A functional analysis of carnassial biting

The jaw mechanism of carnivores is studied using an idealized model (Greaves, 1978). The model assumes: (i) muscle activity on both sides of the head, and (ii) that the jaw joints and the carnassial teeth are single points of contact between the skull and the lower jaw during carnassial biting. The model makes the following predictions: (i) in carnivores with carnassial teeth the resultant force of the jaw muscles will be positioned approximately 60% of the way from the jaw joint to the tooth—this arrangement delivers the maximum bite force possible together with a reasonably wide gape (remembering that bite force and gape cannot both be maximized); (ii) in an evolutionary sense, if greater bite force is required at the carnassial tooth, either the animal will get larger so as to deliver an absolutely larger bite force or the architecture of the muscles may change, becoming more pinnate, for example, but jaw geometry (i.e. the relative positions of the jaw joints, the carnassial tooth, and the muscle resultant force) will not change; (iii) if greater gape is required, the animal will get larger so as to have longer jaws and therefore an absolutely wider gape or change its muscle architecture allowing for greater stretch while the geometry remains unchanged; and (iv) in animals with a longer shearing region (e.g. the extinct hyaenodonts) the shearing region will be approximately 20% of jaw length and the muscle resultant force will be positioned approximately 60% of the way from the jaw joint to the most anterior shearing tooth.     

Gape and bite force in the rodents Onychomys leucogaster and Peromyscus maniculatus: Does jaw‐muscle anatomy predict performance?

Compared with the deer mouse, Peromyscus maniculatus, the grasshopper mouse, Onychomys leucogaster, exhibits modifications in its jaw‐muscle architecture that promote wide gapes and large bite forces at wide gapes to prey upon large vertebrate prey. In this study, we determine whether jaw‐muscle anatomy predicts gape and biting performance in O. leucogaster, and we also assess the influence of gape on bite force in the two species. Although O. leucogaster has an absolutely longer jaw, which facilitates larger gapes, maximum passive gape is similar in both species, averaging ～12.5 mm. Thus, when scaled to jaw length, O. leucogaster has a smaller maximum passive gape. These results suggest that predatory behaviors of O. leucogaster may not require remarkably large gapes. On the other hand, both absolute and relative bite forces exerted by O. leucogaster are significantly larger than those of P. maniculatus. The largest bite forces in both species occur at 5.0 mm of gape at the incisors, or 40% of maximum gape. Although bite force in both species decreases at larger gapes, O. leucogaster does maintain a larger percentage of maximum bite force at gapes larger than 40% of maximum passive gape. Therefore, although structural modifications in the masticatory apparatus of O. leucogaster may constrain gape, they may help to maintain bite force at large gapes. These results suggest that increases in gape differentially influence the length‐tension properties of the jaw muscles in the two species. Finally, these results highlight the importance of considering the effect of muscle stretch on force production in comparative studies of bite force. As a first approximation, it appears that gapes of 40–50% of maximum gape in rodents optimizes bite force production at the incisors. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

Functional implications of craniomandibular morphology in African mole‐rats (Rodentia: Bathyergidae)

African mole‐rats are subterranean rodents from the family Bathyergidae. The family consists of six genera, five of which (Cryptomys, Fukomys, Georychus, Heliophobius and Heterocephalus) are chisel‐tooth diggers, meaning they dig underground using procumbent incisors. The remaining genus of mole‐rat (Bathyergus) is a scratch digger, which digs using its forelimbs. Chisel‐tooth digging is thought to have evolved to enable exploitation of harder soils. It was hypothesized that to dig successfully using incisors, chisel‐tooth digging mole‐rats will have a craniomandibular complex that is better able to achieve a large bite force and wide gape compared with scratch digging mole‐rats. Linear measurements of morphological characteristics associated with bite force and gape were measured in several chisel‐tooth digging and scratch digging mole‐rats. Chisel‐tooth diggers have increased jaw and condyle lengths relative to their size (characteristics associated with larger gape). They also have relatively wider and taller skulls (characteristics associated with larger bite force). The mechanical advantage of three masticatory muscles of each specimen was also calculated. The mechanical advantage of the temporalis muscle was significantly larger in chisel‐tooth digging mole‐rats than scratch digging genus. The results demonstrate that chisel‐tooth digging bathyergids have a craniomandibular morphology that is better able to facilitate high bite force and wide gape than scratch digging mole‐rats.     

Functional links between canine height and jaw gape in catarrhines with special reference to early hominins

This study tests the hypothesis that decreased canine crown height in catarrhines is linked to (and arguably caused by) decreased jaw gape. Associations are characterized within and between variables such as upper and lower canine height beyond the occlusal plane (canine overlap), maximum jaw gape, and jaw length for 27 adult catarrhine species, including 539 living subjects and 316 museum specimens. The data demonstrate that most adult male catarrhines have relatively larger canine overlap dimensions and gapes than do conspecific females. For example, whereas male baboons open their jaws maximally more than 110% of jaw length, females open about 90%. Humans and hylobatids are the exceptions in that canine overlap is nearly the same in both the sexes and so is relative gape (ca. 65% for humans and 110% for hylobatids). A correlation analysis demonstrates that a large portion of relative gape (maximum gape/projected jaw length) is predicted by relative canine overlap (canine overlap/jaw length). Relative gape is mainly a function of jaw muscle position and/or jaw muscle‐fiber length. All things equal, more rostrally positioned jaw muscles and/or shorter muscle fibers decrease gape and increase bite force during the power stroke of mastication, and the net benefit is to increase the mechanical efficiency during chewing. Similarly, more caudally positioned muscles and/or longer muscle fibers increase the amount of gape and decrease bite force. Overall, the data support the hypothesis that canine reduction in early hominins is functionally linked to decreased gape and increased mechanical efficiency of the jaws. Am J Phys Anthropol, 2013. © 2012 Wiley Periodicals, Inc.     

Mechanics of biting in great white and sandtiger sharks

Although a strong correlation between jaw mechanics and prey selection has been demonstrated in bony fishes (Osteichthyes), how jaw mechanics influence feeding performance in cartilaginous fishes (Chondrichthyes) remains unknown. Hence, tooth shape has been regarded as a primary predictor of feeding behavior in sharks. Here we apply Finite Element Analysis (FEA) to examine form and function in the jaws of two threatened shark species, the great white (Carcharodon carcharias) and the sandtiger (Carcharias taurus). These species possess characteristic tooth shapes believed to reflect dietary preferences. We show that the jaws of sandtigers and great whites are adapted for rapid closure and generation of maximum bite force, respectively, and that these functional differences are consistent with diet and dentition. Our results suggest that in both taxa, insertion of jaw adductor muscles on a central tendon functions to straighten and sustain muscle fibers to nearly orthogonal insertion angles as the mouth opens. We argue that this jaw muscle arrangement allows high bite forces to be maintained across a wider range of gape angles than observed in mammalian models. Finally, our data suggest that the jaws of sub-adult great whites are mechanically vulnerable when handling large prey. In addition to ontogenetic changes in dentition, further mineralization of the jaws may be required to effectively feed on marine mammals. Our study is the first comparative FEA of the jaws for any fish species. Results highlight the potential of FEA for testing previously intractable questions regarding feeding mechanisms in sharks and other vertebrates.     

Using linkage models to explore skull kinematic diversity and functional convergence in arthrodire placoderms

Biomechanical models offer a powerful set of tools for quantifying the diversity of function across fossil taxa. A computer‐based four‐bar linkage model previously developed to describe the potential feeding kinematics of Dunkleosteus terrelli is applied here to several other arthrodire placoderm taxa from different lineages. Arthrodire placoderms are a group of basal gnathostomes showing one of the earliest diversifications of jaw structures. The linkage model allows biomechanical variation to be compared across taxa, identify trends in skull morphology among arthrodires that potentially influence function and explore the role of linkage systems in the early evolution of jaw structures. The linkage model calculates various kinematic metrics including gape angle, effective mechanical advantage, and kinematic transmission coefficients. Results indicate that the arthrodire feeding system may be more diverse and complex than previously thought. A range of potential kinematic profiles among arthrodire taxa illustrate a diversity of feeding function comparable with modern teleost fishes. Previous estimates of bite force in Dunkleosteus are revised based on new morphological data. High levels of kinematic transmission among arthrodires suggest the potential for rapid gape expansion and possible suction feeding. Morphological comparisons indicate that there were several morphological solutions for obtaining these fast kinematics, which allowed different taxa to achieve similar kinematic profiles while varying other aspects of the feeding apparatus. Mapping of key morphological components of the linkage system on a general placoderm phylogeny illustrates the potential importance of four‐bar systems to the early evolution of jaw structures. J. Morphol. 271:990–1005, 2010. © 2010 Wiley‐Liss, Inc.     

Computer simulation of feeding behaviour in the thylacine and dingo as a novel test for convergence and niche overlap

The extinct marsupial thylacine (Thylacinus cynocephalus) and placental grey wolf (Canis lupus) are commonly presented as an iconic example of convergence. However, various analyses suggest distinctly different behaviours and specialization towards either relatively small or large prey in the thylacine, bringing the degree of apparent convergence into question. Here we apply a powerful engineering tool, three-dimensional finite element analysis incorporating multiple material properties for bone, to examine mechanical similarity and niche overlap in the thylacine and the wolf subspecies implicated in its extinction from mainland Australia, Canis lupus dingo. Comparisons of stress distributions not only reveal considerable similarity, but also informative differences. The thylacine's mandible performs relatively poorly where only the actions of the jaw muscles are considered, although this must be considered in the light of relatively high bite forces. Stresses are high in the posterior of the thylacine's cranium under loads that simulate struggling prey. We conclude that relative prey size may have been comparable where both species acted as solitary predators, but that the dingo is better adapted to withstand the high extrinsic loads likely to accompany social hunting of relatively large prey. It is probable that there was considerable ecological overlap. As a large mammalian hypercarnivore adapted to taking small-medium sized prey, the thylacine may have been particularly vulnerable to disturbance.     

The vector of jaw muscle force as determined by computer-generated three dimensional simulation: a test of Greaves' model

We present results from a detailed three-dimensional finite element analysis of the cranium and mandible of the Australian dingo (Canis lupus dingo) during a range of feeding activities and compare results with predictions based on two-dimensional methodology [Greaves, W.S., 2000. Location of the vector of jaw muscle force in mammals. Journal of Morphology 243, 293-299]. Greaves showed that the resultant muscle vector intersects the mandible line slightly posterior to the lower third molar (m3). Our work demonstrates that this is qualitatively correct, although the actual point is closer to the jaw joint. We show that it is theoretically possible for the biting side of the mandible to dislocate during unilateral biting; however, the bite point needs to be posterior to m3. Simulations show that reduced muscle activation on the non-biting side can considerably diminish the likelihood of dislocation with only a minor decrease in bite force during unilateral biting. By modulating muscle recruitment the animal may be able to maximise bite force whilst minimising the risk of dislocation.     

Comparison between in vivo and theoretical bite performance: Using multi-body modelling to predict muscle and bite forces in a reptile skull

In biomechanical investigations, geometrically accurate computer models of anatomical structures can be created readily using computed-tomography scan images. However, representation of soft tissue structures is more challenging, relying on approximations to predict the muscle loading conditions that are essential in detailed functional analyses. Here, using a sophisticated multi-body computer model of a reptile skull (the rhynchocephalian Sphenodon), we assess the accuracy of muscle force predictions by comparing predicted bite forces against in vivo data. The model predicts a bite force almost three times lower than that measured experimentally. Peak muscle force estimates are highly sensitive to fibre length, muscle stress, and pennation where the angle is large, and variation in these parameters can generate substantial differences in predicted bite forces. A review of theoretical bite predictions amongst lizards reveals that bite forces are consistently underestimated, possibly because of high levels of muscle pennation in these animals. To generate realistic bites during theoretical analyses in Sphenodon, lizards, and related groups we suggest that standard muscle force calculations should be multiplied by a factor of up to three. We show that bite forces increase and joint forces decrease as the bite point shifts posteriorly within the jaw, with the most posterior bite location generating a bite force almost double that of the most anterior bite. Unilateral and bilateral bites produced similar total bite forces; however, the pressure exerted by the teeth is double during unilateral biting as the tooth contact area is reduced by half.     

Application and validation of a three-dimensional mathematical model of the human masticatory system in vivo.

A previously described three-dimensional mathematical model of the human masticatory system, predicting maximum possible bite forces in all directions and the recruitment patterns of the masticatory muscles necessary to generate these forces, was validated in in vivo experiments. The morphological input parameters to the model for individual subjects were collected using MRI scanning of the jaw system. Experimental measurements included recording of maximum voluntary bite force (magnitude and direction) and surface EMG from the temporalis and masseter muscles. For bite forces with an angle of 0, 10 and 20 degrees relative to the normal to the occlusal plane the predicted maximum possible bite forces were between 0.9 and 1.2 times the measured ones and the average ratio of measured to predicted maximum bite force was close to unity. The average measured and predicted muscle recruitment patterns showed no striking differences. Nevertheless, some systematic differences, dependent on the bite force direction, were found between the predicted and the measured maximum possible bite forces. In a second series of simulations the influence of the direction of the joint reaction forces on these errors was studied. The results suggest that they were caused primarily by an improper determination of the joint force directions.     

Factors influencing the anterior component of occlusal force

We hypothesized that the anterior component of occlusal force (ACF) generated by mandibular molars was a function of molar inclination, height of the transverse condylar axis above the occlusal plane, steepness of the occlusal plane, gape, molar root dimensions, interproximal tooth contact force when not biting, and bite force. Our research aim was to identify those biomechanical factors which determine ACF. Mandibular second molars were axially loaded with a 90 N force (10 mm second molar gape) in 15 subjects, and the resulting ACF was measured at the mandibular first molar-second premolar contact using a recording technique based on interproximal frictional forces. Morphologic measurements were obtained from lateral cephalometric radiographs of each subject and included: Frankfort mandibular plane angle, occlusal plane angle, angles formed by the longitudinal axis of the second molar and the occlusal and mandibular planes, perpendicular distance from the top of the condyle to the occlusal plane, and second molar root width and root length. For ten subjects, ACF resulting from axial loads of 50, 100, 150, and 200 N was measured. For ten subjects, ACF resulting from an axial load of 50 N and second molar gapes of 10 mm, 14 mm, 18 mm, and 22 mm were measured. ACF increased with increasing gape and increased proportionally to increasing bite force. Correlation and stepwise regression analyses revealed that ACF varies with interproximal tooth contact force when not biting (contact ‘tightness’) and molar root width (model R² = 0.71, p < 0.01). The hypothesis that ACF is a function of bite force, gape, molar root width, and interproximal contact tightness has been supported, and the hypothesis that ACF is a function of molar inclination, occlusal plane steepness, condylar axis height, and root length was rejected.     

A dynamic model of mouth closing movements in clariid catfishes: the role of enlarged jaw adductors

Some species of Clariidae (air breathing catfishes) have extremely large (hypertrophied) jaw closure muscles. Besides producing higher bite forces, the enlarged muscles may also cause higher accelerations of the lower jaw during rapid mouth closure. Thus, jaw adductor hypertrophy could potentially also enable faster mouth closure. In this study, a forward dynamic model of jaw closing is developed to evaluate the importance of jaw adductor hypertrophy on the speed of mouth closure. The model includes inertia, pressure, tissue resistance and hydrodynamic drag forces on the lower jaw, which is modelled as a rotating half-ellipse. Simulations are run for four clariid species showing a gradual increase in jaw adductor hypertrophy (Clarias gariepinus, Clariallabes longicauda, Gymnallabes typus and Channallabes apus). The model was validated using data from high-speed videos of prey captures in these species. In general, the kinematic profiles of the fastest mouth closure from each species are reasonably well predicted by the model. The model was also used to compare the four species during standardized mouth closures (same initial gape angle, travel distance and cranial size). These simulations suggest that the species with enlarged jaw adductors have an increased speed of jaw closure (in comparison with the non-hypertrophied C. gariepinus) for short lower jaw rotations and when feeding at high gape angles. Consequently, the jaw system in these species seems well equipped to capture relatively large, evasive prey. For prey captures during which the lower jaw rotates freely over a larger distance before impacting the prey, the higher kinematic efficiency of the C. gariepinus jaw system results in the fastest jaw closures. In all cases, the model predicts that an increase in the physiological cross-sectional area of the jaw muscles does indeed contribute to the speed of jaw closure in clariid fish.     

A functional analysis of food procurement in two surgeonfish species,Acanthurus nigrofuscus andCtenochaetus striatus (Acanthuridae)

Synopsis The mechanisms of food procurement in the surgeonfishesCtenochaetus striatus andAcanthurus nigrofuscus from the Great Barrier Reef were determined by functional analyses of the jaws and associated structural elements (based on myological and osteological examinations and X-ray photographs) and by video analyses of actions of the mouth and body during feeding.Acanthurus nigrofuscus has relatively robust jaw bones. The movement of the elements during mouth opening is limited with a mean maximum gape angle of 112.8°. Each bite is relatively fast and is characterized by a quick nip at algal filaments, usually followed by a sidewads flick of the head. The jaws bear several broad multidenticulate teeth. It appears that these teeth engage turf algal strands which are either sheared during mouth closure or torn off as the head flicks sideways. InC. striatus, the jaw bones are considerably lighter than those ofA. nigrofuscus. There is much greater movement of the elements during mouth opening, resulting in a mean maximum gape angle of 177.6°. Each bite is slower than inA. nigrofuscus and is characterized by a wide gape as the mouth is applied to the substratum followed by a quick, upward flick of the lower jaw, with no sideways flick of the head. The jaws bear numerous elongate flexible teeth, with expanded incurved denticulate tips; those on the dentary often possessing a pointed blade-like process. It appears that these teeth brush particulate and epiphytic material from the surface of the turf algal strands and other substrata. These observations demonstrate howA. nigrofuscus andC. striatus are able to remove microalgae and detritus, respectively, from the same substratum. The results also demonstrate how relatively small differences in morphology can have a profound influence on the feeding abilities and trophic ecology of fishes.     

Bite performance in clariid fishes with hypertrophied jaw adductors as deduced by bite modeling

Within clariid fishes several cranial morphologies can be discerned. Especially within anguilliform representatives an increase in the degree of hypertrophy of the jaw adductors occurs. The hypertrophy of the jaw adductors and skeletal modifications in the cranial elements have been linked to increased bite force. The functional significance of this supposed increase in bite force remains obscure. In this study, biomechanical modeling of the cranial apparatus in four clariid representatives showing a gradual increase in the hypertrophy of the jaw adductors (Clarias gariepinus, Clariallabes melas, Channallabes apus, and Gymnallabes typus) is used to investigate whether bite force actually increased. Static bite modeling shows that the apparent hypertrophy results in an increase in bite force. For a given head size, the largest bite forces are predicted for C. apus, the lowest ones for C. gariepinus, and intermediate values are calculated for the other species. In addition, also in absolute measures differences in bite force remain, with C. apus biting distinctly harder than C. gariepinus despite its smaller head size. This indicates that the hypertrophy of the jaw adductors is more than just a correlated response to the decrease in absolute head size. Further studies investigating the ecological relevance of this performance difference are needed.     

Size ratios among sympatric neotropical cats

Summary Data were gathered on body weight, body length, relative maximum bite force and relative maximum gape for six sympatric species of neotropical cats (Felidae) to see if constant size ratios occur between adjacent species or if the minimum ratio in a series is greater than expected by chance. Although clearly likely to be correlated, these four parameters were thought to have potential for some independent variation and independent influence on prey capture abilities. None of the four sets of ratios was statistically distinguishable from random when all six species were included in the analysis; however, the ratios for relative maximum gape among just the four largest species were significantly more even and the minimum ratio significantly greater than expected by chance among four species. This constancy occurs because of departures of jaw lengths from what would be expected by the average allometric relationship between this parameter and total body size. Competitive character displacement is a possible explanation for the constant ratios in maximum gape of the larger species, but it is not the only possibility.     

Efficiency as a predictor of human jaw design in the sagittal plane

The effects of changing the direction of the bite force and of the mandibular joint reaction have been studied with a mathematical model assisted by a computer using the technique of linear programming. We conclude the following: In the sagittal plane the long axes of lower molars are each tilted in the direction that most efficiently converts muscle force into work at the bite point rather than in the direction that would maximize static bite force. These genetically determined angles are referred to as the most 'work efficient' angles. Collectively they lead to the appearance of the curve of Spee associated with the postcanines. Given the most work efficient angle of the first molar, the model indicates for bite forces generated in this direction the joint reaction is least when tilted forward from the vertical at between 20 degrees and 30 degrees. The joint reaction is normal to the articular surface of the condyle which is itself tilted forward 20-30 degrees from the occlusal plane. We conclude the condyle and articular eminence are remodelled to the angle that minimizes the joint reaction. The direction of the bite force may be controlled via neuronal circuitry connecting mechanoreceptors of the periodontal ligament with motor nerves supplying the jaw-closing muscles. The height of the occlusal plane in the molar region has little effect on jaw efficiency.     

Masticatory Muscle Anatomy and Feeding Efficiency of the American Beaver, <Emphasis Type="Italic">Castor canadensis</Emphasis> (Rodentia,Castoridae)

Beavers are well-known for their ability to fell large trees through gnawing. Yet, despite this impressive behavior, little information exists on their masticatory musculature or the biomechanics of their jaw movements. It was hypothesized that beavers would have a highly efficient arrangement of the masticatory apparatus, and that gnawing efficiency would be maintained at large gape. The head of an American beaver, Castor canadensis, was dissected to reveal the masticatory musculature. Muscle origins and insertions were noted, the muscles were weighed and fiber lengths measured. Physiological cross-sectional areas were determined, and along with the muscle vectors, were used to calculate the length of the muscle moment arms, the maximum incisor bite force, and the proportion of the bite force projected along the long axis of the lower incisor, at occlusion and 30° gape. Compared to other sciuromorph rodents, the American beaver was found to have large superficial masseter and temporalis muscles, but a relatively smaller anterior deep masseter. The incisor bite force calculated for the beaver (550–740 N) was much higher than would be predicted from body mass or incisor dimensions. This is not a result of the mechanical advantage of the muscles, which is lower than most other sciuromorphs, but is likely related to the very high percentage (>96 %) of bite force directed along the lower incisor long axis. The morphology of the skull, mandible and jaw-closing muscles enable the beaver to produce a very effective and efficient bite, which has permitted beavers to become highly successful ecosystem engineers.     

A dynamic model for the evolution of sabrecat predatory bite mechanics

The ability of sabretoothed felids to achieve sufficiently high bite forces for predation at extreme gape angles has been the subject of decades of debate. Previous studies have indicated that bite forces in derived sabretoothed felids would have been low, but that they were probably augmented by head depressing muscles. However, bite mechanics is a dynamic process, and mechanical properties change with changes in gape angles. In this study, I present the first comprehensive model of bite mechanics, vector angles, and forces about the temporomandibular joint at gape angles from occlusion to maximal inferred gape in sabretoothed felids. Primitive sabrecats (Machairodus, Paramachairodus) appear broadly comparable to extant large felids (Panthera, Puma), but derived sabrecats in the groups Homotherini (Amphimachairodus, Homotherium, Xenosmilus) and Smilodontini (Megantereon, Smilodon) are often substantially different from either of the former. The ability of the mandibular adductors to generate torque changes with gape angle, indicating that previous models fail to capture potentially important differences in bite function. Inferred muscle sizes and the angles of effective torque from individual adductor fibres in derived sabrecats are different from those of primitive sabrecats and extant large felids, but they had evolved a number of compensatory adaptations for maximizing force output at the canine and carnassial, primarily changes in muscle fibre angles and more compact crania. Inferred outforces at the canines and carnassials were comparable amongst all groups at low gape angles, but at extreme gape angles outforces would have been low, supporting previous hypotheses of head flexor contribution during initial stages of the killing bite in sabrecats. Mandibular adduction in extant carnivores is a complicated pattern of differences in twitch tension and electromyographical activity at different gape angles, and inference of maximal isotonic bite forces from reconstructed mandibular adductor sizes in fossils will give estimates primarily suitable for comparative purposes. Potentially, derived sabrecats could have evolved differences from extant felids in adductor histochemistry or pinnation angle of individual fibres. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 162 , 220–242.     

Allometry and performance: the evolution of skull form and function in felids

Allometric patterns of skull‐shape variation can have significant impacts on cranial mechanics and feeding performance, but have received little attention in previous studies. Here, we examine the impacts of allometric skull‐shape variation on feeding capabilities in the cat family (Felidae) with linear morphometrics and finite element analysis. Our results reveal that relative bite force diminishes slightly with increasing skull size, and that the skulls of the smallest species undergo the least strain during biting. However, larger felids are able to produce greater gapes for a given angle of jaw opening, and they have overall stronger skulls. The two large felids in this study achieved increased cranial strength by increasing skull bone volume relative to surface area. Allometry of skull geometry in large felids reflects a trade‐off between the need to increase gape to access larger prey while maintaining the ability to resist unpredictable loading when taking large, struggling prey.     

The growth of the skull and jaw muscles and its functional consequences in the New Zealand rabbit (Oryctolagus cuniculus)

Between weaning and adulthood, the length and height of the facial skull of the New Zealand rabbit (Oryctolagus cuniculus) double, whereas much less growth occurs in the width of the face and in the neurocranium. There is a five-fold increase in mass of the masticatory muscles, caused mainly by growth in cross-sectional area. The share of the superficial masseter in the total mass increases at the cost of the jaw openers. There are changes in the direction of the working lines of a few muscles. A 3-dimensional mechanical model was used to predict bite forces at different mandibular positions. It shows that young rabbits are able to generate large bite forces at a wider range of mandibular positions than adults and that the forces are directed more vertically. In young and adult animals, the masticatory muscles differ from each other with respect to the degree of gape at which optimum sarcomere length is reached. Consequently, bite force can be maintained over a range of gapes, larger than predicted on basis of individual length-tension curves. Despite the considerable changes in skull shape and concurrent changes in the jaw muscles, the direction of the resultant force of the closing muscles and its mechanical advantage remain stable during growth. Observed phenomena suggest that during development the possibilities for generation of large bite forces are increased at the cost of a restriction of the range of jaw excursion.