Properties enhancing aerobic capacity of calling muscles in gray tree frogs Hyla versicolor.

The muscle features that accommodate the extraordinarily high aerobic respiration during calling by the gray tree frog Hyla versicolor were examined. We compared the muscles used for calling by males (external and internal obliques and laryngeal muscles) with the homologous muscles of females and with the leg muscles of males and females. The leg muscles consisted of 75% by volume fast glycolytic fibers, a composition typical of other muscles described in anuran amphibians. In contrast the calling muscles of males consisted of 100% fast oxidative fibers and had citrate synthase (CS) activities among the highest recorded for ectothermic vertebrates, 65-80 mumol X min-1 X g fresh mass-1. We also noted a strong sexual dimorphism in size and oxidative capacity of these muscles. The external and internal obliques of females weighed an order of magnitude less than the corresponding muscles of males and had CS activities of only 6 mumol X min-1 X g-1. Morphometric measurements of transmission electron micrographs revealed that the calling muscles of males contained high mitochondrial densities (approximately 20% of fiber volume) and capillary densities (approximately 700 mm-2) compared with a representative hindlimb muscle, the sartorius (mitochondrial density, 6% of fiber volume; capillary density, 230 mm-2). These frog muscles, which operate at approximately 20 degrees C, have lower capillary densities per mitochondrial volume than are found in mammalian muscles that function at higher temperatures.

Water-vapor conductance of testudinian and crocodilian eggs (class reptilia).

Flexible-shelled eggs of snapping turtles (Chelydra serpentina) have conductances to water vapor that are 55 times higher than predicted for avian eggs of similar size, whereas rigid-shelled eggs of softshell turtles (Trionyx spiniferus) and American alligators (Alligator mississippiensis) have conductances that are only five times higher than expected for comparable eggs of birds. The differences between empirical and predicted values result from the much higher effective pore areas in reptilian eggshells than in those of birds. The relatively high porosities of these reptilian eggs presumably facilitate the transport of oxygen and carbon dioxide eggshells in later stages of incubation when air trapped inside nest chambers may become hypoxic and hypercapnic, yet seem not to lead to excessive transpiration of water vapor owing to the high humidities in nests where incubation occurs.