Australasian Science: Australia's authority on science since 1938

Flat Out Like a Thorny Devil Drinking

Thorny devils scampering across the baking red Australian sand in search of ants to feed on look almost invincible in their coat of spiky armour, but their choice of diet may make survival even more challenging in the harsh environment. The lizards’ mouths are so well-adapted to consuming ants that they are unable to lick water to drink.

However, they do have a remarkable method to overcome their drinking problem: they have effectively turned the entire surface of their skin into a drinking straw. The reptile’s skin is covered with microscopic channels that take up water by capillary action. The lizards then suck the water along the channels and into their mouths.

However, it hasn’t been clear how thorny devils access water in their arid environment. They rarely encounter puddles, and the dew that falls at sunrise only dampens the ground.

To solve this problem, Dr Philipp Comanns of RWTH Aachen University in Germany arranged to visit Dr Philip Withers – who originally discovered the skin capillary phenomenon – at The University of Western Australia. Working with six thorny devils that had been caught in the bush, Comanns says 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 seconds 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-gram reptiles had opened and closed their mouths almost 2500 times during an hour-long drinking session and downed as much as 1.28 grams of water (3.3% of bodyweight) in 0.7 μL sips. Meanwhile, the channels on the surface of the skin could hold an additional 1.32 grams of water.

But Comanns was still none the wiser about which water sources 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 quench 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, with 22% water content, only saturated 59% of the capillaries. While this could be sufficient to satisfy the lizards, Comanns never saw them drink.

However, the animals like to cover their backs in damp sand, and Comanns 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 and humid room, the condensation that formed on the lizards’ bodies was only sufficient to wet the surface of the skin but not enough to charge the water capillaries. The findings have been published in the Journal of Experimental Biology.

Since thorny devils can extract significant quantities of water from soggy sand but not enough from dew at sunrise, Comanns suggests that the lizards and other animals that resort to capillary action to get water should be referred to as “moisture harvesters because it is not always about rain”.