Functional morphology and fiber types of the masseter muscles of two caviomorph rodents with contrasting lifestyles, Ctenomys talarum (Ctenomyidae) and Cavia aperea (Caviidae)

The aim of this work is the analysis of histochemical and morphometric properties of the masseter muscles of Ctenomys talarum and Cavia aperea. The former belongs to a subterranean rodent clade, Ctenomyidae, which has evolved a robust masticatory apparatus adapted to chisel-tooth digging and processing of abrasive grasses; C. aperea belongs to the family Caviidae, with relatively graceful jaws and mandibular musculatures, consistent with less mechanically challenging diets. Adult males were captured, immediately transported to the laboratory, and euthanized in a CO₂ chamber. The musculus masseter superficialis and musculus masseter profundus on the left side of the animals were used to analyze the histochemical composition of the fiber types treated with myosin adenosine triphosphatase, succinate dehydrogenase and periodic acid Schiff. The mean fiber diameters, relative areas, and frequencies of each muscle fiber type were calculated. The mm. masseter superficialis and masseter profundus on the right side were used to measure the physiological cross-sectional area (PCSA). Based on this measurement, the internal force (F) was estimated. In the m. masseter profundus of both species and in the m. masseter superficialis of C. aperea intermediate fast oxidative-glycolytic fibers (FOGi) predominated. In the mm. masseter superficialis and masseter profundus of C. talarum the relative area of fast glycolytic (FG) fibers was greater than that of the muscles of C. aperea, whose main muscle fiber component is FOGi fibers. When corrected for body mass differences, PCSA was higher for the mm. masseter superficialis of C. talarum. This and the larger relative area of FG fibers, probably contributes to the exertion of large bite forces in C. talarum, as measured in previous studies.