Geckos are incredible creatures. They're able to easily maneuver on land or in the air, and now it turns out they can walk on water, too. Given their size, it's something they shouldn't be able to do.
"They can run up a wall at a meter per second, they can glide, they can right themselves in midair with a twist of their tail and rapidly invert under a leaf running at full speed. And now they can run at a meter per second over water. Nothing else can do that; geckos are superheroes," Robert Full, a professor of integrative biology at University of California, Berkeley, explained in a university statement.
Jack of all trades
If Full is impressed, you know it's a big deal. Full was the one who discovered that toe hairs help geckos climb up smooth vertical surfaces and hang from ceilings, after all. While he was the senior author on the paper published in Current Biology that outlines this latest feat of gecko engineering, much of the research was conducted by Jasmine Nirody, a biophysicist at the University of Oxford and Rockefeller University, with help from Judy Jinn when they were both Ph.D. students at Berkeley.
Nirody first became curious about this water-running skill after she observed video footage taken by co-author and Berkeley Ph.D. Ardian Jusufi, now a biophysicist at the Max Planck Institute for Intelligent Systems in Stuttgart, Germany, that showed the geckos running across puddles both in the lab and in the wild to escape predators. Indeed, they could cover 3 feet (1 meter) in a second and then, without missing a beat, scurry along solid ground, including up a vertical surface. This speed outpaced animals that are more at home in water, including ducks and baby alligators.
Being able to walk on water isn't rare in the wild. Lots of insects like spiders and water striders can stand and walk on water because their light weight doesn't disturb the water's surface tension, which is the force that allows water molecules to cling together. Some larger animals, like swans during takeoffs or the basilisk lizard, will slap and stroke the water with a lot of force as they run across, helping to keep them afloat.
Geckos, however, are in the middle. They're too heavy to rely on surface tension alone, and they're neither heavy or strong enough to slap their way across. "The gecko's size places them in an intermediate regime, a middle ground," Nirody said. "They can't generate enough force to run along the surface without sinking, so the fact they can race across water is really surprising."
Nirody and her team constructed a long water tank and placed flat-tailed house geckos (Hemidactylus platyurus) in it. Startling them by touching their tails, the researchers recorded the geckos running across the water using high-speed video. This allowed them to easily slow down the video later and observe the process by which the lizards skitter across the water. It turns out that there's not one strategy but four.
First, like insects, they rely on surface tension to keep themselves moving. Even when Nirody added soap to loosen up the water molecules, the geckos, while slowed to half speed, could still make it across. Second, like the bigger animals, they slap and stroke the water to create air cavities that stop them from fully submerging. Third, they rely on their water-repellent skin to semi-plane through the water, like muskrats do. Finally, they swish their tails like alligators do, and this provides lift, stabilization and propulsion.
In short, geckos are water-walking chimeras, utilizing strategies from other organisms to achieve the same goals. A deeper understanding of how geckos run across the water could help us develop robots that do the same thing.
"Even knowing the extensive list of locomotive capabilities that geckos have in their arsenal, we were still very surprised at the speed at which they could dart across the water's surface," Nirody said. "The way that they combine several modalities to perform this feat is really remarkable."