A Desert Lizard That Drinks From Sand

The thorny devil stays hydrated thanks to its skin, which pulls water away from moist grains, against gravity and into its mouth.

A thorny devil (Christopher Watson / Wikimedia )

It’s early morning in the Australian desert, and a squat, palm-sized lizard called the thorny devil is having a drink. It hasn’t rained for weeks, and there’s no water in sight. The lizard’s body is still and its head is raised. And yet, through almost no effort, it is quenching its thirst.

Its secret lies in its extraordinary skin. Between the intimidating and ostentatious spikes, there’s a subtle network of microscopic grooves. These can yank water out of moist sand, drawing the fluid up against the pull of gravity, across the lizard’s body, and into its waiting mouth. All it needs to do is to stand in the right spot and without flexing a muscle, it can drink with its skin.

Though long known to Aboriginal Australians, the lizard was first described by Western zoologists in 1841. Its fearsome appearance earned it sinister names—the thorny devil, or Moloch horridus. In truth, the creature eats only ants, and otherwise moves slowly and placidly. “They’re like Swiss people—very relaxed,” says Philippe Comanns, from RWTH Aachen University.

In 1923, one biologist wrote that the thorny devil “has the power of absorbing water through the skin after showers of rain.” But that didn’t make sense—a desert reptile with permeable skin would quickly dry out. In 1962, Two herpetologists discovered what really happens by putting one on the creatures in belly-deep water. They noticed “an advancing water front” moving over its skin and towards its mouth, which it opened and closed.

The liquid was clearly traveling through a phenomenon called capillary action, where water can flow unassisted through narrow tubes, propelled by the natural attraction that water molecules have to the surface of that tube and to each other. That phenomenon drives water up the stems and trunks of plants, into sponges and paper towels, through the nibs of fountain pens, and apparently across the skin of the thorny devil.  Essentially, the lizard’s body is like a paper towel wrapped around a rubber tube—a highly absorbent layer stretched over a totally impermeable one.

But where does the water come from? Rain, puddles, and fog are scarce and fleeting in the outback. Some researchers suggested that the thorny devil licks dew from rocks and plants, but it doesn’t really have the mechanics do that; its jaws and tongue are specialized for grabbing ants. Others suggested that its spines, like those of a cactus, might provide points for water to condense on its own body, but there’s no evidence that this actually happens.

In 1993, Philip Withers suggested a somewhat counterintuitive idea—perhaps the lizard stands on moist sand. In the early morning, when dew condenses and falls to the ground, the sand becomes a little damp. It doesn’t last for long, but it provides a temporary daily oasis for a thorny devil. Two decades later, Comanns has checked to see if this actually works.

By placing thorny devils in shallow puddles, he showed that their skin can hold up to 3.2 percent of their body weight in water—the equivalent of an average American adult carrying five pints. If the lizard drinks continuously, it can fully satisfy its thirst in under an hour. While it can indeed absorb water from the moist sand, the grains actually don’t provide enough water—only up to 60 percent of its full capacity.

Yet thorny devils have a way of making their skins even more absorbent than usual. In the wild, beyond simply standing in sand, they have been known to rub their bellies into it and to flick it onto their backs. This trick loads their channels with small traces of water. And since water attracts water, already-wet surface will draw in more water than totally dry one.

Comanns confirmed this by making resin casts of the thorny devil’s skin. If he first placed moist sand on the casts, and then brushed it off, he found that new droplets were more likely to be pulled into the grooves, rather than simply beading on the surface. “Once you fill the channels, water immediately spreads on the skin,” says Comanns.

“This provides excellent support for the hypothesis that sand-shoveling behavior plays a role in the water-harvesting process,” says Lisa Hazard, from Montclair State University. The trick might help to pre-wet the skin,  or it might give the lizards enough water in itself.

Either way, “it is exciting to discover new means by which animals cope with what we see as ‘harsh’ environments,” says Dale DeNardo, from Arizona State University. Many reptiles, he adds, use their skin to collect water. Rattlesnakes will coil tightly in light rain, and then drink the water that collects between their coils. Horned lizards and thorny devils go one step further by using grooved skin. “The value of this is that rain and any resulting puddles do not last long in the desert,” DeNardo adds. “Being able to get drinking water from moist sand increases the opportunities for thorny devils to drink.”