The muscles applied to the finite element models in this study were limited to the muscles primarily responsible for closing the jaw during biting: anterior temporalis, superficial and deep masseter and medial pterygoid. The muscle force data were derived from physiological cross sectional area data (PCSA), which were obtained from the dissection of a female chimpanzee (see Table 3 , Strait et al., 2009 (link)). These data were not scaled by experimental electromyographic (EMG) data because at the time of this FE study, in vivo feeding experiments had not yet been completed for chimpanzees, and thus these data were not available. Because of this, muscle forces were modeled as bilaterally symmetric and at peak (100%) activity level, which approximates maximal static biting. Resulting strain and reaction force magnitudes therefore represent the maximum values that are allowable physiologically, and likely exceed those present in life. However, prior studies (Ross et al., 2005 ; Strait et al., 2009 (link)) demonstrate that the effect of using bilaterally symmetrical forces has a minimal effect on the spatial distribution of strain concentrations, except insofar as strain magnitudes in certain balancing side regions are disproportionately high; strain distributions on the working side are largely unaffected. Fitton et al. (2012) (link) have further demonstrated that strain and deformation patterns are very conservative in the face of variation in muscle force activity.
Muscle forces were applied in all models by scaling the PCSA values by the bone volume of each model to the 2/3 power. This procedure ensures that larger models experience larger muscle forces; however the purpose of this approach is not to estimate true muscle forces in each of our models, since it is known that muscle PCSA scales with positive allometry to body mass (Perry and Wall, 2008 ). Rather, this scaling procedure allows us to eliminate size as a variable affecting strains. Thus, the differences in strain in our models only reflect differences in cranial shape and do not reflect differences in cranial size (Dumont et al., 2009 ). This allows for an assessment of the effect of shape on structural strength. In order to more closely approximate physiological loading, all of the models were loaded using the tangential plus traction function of BoneLoad, a program that simulates the physiological wrapping of muscle around rigid bony structures and extrapolates muscle forces to vectors (Grosse et al., 2007 ). Areas of muscle origin were modeled as force plates and focal coordinates were chosen by selecting insertion points for the muscles of mastication on the surface file of a mandible attached to the modeled cranium, using muscle markings as a guide. This had the effect of directing the force vectors for each muscle to run from origin to insertion.
Muscle forces were applied in all models by scaling the PCSA values by the bone volume of each model to the 2/3 power. This procedure ensures that larger models experience larger muscle forces; however the purpose of this approach is not to estimate true muscle forces in each of our models, since it is known that muscle PCSA scales with positive allometry to body mass (Perry and Wall, 2008 ). Rather, this scaling procedure allows us to eliminate size as a variable affecting strains. Thus, the differences in strain in our models only reflect differences in cranial shape and do not reflect differences in cranial size (Dumont et al., 2009 ). This allows for an assessment of the effect of shape on structural strength. In order to more closely approximate physiological loading, all of the models were loaded using the tangential plus traction function of BoneLoad, a program that simulates the physiological wrapping of muscle around rigid bony structures and extrapolates muscle forces to vectors (Grosse et al., 2007 ). Areas of muscle origin were modeled as force plates and focal coordinates were chosen by selecting insertion points for the muscles of mastication on the surface file of a mandible attached to the modeled cranium, using muscle markings as a guide. This had the effect of directing the force vectors for each muscle to run from origin to insertion.
