Simulating Squirrel Maximum Bite Forces

To test our first and second hypotheses, maximum bite forces were simulated. In order to obtain maximum bite forces that were comparable at each gape, the temporo-mandibular joint (TMJ) was modelled as a revolute joint (only permitting rotation in one degree of freedom). The DGO was set to follow a motion which opened the jaw to a distance of approximately 15.5 mm between the incisors, and then closed it again. Although no in vivo data on squirrel jaw motion during feeding currently exists in the published literature, studies of other rodents [34 (link),35 (link)] suggest that, during the power stroke of incision, mandibular motion is largely constrained to the vertical axis. Thus, the use of a revolute TMJ, producing a simple hinge movement of the mandible, was felt to be a reasonable approximation of incision in the squirrel. By contrast, mastication at the molar teeth in rodents involves a much wider range of highly complex jaw movements in all three axes. Without further experimental information on squirrels, it was felt that maximal molar biting could not be realistically simulated in our MDA models and so was not included in the analyses here (but see section below on non-maximal molar biting).
To simulate a maximum bite force, the translational spring damper was set with a high stiffness in each orthogonal direction, so that the food bolus did not deform. Maximum incisor bite force was calculated using three different sizes of food bolus: 2, 7.5 and 15 mm. The largest bolus size was chosen to represent the approximate diameter of a hazelnut. The smaller sizes are an acknowledgement that squirrels do not generally bite across the widest point of a nut, but rather gnaw with multiple, smaller bites.
In order to investigate how each masticatory muscle contributes to incisor biting, maximum incisor bites were calculated in a series of simulations, whereby in each simulation the activation of each bilateral pair of muscles was set to zero. This represented a ‘virtual ablation’ of each pair of muscles, as has been undertaken previously in finite-element analysis studies (e.g. [13 ,61 (link),62 (link)]). The maximum incisor bite force calculated in each simulation was then used to determine the percentage reduction in force, when compared to the maximum incisor bite force generated with all muscles active. The percentage reduction in bite force was compared across gapes to determine whether each pair of muscles performs better at narrow or wide gapes. In addition, the percentage reduction in bite force was also compared to the muscle's percentage contribution to total adductor force (table 1, as determined from muscle PCSA) to investigate if each muscle ‘overperforms’ or ‘underperforms’ relative to the theoretical maximum force it can produce.