PETERS, S.E.*; CLARK, D.L.; Univ. of North Carolina at Charlotte: Comparison of forelimb muscle properties in two species of anurans: continuity of muscle function in amplexus

Forelimb muscles of frogs provide a model to study diversity of skeletal muscle structure and function among the sexes. Males differentially use forelimb flexor muscles to establish and maintain amplexus. The sexual dimorphism in size of these muscles has long been known, but few direct measurements of contractile properties have been made. In an earlier study, we found that dimorphic muscles in male bullfrogs (Rana catesbeiana) produce greater force at low stimulus frequencies, have slower relaxation times and fatigue more slowly than in females. In an effort to see whether these functional differences are unique to bullfrogs or represent common adaptive features of male frogs, we examined the same contractile properties in forelimb muscles of the marine toad (Bufo marinus). We found that marine toads have similar modifications to produce slow relaxation times and low fatigability in males as were found in bullfrogs. Most importantly, they display the same extreme elongation of relaxation times during intermittent stimulation as was found in bullfrogs. This results in high levels of force maintained with low stimulus input and accounts for the low fatigability in both species. Since bullfrogs and marine toads are distantly related neobatrachians, evidence of common muscle properties supports the hypothesis that amplexus is a sympleisiomorphic feature of the entire group.