Thorny devil, in the reptile house at Alice Springs Desert Park, Australia. Stu's Images [CC BY-SA 4.0], via Wikimedia Commons.

Thorny devil, in the reptile house at Alice Springs Desert Park, Australia. Stu's Images [CC BY-SA 4.0], via Wikimedia Commons.

Thorny devils scampering across the baking red Australian sand in search of an ant dinner look almost invincible in their coat of spikey armour, but their choice of diet may make survival even more challenging in the harsh environment. Philipp Comanns from RWTH Aachen University, Germany, explains that the lizards’ mouths are so well adapted to consuming ants that they are unable to lick water to drink. However, the resourceful animals have a remarkable alternative strategy to overcome the drinking challenge: they have effectively turned the entire surface of their skin into a drinking straw.

Comanns describes how the reptile's skin is covered with microscopic channels that take up water by capillary action. The lizards then suck the water through the channels into their mouths: ‘It is very cool seeing these lizards standing in a puddle and finally start to move their mouths as they drink’, he says. However, it wasn't clear how thorny devils and other so-called ‘rain harvesters’ access water in one of the most arid environments in the world. They rarely encounter puddles and the dew that falls at sunrise only dampens the ground. Fascinated by the lizard's ability to extract water from the most parched locations, Comanns arranged to visit Philip Withers – who originally discovered the skin phenomenon – at the University of Western Australia, to find out more about how the mysterious creatures extract water from their desiccated surroundings.

Working with six thorny devils that had been caught in the bush by Graham Thompson, Comanns recalls that the animals were content to have their feet immersed in a puddle of room temperature water for an hour. Some even began opening and closing their mouths to drink within 10 s of being dipped into the water. Having weighed the lizards before they began drinking, an hour later (when they had drunk their fill and their skins were fully charged) and then an hour after that (when the skins had dried and any additional mass accounted for the water consumed), Comanns discovered that the 40 g reptiles opened and closed their mouths almost 2500 times during an hour-long drinking session and downed as much as 1.28 g of water (3.3% of body weight) in 0.7 μl sips. Meanwhile, the channels on the surface of the skin could hold an additional 1.32 g of water.

But Comanns was still none the wiser about which water sources that the animals depend on. Were they extracting water from damp sand, or could they gather enough dew on their chilly bodies from air warming in the early morning to slake their thirst? After allowing the lizards to stand in the damp sand and measuring how much water wicked up into the skin, Comanns found that even the wettest sand (22% water content) only saturated 59% of the capillaries and although Comanns assumes that this could be sufficient to satisfy the lizards, he never saw them drink. However, the animals like to cover their backs in damp sand and he suspects that this may allow them to extract more water. Also, when he cooled the lizards to 22°C and placed them in a warm humid room, the condensation that formed on the lizards’ bodies was only sufficient to wet the surface of the skin and not enough to charge the water capillaries.

So, thorny devils can extract significant quantities of water from soggy sand but not enough from dew at sunrise and Comanns suggests that the lizards and other animals that resort to capillary action to get water shots should be rechristened moisture harvesters, ‘Because it is not always about rain’, he says.

P. C.
F. J.
Cutaneous water collection by a moisture-harvesting lizard, the thorny devil (Moloch horridus)
J. Exp. Biol.