ULTRASTRUCTURAL PROPERTIES OF MUSCLES USED FOR CALL PRODUCTION IN NEOTROPICAL FROGS

On the basis of structure-function analyses of mammalian muscle, an ax iom for the ultrastructural design of vertebrate skeletal muscle has e merged that predicts that capillary supply should match the mitochondr ial oxygen demand set by levels of aerobic muscle activity. To test th is prediction in frogs, I quantified the ultrastructure of male trunk muscles in relation to different levels of vocal activity, an aerobica lly supported natural behavior. Mitochondrial volume was positively co rrelated with mean calling rate among seven species of Neotropical fro gs from two families (Hylidae and Leptodactylidae) that varied approxi mately 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 posit ively 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 mamma lian muscle. These data support the hypothesis that anuran trunk muscl es may exhibit a respiratory, design in which the microvasculature con forms to reduced mitochondrial respiration rates at tissue temperature s that are significantly lower than those of mammalian internal temper atures 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 whi ch 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 tr unk muscles may reflect the aerobic workload associated with a specifi c level of vocal activity and the thermal environment in which male fr ogs vocalize.