PETERS, S.E.: Muscle fiber types in sexually dimorphic forelimb muscles of the bullfrog, Rana catesbeiana
Anuran forelimb muscles associated with medial and dorsiflexion of the wrist, abduction of the thumb, and elbow flexion are larger in males than in females. It is likely that these muscles are enlarged in males due to their use in amplexus. An earlier study found that several muscles of the bullfrog forelimb are sexually dimorphic in total isometric force, with the male muscles producing much larger forces than the females’. However, the force/cm2 of muscle cross-section did not differ between sexes, indicating that force differences are a direct result of size differences. Other results, however, notably slower contraction and relaxation times and lower fatigability in male muscles, showed that differences are not in size alone. The current study found that the most sexually dimorphic muscle, flexor carpi radialis (FCR), differs in muscle fiber types and muscle fiber areas between the sexes. Histochemical and immunohistochemical staining showed that males have a larger number of putatively tonic muscle fibers. The larger average size of fibers in the male resulted not from unusually large male fibers, but because there is a well-defined compartment within the female FCR composed of much smaller fibers than in the males. These fibers were also smaller on average than fibers throughout the rest of the female muscle and to those found in a non-dimorphic control. In addition, the mean fiber areas did not differ between males and females in areas outside this isolated compartment. Preliminary results of citrate synthase and lactate dehydrogenase activities show higher oxidative capacity in the FCR of the male. Thus, differences in fiber types with their associated functional diversity may explain the physiological aspects of sexual dimorphism in forelimb muscles of bullfrogs.
