Results In all our FE models the highest concentration of von Mises stresses occurred in the most anterior aspect of the skull (Figs. 4 and 5 and Table 1). The anterior connective tissue partitions within the junk were subjected to higher tensile loading than the posterior portions (Fig. 6). Tension in the connective tissue partitions redistributed compressive stresses across the skull (Models A and B) and the absence of the partitions (Model C) raised anterior skull stresses (Figs. 4 and 5 and Table 1). Figure 4: Von Mises stress distribution results. Figure 5: Region definitions (blue vertical bars). % increase 1 2 3 4 5 Max. Mean Min. Max. Mean Min. Max. Mean Min. Max. Mean Min. Max. Mean Min. Model A– Model C 42.9 45.7 3.6 25 24.1 9.4 −8.8 6.1 60.8 −6.5 −0.9 −27.3 1.4 −0.3 −62.8 Model B– Model C 10.1 15.5 12.8 −7.3 4.1 35.7 0.6 0.8 −6.9 −6.7 −1.4 −27.4 1.4 −0.2 −59.6 DOI: 10.7717/peerj.1895/table-1 Figure 6: Maximum principal stress distributions across the connective tissue partitions. Positive and negative stresses indicate areas of tension and compression respectively. Figure 7: Probabilistic FEA simulation. Z statistic distributions depicting mean elemental von Mises stress differences divided by elemental standard deviation under an assumed population material stiffness variance of 10%. Data are thresholded at alpha = 0.01. A reduced number of partitions (Model B) did reduce stresses in the anterior skull, but stress reduction was not as effective as Model A (Figs. 4 and 5 and Table 1). The skull stress difference distributions resulting from Monte Carlo simulations suggest that our main finding regarding the load-redistribution functionality of the connective tissue is insensitive to relatively large changes in both material parameters (Fig. 7) and, indirectly, load magnitudes. Our sensitivity analysis of the load location further supported our findings that the connective tissue partitions reduce stresses on the anterior skull; however, stress magnitudes on the skull and the connective tissue case were sensitive to variations of the load location (Figs. S2–S6). In all comparisons, the highest stresses were found to occur in Model C, which lacked the connective tissue partitions and the lowest stresses occurred in Model A, with the twelve connective tissue partitions (Figs. S2–S6). Load application on the spermaceti organ generated lower skull stresses than an impact load on the junk (Figs. S2–S6), yet it increased stress concentrations on the superior aspect of the connective tissue case and the rostral end of the spermaceti organ, where the sound generator of the sonar system (monkey lips) is housed (Fig. 8). Although, impact load on the junk created higher skulls stresses (Figs. S2–S6), it created lower connective case stresses (Fig. 8) than an impact load on the spermaceti organ. In addition, as the loading site on the anterior junk moved inferiorly, the connective case stresses were reduced, whereas the skull stresses increased and the connective tissue partitions underwent increased tension. From the top of the anterior junk to the bottom, variations in the angle of the applied force on the junk increased skull stresses on the posterior aspect of the skull in all models, yet the model without partitions (Model C) showed the highest stress concentration across the whole skull and thus the least resistance to bending (Fig. S6). Figure 8: Von Mises stress distribution results following variations in the topological application of the impact force. (A) Impact force on the spermaceti organ; (B) Impact force on the superior aspect of the spermaceti junk; (C) Impact force on the mid spermaceti junk; (D) Impact force on the inferior aspect of the spermaceti junk; (E) Impact force on the entire anterior aspect of the spermaceti junk. Warm (red) and cold (blue) colors show higher and lower von Mises stresses respectively. The results of our sensitivity analysis on the partition thickness showed that the skull stresses are insensitive to variations in the partitions’ dimensions (Fig. S7). Model A with the simplified partitions (Fig. S7) showed slightly decreased stresses on the mid-anterior aspect of the skull than Model A2, with partition thickness according to Clark’s observations. Nevertheless, the skull von Mises differences were minimal and do not influence the general comparisons between Models A (with twelve connective tissue partitions) and Model C (with no connective tissue partitions). This raises confidence that the model simplifications on the partition design for Models A and B did not compromise the biological results of our study. From solely an engineering perspective, Model A with the higher number of connective tissue partitions had higher overall stiffness and potentially lower stress response than Model B with fewer partitions and Model C with no partitions. To test the potential effect of system stiffness on our model comparisons, we measured the reaction forces at the back of the skull (location of the constraints) in all models, after applying a uniform horizontal displacement 0.1 m at the load application location on the junk. The results gave a reaction forces of 9.56E + 06, 9.16E + 06 and 7.8E + 06N for Models A, B and C respectively (Supplemental Information 2 and 3). This suggested that whilst the stiffness between Models A and B was quite similar, Model C was substantially different. To ensure that the comparisons between the models reflected a true effect of architectural design rather than solely being due to the changes in the system stiffness, we varied the stiffness of Models B and C by changing the Young’s modulus of the connective tissue until we obtained similar reaction force for all models (Fig. S8). Comparisons of von Mises stress between Models A, B, and C with equivalent stiffnesses further supported our findings that Model C increases stress on the anterior aspect of the skull (Fig. S8).