Cocoon forming desert frogs

Cocooning frogs burrow into the ground after the surface water has receded but most often before the ponds have dried out. They burrow into sandy-clay soils, and probably dig down as deep as 1.2m, but most often much less than that.

We studied the Northern Burrowing frog (Neobatrachus aquilonius) in the Gibson Desert, and with advice from our friends from the Kiwirrkurra Community we dug up frogs that had been buried for approximately 18 months (Cartledge et al. 2006). We determined the period that they had been buried by the number of layers in their cocoon. Layers of epidermis are shed to form a cocoon at the rate of one layer every 2-3.5 days (Withers 1995), so by using this information and local rainfall records that coincided with the number of cocoon layers, we can estimate how long they have been buried.

We have dug cocooned frogs from many locations (Plates 1 and 2) in the sandy deserts and the Pilbara (Plates 1, 2 and 3). On one occasion we were able to remove the cocoon from a Northern Burrowing frog and it remained largely intact showing the shape of the frog (Plate 3).

Plate 1. Northern burrowing frog in its cocoon	Plate 2. Main’s frog with its cocoon coming off

Digging out cocooning species is generally more difficult than digging out the non-cocooning species as the ground is often much harder as it contains a higher proportion of clay. Plate 3  shows where a Northern Burrowing frog has been dug from the ground and the burrow is still obvious. Plate 5 shows the surface evidence of a Northern Burrowing Frog that had been buried for about 18 months. Plate 6 show the surface evidence of a Main’s frog (Cyclorana maini) buried in the Pilbara.

Plate 3. Northern burrowing frog cocoon	Plate 4. After excavating a Northern burrowing frog from its burrow which is in the background

Plate 5. Surface evidence of a Northern burrowing frogs burrow in the Gibson Desert	Plate 6. Surface evidence of a Main’s frog burrow in the Pilbara

A frogs cocoon seals off its mouth and cloaca with the only opening being its nares, through which it breathes at a reduced rate because it has entered into a state of aestivation. Aestivation enables a frog to lower its metabolism to about one-fifth of its resting rate on the surface (Withers 1993). This means that its stored energy will last five times longer. The consequence is that some cocooned frogs can probably remain underground for at least a couple of years. A cocooned and aestivating frog is unable to excrete its body waste, which therefore must accumulate and must be tolerated until it comes to the surface after rain (Cartledge et al. 2006).

Fauna survey for environmental impact assessments

Arid adapted frogs are seldom sampled in fauna surveys for environmental impact assessments (EIAs), and when they are it is normally as a consequence of sampling during or immediately after rain (Thompson 2006). Wet and muddy conditions are not good for undertaking generic fauna surveys for reptiles and small mammals, and roads and tracks often become impassable. The consequence is that often the most abundant vertebrate fauna in some arid areas are not sampled or reported in fauna assessments undertaken to support EIAs.

References

Cartledge, V. A., P. C. Withers, K. A. McMaster, G. G. Thompson, and S. D. Bradshaw. 2006. Water balance of field-excavated aestivating Australian desert frogs, the cocoon-forming Neobatrachus aquilonius and the non-cocooning Notaden nichollsi (Amphibia-Reptilia). Journal of Experimental Biology 209:3309-3321.

Thompson, G. G. 2006. The impact of land clearing and mining on arid-adapted frogs. Pages 18-35 in Goldfields Environmental Management Workshop 2006 Proceedings. Goldfields Environmental Management Group, Kalgoorlie.

Withers, P. C. 1993. Metabolic depression during aestivation in the Australian frogs, Neobatrachus and Cyclorana. Aust. J. Zool. 41:467-473.

Withers, P. C. 1995. Cocoon formation and structure in the aestivating Australian desert frogs, Neobatrachus and Cyclorana. Aust. J. Zool. 43:429-441.