The thorny devil is looking sharp. Rows of cactus-like spikes and thorns give this Australian reptile a formidable armor and a don’t-even-think-about-eating-me swagger. But its exterior is remarkable for another reason too: This lizard drinks with its skin.
Australia's deserts are among the driest places on Earth, with rain falling just a couple times a year. In that kind of arid environment, animals need every drop they can get. So nature has gotten creative. Tucked under this lizard's scales is a network of tiny tunnels, which collect and channel water up to its mouth like a built-in irrigation system.
During the rare rainfall, the lizard drinks up when its skin soaks up water. Now, lab experiments suggest it can also quench its thirst from wet sand. "A very likely regular water source appears to be wet sand," says Philipp Comanns, a biologist at RWTH Aachen University in Germany and author of a new study on the lizard's unlikely drinking system in this week's issue of the Journal of Experimental Biology. "Almost every morning, we have this dew-wetted sand."
The thorny devil's drinking style is a rarity. When water accumulates on its body, the small channels draw in the water via capillary action—the same way a nurse draws blood when pricking your finger. The water sticks to the inner walls of the channels, and is pulled in. So far, only the Texas horned lizard in North America and Horvath's toad-headed agama in Turkey are known to have this network of water-collecting channels within its skin.
What's great about water-collecting skin is that it enables a diversity of drinking techniques. Unlike boring humans, the lizard can guzzle water through its legs while standing in a puddle. (Though that probably doesn't occur much in the wild, since puddles rarely exist in the lizard's sandy, arid habitat.) It might sip the condensation that forms on its skin when the rising sun rapidly heats the desert. Or, it might suck the moisture out of sand that's wet from morning dew.
But which approach was more likely? To find out, Comanns and colleagues put the lizards in all three situations in the lab: in a puddle, in a humid environment where condensation would form on the lizard, and in sand with varying levels of moisture. By weighing the reptiles after each session and then again after air drying, the researchers determined how much water the lizards had taken in.
Not all that water got drunk, though. To actually imbibe the liquid, the lizard opens and closes its jaws—perhaps to squeeze the water into its mouth, although no one knows for sure. But even when it's not moving its mouth, the capillary system still takes in water. About half of the lizards in the puddle actively drank, but the lizards only drank while in the puddle, and not in humidity or wet sand.
By comparing the water intake of a lizard that actively drank from the puddle and one that didn't—but still pulled in water—the researchers could determine how much water the lizard's capillary system could hold: about 3 percent of its body weight. That's important, because water can only reach the lizard's mouth when the channel system is flush with water.
This wasn't the case in the humid environment. Condensation couldn't fill the capillary systems at all, ruling that out as a water source—an idea that had persisted for decades. "That's the most exciting part for me," says Wade Sherbrooke, a biologist at the Southwest Research Station of the American Museum of Natural History in Arizona who was not involved in the study. "They dismiss a number of hypotheses that were put out earlier."
Things got interesting with moist sand. The lizards could extract water from the sand, but even from the wettest sand, they could only fill up to 59 percent of their capillary system. Still, Comanns says, that's not necessarily a deal-breaker, pointing to a curious observation made more than 25 years ago.
In 1990, after a light rain, Sherbrooke noticed sand on a lizard's back and markings in the sand: The lizard it appeared to have kicked up wet sand onto its back. He had no idea why. The researchers now hypothesize the lizard may have been drinking.
To test their idea, the researchers placed wet sand on an artificial replica of the lizard's skin. They found that gravity helped pull more water into the lizard's capillary channels, and that the wet sand moistened the skin, which boosted the capillary action. Both factors mean tossing wet sand on the back could be a viable way of drinking. "We're 95 percent confident that wet sand is one of the major water sources for thorny devils," Comanns says.
Sherbrooke, however, is skeptical. On the lizards he saw, there wasn't much sand at all. "It's an inadvertent thing that happens," he says. "I'm not convinced that's how they get any water for drinking." In the experiments, the researchers placed about a centimeter-thick layer of sand on the fake skin—much more than what Sherbrooke thinks is likely.
Before the lizards tossed the sand, Sherbrooke had seen them rubbing their bellies in the wet sand. Having filled their capillary system during the rain, they might have been trying to squeeze out another sip, he says. But to do so, they needed leverage. "So they're moving around, pushing and pushing, trying to get that belly down in the sand as far as they can," he explains. "In the process, they kick up some sand and some of it lands on their back—that's my guess."
Which would leave rain, however rare, as the thorny devil's main source of water. Sherbrooke, who has seen the lizard lick droplets off plants after rain, hypothesizes that these animals might also get rub up against wet plants. Even when there's but a drizzle, the lizard takes advantage.
"They've been going without water for six to eight months," he says. "If they can pick up some of that, it may mean life or death."