Skull Shape Research Articles

What Makes a Rabbit Cute? Preference for Rabbit Faces Differs according to Skull Morphology and Demographic Factors.

Domesticated rabbits typically exhibit shorter, flatter skulls than their wild counterparts (brachycephalism). However, brachycephaly is associated with considerable health problems, including problems with dentition. The aim of this study was to establish which type of rabbit face people prefer, with a particular emphasis on skull morphology and brachycephaly. We grouped 25 images of rabbit faces by cephalic degree based on ratings assigned by 134 veterinary professionals. An online questionnaire was then launched, in which people could rate each of the 25 images according to preference for the rabbits' faces, and a total of 20,858 questionnaires were completed globally. Repeated-measure, multi-level general linear modelling revealed mildly-brachycephalic rabbits to be the most preferred type of rabbit, and moderately-dolichocephalic (longer skulled) rabbits to be the least preferred. The preference for brachycephalic rabbits was stable across continents, and as such it is highly plausible that human preference has been a driver for the shortening of the skull typically seen in domestic rabbits, perhaps as a result of the 'baby-schema'. Additional features of rabbit faces that were preferred include a soft, medium-light fur appearance and being generally short-furred. These novel insights may prove useful in the improvement of the public understanding of rabbit health and welfare. The relationship between preference and skull shape is particularly pertinent to future work concerning rabbit health, given the cross-species evidence that having a flat face is associated with chronic health conditions.

Phenotypic integration and modularity drives skull shape divergence in the Arctic fox (Vulpes lagopus) from the Commander Islands.

Phenotypic integration and modularity influence morphological disparity and evolvability. However, studies addressing how morphological integration and modularity change for long periods of genetic isolation are scarce. Here, we investigate patterns of phenotypic integration and modularity in the skull of phenotypically and genetically distinct populations of the Artic fox (Vulpes lagopus) from the Commander Islands of the Aleutian belt (i.e. Bering and Mednyi) that were isolated ca 10 000 years by ice-free waters of the Bering sea. We use three-dimensional geometric morphometrics to quantify the strength of modularity and integration from inter-individual variation (static) and from fluctuating asymmetry (random developmental variation) in both island populations compared to the mainland population (i.e. Chukotka) and we investigated how changes in morphological integration and modularity affect disparity and the directionality of trait divergence. Our results indicate a decrease in morphological integration concomitant to an increase in disparity at a developmental level, from mainland to the smallest and farthest population of Mednyi. However, phenotypic integration is higher in both island populations accompanied by a reduction in disparity compared to the population of mainland at a static level. This higher integration may have favoured morphological adaptive changes towards specific feeding behaviours related to the extreme environmental settings of islands. Our study demonstrates how shifts in phenotypic integration and modularity can facilitate phenotypic evolvability at the intraspecific level that may lead to lineage divergence at macroevolutioanry scales.

Why the long face? Comparative shape analysis of miniature, pony, and other horse skulls reveals changes in ontogenetic growth.

BackgroundMuch of the shape variation found in animals is based on allometry and heterochrony. Horses represent an excellent model to investigate patterns of size-shape variation among breeds that were intentionally bred for extreme small and large sizes.MethodsWe tested whether ponies (wither height < 148 cm) have a diverging size-shape relationship in skull shape as compared to regular-sized horse breeds (wither height > 148 cm, here-after called horses) during ontogenetic growth. We used a dataset of 194 specimens from 25 horse and 13 pony breeds, two of which are miniature breeds (wither height < 96.5 cm)—Falabella, Shetland. We applied three-dimensional geometric morphometrics, linear measurements, and multivariate analyses (Procrustes ANOVAs) to quantitatively examine and compare the ontogenetic trajectories between pony and horse breeds with an emphasis on the miniature breeds as an extreme case of artificial selection on size. Additionally, we tested for juvenile characteristics in adult horse and miniature breeds that could resemble “paedomorphosis”—retention of juvenile characteristics in adult stage; e.g. large eyes, large braincase-to-face-relationship, and large head-to-body relationship.ResultsAllometric regression of size on shape revealed that 42% of shape variation could be explained by variation in size in all breeds. The ontogenetic trajectories of ponies and horses vary in slope and therefore in rate of change per unit size, and length. The differences in trajectory lengths and slopes result in ponies having a similar skull shape in an older age stage than horses of the same size in a younger age stage. This pattern could cause the generally perceived “paedomorphic” appearance of ponies. Miniature breeds have larger heads in relation to wither height compared to horses, a non-paedomorphic feature in horses specifically. Also, rostra (faces) are longer in adult individuals than in juveniles across all kinds of breeds. This pattern can be explained by the long-face hypothesis for grazing ungulates and could possibly be caused by the mismatch of selection by humans for shorter rostra and the dentition of ruminants.ConclusionsMiniature breed specimens do not exhibit any of the classical mammalian “paedomorphic” features (large orbits, large heads), except for the adult Falabella that has enlarged orbits, possibly because they are herbivorous ungulates that are affected by functional and metabolic constraints related to low nutrient-food consumption. Instead ponies, including miniature breeds, have faster and shorter ontogenetic growth compared to horses, resulting in adult pony skulls looking in part like juvenile horse skulls.

ADAPTIVE REGION GROWING FOR SKULL, BRAIN, AND SCALP SEGMENTATION FROM 3D MRI

Many of the diseases that are related to the nervous system are serious and can lead to pain, headaches, and many other symptoms. Magnetic resonance imaging (MRI), which produces detailed medical images for the diagnosis of nervous system-related diseases, offers an advanced approach to the study of these diseases. In this paper, a method for the automatic segmentation of the skull, scalp, and brain of 3D MRI imagery is proposed to help doctors diagnose numerous injuries and diseases. For the proposed method, the 3D MRI dataset was first transformed to 2D axial slices, which were grouped based on the skull shape. Second, the histogram method was used to compute the fitting rectangle boundary around the object. Third, the segmentation of the skull was performed using the proposed adaptive region growing method, depending on the slice groups. Finally, the brain and the scalp were segmented. The proposed method was tested and validated on 20 subjects from the BrainWeb database and seven adults from the Neurodevelopmental MRI database with the use of the tissues for the ground truth data. The performance of the proposed method was evaluated using the Dice percentage and a comparison with the other methods.

Taxonomic revision of the long-tailed shrew, Sorex dispar Batchelder, 1911, from the Appalachian Region of North America, with the description of a new subspecies

Abstract The long-tailed shrew, Sorex dispar Batchelder, 1911, and Gaspe shrew, S. gaspensis Anthony and Goodwin, 1924, from the Appalachian Mountains of North America have been characterized as genetically highly similar, and that one is morphologically a clinal variate of the other, i.e., there is a single species. I measured 24 characters of the skull on 196 shrews from throughout the range of the species. Geographic variation in skull shape and size was not gradual or continuous, but abrupt. These abrupt changes in morphology are associated with major water barriers, primarily the Connecticut River, middle Saint John River, and the Strait of Canso, which separates mainland Nova Scotia from Cape Breton Island. The morphological analyses presented here and previous genetic studies indicate that S. dispar and S. gaspensis are likely conspecific. Shrews with the largest skull occur from North Carolina north to Vermont and are referable to S. d. dispar with S. d. blitchi as a synonym. Shrews from New Hampshire northeast to southern New Brunswick and mainland Nova Scotia have a medium-sized skull and are referable to a new subspecies. Those from northern New Brunswick, Gaspe Peninsula of Quebec, and Cape Breton Island have a small skull and are referable to S. d. gaspensis. The skull morphology of S. d. gaspensis and the new subspecies are more similar to each other than to S. d. dispar. Results of this study differ from those of previous morphological studies because measurement error and within-group variation were reduced, which allowed for visibility of otherwise “hidden” between-group differences, or geographic variation.

Craniosynostosis: A Reversible Pathology?

The formation of the cranial sutures, in utero, occurs when the ossification of the skull bones reaches predestined positions around gestational week 15 to 20. Craniosynostosis, and the consequent skull shape deformities, is treated with surgery including osteotomies of the fused sutures. The occasional appearance of a new suture in the osteotomy lines has previously been described as sporadic events. In this retrospective study, a 4-year consecutive series of osteotomies combined with springs for craniosynostosis were systematically analysed regarding the appearance of neosutures. In total, 84 patients were included and in 16 patients (19%) a new radiologically normal suture appeared in a part of the suture that was completely closed preoperatively. Additionally, in 7 patients (8%) a new suture appeared in a part of the suture that had a discernible suture prior to surgery.In conclusion, in this consecutive and well-defined patient cohort operated for craniosynostosis, the formation of a neosuture is not a rare, and speculatively not a random, event. The appearance of a new suture long after the normal time period for suture formation in utero indicates that the craniosynostosis may just as well be caused by disturbed formation of the suture as actual premature closure.

Variación no alométrica en el cráneo del perezoso bayo &lt;em&gt;Bradypus variegatus&lt;/em&gt; (Schinz, 1825) (Mammalia, Pilosa, Bradypodidae)

El perezoso bayo Bradypus variegatus (Schinz, 1825) es un mamífero monomórfico, de ontogenia craneal poco conocida. En este estudio analizamos la alometría estática entre tamaño y forma, utilizando 21 especímenes diferentes de edades diversas. El tamaño y la forma fueron determinados mediante técnicas de morfometría geométrica. De los resultados obtenidos se desprende que la variación en la forma craneal queda muy poco explicada por la variación en el tamaño. Muchos estudios han señalado características morfológicas únicas en los perezosos en relación al resto de mamíferos, características fenotípicas que van de la estructura esquelética a tejidos blandos. En este caso, el escalado no alométrico del cráneo debería ser visto como otra característica única de este grupo taxonómico.

S7A-01 SESSION 7A: SAGITTAL SYNOSTOSIS PROGRESSION OF SCAPHOCEPHALY IN UNTREATED SAGITTAL SYNOSTOSIS OVER THE FIRST YEAR OF AGE

Introduction: The timing of surgical treatment of sagittal craniosynostosis (SC) within the first year of life varies by technique and patient presentation. It is important to determine whether scaphocephaly associated with SC is static in this first year, or if there is progression that should be included in surgical decision-making. The purpose of this study was to perform quantitative analysis of the mean changes in pre-operative SC skull shape to determine which characteristics improve, and which deteriorate. Methods: We compared CT scans from consecutive untreated isolated SC cases who were less than one year old with an age matched normal cohort. Craniofacial landmarks were marked using 3D Slicer, and standardized measures of shape were calculated using custom software developed from open source image analysis libraries (ITK, VTK and Morpho). Shape measures were compared between SC and normal cohorts across three age groups: 0–3 months, 3–6 months, and 6–12 months. Results: We included 109 patients with SC from 2003 to 2016 and 50 control patients. Over the first year, cephalic index in SC remained stable and lower than normal. Frontal bossing was highest in the young age groups, and approached normal by one year. Occipital ratio and coronal constriction both became narrower with time, whereas they were stable in normal patients. Conclusion: Scaphocephaly and pterion constriction remains abnormal through the first year of life in patients with SC and need to be addressed in any surgical technique. The worsening of occipital ratio after six months of age implies that techniques performed in this later window specifically need to address occipital constriction. Frontal bossing appears to continue to normalize in untreated SC cases without surgery, indicating surgery should focus on the middle and posterior cranial vaults.

Postmetamorphic ontogenetic allometry and the evolution of skull shape in Nest-building frogs Leptodactylus (Anura: Leptodactylidae).

Allometry constitutes an important source of morphological variation. However, its influence in head development in anurans has been poorly explored. By using geometric morphometrics followed by statistical and comparative methods we analyzed patterns of allometric change during cranial postmetamorphic ontogeny in species of Nest-building frogs Leptodactylus (Leptodactylidae). We found that the anuran skull is not a static structure, and allometry plays an important role indefining its shape in this group. Similar to other groups with biphasic life-cycle, and following a general trend in vertebrates, ontogenetic changes mostly involve rearrangement in rostral, otoccipital, and suspensorium regions. Ontogenetic transformations are paralleled by shape changes associated with evolutionary change in size, such that the skulls of species of different intrageneric groups are scaled to each other, and small and large species show patterns of paedomorphic/peramorphic features, respectively. Allometric trajectories producing those phenotypes are highly evolvable though, with shape change direction and magnitude varying widely among clades, and irrespective of changes in absolute body size. These results reinforce the importance of large-scale comparisons of growth patterns to understand the plasticity, evolution, and polarity of morphological changes in different clades.

Ecomorphological diversification in squamates from conserved pattern of cranial integration

Factors intrinsic and extrinsic to organisms dictate the course of morphological evolution but are seldom considered together in comparative analyses. Among vertebrates, squamates (lizards and snakes) exhibit remarkable morphological and developmental variations that parallel their incredible ecological spectrum. However, this exceptional diversity also makes systematic quantification and analysis of their morphological evolution challenging. We present a squamate-wide, high-density morphometric analysis of the skull across 181 modern and extinct species to identify the primary drivers of their cranial evolution within a unified, quantitative framework. Diet and habitat preferences, but not reproductive mode, are major influences on skull-shape evolution across squamates, with fossorial and aquatic taxa exhibiting convergent and rapid changes in skull shape. In lizards, diet is associated with the shape of the rostrum, reflecting its use in grasping prey, whereas snakes show a correlation between diet and the shape of posterior skull bones important for gape widening. Similarly, we observe the highest rates of evolution and greatest disparity in regions associated with jaw musculature in lizards, whereas those forming the jaw articulation evolve faster in snakes. In addition, high-resolution ancestral cranial reconstructions from these data support a terrestrial, nonfossorial origin for snakes. Despite their disparate evolutionary trends, lizards and snakes unexpectedly share a common pattern of trait integration, with the highest correlations in the occiput, jaw articulation, and palate. We thus demonstrate that highly diverse phenotypes, exemplified by lizards and snakes, can and do arise from differential selection acting on conserved patterns of phenotypic integration.

Prenatal developmental sequence of the skull of minke whales and its implications for the evolution of mysticetes and the teeth‐to‐baleen transition

Baleen whales (Mysticeti) have an extraordinary fossil record documenting the transition from toothed raptorial taxa to modern species that bear baleen plates, keratinous bristles employed in filter-feeding. Remnants of their toothed ancestry can be found in their ontogeny, as they still develop tooth germs in utero. Understanding the developmental transition from teeth to baleen and the associated skull modifications in prenatal specimens of extant species can enhance our understanding of the evolutionary history of this lineage by using ontogeny as a relative proxy of the evolutionary changes observed in the fossil record. Although at present very little information is available on prenatal development of baleen whales, especially regarding tooth resorption and baleen formation, due to a lack of specimens. Here I present the first detailed description of prenatal specimens of minke whales (Balaenoptera acutorostrata and Balaenoptera bonaerensis), focusing on the skull anatomy and tooth germ development, resorption, and baleen growth. The ontogenetic sequence described consists of 10 specimens of both minke whale species, from the earliest fetal stages to full term. The internal skull anatomy of the specimens was visualized using traditional and iodine-enhanced computed tomography scanning. These high-quality data allow detailed description of skull development both qualitatively and quantitatively using three-dimensional landmark analysis. I report distinctive external anatomical changes and the presence of a denser tissue medial to the tooth germs in specimens from the final portion of gestation, which can be interpreted as the first signs of baleen formation (baleen rudiments). Tooth germs are only completely resorbed just before the eruption of the baleen from the gums, and they are still present for a brief period with baleen rudiments. Skull shape development is characterized by progressive elongation of the rostrum relative to the braincase and by the relative anterior movement of the supraoccipital shield, contributing to a defining feature of cetaceans, telescoping. These data aid the interpretation of fossil morphologies, especially of those extinct taxa where there is no direct evidence of presence of baleen, even if caution is needed when comparing prenatal extant specimens with adult fossils. The ontogeny of other mysticete species needs to be analyzed before drawing definitive conclusions about the influence of development on the evolution of this group. Nonetheless, this work is the first step towards a deeper understanding of the most distinctive patterns in prenatal skull development of baleen whales, and of the anatomical changes that accompany the transition from tooth germs to baleen. It also presents comprehensive hypotheses to explain the influence of developmental processes on the evolution of skull morphology and feeding adaptations of mysticetes.

Analysis of skull morphometric characters in diurnal raptors (Accipitriformes and Falconiformes)

Abstract Diurnal birds of prey (Accipitriformes and Falconiformes) has traditionally been known as comprising a single order. Recently, this classification has been used in the non-taxonomic sense as referring to a convergent group of birds that are largely classified as predatory birds. Although these birds are similar in their morphology, the species differ in their foraging methods and prey preference. The cranial shape and the physical attributes determine the efficiency of the resource use. The aim of this study is to increase our knowledge of the relationship between skull shape, prey preference, and foraging habits. A geometric morphometric approach was used to analyse two-dimensional cranial landmarks. We used principal component (PC) analyses on measurements that may be related to prey preference and foraging habits. The PCs are resulted described the relative height of the skull and beak, the variation in the relative size, the orientation and robustness of the lacrimal bone, the variation in the relative size of the neurocranium compared to the viscerocranium, and the orientation of the palatine bone. The dietary categories significantly overlap. The skull morphology reflects more on foraging habits than diet or prey preference.

Divergent selection along elevational gradients promotes genetic and phenotypic disparities among small mammal populations.

Species distributed along mountain slopes, facing contrasting habitats in short geographic scale, are of particular interest to test how ecologically based divergent selection promotes phenotypic and genetic disparities as well as to assess isolation‐by‐environment mechanisms. Here, we conduct the first broad comparative study of phenotypic variation along elevational gradients, integrating a large array of ecological predictors and disentangling population genetic driver processes. The skull form of nine ecologically distinct species distributed over a large altitudinal range (100–4200 m) was compared to assess whether phenotypic divergence is a common phenomenon in small mammals and whether it shows parallel patterns. We also investigated the relative contribution of biotic (competition and predation) and abiotic parameters on phenotypic divergence via mixed models. Finally, we assessed the population genetic structure of a rodent species (Niviventer confucianus) via analysis of molecular variance and FST along three mountain slopes and tested the isolation‐by‐environment hypothesis using Mantel test and redundancy analysis. We found a consistent phenotypic divergence and marked genetic structure along elevational gradients; however, the species showed mixed patterns of size and skull shape trends across mountain zones. Individuals living at lower altitudes differed greatly in both phenotype and genotype from those living at high elevations, while middle‐elevation individuals showed more intermediate forms. The ecological parameters associated with phenotypic divergence along elevation gradients are partly related to species' ecological and evolutionary constraints. Fossorial and solitary animals are mainly affected by climatic factors, while terrestrial and more gregarious species are influenced by biotic and abiotic parameters. A novel finding of our study is that predator richness emerged as an important factor associated with the intraspecific diversification of the mammalian skull along elevational gradients, a previously overlooked parameter. Population genetic structure was mainly driven by environmental heterogeneity along mountain slopes, with no or a week spatial effect, fitting the isolation‐by‐environment scenario. Our study highlights the strong and multifaceted effects of heterogeneous steep habitats and ecologically based divergent selective forces in small mammal populations.

Development of three-dimensional facial approximation system using head CT scans of Japanese living individuals

Abstract We developed software for three-dimensional (3D) facial approximation from the skull based on computed tomography (CT) scans of the head obtained from 59 Japanese adult volunteers (40 males, 19 females). CT scanning was performed while participants raised their head in an upright position while in a prone body posture. The positional relationships between the skull and head surface shape were analyzed by creating anatomically homologous shape models. Before modeling, skull shapes were simplified by concealing hollow structures of the skull using in-house software. Surficial tissue thickness (STT), represented by the distances between corresponding vertices of the simplified skull and head surface, was calculated for each individual, and averaged for each sex. Although the approximate head shapes of known individuals showed a relatively good resemblance in both the shape of the whole head and facial parts, some errors were identified, particularly in areas with thick surficial tissue. The results revealed sexual dimorphism of STT, with males exhibiting thinner surficial tissue at the cheek, and thicker tissue at the glabella, nose, mouth, and chin. Moreover, we created referential models for craniofacial superimposition from average models of the skull and head surface shape for each sex.

Exploring the effects of character construction and choice, outgroups and analytical method on phylogenetic inference from discrete characters in extant crocodilians

Abstract Phylogenies for fossil taxa must be inferred from morphology, but accuracy of inference is questionable. Here, morphological characters for extant crocodilians are investigated to assess how to improve inference accuracy. The homoplasy of characters is assessed against a DNA-based phylogenetic tree. Cranial characters are significantly less homoplastic, but this result is perhaps confounded by research effort. Meristic characters are significantly more homoplastic and should be used with caution. Characters were reassessed first hand and documented. Those characters passing tests of robust construction are significantly less homoplastic. Suggestions are made for means to improve coding of discrete characters. Phylogenies inferred using only robust characters and a reassessed matrix, including corrected scorings, were not overall closer to the DNA tree, but did often place the gharial (Gavialis) in a position agreeing with or closer to it. The effects of the choice of analytical method were modest, but Bayesian analysis of the reassessed matrix placed Gavialis and Mecistops (slender-snouted crocodile) in DNA-concordant positions. Use of extant rather than extinct outgroups, even with the original matrix, placed Gavialis in a more DNA-concordant position, as did factoring out 3D skull shape. The morphological case for placement of Gavialis outside other extant crocodilians is arguably overstated, with many characters linked to skull shape.

The multifactorial nature of beak and skull shape evolution in parrots and cockatoos (Psittaciformes)

BackgroundThe Psittaciformes (parrots and cockatoos) are characterised by their large beaks, and are renowned for their ability to produce high bite forces. These birds also possess a suite of modifications to their cranial architecture interpreted to be adaptations for feeding on mechanically resistant foods, yet the relationship between cranial morphology and diet has never been explicitly tested. Here, we provide a three-dimensional geometric morphometric analysis of the developmental and biomechanical factors that may be influencing the evolution of psittaciformes’ distinctive cranial morphologies.ResultsContrary to our own predictions, we find that dietary preferences for more- or less- mechanically resistant foods have very little influence on beak and skull shape, and that diet predicts only 2.4% of the shape variation in psittaciform beaks and skulls. Conversely, evolutionary allometry and integration together predict almost half the observed shape variation, with phylogeny remaining an important factor in shape identity throughout our analyses, particularly in separating cockatoos (Cacatuoidea) from the true parrots (Psittacoidea).ConclusionsOur results are similar to recent findings about the evolutionary trajectories of skull and beak shape in other avian families. We therefore propose that allometry and integration are important factors causing canalization of the avian head, and while diet clearly has an influence on beak shape between families, this may not be as important at driving evolvability within families as is commonly assumed.

Signatures of echolocation and dietary ecology in the adaptive evolution of skull shape in bats

Morphological diversity may arise rapidly as a result of adaptation to novel ecological opportunities, but early bursts of trait evolution are rarely observed. Rather, models of discrete shifts between adaptive zones may better explain macroevolutionary dynamics across radiations. To investigate which of these processes underlie exceptional levels of morphological diversity during ecological diversification, we use modern phylogenetic tools and 3D geometric morphometric datasets to examine adaptive zone shifts in bat skull shape. Here we report that, while disparity was established early, bat skull evolution is best described by multiple adaptive zone shifts. Shifts are partially decoupled between the cranium and mandible, with cranial evolution more strongly driven by echolocation than diet. Phyllostomidae, a trophic adaptive radiation, exhibits more adaptive zone shifts than all other families combined. This pattern was potentially driven by ecological opportunity and facilitated by a shift to intermediate cranial shapes compared to oral-emitters and other nasal emitters.

Acherontiscus caledoniae: the earliest heterodont and durophagous tetrapod.

The enigmatic tetrapod Acherontiscus caledoniae from the Pendleian stage of the Early Carboniferous shows heterodontous and durophagous teeth, representing the earliest known examples of significant adaptations in tetrapod dental morphology. Tetrapods of the Late Devonian and Early Carboniferous (Mississippian), now known in some depth, are generally conservative in their dentition and body morphologies. Their teeth are simple and uniform, being cone-like and sometimes recurved at the tip. Modifications such as keels occur for the first time in Early Carboniferous Tournaisian tetrapods. Acherontiscus, dated as from the Pendleian stage, is notable for being very small with a skull length of about 15 mm, having an elongate vertebral column and being limbless. Cladistic analysis places it close to the Early Carboniferous adelospondyls, aïstopods and colosteids and supports the hypothesis of ‘lepospondyl’ polyphyly. Heterodonty is associated with a varied diet in tetrapods, while durophagy suggests a diet that includes hard tissue such as chitin or shells. The mid-Carboniferous saw a significant increase in morphological innovation among tetrapods, with an expanded diversity of body forms, skull shapes and dentitions appearing for the first time.

Temporomandibular Joint Condyle–Disc Morphometric Sexual Dimorphisms Independent of Skull Scaling

Approximately 2 to 4% of the US population have been estimated to seek treatment for temporomandibular symptoms, predominately women. The study purpose was to determine whether sex-specific differences in temporomandibular morphometry result from scaling with sex differences in skull size and shape or intrinsic sex-specific differences. A total of 22 (11 male [aged 74.5±9.1years]; 11 female [aged 73.6±12.8years]) human cadaveric heads with no history of temporomandibular disc derangement underwent cone beam computed tomography and high-resolution magnetic resonance imaging scanning to determine 3-dimensional cephalometric parameters and temporomandibular morphometric outcomes. Regression models between morphometric outcomes and cephalometric parameters were developed, and intrinsic sex-specific differences in temporomandibular morphometry normalized by cephalometric parameters were determined. Subject-specific finite element (FE) models of the extreme male and extreme female conditions were developed to predict variations in articular disc stress-strain under the same joint loading. In some cases, sex differences in temporomandibular morphometric parameters could be explained by linear scaling with skull size and shape; however, scaling alone could not fully account for some differences between sexes, indicating intrinsic sex-specific differences. The intrinsic sex-specific differences in temporomandibular morphometry included an increased condylar medial length and mediolateral disc lengths in men and a longer anteroposterior disc length in women. Considering the extreme male and female temporomandibular morphometry observed in the present study, subject-specific FE models resulted in sex differences, with the extreme male joint having a broadly distributed stress field and peak stress of 5.28MPa. The extreme female joint had a concentrated stress field and peak stress of 7.37MPa. Intrinsic sex-specific differences independent of scaling with donor skull size were identified in temporomandibular morphometry. Understanding intrinsic sex-specific morphometric differences is critical to determining the temporomandibular biomechanics given the effect of anatomy on joint contact mechanics and stress-strain distributions and requires further study as one potential factor for the increased predisposition of women to temporomandibular disc derangement.

Skull shape and the demands of feeding: a biomechanical study of peccaries (Mammalia, Cetartiodactyla)

A primary requirement of the mammalian skull is to exert forces on different foods and to resist the forces imposed on it during feeding. Skull shape patterns within and among mammals are generally well known, but the biomechanical relevance of this variation remains limited for some groups. By integrating geometric morphometric and biomechanical analyses, we test the hypothesis that skull shape in peccaries reflects biomechanical attributes to generate and dissipate powerful forces, presumably in response to tough foods. We obtained skull shape and size from 213 specimens of the three living peccary species and estimated bite force, bite stress at molars, bending and shear stress on the mandibular corpus, and condylar stress. We found larger estimated bite forces, greater resistance to bending loads, and lower stress emerging from the larger muscle attachment areas and shorter and deeper mandibular corpora for both Pecari tajacu and Tayassu pecari relative to Parachoerus wagneri. Peccaries (P. tajacu and T. pecari) with more powerful biomechanical attributes feed mainly on tougher foods (e.g., palm fruits). These results support the hypothesis that species eating tough foods tend to have a feeding morphology mechanically adapted to stronger bites and greater biting resistance, which must be closely reflected in their craniomandibular shape.