Ultrastructural properties of muscles used for call production in neotropical frogs

On the basis of structure-function analyses of mammalian muscle, an axiom for the ultrastructural design of vertebrate skeletal muscle has emerged that predicts that capillary supply should match the mitochondrial oxygen demand set by levels of aerobic muscle activity. To test this prediction in frogs, I quantified the ultrastructure of male trunk muscles in relation to different levels of vocal activity, an aerobically supported natural behavior. Mitochondrial volume was positively correlated with mean calling rate among seven species of Neotropical frogs from two families (Hylidae and Leptodactylidae) that varied approximately 22-fold in levels of vocal activity. This correlation suggests that mitochondrial volume is the basic structural descriptor of trunk-muscle oxidative capacity. Capillary length per fiber volume was positively correlated with mitochondrial volume across species, as seen in mammalian muscle. Capillary-to-mitochondria ratio in trunk muscles was, on average, 47% lower than that observed in and predicted from mammalian muscle. These data support the hypothesis that anuran trunk muscles may exhibit a respiratory, design in which the microvasculature conforms to reduced mitochondrial respiration rates at tissue temperatures that are significantly lower than those of mammalian internal temperatures Intracellular lipid volume increased linearly with mean calling rate across species, which suggests that trunk-muscle lipids serve as an important endogenous fuel source during calling. The extent to which lipids enhance oxygen diffusion rates in trunk muscles of tropical frogs remains speculative because of potentially high depletion rates of these intramuscular lipid stores during calling. Results from this study suggest that the ultrastructural respiratory design of anuran trunk muscles may reflect the aerobic workload associated with a specific level of vocal activity and the thermal environment in which male frogs vocalize.