Aestivation Ecology

Aestivation is an act of dormancy undergone by many amphibian species to survive harsh summer conditions. Of the twelve species of Cyclorana, five are known to aestivate during the summers in Australia (Tracy et al. 2007). Aestivation usually begins in April and May and ends in October or November. The timing of aestivation may be influenced by soil conditions rather than time of year. Aestivation occurs soon after heavy rainfall ends, before soil at 10 cm falls below threshold levels for water absorption (2007). However, aestivation occurred well before all light rains ended. This indicates that Cyclorana may act on a circadian rhythm, or begin once they have amassed enough energy (2007).

The first indication that aestivation is occurring is when a frog assumes a 'water-conserving' posture: the front and back legs are tucked under the body, the head is bent forwards and downwards, and the eyes are closed and retracted (Withers 1995). Cocoon formation varies among species of Cyclorana. Early signs of cocoon formation can show up within a week of burrow formation (1995), but the formation of a distinct cocoon can take up to three to six weeks (Tracy et al., 2007). The cocoon covers all of a frog's body, including the mouth, eyes, and cloaca, except for the external nares, which remain open for ventilation. A cocoon is made up of between 20 and 30 layers of shed skin cells, and epidermal shedding is highly variable among species of Cyclorana. At first the cocoon is thin and clear, but it eventually becomes thick (between 10 and 30 micrometers), hard, and opaque (Withers and Thompson, 2007).

(Figure 1 from Withers, P.C. and Thompson, G.G. 2000; permission given by Philip O'Brien, President of RSWA)

Depression of metabolic rate varies among species of Cyclorana. While mass dependent, depression of SMR typically is between 70 and 80% (metabolic rate reaches 20-30% of SMR) (2007).

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