Fig. 6. The effect of gape angle and adductor mandibula geometry on regional strain variation (S). In all panels, gape angle is on the x-axis, theoretical manipulations of morphology are on the y-axis and S is on the z-axis. (A,B) Strain fields resulting from an MTC that is 24.5% muscle and (C,D) an MTC that is 100% muscle. (A,C) Modifications in the width of the muscle insertion; (B,D) the distance of the posterior insertion of muscle from the jaw joint. z-axis variables represent strain differences (S) between the anterior and posterior faces of the muscle, forming a topographic representation of strain heterogeneity. The shaded plateau region of each graph indicates morphologies where strains in the anterior and posterior muscle faces are nearly similar (±1.0% S in A,B; ±0.5% S in C,D). The broken green line represents natural morphology/geometry of the adductor mandibula of the ratfish. This natural morphology generally corresponds to regions of low strain variation across a broad range of gape angles.