Cranial strength in relation to estimated biting forces in some mammals

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.

Bone strain gradients and optimization in vertebrate skulls

“…Closer optimality of the mandible to feeding forces is also suggested by the high in vivo bone strain magnitudes recorded from the mandibles of a range of nonprimate mammals (opossums, Crompton 1995;rabbits, Weijs and De Jong 1977;dogs, Kakudo et al 1973), and by the many aspects of mandibular and dental morphology in extant and fossil mammals related to loading regimes deduced from in vivo studies (e. g., Bicknevicius and Ruff 1992;Bouvier 1986;Bouvier and Hylander 1996;Daeglin 2001Daeglin , 2002Jenkins et al 2002;Herring and Liu 2000;Hogue and Ravosa 2001;Hylander a, b, 1985Hylander , 1988Hylander and Bays 1979;Hylander and Johnson 1992;Liu and Herring 2000 a, b;Ravosa 1990Ravosa a, b, 1996Vinyard and Ravosa 1998;Wolff 1984). This hypothesis is also supported by Thomason's demonstration that the carnivoran mandible is better optimized for resisting feeding forces than the cranium (Thomason 1991). In addition, although bone strain data from the tetrapod rostrum are scant, crocodilians resemble mammals in their patterns of optimization, with the cross-sectional distribution of bony material in their mandible and rostrum being suggestive of fairly evenly distributed stresses under bending, torsion and shear, with decreased stresses in the back of the rostrum, around the orbits, and in the braincase (van Drongelen and Dullemeijer 1982;.…”

Using finite‐element analysis to investigate suture morphology: A case study using large carnivorous dinosaurs

“…An upper and lower estimate of muscle stress was used [147 and 392 kPa, respectively (Carlson and Wilkie, 1974;Thomason et al, 1990)] to bracket a perceived range of adductor force values. Pennation was not considered; therefore, muscle stress values may underestimate bite forces up to a factor of 2, as Thomason (1991) found using a similar method on dry mammalian skulls. Lever arm calculations were employed to convert adductor force values into upper and lower estimates of bite force and condylar force generated on adductor contraction [assuming all adductors were contracting maxi- , dorsal, and ventral view).…”

Bite Force Estimation and the Fiber Architecture of Felid Masticatory Muscles

“…We realize these are unrealistic assumptions but they are necessary given the state of the data. Furthermore, this type of estimate is common practice (e.g., Thomason, 1991;Wroe et al, 2005) and is considered to represent the bite force (BF) when the muscles are contracting maximally. We have also refined these estimates by incorporating EMG data on the working-side and balancing-side masticatory muscles (from Gorniak and Gans, 1980).…”