Greater Bite Force Research Articles

Descriptive anatomy and three-dimensional reconstruction of the skull of the tetrapod Eoherpeton watsoni Panchen, 1975 from the Carboniferous of Scotland

ABSTRACT The early tetrapod Eoherpeton watsoni is known from the mid- to late Carboniferous (late Viséan to Namurian, approximately 346–313 Ma) of Scotland. The holotype is made up of a nearly complete but crushed skull with postcranial fragments. The skull anatomy of Eoherpeton was first described over 40 years ago; however, many details are obscured due to deformation of the specimen, including internal bone surfaces, the palatal bones and dentition, and suture morphology. Most phylogenetic analyses place Eoherpeton as an embolomere/reptilomorph on the lineage leading to amniotes, making it a key taxon for understanding anatomical changes during the fish-tetrapod transition. In this paper, we scanned the holotype using micro-computed tomography and digitally prepared the specimen. Based on these data, we present a revised description of the skull, including sutural morphology, that supplements and amends previous descriptions. New anatomical findings include the presence of a previously unknown tooth-bearing vomer, additional information on the shape of the basipterygoid processes and jaw joint, the ability to visualise the full extent of the pterygoid, and confirmation of the arrangement of the coronoid series. We also note the size of the pterygoid flange, which is larger than previously described for Eoherpeton. The pterygoid flange is widely considered to be characteristic of amniotes and serves as the origin of the medial pterygoideus muscle. The differentiation of the adductor muscles and appearance of medial pterygoideus are thought to have permitted a static pressure bite in amniotes, potentially resulting in greater bite forces and increased dietary range. Thus, the presence and extent of the pterygoid flange in Eoherpeton suggests this feature (and associated changes in feeding mechanism) may have evolved earlier than previously thought. Finally, the skull was digitally repaired and retrodeformed to create a new, hypothetical three-dimensional reconstruction of the skull of Eoherpeton.

Comparative cranial biomechanics reveal that Late Cretaceous tyrannosaurids exerted relatively greater bite force than in early-diverging tyrannosauroids.

Tyrannosaurus has been an exemplar organism in feeding biomechanical analyses. An adult Tyrannosaurus could exert a bone-splintering bite force, through expanded jaw muscles and a robust skull and teeth. While feeding function of adult Tyrannosaurus has been thoroughly studied, such analyses have yet to expand to other tyrannosauroids, especially early-diverging tyrannosauroids (Dilong, Proceratosaurus, and Yutyrannus). In our analysis, we broadly assessed the cranial and feeding performance of tyrannosauroids at varying body sizes. Our sample size included small (Proceratosaurus and Dilong), medium-sized (Teratophoneus), and large (Tarbosaurus, Daspletosaurus, Gorgosaurus, and Yutyrannus) tyrannosauroids, and incorporation of tyrannosaurines at different ontogenetic stages (small juvenile Tarbosaurus, Raptorex, and mid-sized juvenile Tyrannosaurus). We used jaw muscle force calculations and finite element analysis to comprehend the cranial performance of our tyrannosauroids. Scaled subtemporal fenestrae areas and calculated jaw muscle forces show that broad-skulled tyrannosaurines (Tyrannosaurus, Daspletosaurus, juvenile Tyrannosaurus, and Raptorex) exhibited higher jaw muscle forces than other similarly sized tyrannosauroids (Gorgosaurus, Yutyrannus, and Proceratosaurus). The large proceratosaurid Yutyrannus exhibited lower cranial stress than most adult tyrannosaurids. This suggests that cranial structural adaptations of large tyrannosaurids maintained adequate safety factors at greater bite force, but their robust crania did not notably decrease bone stress. Similarly, juvenile tyrannosaurines experienced greater cranial stress than similarly-sized earlier tyrannosauroids, consistent with greater adductor muscle forces in the juveniles, and with crania no more robust than in their small adult predecessors. As adult tyrannosauroid body size increased, so too did relative jaw muscle forces manifested even in juveniles of giant adults.

The Evolution of Diet and Morphology in Insular Lizards: Insights from a Replicated Island Introduction Experiment.

Resource-limited environments may drive the rapid evolution of phenotypic traits and ecological preferences optimizing the exploitation of resources. Very small islands are often characterized by reduced food availability, seasonal fluctuations in resources and strong unpredictability. These features may drive the evolution of phenotypic traits such as high bite forces, allowing animals to exploit a wider variety of the available resources. They may also lead to more generalist dietary patterns in response to food scarcity. However, the lack of predators and competitors on such small islands often also leads to high densities and the evolution of strong sexual dimorphism, which may also drive the evolution of bite force. Here, we take advantage of a unique replicated introduction experiment to test whether lizards introduced into very small islands alter their feeding ecology and use different resources, resulting in the evolution of a large body size, large head size and large bite forces. Our results show that three years after their introduction, the island lizards were larger and had greater bite forces and more pronounced sexual dimorphism. However, the diets were only marginally different between animals from the source population on a very large nearby island and those on the islets. Moreover, distinct differences in diet between animals on the different islets were observed, suggesting that the local environment is a strong driver of resource use. Overall, lizards with absolutely and relatively (adjusted for body size) large bite forces did eat larger and harder prey. Taken together, our data suggest that intraspecific competition is an important driver of the rapid evolution of bite force, which may allow these lizards to exploit the scarce and fluctuating resources on the islets. Whether or not lizards will evolve to include other types of food such as plants in their diet, facilitated by their large bite forces, remains to be explored in future studies.

Colour Variation in the Crocodile Lizard (Shinisaurus crocodilurus) and Its Relationship to Individual Quality.

Simple SummaryThis study examines colour variation in the highly endangered crocodile lizard, Shinisaurus crocodilurus. Both males and females vary in the extent to which their throats and venters are red. Their colouration is easily visible to a lizard receiver, and we found evidence that colour signals individual quality. Females with red venters had larger heads while females with red throats had greater bite force. In males, redder individuals were older. Finally, we found links between colour and fitness in males but not females. Aspects of male colouration were linked to reproductive output such that they sired offspring from heavier litters. The potential fitness consequences of colour should be considered in captive breeding and release programs.Colour plays a key role in animal social communication including as an indicator of individual quality. Using spectrophotometry, we examined colour variation in the throat and venter of the crocodile lizard (Shinisaurus crocodilurus), an endangered species native to southern China and northern Vietnam. We detected two broad colour variants, individuals with and without red, for each body region and each sex. A cluster analysis of spectral colour measurements (hue, chroma, luminance) revealed discrete throat and ventral morphs when measured in a single snapshot in time. However, photographic evidence revealed that the amount of red relative to body size increased as they got older. Individuals with red were equally likely to be male or female and throat colour was unrelated to ventral colour. Therefore, it is premature to claim that crocodile lizards have discrete colour morphs. We used visual modelling to show that the throat and venter were easily discriminable to a lizard visual system, suggesting they function in social communication. We also asked whether colour variation signalled individual quality. Females with red throats had greater bite force while males with red throats were older. In addition, females with red venters had larger heads. We also detected differences in morphology linked to colour. Females with red throats had slender bodies and longer tails, while individuals lacking red on their throats were stouter and had shorter tails. Finally, throat and ventral colour were unrelated to reproductive output (litter size and mass) in females. Males with greater ventral luminance contrast sired offspring from litters with greater litter mass (including stillborns), while males with greater ventral chromatic contrast sired offspring whose collective live mass (excluding stillborns) was greater. Males with greater luminance contrast also sired more live offspring (excluding stillborns). Collectively, these results suggest that male ventral colour signals individual quality in males. Conservation initiatives should take colour variation into account when planning future captive breeding and release programs for this endangered species.

Comparison of head size and bite force in two sympatric munia species

Brain size is a key factor determining the performance and fitness of an organism; birds with relatively larger brains might have advantages in the utilization of resources and dispersal. Bite force is another important indicator of fitness, and plays an essential role in intra- and inter-specific competition. In this study, a comparative analysis was conducted on the head size and bite force of the two sister species, the white-rumped munia ( Lonchura striata) and scaly-breasted munia ( L. punctulata) that differ in distribution range. The results showed that the scaly-breasted munia, which has a wider distribution range, had a significantly larger head size than that of the white-rumped munia. In addition, scaly-breasted munias had a greater bite force; however, the difference was not significant. One possible explanation could be that the smaller head to body ratio in scaly-breasted munias reduced their bite-force slightly. Further, this finding could be attributed to the fact that similar food resources and foraging habitats of the two species lead to a similar bite force. Our study supported the argument that head size, rather than bite force, plays a more important role in the dispersal and evolutionary adaptation of birds.

Correlation of skull morphology and bite force in a bird-eating bat (Ia io; Vespertilionidae)

BackgroundGenetic and ecological factors influence morphology, and morphology is compatible with function. The morphology and bite performance of skulls of bats show a number of characteristic feeding adaptations. The great evening bat, Ia io (Thomas, 1902), eats both insects and birds (Thabah et al. J Mammal 88: 728-735, 2007), and as such, it is considered to represent a case of dietary niche expansion from insects to birds. How the skull morphology or bite force in I. io are related to the expanded diet (that is, birds) remains unknown. We used three-dimensional (3D) geometry of the skulls and measurements of bite force and diets from I. io and 13 other species of sympatric or closely related bat species to investigate the characteristics and the correlation of skull morphology and bite force to diets.ResultsSignificant differences in skull morphology and bite force among species and diets were observed in this study. Similar to the carnivorous bats, bird-eaters (I. io) differed significantly from insectivorous bats; I. io had a larger skull size, taller crania, wider zygomatic arches, shorter but robust mandibles, and larger bite force than the insectivores. The skull morphology of bats was significantly associated with bite force whether controlling for phylogeny or not, but no significant correlations were found between diets and the skulls, or between diets and residual bite force, after controlling for phylogeny.ConclusionsThese results indicated that skull morphology was independent of diet, and phylogeny had a greater impact on skull morphology than diet in these species. The changes in skull size and morphology have led to variation in bite force, and finally different bat species feeding on different foods. In conclusion, I. io has a larger skull size, robust mandibles, shortened dentitions, longer coronoid processes, expanded angular processes, low condyles, and taller cranial sagittal crests, and wider zygomatic arches that provide this species with mechanical advantages; their greater bite force may help them use larger and hard-bodied birds as a dietary component.

New fossils from Tadkeshwar Mine (Gujarat, India) increase primate diversity from the early Eocene Cambay Shale

Several new fossil specimens from the Cambay Shale Formation at Tadkeshwar Lignite Mine in Gujarat document the presence of two previously unknown early Eocene primate species from India. A new species of Asiadapis is named based on a jaw fragment preserving premolars similar in morphology to those of A. cambayensis but substantially larger. Also described is an exceptionally preserved edentulous dentary (designated cf. Asiadapis, unnamed sp. nov.) that is slightly larger and much more robust than previously known Cambay Shale primates. Its anatomy most closely resembles that of Eocene adapoids, and the dental formula is the same as in A. cambayensis. A femur and calcaneus are tentatively allocated to the same taxon. Although the dentition is unknown, exquisite preservation of the dentary of cf. Asiadapis sp. nov. enables an assessment of masticatory musculature, function, and gape adaptations, as well as comparison with an equally well-preserved dentary of the asiadapid Marcgodinotius indicus, also from Tadkeshwar. The new M. indicus specimen shows significant gape adaptations but was probably capable of only weak bite force, whereas cf. Asiadapis sp. nov. probably used relatively smaller gapes but could generate relatively greater bite forces.

Drivers of in vivo bite performance in wild brown mouse lemurs and a comparison with the grey mouse lemur

AbstractPhysical performance is crucial for animal survival and fitness. In this context, greater bite forces can provide advantages and may allow an individual to gain access to reproductive partners and/or different food resources. Here, we explored the determinants of bite force in a wild population of the brown mouse lemur (Microcebus rufus). Our objectives were to elucidate (1) if sex, head width, heart rate (as an indicator of overall physical fitness) and body condition drive variation in bite force in this population of wild mouse lemurs; and (2) the relative importance of the ecological niche in determining bite force by comparing results from this wild population with previously published results on bite force, body mass and head width from a laboratory colony of the grey mouse lemur (Microcebus murinus). We captured 32 wild brown mouse lemurs at night in the Ranomafana National Park in Madagascar during the beginning of the rainy season from 1st to 31st October 2016. We measured bite force, heart rate, body mass and head width of all individuals, and assigned sex and body condition (estimated as the unstandardized residual of a regression of body mass against head size). Although maximum bite force was positively correlated with body mass, it was not correlated with body condition. Residual bite force was highly correlated with residual head width and heart rate. The mean bite force of wild brown mouse lemurs was much lower than that of grey mouse lemurs in captivity, but showed similar relationships to head dimension and body mass. Even when corrected by body condition, grey mouse lemurs bit significantly harder than brown mouse lemurs. The difference in bite force between species could be explained by differences in head size and niche divergence with brown mouse lemurs eating mostly soft fruits and grey mouse lemurs eating more hard insects.

Sex-specific thermal sensitivities of performance and activity in the asian house gecko, Hemidactylus frenatus.

Studies of sexual selection primarily focus on morphological traits such as body size and secondary trait dimorphism, with less attention been given to the functional differences between the sexes and even more so their thermal performance capacities. Each sex may benefit from possessing different thermal performance capacities that would allow them to maximise their fitness relative to their different reproductive roles; especially when performances are closely related to reproductive success. Here, we examine sexual divergence in thermal sensitivities of performance across three populations of the Asian house gecko (Hemidactylus frenatus) over an extensive latitudinal cline. Using analyses of the thermal sensitivity of routine activity, bite force and sprint speed, we explored whether: (i) males and females differed in their optimal temperatures for performance, (ii) the sexes differed in their thermal sensitivities of performance, and (iii) the degree of sexual divergence in thermal sensitivity varied among the populations. Because male H. frenatus are highly aggressive and frequently engage in combat to gain territories and mating opportunities, we expected males would be active over a wider range of temperatures than females and this would favour broad thermal sensitivity curves for males. In addition, we expected a greater divergence between the sexes in thermal sensitivities for the temperate populations that experience greater daily and seasonal thermal variation. We found that males were more active, and had greater bite forces and faster sprint speeds than females, independent of body size. In addition, we found differences between the sexes in thermal sensitivities for the tropical population; female H. frenatus were less active and possessed lower sprint speeds at higher temperatures than males. Although H. frenatus from the most variable thermal environments also displayed the broadest thermal performance range, it was the more stable tropical population that exhibited the greatest divergence between the sexes in thermal sensitivity of performance. The divergence in thermal physiology that we detected between the sexes of H. frenatus is consistent with the idea that males will derive mating and territorial advantages for maintaining function over a broader range of temperatures.

Comparative cranial myology and biomechanics of Plateosaurus and Camarasaurus and evolution of the sauropod feeding apparatus

AbstractSauropodomorpha represents an important group of Mesozoic megaherbivores, and includes the largest terrestrial animals ever known. It was the first dinosaur group to become abundant and widespread, and its members formed a significant component of terrestrial ecosystems from the Late Triassic until the end of the Cretaceous. Both of these factors have been explained by their adoption of herbivory, but understanding the evolution of sauropodomorph feeding has been hampered by the scarcity of biomechanical studies. To address this, the jaw adductor musculature of the basal sauropodomorph Plateosaurus and the sauropod Camarasaurus have been reconstructed. These reconstructions provide boundary conditions for finite element models to assess differences in structural performance between the two taxa. Results demonstrate that Camarasaurus was capable of much greater bite forces than Plateosaurus, due to greater relative adductor muscle mass and shape changes to the mandible. The skull and mandible of Camarasaurus are also ‘stronger’ under static biting. The Plateosaurus mandible appears to compromise structural efficiency and force transmission in order to maintain relatively high jaw closure speed. This supports suggestions of facultative omnivory in basal sauropodomorph taxa. The expanded mandibular symphysis and ‘lateral plates’ of sauropods each lead to greater overall craniomandibular robustness, and may have been especially important in accommodating forces related to asymmetric loading. The functional roles of these characters, and observed general shape changes in increasing skull robustness, are consistent with hypotheses linking bulk‐herbivory with the origin of Sauropoda and the evolution of gigantism.

Evaluation of masseter muscle morphology in different types of malocclusions using cone beam computed tomography

To evaluate the length and orientation of masseter in different types of malocclusions using Cone Beam Computed Tomography (CBCT). Samples of 180 patients seeking orthodontic treatment at the University of Nevada, Las Vegas School of Dentistry, were included in the study. Pre-treatment multi-slice CBCT scans of these patients were divided into three anteroposterior groups: Class I subjects with ANB angle 0° to 5°; Class II subjects with ANB angle >5°; and Class III subjects with ANB angle <0°. CBCT scans were also divided into three vertical groups: normodivergent subjects with mandibular plane angle 22° to 30°; hyperdivergent subjects with mandibular plane angle >30°, hypodivergent subjects with mandibular plane angle <22°. The masseter was identified and landmarks were placed on the anterior border, at the origin and insertion of the muscle in 3-D mode of the Dolphin Imaging 10.5 Premium software. The Frankfort Horizontal Plane was used as a reference plane and an angular measurement was obtained by intersection of a line produced by the masseter landmarks to calculate the orientation of the muscles. The length of the masseter was measured and data were analyzed using ANOVA and matched pairs test. ANOVA found significant differences in muscle length among the three vertical groups for both the left and right muscles. Paired t test showed significantly shorter muscle length for the hypodivergent group (43.3 ± 4.0 mm) compared to the normodivergent group (45.6 ± 4.5 mm, P < 0.05) and shorter muscle length for the hyperdivergent group (42.3 ± 4.7 mm) compared to the hypodivergent group, P < 0.05. No significant differences were found in muscle length among the three anteroposterior groups. However, significant differences in muscle orientation angle were found among the three anteroposterior groups (P < 0.05). Class II subjects were found to have the most acute orientation angle (67.2 ± 6.6°) and Class III had the most obtuse orientation angle (81.6 ± 6.8°). These results suggest that certain types of malocclusion may have different masseter lengths and orientations and these differences may have implications for the mechanical advantage in bite force. For example, Class III individuals may have greater bite force than Class II individuals because the muscle fibers are oriented more along the arch of closure.

Comparison of Jaw Muscle Morphology in Two Sympatic Callosciurine Squirrels (Callosciurus ErythraeusandDremomys Rufigenis) in Vietnam

Laboratory of Anatomy, Department of Basic Veterinary Sciences, Obihiro University of Agriculture and Veterinary Medicine, Obihiro 080-8555, JapanIn this study, we focus on the jaw muscle morphol-ogy of two callosciurine squirrel species, Callosciuruserythraeus and Dremomys rufigenis. Callosciuruserythraeus (Callosciurini: Sciuridae: Callosciurinae) isan arboreal squirrel that inhabits Southeast Asia fromeastern India to the Malayan archipelago, Indochina,southern China, and Taiwan (Nowak 1999; Can et al.2008; Francis 2008; Smith and Xie 2008). D. rufigenis,another member of the tribe Callosciurini, is found sym-patrically with C. erythraeus throughout Southeast Asia(Nowak 1999; Can et al. 2008; Francis 2008; Smith andXie 2008). Koyabu et al. (2009) previously reported thatthe sympatric C. erythraeus and D. rufigenis displaynotable difference in craniodental mechanics. Both C.erythraeus and D. rufigenis are known to prefer fruitsand insects, as generally seen in other tree squirrels.However, C. erythraeus also feeds extensively on hardseeds and tough tree bark, which are rarely included inthe diet of D. rufigenis. For example, C. erythraeus isreported to frequently forage on the hard seeds ofPinus, Camellia, and Castanopsis tree (Chou et al.1985; Sonoda et al. 2001; Lu 2003; Men et al. 2007).Callosciurus erythraeus is also known to often strip toughbark to obtain sugar from sweet tree saps (Tamura andOhara 2005), whereas this dietary behavior seems to berare or not reported in D. rufigenis (Medway 1969;Nowak 1999; Smith and Xie 2008). Koyabu et al.(2009) demonstrated that C. erythraeus possesses agreater mechanical advantage of the masseter and tem-poralis muscles for chewing compared to D. rufigenis.Callosciurus erythraeus was also found to display greaterleverage of the masseter and temporalis for incision.Given these findings, it was suggested that C. erythraeusshould be capable of generating greater bite force for agiven muscle force at the cheek teeth and that the cranio-dental divergence between C. erythraeus and D. rufigenismay facilitate their trophic separation under sympatry.In their study, however, only the analysis of mechani-cal leverages was conducted, and comparisons of jawmuscle morphology were not incorporated. Althoughthe craniodental arrangements and mechanical advantageof the masticatory muscles are critical parameters thataffect the amount of force production, muscle mass isalso another essential contributor to the bite forces ananimal can generate (Raadsheer 1999). Since the biteforces an organism can generate should constrain therange of food items it can eat (Lucas 2004), variation inmasticatory muscle mass has undoubtedly been funda-mental for the dietary diversification of rodents. Giventhe great range of diet and phenomenal level of morpho-logical diversity found across rodents, studies on rodentmuscle mass variation should provide promising avenuesto study the ecomorphological patterns of mammals ingeneral. To date, reports on muscle mass are availablefor some muroids (e.g., Hiiemae and Houston 1971;Weijs and Dantuma 1975; Satoh 1997; Satoh and Iwaku2004) and New World squirrels (Ball and Roth 1995).Although Thorington and Darrow (1996) provideddetailed anatomical descriptions of jaw muscles for someOld World squirrel species, still very little is describedquantitatively for them. Here, to bridge the gaps in ourknowledge of Old World squirrel muscle morphology

Built to Bite: Feeding Kinematics, Bite Forces, and Head Shape of a SpecializedDurophagous Lizard,Dracaena Guianensis(Teiidae)

Most lizards feed on a variety of food items that may differ dramatically in their physical and behavioral characteristics. Several lizard families are known to feed upon hard-shelled prey (durophagy). Yet, specializations toward true molluscivory have been documented for only a few species. As snails are hard and brittle food items, it has been suggested that a specialized cranial morphology, high bite forces, and an adapted feeding strategy are important for such lizards. Here we compare head and skull morphology, bite forces, and feeding kinematics of a snail-crushing teiid lizard (Dracaena guianensis) with those in a closely related omnivorous species (Tupinambis merianae). Our data show that juvenile D. guianensis differ from T. merianae in having bigger heads and greater bite forces. Adults, however, do not differ in bite force. A comparison of feeding kinematics in adult Dracaena and Tupinambis revealed that Dracaena typically use more transport cycles, yet are more agile in manipulating snails. During transport, the tongue plays an important role in manipulating and expelling shell fragments before swallowing. Although Dracaena is slow, these animals are very effective in crushing and processing hard-shelled prey.

Digitalized measurement of maximum bite force in Iraqi adult sample aged 18 - 25 years with different malocclusion groups

Information concerning the maximum bite force in human population is important to clinical orthodontics. Additionally, the influence of bite force on the vertical stability of any treatment result is important. The new position of the dentition should be compatible with the dynamics of the muscular and occlusal forces in all planes. This study was conducted to assess the maximum bite force in the molar (left and right) and incisor region of Iraqi adult persons aged 18-25 years. Materials and method: The total sample size is (150) persons (75 male, 75 female) of untreated Iraqi subjects divided according to the class of malocclusion. The maximum bite force measurements were performed by a digital device (GM10) placed in the first molar area unilaterally on both the left and right sides which was used especially for this study. After that measured the maximum bite in the right central incisor region. Results: The maximum molar force was higher in class III followed by class II and then class I. The maximum incisal force was higher in class I followed by class II then class III. Conclusions: there was a very high significant difference between molar and incisal bite force. There was non significant difference between right and left side in all classes of malocclusion. Generally there was a very high significant difference regarding the genders, males have a greater bite force than females for class I, II, III

Cranial mechanics compared in extinct marsupial and extant African lions using a finite‐element approach

AbstractFew species have generated more or longer running controversy than Australia's extinct marsupial lion Thylacoleo carnifex (Owen, 1859). Over the last century and a half, feeding behaviours as disparate as osteophagy and specialized herbivory have been suggested and T. carnifex has been placed in both phalangeriform and vombatiform clades. Phylogenetic placement remains uncertain, but broad consensus has been achieved regarding diet, with all recent authors agreeing that T. carnifex was a carnivore. However, the marsupial lion's extraordinary cranial and dental morphologies remain without clear analogy, leaving many questions unanswered regarding how this most atypical mammalian predator killed its prey. Here I apply a rapidly emerging new approach in comparative biology, finite‐element analysis, to the examination of cranial mechanics in T. carnifex. Comparisons are made with an extant lion Panthera leo (Linnaeus, 1758) under simulations designed to model stress distributions imposed by biting (intrinsic loads) and interaction with struggling prey (extrinsic loads). Modelling that approximates the 3‐D architecture of jaw adductors suggests that both the placental and marsupial lions could generate considerably greater bite forces than has been predicted using 2‐D approaches, but with relatively greater forces in the marsupial. The distribution of cranial stress is in many respects similar in both species, but results from simulations of extrinsic forces suggest that the marsupial was particularly well adapted to resist the high stresses that would be expected in dealings with relatively large prey. On the other hand, relatively high stress recorded in the rostrum of T. carnifex under intrinsic loadings suggests that it may have deployed a very different modus operandi, wherein the carnassial teeth played an active role in effecting a kill.

Biomineralization and adaptive plasticity of the temporomandibular joint in myostatin knockout mice

Mice lacking myostatin (GDF-8), a negative regulator of skeletal muscle growth, show a significant increase in muscle mass versus normal mice. We compared wild-type and myostatin deficient mice to assess the postnatal effect of elevated masticatory loads due to increased jaw-adductor muscle activity and greater bite forces on mandibular condyle morphology. Microcomputed tomography (microCT) was used to provide details of internal condylar morphology and quantify bone density in three condylar regions. Biomineralization levels, as well as external mandibular dimensions, were used to characterize within-slice, within-joint, within-group and between-group variation. Dimensions of the mandible and mandibular condyle were similar between the myostatin knockout and normal mice. Knockout mice exhibited significantly more biomineralization on the outer surface of the condylar subchondral bone and along the condylar neck, most notably on the buccal side of the condylar neck. The buccal side of the inner aspect of the condyle was significantly less biomineralized in knockout mice, both for the pooled data and for the posterior and anterior condylar slices. Whilst normal mice had symmetric subchondral bone surfaces, those of knockout mice were asymmetric, with a lower, less convex surface on the buccal side versus the lingual side. This appears related to the ontogenetic effects of increased masticatory stress in the mandibles of knockout mice as compared to normal mice. Significant differences in biomineralization between normal and myostatin knockout mice, coupled with the lack of significant differences in certain external dimensions, underscores a need for information on the external and internal morphology of mineralized tissues vis-à-vis altered or excessive mechanical loads.

Comparative data from young men and women on masseter muscle fibres, function and facial morphology

The primary aim was to relate information about masseter muscle fibres and function to aspects of facial morphology in a group of healthy young men. The secondary aim was to investigate possible sex differences using data previously obtained from a comparable group of age-matched, healthy women. Dental status and facial morphology were recorded in 13 male students aged 20–26 years. Functional examinations included bite-force measurements and electromyographic recordings of masseter activity. A biopsy was removed from the masseter of each participant during surgical extraction of a wisdom tooth, and the tissue examined for myosin ATPase activity. Further, the cross-sectional areas of the different fibre types were measured. In spite of using age-matched healthy men and women with a full complement of teeth, statistically significant sex differences were found among measures related to muscle function and some measures of facial morphology. Thus data from men and women should not be pooled uncritically. The greater bite force in men than women corresponded with the greater diameter and cross-sectional area of type II fibres. Further, the males had more anteriorly inclined mandibles and shorter anterior facial height, suggesting a relation between the greater muscle force and the shape of the face. However, linear regression analysis failed to demonstrate any significant association between bite force and facial morphology among men and women. Thus, craniofacial morphology could be a result of far more contributing factors than previously believed.

A study of maximum bite force during growth and development.

Bilateral bite force was measured in a sample of 457 subjects (231 males and 226 females) from 6 years through 20 years. The mean maximum bite force was found to increase from 78 Newtons at 6 to 8 years to 176 Newtons at 18 to 20 years. While earlier studies have shown adult males have a greater mean bite force than females, this difference is not evident during growth and development. Gender-related bite force difference likely develops during the postpubertal period in association with greater muscle mass development in males.