Head Myology of Wild Cavies (caviidae, Caviomorpha) and Functional Implications of Hystricomorphous and Hystricognathous Configurations

Cavies are one of the most-specialized grinding rodents, with folivorous and grass-based diets. Antecedents on head myology are focused mainly on domestic guinea pig. Here, we analyze three specimens of wild caviines (two of Cavia aperea and one of Galea leucoblephara), describing, illustrating, mapping, and weighing facial and masticatory muscles. Overall, facial musculature in caviines is simpler, and some muscles are smaller than other rodents. This could be linked to lower demands on precise oronasal movements and lesser snout sensitivity during searching and prehension of food. Conversely, mm. malaris and buccinator are enlarged, suggesting intense processing of the bolus during chewing. Cavies, particularly the grass-eater Cavia, have highly modified osteo-myological masticatory apparatuses. The length and horizontalization of mm. masseter superficialis, profundus, and pterygoideus medialis are exacerbated. The latter two muscles are more developed than in other caviomorphs and most non-caviomorph rodents, enhancing horizontal grinding. Although framed in a rodent plan, functional characterization of facial and masticatory muscles partially resembles that of ungulates (especially grazers) rather than other derived rodent morphotypes. Implications and variations of hystricomorphy and hystricognathy for caviines and other ctenohystricans are discussed. Caviines are hystricomorphous rodents with a particularly short m. zygomaticomandibularis and a small infraorbital part, which would limit gape and ensure homogeneous occlusal pressures. The m. masseter superficialis configuration, with a long origin tendon and the extended fleshy bundles that wrap the mandible posteriorly, is here recognized as a key factor for understanding hystricognathy and propalinal grinding. Myology of cavies allows arguing that the "cavioid type" is a modification but not a reversion of hystricognathy.