Mandibular movements of the albino rat during feeding

Jaw movements of albino rats during biting and mastication of relatively hard food were recorded by means of conventional and X-ray cinematography. Mandibular kinetics have been analysed in the context of passive mechanical limits imposed by jaw morphology, particularly of the joints, and by the food itself. Movements have been described in terms of degrees of gape, condylar translation and horizontal rotation of the rami about the symphysis. During biting the condyle remains in the anterior two-thirds of the fossa, moves forward as the jaw opens and the converse. The rami usually spread well apart; the lower incisors are usually approximated. Incised food particles are transported toward the molars by means of coordinated jaw and tongue movements. The prominent palatal rugae of the diastemal region abet this process. In the power stroke of mastication, the mandible shifts forward as the lower toothrows move a little inward; the condyles occupy the posterior two-thirds of the fossa. All movements seen were bilaterally symmetrical. Simultaneous chewing occurred on both sides. It is suggested that the lingual components in the primarily anterior power stroke enhance grinding efficiency. A movable symphysis appears to be of critical importance in facilitating this type of mastication.     

A cineradiographic and electromyographic study of mastication in Tenrec ecaudatus

Regular chewing was studied in the specialized Malagasy insectivore Tenrec ecaudatus with the aid of precisely correlated electromyography of the main adductors, digastrics, and two hyoid muscles and cineradiography for which metallic markers were placed in the mandibles, tongue, and hyoid bone. During the power stroke the body of the mandible moves dorsally and medially. The medially directed component of movement at this time is greatly increased by simultaneous rotation of the mandible about its longitudinal axis. The highly mobile symphysis, spherical dentary condyle, loss of superficial masseter muscle and zygoma, and the simplified zalamnodont molars all appear to be related to the large amount of mandibular rotation that occurs during occlusion. The balancing side lateral pterygoid muscle (inferior head) apparently shifts the working side mandible laterally during the last part of opening and the first part of closing. The working side temporalis and the superficial masseter muscle are both responsible for the shift back to the midline. The temporalis is usually active to the same extent on the working and balancing sides during the power stroke. The level of activity (amplitude) of the temporalis and duration of the power stroke increase with harder foods. Whenever soft foods are chewed, the superficial masseter is only active on the working side; whenever foods of increasing hardness are chewed, its level of activity on the balancing side increases to approach that of the working side. Mandibular rotation is greatly reduced when hard foods are chewed.     

The instantaneous center of rotation during human jaw opening and its significance in interpreting the functional meaning of condylar translation

Mandibular condyles translate back and forth during mouth closing and opening in primates and most other mammals. To account for the functional significance of this phenomenon, several hypotheses have been proposed. The sarcomere-length hypothesis holds that condylar translation provides a mechanical advantage by minimizing sarcomere-length changes in the masseter-medial pterygoid complex throughout a wide range of jaw openings. As the hypothesis is inherently associated with the locations of the instantaneous centers of rotation (ICRs) of the mandible, a more accurate determination of this variable would help test this hypothesis. This study investigated ICRs in the sagittal plane during human symmetrical mandibular opening based on a recently developed analytical method. The results confirmed that, with inter- and intraindividual variation, the natural opening was a simultaneous rotational and translational motion. In addition, the ICR was found to lie closer to the condyle during the first 10° than during the rest of the rotation. This suggests that for the condyles the rotational component is somewhat more significant at the early phase than at the late phase of the opening stroke. For the whole range of the natural opening, the grossly approximated centers of rotation (CRs) scattered below the palpable lateral condylar poles in the superior half of the ramus. This study supports neither the ICR path determined by Grant ([1973], J. Biomech. 6:109–113) nor the conclusions reached by recording manually operated jaw movements in human cadavers (Rees [1954] Br. Dent. J. 6:125–133). Moss's suggestion ([1960] Disorders of the Temporomandibular Joint (Philadelphia: W.B. Saunders), pp. 73–88) that the center of rotation lies at the lingula is also not confirmed. Although the new data cannot reject the sarcomere-length hypothesis, they do not strongly support it either. Another hypothesis is proposed in this study as plausible. With this hypothesis, translation is regarded as an adaptation to the use of the inferior head of the lateral pterygoid as a jaw depressor in noncarnivorous mammals. Potential functional advantages of this portion of the muscle are also discussed. Am J Phys Anthropol 106:35–46, 1998. © 1998 Wiley-Liss, Inc.     

Why fuse the mandibular symphysis? A comparative analysis

Fused symphyses, which evolved independently in several mammalian taxa, including anthropoids, are stiffer and stronger than unfused symphyses. This paper tests the hypothesis that orientations of tooth movements during occlusion are the primary basis for variations in symphyseal fusion. Mammals whose teeth have primarily dorsally oriented occlusal trajectories and/or rotate their mandibles during occlusion will not benefit from symphyseal fusion because it prevents independent mandibular movements and because unfused symphyses transfer dorsally oriented forces with equal efficiency; mammals with predominantly transverse power strokes are predicted to benefit from symphyseal fusion or greatly restricted mediolateral movement at the symphysis in order to increase force transfer efficiency across the symphysis in the transverse plane. These hypotheses are tested with comparative data on symphyseal and occlusal morphology in several mammals, and with kinematic and EMG analyses of mastication in opossums (Didelphis virginiana) and goats (Capra hircus) that are compared with published data on chewing in primates. Among mammals, symphyseal fusion or a morphology that greatly restricts movement correlates significantly with occlusal orientation: species with more transversely oriented occlusal planes tend to have fused symphyses. The ratio of working- to balancing-side adductor muscle force in goats and opossums is close to 1:1, as in macaques, but goats and opossums have mandibles that rotate independently during occlusion, and have predominantly vertically oriented tooth movements during the power stroke. Symphyseal fusion is therefore most likely an adaptation for increasing the efficiency of transfer of transversely oriented occlusal forces in mammals whose mandibles do not rotate independently during the power stroke.     

Kinematic analysis of jaw protrusion in orectolobiform sharks: A new mechanism for jaw protrusion in elasmobranchs

Jaw protrusion is an important component of prey capture in fishes, although the mechanics of protrusion have thus far been studied largely in teleosts. Elasmobranchs are also able to protrude their jaws (Tricas and McCosker [1984] Proc. Cal. Acad. Sci. 43: 221–238; Tricas [1985] Mem. S. Calif. Acad. Sci. 8:81–91.; Frazzetta and Prange [1987] Copeia 4:979–993). Several related features of the feeding apparatus contribute to jaw protrusion in sharks. Labial cartilages form an extendible series attached dorsally to the anterolateral face of the palatoquadrate and ventrally to the anteroventral surface of Meckel's cartilage. The labial cartilage chain swings anterolaterally as the lower jaw is depressed, thrusting the labial margins forward to form a circular oral opening and displacing the jaw apparatus towards the food; this pattern is analogous to halecomorph and primitive actinopterygian fishes in which the maxilla swings forward (Lauder [1979] J. Zool. Lond. 187:543–578). The palatoquadrate and Meckel's cartilage also project anteriorly and represent the major contribution to protrusion. These movements occur simultaneously with enlargement of the oral cavity to generate suction. The wobbegong sharks (Orectolobidae) are specialized for jaw protrusion. The spotted wobbegong protrudes its jaw by 33% of its chondrocranial length using two different mechanical systems. In the first mechanism of jaw protrusion, the intermandibularis and interhyoideus muscles medially compress the lower jaw and hyomandibulae. Compression of the lower jaw results in a more acute symphyseal angle so that the anteroposterior alignment of the lower jaw increases due to the rotation of each lower jaw towards a saggital orientation. Distal compression of the hyomandibulae at their attachments to the jaws swings the jaws forward. The second mechanism involves rotation of the ceratohyal around a posterior process of the lower jaw, pushing the hyomandibulae anteroventrally, thereby pushing the jaw articulation ventrally and anteriorly to protrude the jaws. © 1994 Wiley-Liss, Inc.     

A model relating patterns of human jaw movement to biomechanical constraints

After testing the effects of the ways in which several different ligaments and bony constraints would influence movements of the human mandible in three dimensions, a mathematical model based on constraints due to the articular eminences, temporomandibular ligaments and sphenomandibular ligaments has been constructed. The effects of these constraints on jaw movements during opening and lateral movements are analysed. The model predicts the observed translation of the human condyle during jaw opening and Bennett shift during lateral jaw movements. The model is refined to account for observed irregular movements of the condyle during opening and to predict a locus for the instantaneous centre of rotation. The model can also be used to predict the new position taken up by any point on the mandible after the jaw has been opened and/or moved laterally a given amount.     

Functional significance of the coupling between head and jaw movements

When humans open or close the jaw they also move the head. Unintentionally, it rotates backwards when the jaw opens and returns upon jaw closure. We hypothesized that this mutual movement coupling is related to the muscles in the floor of the mouth. A biomechanical model was applied to comprehend the functional significance of this movement coupling. As the jaw opened the jaw opening muscles shortened and became less forceful. Meanwhile they had to stretch the jaw closing muscles. The simulations showed that a simultaneous head extension facilitated jaw opening. A possible functional significance for the coupling between head and jaw movements is that it can extend jaw gape. Head extension can contribute to a wider jaw gape by on the one hand a reduced shortening of the jaw opening muscles and on the other hand by a reorientation of these muscles so that they obtain a more favorable position for jaw opening.     

Unusual kinematics and jaw morphology associated with piscivory in the poeciliid,Belonesox belizanus

Piscivory in fishes is often associated with the evolution of highly elongate jaws that achieve a large mouth opening, or gape. Belonesox belizanus, the pike killifish, has independently evolved this morphology, which is derived from short-jawed poeciliids within the Cyprinodontiformes. Using kinematic analysis of high-speed video footage, we observed a novel aspect of the elongate jaws of Belonesox; the premaxilla rotates dorsally during mouth opening, while the lower jaw rotates ventrally. Anatomical study revealed that this unusual motion is facilitated by the architecture of the premaxillomandibular ligament, prominent within cyprinodontiforms. In Belonesox, it allows force to be transferred from the lower jaw directly to the premaxilla, thereby causing it to rotate dorsally. This dorsal rotation of the premaxilla appears to be assisted by a mediolateral twisting of the maxilla during jaw opening. Twisting maxillae are found in members of the group such as Fundulus, but are lost in Gambusia. Models revealed that elongate jaws partially account for the enlarged gape, but enhanced rotation at the quadrato-mandibular joint was equally important. The large gape is therefore created by: (i) the convergent evolution of elongate jaws; (ii) enhanced jaw rotation, facilitated by loss of a characteristic cyprinodontiform trait, the lip membrane; and (iii) premaxilla rotation in a novel direction, facilitated by the retention and co-option of additional cyprinodontiform traits, the premaxillomandibular ligament and a twisting maxilla.     

Transverse masticatory movements, occlusal orientation, and symphyseal fusion in selenodont artiodactyls

Based on extensive experimental work on primates, two masticatory loading regimes have emerged as the likely determinants of mandibular symphyseal fusion-dorsoventral shear and lateral transverse bending (wishboning) (Ravosa and Hylander, 1994; Hylander et al., 1998, 2000). Recently, however, it has been argued that, rather than functioning to strengthen the symphysis during mastication, fusion serves to stiffen the symphyseal joint so as to facilitate increased transverse jaw movements during occlusion (Lieberman and Crompton, 2000). As part of this transverse stiffness model, it has been suggested that taxa with fused symphyses should also exhibit more horizontally oriented occlusal wear facets. Using a series of univariate and bivariate analyses, we test predictions of these three models in a sample of 44 species of selenodont artiodactyls. Consistent with the wishboning and transverse stiffness models, taxa with fused symphyses (camelids) have more horizontally oriented M(2) and M(2) occlusal wear facets, anteroposteriorly (AP) elongate symphyses, and relatively wider corpora. Contrary to the dorsoventral shear model, camelids do not have relatively deeper corpora (due to greater parasagittal bending). While taxa with ossified symphyses have relatively larger symphysis cross-sectional areas, this appears to be the byproduct of an increase in AP symphysis length due to greater lateral transverse bending of the mandible. Theoretical consideration of the biomechanics of mastication further suggests that strength, not stiffness, is the critical factor in determining symphyseal ossification. Thus, like anthropoid primates, fusion in selenodont artiodactyls appears to function in resisting increased wishboning stresses arising from an emphasis on transverse occlusal/mandibular movements and loads.     

Condylar translation and the function of the superficial masseter muscle in the rhesus monkey (M. mulatta).

The relationship between translation of the mandibular condyle during symmetrical mandibular rotation, i.e., symmetrical jaw depression and elevation, and the function of the superficial masseter muscle was examined in light of relative torque and the length-tension relationship for muscle. Lateral cephalograms of live adult rhesus monkeys (Macaca mulatta) were analyzed using two models: (1) Model A, normal symmetrical jaw rotation accompanied by condylar translation; and (2) Model B, mandibular rotation about an axis fixed at the position of the condyles during centric occlusion. The decrease in relative torque and the excursion of the superficial masseter at mouth-open positions are significantly greater in Model B than in Model A. Symmetrical rotation of the jaw about a fixed axis would result in a 35% greater loss of maximum producible tension at maximum gape than rotation associated with condylar translation. These results suggest that condylar translation during mandibular depression and elevation functions to minimize reduction in relative torque and excursion of superficial masseter muscle, thereby maintaining optimal potential for exerting maximum tension during jaw closure.     

A dynamic model of jaw and hyoid biomechanics during chewing

Our understanding of human jaw biomechanics has been enhanced by computational modelling, but comparatively few studies have addressed the dynamics of chewing. Consequently, ambiguities remain regarding predicted jaw-gapes and forces on the mandibular condyles. Here, we used a new platform to simulate unilateral chewing. The model, based on a previous study, included curvilinear articular guidance, a mobile hyoid apparatus, and a compressible food bolus. Muscles were represented by Hill-type actuators with drive profiles tuned to produce target jaw and hyoid movements. The cycle duration was 732 ms. At maximum gape, the lower incisor-point was 20.1mm down, 5.8mm posterior, and 2.3mm lateral to its initial, tooth-contact position. Its maximum laterodeviation to the working-side during closing was 6.1mm, at which time the bolus was struck. The hyoid's movement, completed by the end of jaw-opening, was 3.4mm upward and 1.6mm forward. The mandibular condyles moved asymmetrically. Their compressive loads were low during opening, slightly higher on the working-side at bolus-collapse, and highest bilaterally when the teeth contacted. The model's movements and the directions of its condylar forces were consistent with experimental observations, resolving seeming discordances in previous simulations. Its inclusion of hyoid dynamics is a step towards modelling mastication.     

Mandibular form and function in North American and European Adapidae and Omomyidae

Previous experimental and comparative studies among a wide variety of primate and nonprimate mammals provide a unique source of information for investigating the functional and phylogenetic significance of variation in the masticatory apparatus of Eocene primates. To provide a quantitative study of mandibular form and function in Eocene primates, the scaling of jaw dimensions and the development of symphyseal fusion was considered in a broad sample of North American and European Adapidae and Omomyidae. Statistical analyses indicate a significant size-related pattern of symphyseal fusion across Eocene primates, with larger taxa often having a greater degree of fusion than smaller species; this trend is also evident at the family level. As adapids are mostly larger than omomyids and these taxa show allometry of symphyseal fusion, this may explain why no omomyids evince complete fusion. Controlling for jaw size, species with greater symphyseal fusion tend to have more robust jaws than those with a lesser amount of fusion. Upon further examination, a primary reason why adapids have more robust mandibles than omomyids is associated with the presence of taxa with fused symphyses, and thus more robust jaws, in the adapid sample, whereas no omomyids have fused symphyses. In addition, there is little indication of a dietary effect, as measured by molar shear-crest development, on symphyseal fusion. Moreover, as there is no correlation between molar shear-crest development and skull size, this also points to the absence of a size-related pattern of dietary preference underlying the allometry of symphyseal fusion. Based on the interspecific and ontogenetic allometry of symphyseal ossification in Eocene primates, jaw-scaling patterns are used to further examine the functional determinants of fusion in this group. This study indicates that greater dorsoventral shear during mastication is a more likely factor than lateral transverse bending (“wishboning”) in the evolution of symphyseal fusion among “late-fusing” mammals like adapids and omomyids. Given that wishboning is an important functional determinant of symphyseal form in recent anthropoids, apparently the evolutionary development of marked wishboning occurs only in taxa that shift the timing of fusion to a growth stage preceding the onset of weaning (before adult masticatory patterns are fully developed) and perhaps first ossified the symphysis to counter elevated dorsoventral shear stress. As early anthropoids probably consisted of members varying interspecifically and ontogenetically in the degree of ossification, it is especially informative to analyze the adaptive setting in which anthropoid symphyseal fusion evolved from a similar primitive “prosimian” perspective. © 1996 Wiley-Liss, Inc.     

Do Changes in Morphology and Prey-Capture Movements Facilitate a Dietary Transition in Juvenile Colorado pikeminnow, Ptychocheilus lucius?

Ontogenetic diet shifts in juvenile fishes are sometimes associated with proportional changes to the feeding mechanism. In addition, many piscivorous teleosts transition from invertebrate-prey to fish-prey when the mouth attains a specific diameter. Allometric (disproportionate) growth of the jaws could accelerate a young fish’s ability to reach a critical gape diameter; alternately by opening the lower jaw to a greater degree, a fish might increase gape behaviorally. We investigated the ontogeny of feeding morphology and kinematics in an imperiled piscivore, the Colorado pikeminnow (Ptychocheilus lucius) in a size range of individuals across which a diet shift from invertebrate-prey to prey-fishes is known to occur. We predicted that: (1) the feeding apparatus of the fish would grow proportionally with the rest of the body (isometric growth), that (2) anatomical gape diameter at the known diet transition would be a similar gape diameter to that observed for other piscivorous juvenile fishes (15–20 mm) and (3) feeding kinematic variables would scale isometrically (that is, change in direct proportion to body length) as juvenile pikeminnow became larger. Furthermore, we also asked the question: if changes in feeding morphology and kinematics are present, do the changes in morphology appear to generate the observed changes in kinematics? For juvenile Colorado pikeminnow, the majority of the morphological variables associated with the skull and jaws scale isometrically (that is, proportionally), but seven of eight kinematic variables, including functional gape, scale with negative allometry (that is, they became disproportionately smaller in magnitude). In contrast with the overall trend of isometry, two key aspects of feeding morphology do change with size; the lower jaw of a young Colorado pikeminnow becomes longer (positive allometry), while the head becomes shallower (negative allometry). These findings do not support the hypothesis that morphological ontogenetic changes directly generate changes in feeding kinematics; in fact, allometric jaw growth would, a priori, be expected to generate a larger gape in older fish—which is the opposite of what was observed. We conclude that ontogenetic morphological changes produce a more streamlined cranium that may reduce drag during a rapid, anteriorly directed strike, while concomitant behavioral changes reduce the magnitude of jaw movements—behavioral changes that will facilitate a very rapid opening and closing of the jaws during the gape cycle. Thus, for juvenile pikeminnow, speed and stealth appear to be more important than mouth gape during prey capture.     

Evolution of the mandibular symphysis in Notharctinae (Adapidae,Primates)

The Eocene Notharctinae provide a record of increasing fusion of the mandibular symphysis. The two sympatric genera,Notharctus andSmilodectes, differed through time in two respects.Notharctus increased in body size and evolved a partially fused mandibular symphysis.Smilodectes changed little in body size and retained an unfused symphysis. Similarities in molar morphology between these two genera and extant leaf-eating mammals suggest thatNotharctus andSmilodectes were specialized for folivory, a dietary regime correlated with partial symphyseal fusion in many extant mammals. It is concluded that the presence and the extant of symphyseal fusion is a function of body size, diet, and jaw mechanics, complicated by lineagespecific factors that vary among higher mammalian taxa.     

Three-dimensional analysis of mandibular morphology in Otolemur

Euclidean distance matrix analysis (EDMA) of three-dimensional data is used here to examine mandibular morphology between two species of galagos. Otolemur crassicaudatus consumes primarily exudates, while O. garnettii is more frugivorous. Acquisition of exudates involves either gouging or scraping tree bark, and may involve different forces at the mandible than incising fruits. Previous studies of mandibular morphology in exudate-feeding primates produced mixed results, some suggesting that morphological specializations reflect adaptations for greater force at the anterior dentition, while others suggest specializations for producing a large gape. This study addresses these controversies by testing predictions associated with O. crassicaudatus generating greater force at the anterior dentition or producing a larger gape relative to O. garnettii. In addition, this study tests predictions associated with specializations of the anterior dentition in O. crassicaudatus as related to exudate-feeding. Crania and mandibles from 28 O. crassicaudatus and 17 O. garnettii were digitized in three dimensions, using 18 landmarks that summarize the shape of the jaw. Two-dimensional measurements were taken to assess incisor robusticity. All three-dimensional data were analyzed using EDMA, and bootstrap tests were executed to identify specific interlandmark differences that were driving any significant (P < 0.05) overall shape differences. Two-dimensional data were analyzed using Student's t-test for independent measures. Results revealed that there was a significant shape difference in mandibles between species, and that mandibles of O. crassicaudatus showed higher condyles, longer mandibles, decreased incisor procumbency, and greater incisor robusticity relative to O. garnettii. It is suggested that the results of the present study reflect adaptations for scraping in O. crassicaudatus rather than gouging.     

Functional morphology of prey capture in the sturgeon,Scaphirhynchus albus

Acipenseriformes (sturgeon and paddlefish) are basal actinopterygians with a highly derived cranial morphology that is characterized by an anatomical independence of the jaws from the neurocranium. We examined the morphological and kinematic basis of prey capture in the Acipenseriform fish Scaphirhynchus albus, the pallid sturgeon. Feeding pallid sturgeon were filmed in lateral and ventral views and movement of cranial elements was measured from video sequences. Sturgeon feed by creating an anterior to posterior wave of cranial expansion resulting in prey movement through the mouth. The kinematics of S. albus resemble those of other aquatic vertebrates: maximum hyoid depression follows maximum gape by an average of 15 ms and maximum opercular abduction follows maximum hyoid depression by an average of 57 ms. Neurocranial rotation was not a part of prey capture kinematics in S. albus, but was observed in another sturgeon species, Acipenser medirostris. Acipenseriformes have a novel jaw protrusion mechanism, which converts rostral rotation of the hyomandibula into ventral protrusion of the jaw joint. The relationship between jaw protrusion and jaw opening in sturgeon typically resembles that of elasmobranchs, with peak upper jaw protrusion occurring after peak gape.     

Functional significance of intramandibular bending in Poeciliid fishes

Substrate-feeding teleosts show multiple, independent evolutionary acquisitions of intramandibular bending (bending within the lower jaw)—a behavior that likely enhances performance when feeding on attached or encrusting food items. However, intramandibular bending has only been quantified for marine teleosts. Here, we examine substrate feeding in eight species from the order Cyprinodontiformes and quantify movements produced by the anterior jaws of four target species selected from the family Poeciliidae to represent a variety of trophic strategies. Intramandibular bending, defined here as bending between the dentary and angular–articular bones of the lower jaw, is not present in some poeciliids (i.e. Gambusia affinis), nor is it present in outgroup cyprinodontiforms (i.e. Fundulus rubrifrons). However, intramandibular bending is present in certain poeciliids (i.e. Poecilia sphenops), and can exceed 90°. Such jaw bending enables the production of a gape angle that approaches 120°, which likely allows the fish to maximize contact between the toothed tips of the jaws and the substrate during the bite. Intramandibular bending in poeciliid species is associated with specific trophic shifts: the greater the intramandibular bending in a given species, the more attached algae (periphyton) reported in its diet. This result supports the hypothesis that intramandibular bending enhances performance when feeding on encrusting food items. We predict that additional examples of functional convergence are likely to be documented in freshwater teleosts as more herbivorous species are examined, and we propose that intramandibular bending represents an excellent model system in which to examine the functional processes that underlie convergent evolution. An erratum to this article can be found at     

Modeling the jaw mechanism of Pleuronichthys verticalis: The morphological basis of asymmetrical jaw movements in a flatfish

Several flatfish species exhibit the unusual feature of bilateral asymmetry in prey capture kinematics. One species, Pleuronichthys verticalis, produces lateral flexion of the jaws during prey capture. This raises two questions: 1) How are asymmetrical movements generated, and 2) How could this unusual jaw mechanism have evolved? In this study, specimens were dissected to determine which cephalic structures might produce asymmetrical jaw movements, hypotheses were formulated about the specific function of these structures, physical models were built to test these hypotheses, and models were compared with prey capture kinematics to assess their accuracy. The results suggest that when the neurocranium rotates dorsally the premaxillae slide off the smooth, rounded surface of the vomer (which is angled toward the blind, or eyeless, side) and are “launched” anteriorly and laterally. The bilaterally asymmetrical trajectory of the upper jaw is determined by the orientation of the “launch pad,” the vomer. During lower jaw depression, the mandibles rotate about their articulations with the quadrate bones of the suspensoria. The quadrato‐mandibular joint is positioned farther anteriorly on the eye side than on the blind side, and this asymmetry deflects the lower jaw toward the blind side. Asymmetry in the articular surfaces of the lower jaw augments this effect. Thus, it appears that fish with intermediate forms of this asymmetrical movement could have evolved from symmetrical ancestors via a few key morphological changes. In addition, similar morphological modifications have been observed in other fish taxa that also produce jaw flexion during feeding, which suggests that there may be convergence in the basic mechanism of asymmetry. J. Morphol. 256:1–12, 2003. © 2003 Wiley‐Liss, Inc.     

The functional morphology of the anterior masticatory apparatus in tree‐gouging marmosets (cebidae,primates)

Although all genera of Callitrichinae feed on tree exudates, marmosets (Callithrix and Cebuella) use specialized anterior teeth to gouge holes in trees and actively stimulate exudate flow. Behavioral studies demonstrate that marmosets use large jaw gapes but do not appear to generate large bite forces (relative to maximal ability) during gouging. Nonetheless, the anterior teeth of marmosets likely experience different loads during gouging compared to nongouging platyrrhines. We use histological data from sectioned teeth, μCTs of jaws and teeth, and in vitro tests of symphyseal strength to compare the anterior masticatory apparatus in Callithrix to nongouging tamarins (Saguinus) and other cebids. We test the hypotheses that (1) marmoset anterior teeth are adapted to accommodate relatively high stresses linked to dissipating gouging forces and (2) the mandibular symphysis does not provide increased load resistance ability compared with closely related nongouging platyrrhines. Differences in decussation between Callithrix and Saguinus are greatest in the anterior teeth, suggesting an increased load resistance ability specifically in incisor and canine enamel of Callithrix. Callithrix lower incisor crowns are labiolingually thicker suggesting increased bending resistance in this plane and improved wedging ability compared with Saguinus. Anterior tooth roots are larger relative to symphyseal bone volume in Callithrix. Anterior tooth root surface areas also are larger in marmosets for their symphyseal volume, but it remains unclear whether this relative increase is an adaptation for dissipating dental stresses versus a growth‐related byproduct of relatively elongated incisors. Finally, simulated jaw loading suggests a reduced ability to withstand external forces in the Callithrix symphysis. The contrast between increased load resistance ability in the anterior dentition versus relatively reduced symphyseal strength (1) suggests a complex loading environment during gouging, (2) highlights the possibility of distinct loading patterns in the anterior teeth versus the symphysis, and (3) points to a potential mosaic pattern of dentofacial adaptations to tree gouging. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Diet and Mandibular Morphology in African Apes

Investigations seeking to understand the relationship between mandibular form, function, and dietary behavior have focused on the mandibular corpus and symphysis. African apes vary along a gradient of folivory/frugivory, yet few studies have evaluated the morphology of the mandibular corpus and symphysis in these taxa, and the investigations have yielded mixed results. Specifically, studies using external metrics have identified differences in mandibular proportions that analysis of cortical bone distribution has not substantiated. I contribute to the ongoing debate on the relationship between jaw form and dietary behavior by comparing mandibular corporal and symphyseal shapes in African apes. Importantly, and in contrast to previous studies of African ape internal geometry, I include the Virunga mountain gorillas (Gorilla beringei beringei), the ape most specialized toward a folivorous diet. I test the hypotheses that 1) Gorilla beringei beringei always has significantly more robust mandibular corpora and symphyses, relative to mandibular length, than all other African apes and 2) all gorillas have significantly more robust mandibular corpora and symphyses, relative to mandibular length, than Pan. Results demonstrate that the folivorous mountain gorillas consistently exhibit a relatively wider mandibular symphysis and corpus than all other African apes. Furthermore, all gorillas consistently exhibit relatively more robust mandibular corporal and symphyseal dimensions than Pan. The results indicate that among African apes, mountain gorillas are better able to counter lateral transverse bending (wishboning) loads at the symphysis and torsional loads at the corpus. All gorillas are likewise better able to resist wishboning and vertical bending at the symphysis, and sagittal bending and torsion at the corpus, than Pan, findings that are consistent with masticating relatively tougher foods, repetitive loading of the jaws, or both. I offer possible explanations for the lack of concordance in results between studies that have analyzed the biomechanical properties of African ape mandibles and others that have relied on external metrics. More comprehensive study of the internal geometry of the mandible is needed to resolve whether African apes differ morphologically in ways predicted by dietary variation.