Passive force characteristics of an architecturally complex muscle

In architecturally complex muscles with large attachment areas, it can be expected that during movement different muscle regions undergo different amounts of length excursions. As a consequence, the amount of passive force produced by the regions will differ. Therefore, we tested the hypothesis that during movement the vector of the passive force of such a muscle, which defines the magnitude, position and orientation of the resultant force of the various regions, has no fixed position, between the muscle's center of origin and insertion. As a model for an architecturally complex muscle we used the masseter muscle. It was expected that during jaw opening anterior muscle regions are more stretched than posterior regions, leading to an anterior shift of the passive force vector. A three-component force transducer was used to measure both the position and magnitude of passive force in the masseter muscle of 9 rabbits. Forces were recorded during repeated cycles of stepwise opening and closure of the jaw. The muscle exhibited a clear hysteresis: passive force measured during jaw opening was larger than that during jaw closing. With an increase of the jaw gape there was an approximately exponential increase of the magnitude of the passive muscle force, while simultaneously the passive force vector shifted anteriorly. Moment arm length of passive force increased by about 100%. This anterior shift contributed substantially to the increase of the passive muscle moment generated during jaw opening. It can be concluded that in architecturally complex muscles the increase of the passive resistance moment which is associated with muscle lengthening might not only be due to an increase of the magnitude of passive muscle force but also to an increase of the moment arm of this force.     

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.     

Osteology,myology and feeding mechanism of Astronotus ocellatus (Pisces,Perciformes)

Summary Astronotus ocellatus captures its prey by creating a negative pressure in the buccal cavity which is caused by its quick expansion. Once the prey has been accommodated, the buccal cavity undergoes a compression which may propel the prey towards the pharyngeal jaws for mastication. The motion picture recordings indicate retracted premaxillae at the beginning of food intake followed by a maximum attainment of mouth gape and then mastication. During the maximum opening of the mouth the premaxillae are protruded and dentaries are at maximum depression. These events are followed by activities such as buccopharyngeal cavity expansion, bulging on the ventral surface of the head, and prominent curvature on the ventral surface anterior to the urohyal, caused by the upward movement of the glossohyal. Based on the cinematographic results, it may be inferred that the maximum mouth gape is caused by the sternohyoid-hyoid-interopercular-mandible coupling, and not by the opercular apparatus-mandible coupling, as the latter acts after the full descent of the lower jaw. Impression of the expanded buccopharyngeal cavity has been made by a paraffin mold technique, which confirms the displacement of the buccopharyngeal elements during expansion of the cavity.     

The suspensorium of Ctenopoma acutirostre: A comparative functional analysis with Anabas testudineus

The functional and structural aspects of the suspensorium of Ctenopoma acutirostre have been correlated with those of Anabas testudineus. The different parts of the suspensorium are described, as are the muscles that are functionally connected with the suspensorium. Functions were analyzed by observations on living specimens, and by measurements recorded from the movie films. The role played by various bones and muscles to carry out the functions (the respiration, the gulping, and the feeding) has been explained. The different bones and muscles have been considered as functional units which often are connected to form couplings. During the respiration in Ctenopoma the depression of the lower jaw is conducted by the levator operculiopercular apparatus-mandible coupling. The presence of this coupling is indicated by the presence of dorso-ventral movement of the operculum. A remarkable ventro-dorsal and antero-caudal movement in the urhyal during feeding shows in Ctenopoma the presence of the sternohyoideus-hyoid apparatus-interopercular-mandible coupling, which depresses the lower jaw. In Ctenopoma, the suspensorium takes part in respiration, gulping, and feeding, whereas in Anabas it is only involved in gulping and feeding. To carry out its functions, the suspensorium implies three articulations: palatocranial, craniohyomandibular, and quadratomandibular with the cranium and the lower jaw, respectively. Finally, the suspensorium has been analyzed as a part of the architectonic structure of the entire head by using a diagrammatic model (fig. 13) based on mutual influence, integration, and couplings.     

Feeding behavior and kinematics of the lesser electric ray, Narcine brasiliensis (Elasmobranchii: Batoidea)

Jaw protrusion is a major functional motif in fish feeding and can occur during mouth opening or closing. This temporal variation impacts the role that jaw protrusion plays in prey apprehension and processing. The lesser electric ray Narcine brasiliensis is a benthic elasmobranch (Batoidea: Torpediniformes) with an extreme and unique method of prey capture. The feeding kinematics of this species were investigated using high-speed videography and pressure transduction. The ray captures its food by protruding its jaws up to 100% of head length (approximately 20% of disc width) beneath the substrate and generating negative oral pressures (< or = 31 kPa) to suck worms into its mouth. Food is further winnowed from ingested sediment by repeated, often asymmetrical protrusions of the jaws (> 70 degrees deviation from the midline) while sand is expelled from the spiracles, gills and mouth. The pronounced ram contribution of capture (jaw protrusion) brings the mouth close enough to the food to allow suction feeding. Due to the anatomical coupling of the jaws, upper jaw protrusion occurs in the expansive phase (unlike most elasmobranchs and similar to bony fishes), and also exhibits a biphasic (slow-open, fast-open) movement similar to tetrapod feeding. The morphological restrictions that permit this unique protrusion mechanism, including coupled jaws and a narrow gape, may increase suction performance, but also likely strongly constrain dietary breadth.     

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 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.     

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.     

Jaw muscle (EMG) activity and amplitude scaling of jaw movements during eating in pigeon (Columba livia)

During each phase of the pigeon's eating sequence, jaw opening amplitude (gape) is adjusted to the size of the food object; first prior to contact (Grasping), again in positioning the food (Stationing), and finally, during its movement through the oral cavity (Intraoral Transport). Part I of this study examined jaw movement kinematics during ingestion of different size food pellets to determine the relative contribution of velocity and rise time variables. Part II specified the muscle activity patterns mediating each phase of the eating sequence, and determined how these patterns are modulated to produce adjustments of gape size.The relative contribution of velocity and rise time variables to the control of gape differs in each phase of the eating sequence. However, for any pellet size, variations in opening rise time may function in a compensatory manner to minimize gape undershooting. Each phase of the eating sequence is mediated by a characteristic muscle activity pattern. The adjustment of gape size to pellet size involves systematic modulation of this pattern, and the parameters modulated differ in the different phases in a manner which may reflect the functional requirements of each phase.Abbreviations AMEM adductor mandibulae externus muscle - DM depressor mandibulae muscle - EMG electromyographic - PDC/PDR pterygoideus muscle, pars dorsalis caudalis and rostralis - PQP protractor quadrati et pterygoidei muscle - PTP pseudotemporalis profundus muscle - PVL/PVM pterygoideus ventralis muscle, pars lateralis and medialis     

Functional morphology of the head of the anabantoid teleost fish Helostoma temmincki

Osteology, myology and motion analysis of the head of the anabantoid fish Helostoma temmincki, a specialized filter feeder, has revealed six functional units: neurocranium, suspensory apparatus, opercular apparatus, hyoid apparatus, branchial apparatus and pectoral girdle. Interactions between the functional units take place through four couplings involved in opening and protruding the jaws. The first coupling is activated in the beginning of the opening cycle by the levator operculi muscle through the opercular apparatus, interoperculomandibular ligament and mandible. The second is activated during feeding by contraction of the sternohyoideus through the hyoid apparatus, interopercular, interoperculomandibular ligament and mandible. The third coupling is active during feeding and “kissing” by contraction of epaxial muscles through mediation of the neurocranium to the jaw apparatus. The fourth coupling is the only one active during air intake and involves contraction of the levator arcus palatini which abducts and rotates the suspensory apparatus forwards, causing the mandible to drop. The retention of isolated ancestral characters during mosaic evolution are explained in terms of the maintenance of couplings which represent functional associations of seemingly remote structures. When natural selection acts on one component of a functional unit or coupling, it essentially acts on all associated elements simultaneously causing character complexes to evolve in common evolutionary trends. It is feasible that functional analysis can separate primary from secondary evolutionary trends.     

Gape size,its morphological basis,and the validity of gape indices in western diamond‐backed rattlesnakes (crotalus atrox)

Maximum gape is important to the ecology and evolution of many vertebrates, particularly gape‐limited predators, because it can restrict the sizes and shapes of prey that can be eaten. Although many cranial elements probably contribute to gape, it is typically estimated from jaw length or jaw width, or occasionally from a combination of these two measures. We measured maximum gape directly for 18 individuals of the western diamond‐backed rattlesnake, Crotalus atrox. We measured each individual's body length, several external cranial dimensions, several cranial osteological dimensions from cleaned skeletons, and we calculated gape index values from two published gape indices (GI). Cranial bone lengths and gape circumference showed negative allometry with snout–vent length (SVL), indicating that small individuals have relatively larger heads and gapes than their larger conspecifics. We then used Akaike's Information Criterion to determine which external and osteological measurements were the best predictors of gape. Body size (SVL) was the best predictor of maximum gape overall; however, when SVL was excluded from the analysis, quadrate (QL) and mandible lengths (MdLs) were the best predictors of maximum gape using both external and osteological measurements. Quadrate length probably contributes directly to gape; however, the importance of MdL to gape is less clear and may be due largely to its allometric relationships with head length and SVL. The two published GI did not prove to be better indicators of actual gape than the jaw and QLs in this study, and the gape values they produced differed significantly from our empirically determined gapes. For these reasons, we urge caution with the use and interpretation of computed GI in future studies. The extensive variation in quadrate and mandible morphology among lineages suggest that these bones are more important to variation in gape among species and lineages than within a single species. J. Morphol., 2013. © 2012 Wiley Periodicals, Inc.     

Odontocete suction feeding: Experimental analysis of water flow and head shape

The role of cranial morphology in the generation of intraoral and oropharyngeal suction pressures in odontocetes was investigated by manipulating the jaw and hyolingual apparatus of submerged heads of three species presenting varied shapes. Hyoid and gular muscles were manually employed to depress and retract the tongue. Pressures were recorded at three locations in the oral cavity, as gape and site, speed, and force of pull were varied. A biomechanical model was also developed to evaluate pressure data. The species with the shortest, bluntest head and smallest mouth opening generated greater negative pressures. Suction generation diminished sharply as gape increased. Greatest negative pressures attained were around -45 mmHg (-6,000 Pa), a magnitude deemed suitable for capture of small live prey. Odontocetes utilizing this bidirectional flow system should profit by evolution of a rounder mouth opening through progressive shortening and widening of the rostrum and jaws, a trend evident in cranial measurements from fossil and recent odontocetes. Blunt heads correlate with anatomical, ecological, and behavioral traits associated with suction feeding. Small-gape suction (with minimally opened jaws) could be used by odontocetes of all head and oral shapes to draw prey sufficiently close to the mouth for suction ingestion or grasping via dentition. Principal limitations of the experimental and mathematical simulations include assumption of a stationary odontocete with static (open or closed) jaws and potential scaling issues with differently sized heads and gapes.     

Modulatory complexity of the feeding repertoire in scincid lizards

The kinematics of jaws and tongue, and jaw muscle activity patterns were investigated in the omnivorous lizard Tiliqua rugosa, and the herbivorous Corucia zebrata (Scincidae) during feeding. Small metal markers were inserted into different parts of the skull, the jaws, and the tongue. Video and cineradiographic images were digitized and displacements of the head, jaws, and tongue were quantified. Additionally, muscle activity patterns were recorded, digitized and several variables were determined quantitatively. The effect of food type on the jaw and hyolingual movement patterns and the jaw muscle activity patterns was investigated for both species. The kinematic data indicate that distinct aspects of gape and tongue cycles are modulated in response to the food characteristics. Similarly, in both species, muscle activity patterns are altered in response to the type of food eaten. A comparison of kinematic and electromyographic patterns during intraoral transport cycles for both species shows that these can be related to food characteristics such as toughness and mobility. Differences between both species in the response to changes in food characteristics are minor. Clearly both species are able to fine tune the activation of the jaw muscles, resulting in the appropriate movement patterns for the type of food eaten. Accepted: 30 January 1999     

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 morphology of the jaw muscles of two species of imperial pigeons, Ducula aenea nicobarica and Ducula badia insignis

1. The functional morphological study of the jaw muscles of 2 species of Imperial Pigeons, Ducula aenea nicobarica and Ducula badia insignis has revealed that the structural variations of the bill, osteological and connective tissue elements, and muscles of the jaw apparatus may be correlated to functional diversity in the fruit-eating adaptation of these birds. 2. Both the species of Ducula possess moderately long, thick and stout bill with flexion zones inside, elongated orbital process of the quadrate, stout pterygoid, broad palatine and wide mandibular ramus on either side with increased retroarticular space. Such skeletal modifications together with increased orbital space indicate wide attachment-sites for the muscles, aponeuroses, tendons, and ligaments. 3. The morphology of the quadrato-mandibular joints suggests possible 'coupled kinesis' of the upper jaw, along with depression of the lower jaw. However, in a rhynchokinetic upper jaw as possessed by these birds, the kinesis is just moderate. Hence the gape of the mouth is mainly effected by the depression of the lower jaw, rather less so by the protraction of the upper jaw. 4. Among the functional groups of muscles, M. depressor mandibulae, M. adductor mandibulae externus, M. pseudotemporalis profundus, and M. pterygoideus are especially well developed. The various components of these muscles are provided with stiff as well as wide aponeuroses and tendons (much stronger than those observed in Columba), indicating forceful opening and closure of the beaks for plucking off the fruit, grasping it hard and manipulating it with the help of the beaks before swallowing. 5. The fleshy insertion of the outer slip of M. pseudotemporalis profundus extends ventrally over the dorsolateral surface of the mandible much more than it does in Columba. Further, 2 short and stiff aponeuroses at the rostral insertion of the inner slip of the muscle increase the force of adduction on the mandible. 6. M. adductor mandibulae posterior has not only wider origin and insertion, but also greater mass of fibres than that observed in Columba. 7. M. adductor mandibulae externus and M. pterygoideus form muscle-complexes with the predominance of bipinnate and multipinnate arrangements of fibres and with occasional joining fibres between their components. Such arrangements of fibres indicate sustained force-production, rather than faster movements of the jaw apparatus. 8. M. pterygoideus ventralis lateralis has a well developed 'venter externus' slip which has its thick and fleshy insertion on the outer lateral angular and articular mandible.(ABSTRACT TRUNCATED AT 400 WORDS)     

Multiaxial movements at the minke whale temporomandibular joint

Mandibular mobility accompanying gape change in Northern and Antarctic minke whales was investigated by manipulating jaws of carcasses, recording jaw movements via digital instruments (inclinometers, accelerometers, and goniometers), and examining osteological and soft tissue movements via computed tomography (CT)-scans. We investigated longitudinal (α) rotation of the mandible and mediolateral displacement at the symphysis (Ω₁) and temporomandibular joint (Ω₂) as the mouth opened (Δ). Results indicated three phases of jaw opening. In the first phase, as gape increased from zero to 8°, there was slight (<1°) α and Ω rotation. As gape increased between 20 and 30°, the mandibles rotated slightly laterally (Mean 3°), the posterior condyles were slightly medially displaced (Mean 4°), and the anterior ends at the symphysis were laterally displaced (Mean 3°). In the third phase of jaw opening, from 30° to full (≥90°) gape, these motions reversed: mandibles rotated medially (Mean 29°), condyles were laterally displaced (Mean 14°), and symphyseal ends were medially displaced (Mean 1°). Movements were observed during jaw manipulation and analyzed with CT-images that confirmed quantitative inclinometer/accelerometer data, including the unstable intermediate (Phase 2) position. Together these shifting movements maintain a constant distance for adductor muscles stretched between the skull's temporal fossa and mandible's coronoid process. Mandibular rotation enlarges the buccal cavity's volume as much as 36%, likely to improve prey capture in rorqual lunge feeding; it may strengthen and stabilize jaw opening or closure, perhaps via a simple locking or unlocking mechanism. Rotated lips may brace baleen racks during filtration. Mandibular movements may serve a proprioceptive mechanosensory function, perhaps via the symphyseal organ, to guide prey engulfment and water expulsion for filtration.     

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.     

The functional significance of morphological variation of the human mandible and masticatory muscles

A review is given of what is known about the functional significance of variation of the morphology of the human mandible and jaw muscles. First, the mandible is a lever transferring muscular forces to the teeth. The angle between corpus and ramus and the width of the ramus are particularly relevant in this respect as they determine the mechanical advantage of the lever system and the capacity for sagittal (open-close) movement. The stability of the mandible in asymmetric bites is especially affected by the ratio between the intermolar and intercondylar distances. The repertoire of bite forces that can be generated at any tooth and the loading pattern of the temporomandibular joint are strongly dependent on the relative size of the masseter, temporalis and medial pterygoid muscles. Second, executing its function as a lever, the mandible is subjected to shearing, bending and torsional forces. The bony parts harbouring the teeth, joints and muscle attachments serve to counter these forces; additional strength is needed in three areas i.e. in the symphysis, the condylar neck and in the transition area between corpus and ramus. In human populations there are clear-cut patterns of correlation between some facial skeletal traits, jaw joint morphology and strength and line of action of the jaw muscles. As a result, facial morphologies can be distinguished with marked differences in mechanical performance of their masticatory apparatus. It is suggested that they emerge as a result of diverging environmental influences during postnatal growth.     

The effects of opercular linkage disruption on prey-capture kinematics in the teleost fish Sarotherodon melanotheron

The kinematics of prey capture in blackchin tilapia (Sarotherodon melanotheron) subjected to three experimental treatments (control, anesthetization, and opercular linkage disruption) were analyzed using high-speed video to explore the role of the opercular four-bar linkage in depressing the lower jaw in teleost fishes. A series of two-way mixed model analyses of variance (random effects=fish; fixed effects=treatment) revealed that maximum gape, lower jaw angle, gape cycle, and time to lower jaw depression differed among treatments. Tukey post-hoc comparisons revealed that the opercular linkage disruption treatment differed from the control and anesthetization treatments, suggesting that severing the opercular linkage affected the ability of fish to depress the lower jaw. We hypothesize that although the opercular four-bar linkage system may not be the only linkage mechanism involved in depressing the lower jaw, it plays a very important role in opening the mouth during feeding in teleost fishes.