Non-cocooning desert frogs

Following on from our recent information on cocoon forming frogs, we provide a comment on the non-cocoon forming group.

Western Australia is blessed with an abundance of arid-adapted frogs (Tyler and Doughty 2009). Arid-adapted or desert frogs can be placed into two broad categories: cocooning and non-cocooning species.

Almost all frogs will dehydrate if their skin is exposed to dry and moving air. So, the long-term survival of a frog in the desert depends on its strategies to avoid dehydration as well finding sufficient food, a breeding partner and a suitable location to lay eggs, and for the eggs to metamorphose into a tadpole and a frog. Non-cocoon forming species avoid dehydrating in the hot dry arid condition, by two strategies:

• finding a retreat location that minimises water loss, and this is often a crevice;
• or burrowing into the ground to a depth that enables osmotic water uptake.

Crevices dwelling frogs

For those frogs that do not burrow but find a retreat site in a crevice, they are reducing their water loss by not exposing their bodies to dry and moving air. The Little Red Tree Frog (Litoria rubella: Plate 1) fits into this category. It is found throughout the Pilbara, western sandy deserts and Kimberley and finds retreats in rock crevices and under the bark near the base of trees.

Plate 1. Little Red Tree Frog (Litoria rubella)	Plate 2. Little Red Tree Frog occupying an outback toilet

Little Red Tree Frogs are often abundant immediately after heavy rain but disappear as soon as the wet conditions disappear. They happily adapt to living in a human environment (i.e. station homesteads, mining camps and small towns in remote areas) in toilet bowls, laundries and around wet areas (Plate 2). They can even be flushed down the toilet only to reappear and hide under the rim of the bowl or in the cistern if they can find a point of access.

Burrowing species

In arid areas this group include the Desert Spadefoot (Notaden nichollsi; Plate 3) and a number of the Uperoleia species (Plate 4). The Desert Spadefoot is relatively abundant in the sandy areas of the Pilbara and western sandy deserts to the southern edge of the Kimberley. This species avoids desiccation by burrowing into sandy areas down as far as there is sufficient moisture in the soil to enable it to take up water osmotically.

Plate 3. Desert Spadefoot (Notaden nichollsi)	Plate 4. Glandular Toadlet (Uperoleia gladulosa)

With the assistance of our friends from the Kiwirrkurra Community we dug up Desert Spadefoot frogs that had burrowed as deep 2.4m in to dunes (Plates 5 and 6; Thompson et al. 2005). There was always only one Desert Spadefoot in a vertical, poorly-defined, sand-filled burrow, but we occasional found one or more Tanami Toadlets (Uperoleia micromeles) in the same burrow, but never at the bottom with the Desert Spadefoot. It is likely that the Toadlets enter the burrow after it was dug by the Spadefoot, enabling them to dig down through soft disturbed sand.

Plate 5. Digging up Desert Spadefoot frogs	Plate 6. Dug out Desert Spadefoot frog burrow

Our data suggest that Desert Spadefoot frogs will come to the surface after rain, perhaps to feed before burrowing again into the ground. We do not know if these frogs re-use an existing burrow to avoid the effort associated with digging a new burrow or dig a new burrow each time. On one occasion, three women from the Kiwirrkurra Community dug 70 Tanami Toadlets from a small area less than 50cm deep on the top of a sand dune adjacent to the track (Plates 7 and 8).

Plate 7. Women from Kirrikurra digging up Tanami Toadlets	Plate 8. The results of knowing where to dig for Tanami Toadlets

When taught and with a bit of practice it is possible to find the surface evidence of Spadefoot frog burrows in the dunes. Essentially, what you are looking for is a shallow depression 4-5cm in diameter (Plate 9), that when dug out has an obvious burrow full of loose sand below it. Spadefoot frog tracks are similar to most other desert frogs, so it is difficult to distinguish them from other species (Plate 10). However, in the habitat in which Spadefoot frogs are moving around, there are few other frogs of a similar size, so frogs tracks on dunes are almost always those of Spadefoot frogs.

Plate 9. Surface evidence of a Spadefoot burrow	Plate 10. Spadefoot tracks

References

Thompson, G. G., P. C. Withers, K. A. McMaster, and V. A. Cartledge. 2005. Burrows of desert-adapted frogs, Neobatrachus aquilonius and Notaden nichollsi. Journal of the Royal Society of Western Australia 88:17-23.
Tyler, M. J. and P. Doughty. 2009. Field Guide to Frogs of Western Australia. Western Australian Museum, Perth.