The scales of thorny devils are a scientific marvel; these lizards use their skin like a drinking straw. But where does the water come from that allows them to survive in Australia's driest deserts?
- Thorny devils cannot lick water from puddles because their mouth has evolved to specifically feed on ants
- They absorb water through capillaries in their skin, but it is unclear where this water comes from
- They have been observed shovelling sand on their backs after rain
Scientists have studied the water-wicking properties of thorny devil skin for more than 40 years.
Channels between the animals' scales draw moisture towards their mouths, where they squeeze it inside in miniscule gulps.
But in the devils' habitat — the arid deserts and sandy plains of central and western Australia — what hasn't been clear is exactly where they get the water from.
Do they rely on rare puddles, or harvest droplets of condensation, or do they suck water from the sand itself?
A study published today in the Journal of Experimental Biology suggests that one of the main ways thorny devils drink is by burying their bodies in damp sand, and then drawing the moisture out of it.
Philip Withers, a professor of zoology at the University of Western Australia and co-author of the research, said the creatures' remarkable skin is critical to their survival in the harsh conditions of the desert.
"Clearly, it's important for them — this special system has evolved and it's really quite bizarre," he said.
"I think it might be more important for the extreme circumstances. You get a little bit of rain and the sand gets wet, and then it's really important to be able to suck the water out of the sand."
A mouth so specialised it can only eat ants
Thorny devils can't simply lick water from puddles or drops of condensation because their entire mouth — including the jaw and tongue — has evolved specifically to feed on small ants that share their habitat.
So this lizard has evolved a bizarre alternative to drinking with its mouth: the capillary channels on its skin squeeze every last drop of moisture out of the arid landscape and push it towards the animal's mouth.
Professor Withers said although scientists already knew about this, they weren't sure whether thorny devils could fill their capillary channels and drink just by standing on moist sand.
To find out, the researchers studied analysed how much water six thorny devils could suck up in different situations.
The animals began to open and close their mouths to drink just 10 seconds after being placed in a puddle of water.
But when placed on wet sand, only 59 per cent of the devils' capillaries filled up and the lizards didn't start drinking.
"They can certainly stand in a puddle of water and suck it up and drink it," Professor Withers said.
"But [what we discovered is] they probably can't just stand on moist sand and suck it up and drink it.
"They can absorb some water that way, but it's not enough to fill the capillary channels."
The researchers believe that if these channels aren't full enough, water won't be pushed into the mouth for the devil to drink.
"So if they want to get the water out of moist sand they have to do something extra," Professor Withers said.
"And it seems that what they do extra is to shovel it on top of themselves, and get gravity to help. And then it works."
How sand-shovelling could help
Thorny devils have been observed sand-shovelling after periods of rain.
Using replica models of the lizard's skin, the scientists showed that piling sand on top of this unusual surface boosted the amount of moisture that could be sucked into capillary channels. Gravity, they suggest, gives the process a helping hand.
And while they haven't yet measured the results of shovelled sand on live lizards, they believe all the signs point to this self-burial in damp sand as the behaviour that allows devils to fill their capillaries and drink.
By piling it onto their backs, the thorny devils maximise the surface area of their skin touching the sand — as well as getting gravity's help in sucking the moisture from it.
Professor Withers thinks the mechanism isn't necessary for the day-to-day survival of the lizard but may be useful in lengthy periods where there is little or no rainfall.
"I would think that a normal desert lizard can survive in the desert eating ants, without all of this stuff," he said.
"So maybe it's just for when it's really hot and dry, a long drought or something and there's a bit of rainfall — they can harvest it."
The thorny devil's suit of armour
The thorny devil is spiked as a form of defence against predators.
The slow-moving lizard sits on ant trails to feed, picking off ants one by one with its tongue as they walk by.
This makes the devil a stationary target for would-be predators, who are warded off by the spikes that make the lizards difficult to eat.
The other defence mechanism the thorny devil has is its colouring. Its mottled brown and yellow skin helps it blend in with the dirt and sand of its habitat.
"Their colouration, you can stand on one of these things and not even see it's there. It's truly amazing," Professor Withers said.
"I've had radio trackers on them. You can walk out there and you hear the radio going beep, beep, beep, and you know it's within a metre or so. And sometimes it's so hard to even find it."
He said the best place to spot one was on a road.
"The easiest way to find them is to see them when they're walking across a road, because they just show up then like nothing," Professor Withers said.