Effects to Muscle Structure

One of the greatest risks to organisms that undergo extended periods of dormancy is muscle disuse atrophy, or the wasting away of muscle tissue. Muscle disuse atrophy includes "a reduction in muscle cross-sectional area and compromised locomotory function" (Symonds et al., 2007). Numerous studies show that during aestivation there is insignificant loss in muscle mass in Cyclorana or in the number of muscle fibres (2007). Swimming tests show that, immediately after emerging from their burrows, post-aestivation individuals performed the same as control individuals (Hudson and Franklin, 2002a).

Significant differences have been shown between the muscle atrophy of animals that commonly undergo dormancy and those that do not. Even among those animals that do, Cyclorana are just as good if not better at preventing muscle atrophy (Hudson and Franklin, 2002b).

(Adapted Table 1 and Table 2 of Hudson and Franklin 2002b)

Aestivating Cyclorana are able to avoid muscle disuse atrophy a couple of ways. First, the curled position of the frog may help prevent atrophy. During aestivation, the hindlimb muscles are in a lengthened position, which studies have shown can delay the development of muscle atrophy (Symonds et al., 2007). A positive correlation has been found between the severity of muscle atrophy and metabolic depression. Since Cyclorana can depress their SMR by 70-80%, they help decrease the onset of muscle atrophy significantly (Hudson and Franklin, 2002b). Depressing metabolism does so by delaying "the need to combust muscle protein" and, due to lower aerobic flux, "lead[ing] to absolute reductions in ROS [reactive oxygen species] consistent with metabolic rate...thereby atrophy associated with oxidative damage" (Hudson et al. 2006).

Along with muscle size and mass, capillary structure has been shown to be unaffected by aestivation, maintaining a three-dimensional structure (Hudson and Franklin, 2003). This most likely allows for Cyclorana to quickly remobilize skeletal muscles (2003). No significance has been found between the connective tissues of post-aestivation and control individuals, either (Hudson et al., 2007). The little data on this shows that Cyclorana maintain this by keeping the "extracellular matrix components of skeletal muscle" proportional, and thereby preserving the typical, non-aestivating phenotype (2007).

There is some muscle atrophy in aestivating Cyclorana. One study showed that there is a "significant reduction in iliofibularis muscle fibre cross-sectional area" (Symonds et al., 2007). Oxidative iliofibularis muscles are used more frequently and for longer periods of time than glycolytic muscles (2007). Therefore, aestivation is a much greater change from the norm for iliofibularis muscles. Oxidative also have more mitochondria, so the sudden down-regulation of oxidative muscle activity may be due to changes in mitochondrial density (2007).