Frogs aren't the first species that spring to mind when you think of desert dwellers, but according to Victoria Cartledge, the western Australian deserts are peppered with them. However, the amphibians spend most of their time tucked away beneath the surface, aestivating until the next downpour. How frogs weather years without rain has fascinated scientists for decades, and much has been learnt about their survival strategies from lab studies; some frogs encase themselves in protective cocoons to reduce dehydration rates and store water in their large bladders. But how the frogs apply these strategies in their natural environment remained unclear. It was only when hunting for marsupial moles that Philip Withers and Graham Thompson struck up a collaboration with Aboriginal elders in the Kiwirrkurra Community and realised that the elders could distinguish aestivating frog burrows from other depressions in the sand. Withers and Thompson decided to return to Kiwirrkurra to find out more about the elusive amphibians' water management().
Embarking on a 3200 km round trip from Perth to the remote Aboriginal settlement in the Gibson Desert, Cartledge, Withers, Kellie McMaster and Thompson found no shortage of elders keen to help them locate frog burrows. Having been led to the frogs' resting ground in a sand dune, the team began digging up the dormant animals, as well as collecting samples of the damp sand that had surrounded each frog, to evaluate its hydration state. Cartledge remembers that the dune excavation work was backbreaking, but became much worse when the aborigines led them to a rockhard clay pan where the frogs were entombed in individual chambers. After their successful first trip in 2003,the aborigines led the scientists to a swale valley (with particle sizes half way between clay and sand) in 2004 to give the team their third frog site.
After days of digging on their first excursion to the sand dune and clay pan, the team were delighted to realise that the aborigines had led them to two different aestivating frog species; Notaden nichollsi, which doesn't protect itself with a cocoon, buried in the sand dunes, and Neobatrachus aquilonius, which surrounds itself in a protective cocoon to reduce water loss, in the claypan. Returning to the sand dune site on successive years, the team realised that Notaden remains fully hydrated in damp years, but suffers in dry years, becoming relatively dehydrated and increasing their blood and urine concentrations while losing water to the environment. Neobatrachus, entombed in clay, suffered much more from their incarceration despite their protective cocoon. Half of the animals had exhausted their bladder water supplies, and their plasma and urine concentrations were perilously high, reducing the animal's ability to extract water from the bladder store.
However, the biggest surprise came when the team realised that the cocoon building Neobatrachus frogs that they found in the swale depression hadn't constructed a protective cocoon, and were in much better shape than the cocoon-swathed, dehydrated animals in the claypan. It seems that the benefits conferred by the waterproof cocoon in the damper swale could be undone if the cocoon limited the amphibian's access to water from the environment, making it preferable for them to do without and aestivate like the cocoonless Notaden.
