Into the Red Centre part II – Surviving the Dry
In Central Australia the climate is very dry for most of the year. Rains do come, although infrequently. Years of drought are interrupted by massive flooding downpours. At the very least it can be rather unpredictable. Once upon a time it was a much wetter area, and many of the animals and plants that do survive are those that adapted to the drying climate. I traveled through the center of the Northern Territory again after some recent heavy rains and found some remarkable creatures with some equally amazing methods to cope with the normally dry conditions…
Frogs of the dry
Perhaps the most famous of the dry country animals that surface after heavy rains are the frogs. Yes, beneath the dry sandy soils lie frogs in a sort of hibernation, known as aestivation. There are several unrelated species that burrow to escape the heat and lack of water, and they have their own slightly different strategies to cope.
The Water-Holding Frogs (Cyclorana species) are perhaps the most well adapted. When the surface water dries, they shuffle down backwards into the wet sand or mud and continue until they are a foot or two down, where some species shed their skin. The shed skin becomes like a plastic bag and barely loses any water. Meanwhile, the frog as it sleeps draws water from its bladder. It can remain this way perhaps for years until the next flooding rain soaks down deep enough to wet its now dry outer skin and awaken it. This will be its first meal, it stuffs the skin into its mouth with its front legs and emerges above the surface for a quick break from the dry.
Other species do not seem to be quite as elaborate and will not build a skin ‘cocoon’, and in fact many still hold onto their secrets. In fact, most desert frogs do not seem to build a cocoon at all, including the following species.
With a flat ‘pug’ face and a look of perpetual misery are the truly delightful Notaden frogs. True burrowers and inhabitants of the drier areas they apparently specialise on ants and termites. Nicknamed ‘Golfball Frogs’ due to their habit of puffing up when disturbed they will also secrete a nasty latex-like chemical from their skin if pressed further. In some species this chemical is bright yellow, and in others it is white. No matter the colour it is best to avoid contact with the eyes or mouth. As these frogs have very short little legs (they cannot jump far, instead they gallop along with great difficulty) and are fat they no doubt would look appealing to many a predator.
Another group, closely related to the Notadens are the Ornate Burrowing Frogs, the Platyplectrum or Opisthodon species. As with many Australian frogs, they have undergone extensive revision to see where they fit in. Currently they are grouped in the Platyplectrum group… at least for now. There are two species, the coastal Ornate Burrowing Frog (P. ornatum) and the inland Spencer’s Burrowing Frog (P. spenceri) which is the species commonly found around Alice Springs. They are active even without recent rains, presumably because they prefer damp sandy river beds and gorge country where humidity remains high long after the rains have passed. On a warm evening they can be readily found around any of the swimming holes around Alice Springs, usually on the beaches.
Another totally different strategy to dodge dry conditions is used by the desert tree frogs. There are two species in the Red Centre, the Red, or Desert Tree Frog (Litoria rubella) and the Gillen’s Tree Frog (Litoria gilleni). Both species do not burrow but make use of cracks and crevices. The Red Tree Frog makes use of hollow trees, especially on dry watercourses and to a lesser extent rock crevices. The trees retain moisture, and in the hollows it is warm and humid all year round. They cluster in large groups to further conserve water, emerging on warm nights to feed.
This species is small but reduces water loss in part with its body shape. It is a chubby species and has very short little legs. Long legs no doubt increase evaporation due to a larger surface area, so having small legs might mean they do not jump as far but lose less water. This species and the Gillen’s Tree Frog also have a wax coating on their backs which significantly reduces water loss. The Gillen’s Tree Frog is not really an animal of the tress, but appears to be a strict rock inhabitant. Only found in the area immediately around Alice Springs in the gorges it is near identical to the larger and more widespread Green Tree Frog Litoria caerulea. It is slightly smaller and has a more generous coating of white or yellow flecks. Also, this frog almost lacks the large gland on the head of its larger cousin. During dry times it squeezes deep into rock crevices, often sharing its home with others. On humid nights it can be found close to water waiting to ambush any small animal that comes by.
No matter the strategy, desert frogs have priorities when they do emerge- feeding and breeding. With the abundant food that surfaces after rain such as spiders, termites, beetles, crickets, lizards and other frogs they can be assured of a good meal but breeding is a very urgent matter indeed as water does not last long. Oddly enough, in the Red Centre, none of the frogs are direct breeders which lay eggs underground that hatch into frogs and skip the tadpole stage. Related species in the west and southwest of the country breed in such a way, but the desert species must have standing water for their tadpoles. They do, however have methods of ensuring success… speed. Eggs that are laid can hatch in as little as a day and leave the water as frogs in six days. That extremely rapid development is faster than that of the direct breeders and probably uses less energy as the mother does not have to invest anywhere near as much in producing all of that extra yolk.
Crabs of the dry?
But perhaps the most surprising desert dweller is the Desert Crab. Yes, you read correctly- a crab. Holthuisana transversa is the species name for this bizzare animal. Looking much like what you would expect a small crab to, it lives in some of the harshest environments on earth – for a crustacean anyway. As a result it has a bunch of adaptations. Obviously it is a burrower, burrows can easily be found in most creek beds on the Stuart Highway from Tennant Creek to Wycliffe Well. Most people that see the burrows in the dry, hot, dusty creeks assume they are spider or reptile holes, but the trained eye would easily recognize the pile of dry mud all around the entrance that is typical of crabs and crayfish. So how do they survive? Like most desert animals they use every drop of water to their advantage. The humidity of the burrows helps slow water loss, but they have another trick shared by the Thorny Devil. Humid air condenses on the top of the shell and special grooves channel it into the mouth where it is taken in. This remarkable method allows the crab to use moist air rather than having to actually drink. Unlike most crabs which have free swimming larvae, these are direct breeders which have the young, fully formed crabs hatch out from the eggs the mother carries. I found a fascinating article on exactly how these crabs survive. Read more…
While we’re at it I might as well include one mammal- the Spinifex Hopping Mouse, Notomys alexis. This little creature, like many of the small desert mammals breeds rapidly and in good conditions numbers rise quickly, then fall again once food and water runs low. They rarely drink, instead eat moist vegetation. Their urine is so concentrated to conserve water that it comes out as a solid pellet! This year with the abundant rain has seen a big surge in the numbers of Hopping Mice but already the numbers are falling as part of the natural cycle.
So the deserts and arid zones are home to a number of creatures that are normally considered delicate and whose relatives in other areas never stray far from water.