Head myology of wild cavies (Caviidae, Caviomorpha) and functional implications of hystricomorphous and hystricognathous configurations

Abstract

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 would be linked to lower demands on precise oronasal movements and lesser snout sensitivity during 28 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 o ungulates (especially grazers) instead of other derived rodent morphotypes. Implications an variations of hystricomorphy and hystricognathy for caviines and other ctenohystricans ar discussed. Caviines are hystricomorphous rodents with a particularly short m zygomaticomandibularis and a small infraorbital part, which would limit gape and ensur homogeneous occlusal pressures. The m. masseter superficialis configuration, with a long origi tendon, and the fleshy bundles that wrap the mandible posteriorly extended, is here recognized as a key factor to understanding hystricognathy and propalinal grinding. Myology of cavies allows arguing that the “cavioid type” is a modification but not a reversion of hystricognathy.