The thorny devil, named for the sharp spikes that cover its head and body, is a lizard that dwells in the Australian Outback’s hot and arid desert. Though its scary appearance has earned it other equally alarming names like the thorny dragon or the mountain devil, the palm-sized reptile is a harmless, slow-moving animal that has managed to adapt to one of the world’s most inhospitable environments. While researchers know a lot about these hardy lizards, the one mystery that had never been completely solved, is how they keep hydrated.
Given that the structure of their mouths do not allow the reptiles to drink water like most organisms, researchers began by investigating their fluid intake mechanism. The first breakthrough came in 1962, after a team of scientists placed some specimens in shallow water and observed an “advancing waterfront” move over the skin and towards the lizard’s mouth that opened and closed. What they had witnessed is capillary action, where water, propelled by the natural attraction between water molecules and the surface of the tube, flows unassisted through tiny tubes. This is how plants can draw water up from roots through their stems and the reason a sponge or paper towel is able to absorb spills.
While that provided some insight, there was still the question of the thorny devil’s source of water, given that its arid habitat receives less than 250 mm (10-inch) of rain, annually. In 1993, Western Australia University zoology professor Philip Withers suggested that the thorny devil absorbed the moisture condensed in the sand through the network of microscopic grooves situated between its spikes. However, it would be another 20 years before Withers and Philipp Comanns from RWTH Aachen University in Germany tested the theory.
The scientists began by analyzing the volume of water the thorny devils were able to absorb in two different situations. They first placed the lizards in a small puddle of water. With ample moisture to draw upon, the lizards were seen drinking water in just 10 seconds and able to satiate their thirst within an hour. However, when placed on moist sand, they were able to fill just 60% of the grooves’ capacity, which, according to the researchers, is probably not enough to push the water into the lizard’s mouth.
The researchers believe that this may be the reason the lizards are often seen shoveling the desert sand on their backs or even burying themselves in it, especially following a rain shower. They hypothesize that this action increases the reptiles’ surface area for water absorption. That’s because water molecules are strongly attracted to one another, which means that the skin, moistened by the sand is able to draw in more water, than when its dry. This, together with some help from gravity, helps fill the grooves to capacity and allows the lizards to quench their thirst.
Comanns and Withers, who published their findings in the Journal of Experimental Biology on August 27, have not yet measured the results of the shoveled sand, on live lizards. They did, however, test the theory with resin casts of the lizard’s scaly skin. Sure enough, the resin drew in more water after being exposed to moist sand.
This latest discovery is just another in the long list of impressive attributes of these unassuming little lizards. The crafty reptiles are known to maintain their camouflage by transforming from olive-brown in the early mornings, to pale yellow and red, as the temperatures rise. Also, while their sharp spikes are enough to discourage most predators, those brave enough to try may find them a little difficult to swallow, given that the lizards can swell in size by inflating their chests with air.
If that is not enough to impress you, thorny devils possess a false head – a strange knob-like growth at the back of their neck. When in danger, they simply tuck their real head between their forelegs, leaving the predator completely confused! Who would have thought that a lizard which could be easily mistaken for a twig would be so exciting?
Resources: wired.com, jeb.biologists.org, theatlantic.com.