weeks and 8 weeks, respectively. In both experimental models, CT analyses indicated that elevated masticatory loading did not induce differences in cortical bone thickness of the zygomatic arch, though biomineralization was positively affected. Hypotheses were supported regarding bone formation for maxillomandibular and neurocranial elements. Varying osteogenic responses in the arch suggests that skeletal adaptation, and corresponding variation in performance, may reside differentially at one level of bony architecture. Thus, it is possible that phenotypic diversity in the mammalian zygoma is due more singularly to natural selection (vs. plasticity). These findings underscore the complexity of the zygomatic arch and, more generally, determinants of skull form. Anat Rec, 299:1646-1660, 2016. © 2016 Wiley Periodicals, Inc.

Ontogenetic and functional modularity in the rodent mandible

The material properties of diets consumed by juvenile individuals are known to affect adult morphological outcomes. However, much of the current experimental knowledge regarding dietary effects on masticatory form is derived from studies in which individuals are fed a non-variable diet for the duration of their postweaning growth period. Thus, it remains unclear how intra-individual variation in diet, due to ontogenetic variation in feeding behaviors or seasonal resource fluctuations, affects the resulting adult morphology. Furthermore, the mandible is composed of multiple developmental and functional subunits, and the extent to which growth and plasticity among these modules is correlated may be misestimated by studies that examine non-variable masticatory function in adults only. To address these gaps in our current knowledge, this study raised Sprague Dawley rats (n=42) in four dietary cohorts from weaning to skeletal maturity. Two cohorts were fed a stable ("annual") diet of either solid or powdered pellets. The other two cohorts were fed a variable ("seasonal") diet consisting of solid/powdered pellets for the first half of the study, followed by a shift to the opposite diet. Results of longitudinal morphometric analyses indicate that variation in the mandibular corpus is more prominent at immature ontogenetic stages, likely due to processes of dental eruption and the growth of tooth roots. Furthermore, adult morphology is influenced by both masticatory function and the ontogenetic timing of this function, e.g., the consumption of a mechanically resistant diet. The morphology of the coronoid process was found to separate cohorts on the basis of their early weanling diet, suggesting that the coronoid process/temporalis muscle module may have an early plasticity window related to high growth rates during this life stage. Conversely, the morphology of the angular process was found to be influenced by the consumption of a mechanically resistant diet at any point during the growth period, but with a tendency to reflect the most recent diet. The prolonged plasticity window of the angular process/pterygomasseteric muscle module may be related to delayed ossification and muscular maturation within this module. The research presented here highlights the importance of more naturalistic models of mammalian feeding, and underscores the need for a better understanding of the processes of both morphological and behavioral maturation that follow weaning.

Intracranial and hierarchical perspective on dietary plasticity in mammals

The effect of dietary properties on craniofacial form has been the focus of numerous functional studies, with increasingly more work dedicated to the importance of phenotypic plasticity. As bone is a dynamic tissue, morphological variation related to differential loading is well established for many masticatory structures. However, the adaptive osteogenic response of several cranial sites across multiple levels of bony organization remains to be investigated. Here, rabbits were obtained at weaning and raised for 48 weeks until adulthood in order to address the naturalistic influence of altered loading on the long-term development of masticatory and non-masticatory elements. Longitudinal data from micro-computed tomography (μCT) scans were used to test the hypothesis that variation in cortical bone formation and biomineralization in masticatory structures is linked to increased stresses during oral processing of mechanically challenging foods. It was also hypothesized that similar parameters for neurocranial structures would be minimally affected by varying loads as this area is characterized by low strains during mastication and reduced hard-tissue mechanosensitivity. Hypotheses were supported regarding bone formation for maxillomandibular and neurocranial elements, though biomineralization trends of masticatory structures did not mirror macroscale findings. Varying osteogenic responses in masticatory elements suggest that physiological adaptation, and corresponding variation in skeletal performance, may reside differentially at one level of bony architecture, potentially affecting the accuracy of behavioral and in silico reconstructions. Together, these findings underscore the complexity of bone adaptation and highlight functional and developmental variation in determinants of skull form.

Shape Variation in the Craniomandibular System and Prevalence of Dental Problems in Domestic Rabbits: A Case Study in Evolutionary Veterinary Science

In contrast to wild lagomorphs, pet rabbits exhibit a noticeably high frequency of dental problems. Although dietary habits are considered as a major factor contributing to acquired malocclusions, the exact causes and interrelationships are still under debate. In this regard, an important aspect that has not been considered thoroughly to date is the effect of diet-induced phenotypic plasticity in skull morphology. Therefore, we conducted a geometric morphometric analysis on skull radiological images of wild and pet rabbits in order to quantify intraspecific variation in craniomandibular morphology. The statistical analyses reveal a significant morphological differentiation of the craniomandibular system between both groups. Furthermore, the analysis of covariance shows that the force-generating modules (cranium and mandible) vary independently from the force-receiving module (hypselodont teeth) in pet rabbits, which is in contrast to their wild relatives. Our findings suggest that the phenotypic changes in domestic rabbits impact mastication performance and, consequently, oral health. An adequate close-to-nature nutrition throughout the whole life and especially beginning early parallel to weaning (phase of increased phenotypic plasticity) is necessary to ensure a normal strain on the teeth by promoting physiological lateral gliding movements and avoiding direct axial loads.