In hot deserts, animals must get water any way they can. A new study sheds light on what has to be one of nature's most bizarre adaptations to dry environments: Certain lizards have a network of tiny channels in the spaces between their scales that can suck up water from the ground (or from rain falling on their back) and transport it to their mouth for drinking.
Researchers have suspected for decades that some desert lizards can harvest rainwater through their skin. The Australian thorny devil (Moloch horridus), for example, rubs its belly into the wet sand after a rain. In the 1920s, inquisitive researchers put this lizard in a shallow bowl of water and noticed that its entire body soon looked wet. "The initial thought was that they just took the water in directly through their skin," says Wade Sherbrooke, a biologist at the American Museum of Natural History's Southwestern Research Station in Portal, Arizona. But that turned out to be wrong. Unlike amphibian skin, which lets water through, reptile skin keeps precious water inside the body, Sherbrooke says. So how were the lizards transporting water?
Later research suggested that water somehow traveled along the "scale hinges" in between the lizards' scales. In the new study, Sherbrooke and colleagues at James Cook University in Townsville, Australia, used light and electron microscopes to examine the scale hinges in detail. They discovered that the hinges contain tubelike channels about the width of one or two human hairs, a good size for harnessing capillary forces to draw in water. In thorny devils, the network of hinges covers the entire body and appears to funnel water to an area near the corner of the lizards' mouth, the researchers report in this month's issue of Zoomorphology. They found a similar plumbing system in another rain-harvesting lizard, the Texas horned lizard (Phrynosoma cornutum), but not in seven related lizard species that don't transport water.
"It's a really neat paper," says Kurt Schwenk, an evolutionary biologist at the University of Connecticut in Storrs. "This adds one of the big missing pieces and gets at the mechanism of how this cutaneous water transport works." One remaining puzzle, Schwenk says, is how the lizards keep the water flowing in the right direction. He says both species make repetitive tongue movements that might somehow accomplish this--by moving water into the mouth drop by drop so that more liquid gets sucked up to take its place, for instance--but the details are a mystery.