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Cranial strength in relation to estimated biting forces in some Mammals

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Abstract

The mammalian skull has proven to be remarkably plastic during ontogeny and phylogeny in response to the demands of mastication. I examine whether the bending strength of the skull in some mammals correlates with the maximal loads imposed through the masticatory apparatus. The approach is analytical, using the methods of beam theory. Cranial strength is estimated from the second moment of area and other geometrical measurements made from 20–30 transverse CT scans through the skulls of 20 opossums (Didelphis virginiana), and through single skulls of five felid and five canid genera of different sizes. Maximal biting forces were first estimated from areas on the dried skulls bounding the spaces filled in life by the jaw-adducting muscles. These estimates were then adjusted with reference to forces recorded in vivo or, for other specimens, to estimates based on dissections of the jaw muscles. Stress distribution in the face, and peak stresses, were calculated for each animal. Stress levels are low (5–35 MPa) compared with peak stresses in limb bones (40–100 MPa), which correlates with the lower in vivo strains in cranial bones reported in the literature. Stress estimates are in a range that is plausible, which supports the validity of the procedure. Patterns of stress distribution along the face are comparable within each group of animals. Peak stress is independent of size for the carnivorans, but decreases with increasing skull length in D. virginiana. High bending strength of the skull is a consequence of cranial form in mammals; having to enclose the brain, for example, increases the bending strength of the skull. Furthermore, factors such as stiffness or shear and torsional strength may be more important than bending strength. However, bending stress levels appear to be closely regulated, as in other bones that have been studied. The threshold for optimising bending strength and weight is simply at a different level.
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... To evaluate the outlever lengths, 16 landmarks were digitized onto the images using tpsDig version 2.31 (Rohlf, 2017), the configuration of which is shown in Fig. 1. The outlever length has been measured as the anteroposterior distance from the jaw joint to the bite point at the teeth in the lateral view of the cranium in previous studies (Christiansen & Wroe, 2007;Thomason, 1991). We obtained the outlever lengths by measuring the anteroposterior distances from the position of the jaw joint to each of the posterior ends of the molar row (OL-M-post); the anterior end of the molar row (OL-M-ante), which is approximately the same as the posterior end of the fourth (last) premolar (P4) as a carnassial; the anterior end of P4 (OL-P4-ante) and the canine (OL-C). ...
... We obtained the outlever lengths by measuring the anteroposterior distances from the position of the jaw joint to each of the posterior ends of the molar row (OL-M-post); the anterior end of the molar row (OL-M-ante), which is approximately the same as the posterior end of the fourth (last) premolar (P4) as a carnassial; the anterior end of P4 (OL-P4-ante) and the canine (OL-C). This study focused on the outlever length relative to the inlever length because it is a determinant of biting force and speed (Greaves, 1978(Greaves, , 1983Preuschoft & Witzel, 2005;Thomason, 1991;Van Valkenburgh & Ruff, 1987). The outlever and the inlever in the cranium scale with the overall cranial size, although the outlever length can vary substantially with changes in the length of snout or jaw or the positioning, number and size of teeth. ...
... This approach is more appropriate for controlling the effect of a continuous variable on the trait of interest than using the ratio to the variable or taking the residual from the regression against the variable (Freckleton, 2009;Garc ıa-Berthou, 2001). The anteroposterior distance from the jaw joint to the midpoint of the anterior and posterior ends of the infratemporal fossa (from jaw joint to infratemporal fossa) was used as a measure of the inlever length, as this distance roughly corresponds to the inlever length based on the approximate cross-sectional area of the masseter-medial pterygoid in the ventral view of the cranium (Thomason, 1991). To calculate these distances, a perpendicular line was drawn from each landmark to the midline of the cranium (the line connecting landmarks 1 and 16; Fig. 1), and the point of the intersection was used to determine the anteroposterior position of the landmark. ...
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Species in the mammalian order Carnivora have extremely diverse diets. The association between diet and craniodental morphology in carnivorans has been the subject of a number of studies. The distance from the jaw joint to the tooth positions may contribute to the ability to acquire and process food because it corresponds to the outlever arm when the jaw functions as a lever to generate a bite force. A shorter outlever arm relative to the inlever arm of the masticatory muscle generates a higher bite force. This study measured the distances from the jaw joint to different points of the teeth as the outlever lengths in the crania of terrestrial Carnivora species to show that outlever lengths corrected for phylogeny and a measure of the inlever length differ according to dietary habits among carnivorans. The distance from the jaw joint to the last molar was shortest in folivores, followed by aquatic prey specialists, suggesting that consumption of tough plant materials and, to some extent, aquatic prey with hard exoskeletons has favoured the evolution of a shorter outlever to allow stronger bites with enlarged molars. In contrast, among Canidae species, a shorter outlever to canines was associated with feeding on large prey, but this association was not found across carnivorans, suggesting that the correlated evolution of a shorter outlever at the canines and specialization for feeding on large prey depends on foraging and hunting behaviours. Combined, these findings provide some evidence that distances from the jaw joint to different points of the teeth are adapted to different feeding ecologies in carnivorans. In craniodental morphology, the distance from the jaw joint to the teeth corresponds to the outlever arm when the jaw functions as a lever to generate a bite force, with a shorter outlever converting more muscle force into bite force. We performed phylogenetic comparative analyses to show that the outlever lengths at different points of the teeth in the cranium differ according to dietary habits in the mammalian order Carnivora. This study provides evidence for adaptations of the distance from the jaw joint to the teeth to different feeding ecologies in Carnivora.
... In a first step, each muscle was modelled using the new method to create a base model before further refinement. In a second step, the muscle user enters folder and file name for saving data (1) user enters muscle name (2) code creates empty with that muscle name (3) user selects bone on which muscle originates (4,5) user draws on muscle origin by selecting faces (6) code makes origin area and boundary objects (7) user selects bone on which muscle inserts (8) user draws on muscle insertion by selecting faces (8) code makes insertion area and boundary objects code matches counts in boundary loops (9), code creates muscle curve template bevels with cross section of origin (10) *if necessary*, user mirrors the cross section to match the origin (11) user adjusts tilt of curve (12) user adjusts bevel extent (13) user adjusts points of curve to get desired shape (14) code converts curve to mesh (15) user adjusts muscle mesh to match insertion (16) code bridges muscle mesh with origin and insertion (17) code resets add-on for new muscle (18) user can adjust muscle meshes iteratively (19) code calculates volumes of all muscles in scene, adds metric to .csv file (20) in above steps, code generates this hierarchy of new objects royalsocietypublishing.org/journal/rsos R. Soc. ...
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Accurate muscle reconstructions can offer new information on the anatomy of fossil organisms and are also important for biomechanical analysis (multibody dynamics and finite-element analysis (FEA)). For the sake of simplicity, muscles are often modelled as point-to-point strands or frustra (cut-off cones) in biomechanical models. However, there are cases in which it is useful to model the muscle morphology in three dimensions, to better examine the effects of muscle shape and size. This is especially important for fossil analyses, where muscle force is estimated from the reconstructed muscle morphology (rather than based on data collected in vivo). The two main aims of this paper are as follows. First, we created a new interactive tool in the free open access software Blender to enable interactive three-dimensional modelling of muscles. This approach can be applied to both palaeontological and human biomechanics research to generate muscle force magnitudes and lines of action for FEA. Second, we provide a guide on how to use existing Blender tools to reconstruct distorted or incomplete specimens. This guide is aimed at palaeontologists but can also be used by anatomists working with damaged specimens or to test functional implication of hypothetical morphologies.
... 1). Damasceno's and Sakamoto's methods are based on Thomason's initial method (Thomason 1991); however specific alterations have been made. The initial formula used for the calculation of bite force is: F = 2 * [dm * (M * 300KPa)+dt * (T * 300Kpa)] c , where M and T, as shown in Damasceno et al. (2013: fig. ...
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... In this taxon the postorbital half of the skull is particularly low, sloping Table 1. Geometric measurements of reconstructed muscles and estimated contraction force (Fmus = (volume / length) × 0.3 N/mm 2 × 1.5 32,34 ). Insertion angles of muscles measured in the sagittal ( α ) and coronal ( β ) planes used to calculate resultant vertical force acting on mandible ( Fres = Fmus × cosα × cosβ). ...
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... Several functional and behavioural factors could affect one or the other component of these interactions. For example, suckling, gnawing and chewing movements have different functional requirements [22] and different diets will imply a different distribution of stresses [23]. Behaviours in relation to siblings or conspecifics (fights for example, [24]) will also have functional demands on the jaws through biting [25]. ...
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... Given that the endocranium and the musculature of the skull are not independent of each other, we calculated the bite force of each specimen using Thomason's dry method ( Figure S2), which estimates skull musculature relative to body mass 35,66 . Measurements of the cross-sectional areas, in-lever, and out-lever moment arms of the masseter and temporalis muscles were taken on photos of the dorsal, ventral, and lateral views of the cranium and lateral view of the mandible via Fiji 67 . ...
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An experiment to quantitate the effect of compressive stress upon bone remodeling rate was devised. Remodeling herein is described as the renewal process of bone separate from initial formation. A finite element mathematical model of the rabbit calvarium was devised. Actual rabbit calvaria were stressed utilizing a load cell monitored lever type loading device. They were periodically stained with fluorescing bone seeking dyes. Upon necropsy, histologic sections were prepared and the growth rate at known locations in the stressed area was measured by planimetry.