The maximum average bite force for a given jaw length

Major osteological landmarks were used to prepare idealized drawings of mammalian and reptilian lower jaws. Measurements from these drawings allowed the average output or bite force, along the entire jaw, to be calculated for many different anteroposterior positions of the input or muscle force. In the mammalian drawing, the maximum average bite force is exerted when the resultant force is located at about 30% of the way along the jaw from the joints. Because of geometric differences in the reptilian drawing, a resultant positioned at 20% of the way along the jaw exerts the maximum average bite force; a maximum force that is smaller than that in the mammalian case. The estimated location of the muscle resultant in actual cases corresponds to these calculated positions. Therefore, in real animals, the muscles are located in the position that produces the largest average force for any jaw length. The geometric changes necessary to transform the idealized reptilian drawing, with a smaller maximum average bite force, into that of the mammalian drawing, with a larger maximum force, are the same as those changes seen in the fossil record of the reptile/mammal transition. This finding suggests that the morphological changes that occurred in the jaws increased the average bite force in the primitive mammals.     

Influence of bite force on jaw muscle activity ratios in subject-controlled unilateral isometric biting

Ratios of muscle activities in unilateral isometric biting are assumed to provide information on strategies of muscle activation independently from bite force. If valid, this assumption would facilitate experiments as it would justify subject-control instead of transducer-based force control in biting studies. As force independence of ratios is controversial, we tested whether activity ratios are associated with bite force and whether this could affect findings based on subject-controlled force. In 52 subjects, bite force and bilateral masseter and temporalis electromyograms were recorded during unilateral biting on a transducer with varying force levels and with uniform subject-controlled force. Working/balancing and temporalis/masseter ratios of activity peaks were related to bite force peaks. Activity ratios were significantly but weakly correlated with the bite force. The subject-controlled force varied within ±25% around the prescribed force in 95% of all bites. This scatter could cause a variation of group mean activity ratios of at most ±6% because of the weak correlation between bite force and ratios. As this small variation is negligible in most cases, subject-control of bite force can be considered an appropriate method to obtain group means of relative muscle activation in particular when force control with transducers is not feasible.     

Incisal bite force direction in humans and the functional significance of mammalian mandibular translation

Incisal bite force direction was recorded and analyzed in ten human subjects using a specially designed force transducer. In all ten subjects the maxillary incisal bite force was vertically and anteriorly directed both during static biting and during biting associated with simultaneous mandibular translation and rotation. Since the resultant muscle force could not have been equal and opposite to the mandibular bite force, the mandibular condyles must have been loaded. These data demonstrate that the mandible acts as a lever during incisal biting and that there is no consistent relationship between incisal bite force direction and object size. In some individuals the bite force direction was more vertical during biting on a large transducer (30 mm high), while in other subjects it was more vertical during biting on a small transducer (10 mm high).     

Fracture in teeth—a diagnostic for inferring bite force and tooth function

Teeth are brittle and highly susceptible to cracking. We propose that observations of such cracking can be used as a diagnostic tool for predicting bite force and inferring tooth function in living and fossil mammals. Laboratory tests on model tooth structures and extracted human teeth in simulated biting identify the principal fracture modes in enamel. Examination of museum specimens reveals the presence of similar fractures in a wide range of vertebrates, suggesting that cracks extended during ingestion or mastication. The use of ‘fracture mechanics' from materials engineering provides elegant relations for quantifying critical bite forces in terms of characteristic tooth size and enamel thickness. The role of enamel microstructure in determining how cracks initiate and propagate within the enamel (and beyond) is discussed. The picture emerges of teeth as damage‐tolerant structures, full of internal weaknesses and defects and yet able to contain the expansion of seemingly precarious cracks and fissures within the enamel shell. How the findings impact on dietary pressures forms an undercurrent of the study.     

Resistance to change as a function of concurrent reinforcer magnitude

Six pigeons responded on two keys in each of three signalled multiple-schedule components, and resistance to disruption of responding on one (target) key by extinction and by response-independent food presented during inter-component blackouts was studied. Alternative reinforcement of different magnitudes was contingent on pecking a non-target key in two components, and in the third only the target response was reinforced. Resistance to change varied with the overall quantity of reinforcement in the component, regardless of whether reinforcers were contingent on the target or non-target response, but did not differ across the two key locations. These results using different magnitudes of reinforcement confirm previous findings using rate of reinforcement as the variable, and suggest that resistance to change is dependent on stimulus-reinforcer rather than response-reinforcer contingencies.     

The recruitment order of scapular muscles depends on the characteristics of the postural task

Previous studies show that the scapular muscle recruitment order could possibly change according to the characteristics of the postural task. We aimed to compare the activation latencies of serratus anterior (SA), upper, middle, and lower trapezius (UT, MT and LT, respectively) between an unpredictable perturbation (sudden arm destabilization) and a predictable task (voluntary arm raise) and, to determine the differences in the muscle recruitment order in each task. The electromyographic signals of 23 participants were recorded while the tasks were performed. All scapular muscles showed earlier onset latency in the voluntary arm raise than in the sudden arm destabilization. No significant differences were observed in the muscle recruitment order for the sudden arm destabilization (p > 0.05). Conversely, for voluntary arm raise the MT, LT SA and anterior deltoid (AD) were activated significantly earlier than the UT (p < 0.001). Scapular muscles present a specific recruitment order during a predictable task: SA was activated prior to the AD and the UT after the AD, in a recruitment order of SA, AD, UT, MT, and LT. While in an unpredictable motor task, all muscles were activated after the destabilization without a specific recruitment order, but rather a simultaneous activation.     

The orientation of the force of the jaw muscles and the length of the mandible in mammals

Ungulates generally have large masseter and pterygoid muscles and a necessarily large angular process provides attachment surface on the mandible. The temporalis muscle tends to be small. It has been suggested that this is an adaptation for enhanced control of the lower jaw and reduction of forces at the jaw joint. I suggest an additional reason: because of the geometry of the jaw, the length of that segment of the lower jaw that spans the distance from the jaw joint to the most posterior tooth is significantly reduced when the masseler and pterygoid are the dominant muscles; this region is necessarily much longer when the temporalis is large.     

Design and construction of a transducer for bite force registration

This study describes the development of a system for quantification of human biting forces by (1) determining the mechanical properties of an epoxy resin reinforced with carbon fiber, (2) establishing the transducer's optimal dimensions to accommodate teeth of various widths while minimizing transducer thickness, and (3) determining the optimal location of strain gages using a series of mechanical resistance and finite element (FE) analyses. The optimal strain gage location was defined as the position that produced the least difference in strain pattern when the load was applied by teeth with two different surface areas. The result is a 7.3-mm-thick transducer with a maximum load capacity beyond any expected maximum bite force (1500 N). This system includes a graphic interface that easily allows acquisition and registration of bite force by any health-sciences or engineering professional.     

Survivorship of an ant-tended membracid as a function of ant recruitment

I used a host-visitor modeling framework to examine the interaction between the treehopper Publilia concava and ants in the genus Formica. In particular, I tested the functional relationship between ant tending, the spatial distribution of treehoppers, and treehopper density. The per-capita density of ants at each host plant was a decreasing function of treehopper density, distance from the ant nest and the neighborhood density of treehoppers. Treehopper survivorship was proportional to the per-capita density of ants and the duration of ant tending. Consequently, treehoppers in low-density aggregations on isolated host plants near the nest received maximum benefit from ant tending. Treehoppers tended by the ant Formica integra were abandoned as the summer progressed, although many of these treehoppers were re-colonized by other species of ants. While F. integra ultimately abandoned all treehoppers, treehoppers on host plants with fewer initial ants were abandoned first. Results from the present study are consistent with previous findings suggesting that patterns of density-dependent benefit for homopterans are a function of the recruitment response of ants. Additionally, results suggest a tradeoff between maximizing the persistence or probability of ant-tending and minimizing competition for ants when tended. In general, host-visitor models of mutualism may provide a theoretical framework for understanding conditional outcomes in ant-homopteran, and other host-visitor mutualisms.     

The structure and function of the jaw muscles in the rat (Rattus norvegicus L.)

'Tonic' and 'phasic' muscle fibre types can be distinguished histologically, using either histochemical techniques or by staining for lipid with Sudan black B. As muscles of mastication not only move the lower jaw of the rat, a 'phasic' action, but also suspend it from the cranium, a 'tonic' activity, some indication of the contribution of the major muscles to these functions has been gained from an examination of the fibre content of transverse frozen sections stained with Sudan black B. The numbers of 'pale' ('phasic') and 'dark' ('tonic') fibres were counted using a montage at a magnification of 60. Results suggest that the anterior temporal, deep masseter and external pterygoid have an important tonic action in stabilizing the position of the lower jaw as well as contributing to the production of movement; and that the superficial masseter and posterior temporal, in particular, have an almost completely phasic action. These conclusions are entirely consistent with the probable functions of the muscles inferred from their anatomy.     

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.     

Feeding behaviour and bite force of sabretoothed predators

The feeding behaviour of extinct sabretoothed predators (machaeroidines, nimravids, barbourofelids, machairodonts and thylacosmilines) is investigated using beam theory. Because bite force applied along the mandible should be proportional to the external dimension of the mandibular corpus, patterns of variation in these dimensions at interdental gaps will reflect the adaptation of the jaw to specific loads, related to killing methods. Comparison of the mandibular force profiles of sabretooths to those of extant conical‐toothed carnivorans of known feeding behaviour reveals that sabretooths had a powerful bite, as strong or stronger than extant felids of similar mandibular length. Loads exerted at the lower canine were better constrained in the sagittal plane than in extant conical‐toothed carnivorans, indicating that prey was efficiently restrained when the sabre bite was delivered. The mandibular symphysis is generally better buttressed dorsoventrally in dirk‐toothed sabretooths than in scimitar‐toothed sabretooths, implying different killing strategies for the two ecomorphs: dirktooths delivered powerful sabre bites on prey they restrained with their forelimbs, while scimitartooths delivered slashing sabre bites and may have used their incisor battery to subdue their prey. The mandibular symphysis of Smilodon fatalis is less buttressed dorsoventrally than that of other dirk‐toothed sabretooths, possibly as a consequence of the greater torsional stresses induced while feeding rapidly on carcasses in response to intense competition. The mandibular symphysis of Thylacosmilus atrox is better buttressed dorsoventrally in juveniles than in adults, suggesting that young marsupial sabretooths underwent an extended period of parental care as typically observed in modern felids and inferred for eutherian sabretooths. Finally, machaeroidines and the nimravid Nimravus brachyops are exceptional in exhibiting a degree of dorsoventral buttressing of the mandibular symphysis that is intermediate between advanced sabretooths and conical‐toothed felids but similar to the extant Neofelis nebulosa, suggesting that the latter taxon may be close to the ancestral condition of a new sabretooth radiation. © 2005 The Linnean Society of London, Zoological Journal of the Linnean Society, 2005, 145 , 393–426.     

Evolution of bite force in Darwin's finches: a key role for head width

Studies of Darwin's finches of the Galapagos Islands have provided pivotal insights into the interplay of ecological variation, natural selection, and morphological evolution. Here we document, across nine Darwin's finch species, correlations between morphological variation and bite force capacity. We find that bite force correlates strongly with beak depth and width but only weakly or not at all with beak length, a result that is consistent with prior demonstrations of natural selection on finch beak morphology. We also find that bite force is predicted even more strongly by head width, which exceeds all beak dimensions in predictive strength. To explain this result we suggest that head width determines the maximum size, and thus maximum force generation capacity of finch jaw adductor muscles. We suggest that head width is functionally relevant and may be a previously unrecognized locus of natural selection in these birds, because of its close relationship to bite force capacity.     

Ontogenetic scaling of bite force in lizards and turtles

Because selection on juvenile life-history stages is likely strong, disproportionately high levels of performance (e.g., sprint speed, endurance, etc.) might be expected. Whereas this phenomenon has been demonstrated with respect to locomotor performance, data for feeding are scarce. Here, we investigate the relationships among body dimensions, head dimensions, and bite force during growth in lizards and turtles. We also investigate whether ontogenetic changes in bite performance are related to changes in diet. Our analyses show that, for turtles, head dimensions generally increase with negative allometry. For lizards, heads scale as expected for geometrically growing systems. Bite force generally increased isometrically with carapace length in turtles but showed significant positive allometry relative to body dimensions in lizards. However, both lizards and turtles display positive allometric scaling of bite force relative to some measures of head size throughout ontogeny, suggesting (1) strong selection for increased relative bite performance with increasing head size and (2) intrinsic changes in the geometry and/or mass of the jaw adductors during growth. Whereas our data generally do not provide strong evidence of compensation for lower absolute levels of performance, they do show strong links among morphology, bite force, and diet during growth.     

Posterior and anterior components of force during bite loading

Late anterior crowding of teeth has been associated with the anterior component of force (ACF) developed during biting. Possible physiologic mechanisms countering ACF, including the presence of a posterior component of force (PCF), are hypothesized. In this self-controlled study, 60 subjects aged 27.05+/-3.9 years were examined for ACF and PCF that were calculated as the change in tightness of a mandibular dental contact points from non-biting to biting state. Both ACF and PCF were found to develop simultaneously. However, the PCF was 4-7 folds smaller than the ACF (p<0.001). The ACF progressively declined by 10-20 folds (p<0.001) from the posterior to anterior dentition. The lateral incisor-canine contact point had the greatest ACF decline (63-74%). ACF effect on the anterior dentition is counteracted by a protective mechanism consisted of PCF, progressive dissipation of ACF, and canine blockage.     

Double function of aptychi (Ammonoidea) as jaw elements and opercula

Lehmann, U. & Kulicki, C. 1990 10 15: Double function of aptychi (Ammonoidea) as jaw elements and opercula. Lethaia , Vol. 23, pp. 325–331. Oslo. ISSN 0024–1164.
Aptychi are calcitic coverings on the outer surface of organic ammonite lower jaws. They are similar in shape to that of the corresponding ammonite apertures. This observation and additional features of many aptychi are in harmony with their former interpretation as protective opercula. We suggest that they served as opercula in addition to functioning as jaws. The primary function of the lower jaws was thus secondarily extended to that of protective shields when they acquired their calcitic covering, while as lower jaws their importance dwindled to that of a more passive abutment. Phylogenetically, this seems to have started slowly in some anaptychi and became obvious with the first aptychi. ▭ Ammonites, aptychus, operculum, jaw apparatus, evolution, function .     

Changes in gravity rapidly alter the magnitude and direction of a cellular calcium current

In single-celled spores of the fern Ceratopteris richardii, gravity directs polarity of development and induces a directional, trans-cellular calcium (Ca²⁺) current. To clarify how gravity polarizes this electrophysiological process, we measured the kinetics of the cellular response to changes in the gravity vector, which we initially estimated using the self-referencing calcium microsensor. In order to generate more precise and detailed data, we developed a silicon microfabricated sensor array which facilitated a lab-on-a-chip approach to simultaneously measure calcium currents from multiple cells in real time. These experiments revealed that the direction of the gravity-dependent polar calcium current is reversed in less than 25 s when the cells are inverted, and that changes in the magnitude of the calcium current parallel rapidly changing g-forces during parabolic flight on the NASA C-9 aircraft. The data also revealed a hysteresis in the response of cells in the transition from 2g to micro-g in comparison to cells in the micro-g to 2-g transition, a result consistent with a role for mechanosensitive ion channels in the gravity response. The calcium current is suppressed by either nifedipine (calcium-channel blocker) or eosin yellow (plasma membrane calcium pump inhibitor). Nifedipine disrupts gravity-directed cell polarity, but not spore germination. These results indicate that gravity perception in single plant cells may be mediated by mechanosensitive calcium channels, an idea consistent with some previously proposed models of plant gravity perception.