Mastication is a fundamental activity undertaken by mammals and the extent of oral food processing distinguishes them from other vertebrate groups. In order to break down food effectively, the mammals have evolved a complex set of masticatory muscles (Turnbull, 1970). Mammalian jaw-closing musculature comprises three distinct muscle groups: the temporalis, running from the temporal region of the skull posterior to the orbit to the coronoid process of the mandible; the masseter, which originates on the zygomatic arch and inserts on the ventrolateral surface of the mandibular ramus; and the pterygoid muscles, which run between the pterygoid region ventral to the orbit and the ventromedial surface of the mandible (Becht, 1953). The relative proportions of these muscles vary amongst mammals in a manner related to their diet and mode of feeding. Carnivores show an arrangement in which the temporalis is considerably larger than the masseter and pterygoids (Maynard Smith and Savage, 1959). Because the temporalis has a posterodorsal line of action and the squamosal is folded over the condyle to form a robust hinge, carnivores are able to produce powerful bite force at the teeth, whilst resisting the forward pull of struggling prey and thus maintaining the structural integrity of the jaw joint. In addition, the small, aborally positioned masseter muscle allows for a large gape and the ingestion of large food items (Becht, 1953). Conversely, herbivores display a configuration of enlarged masseter and pterygoid muscles and a reduced temporalis (Turnbull, 1970). The masseter extends further forward in herbivores and can thus work as a second-order lever and exert considerable masticatory pressure at the molar teeth (a clear advantage for grinding plant material). Furthermore, the complex, layered nature of the masseter and the open morphology of the temporo-mandibular joint allow mediolateral and anteroposterior movements of the mandible (Maynard Smith and Savage, 1959).
The masticatory musculature of rodents represents an extreme version of the herbivore configuration described above. In many rodents, the temporalis has reduced in size and the masseter has become the overwhelmingly dominant muscle, comprising around 70% of the entire masticatory muscle mass (Turnbull, 1970; Cox and Jeffery, 2011).