Geckos are known for being expert climbers, able to stick to any surface thanks to nearly 500,000 [corrected] tiny hair-like structures on the bottoms of their feet. Now it turns out the little lizards can also zip along the surface of water at high speeds to elude predators. They can't do it for very long; the energy expenditure required is too great. But it's amazing they can do it at all. Scientists think they've pinpointed the mechanisms behind the feat, described in a new paper in Cell Biology.

The project started when co-author Ardian Jusufi, then a postdoc in the lab of University of California, Berkeley biophysicist Robert Full, was doing field work in Singapore during monsoon season. One day, after a big rain storm, he caught a gecko skimming across the water to escape a predator on video. The footage astounded everyone in the lab when he showed it to them. "It was super weird and unexpected, so naturally we had to test this," says co-author Jasmine Nirody, another former Full student who now splits her time between Rockefeller University and the University of Oxford.

There are several creatures in nature capable of walking on water, but they employ different mechanisms depending on their size. Small, lightweight water striders, for instance, rely entirely on surface tension to stay afloat, while the larger, heavier basilisk lizards employ a slapping motion with their feet that creates pockets of air bubbles to keep from sinking. The standard theoretical calculations set very strict boundaries for how small an animal has to be to use surface tension and how large it needs to be before the surface slapping mechanism is viable.

Geckos fall somewhere in between. They are too large to rely solely on surface tension and too small to generate sufficient force to run along the surface of water without sinking. And yet they can still somehow accomplish the feat at lightning speed—almost one meter per second. That's why Full's team decided to investigate further.

They used laser cutters to create entry and exit holes in a large plastic box to make a water tank and then built two wooden ramps so their group of Asian house geckos (Hemidactylus platyurus) could enter and exit the water easily. A pair of high-speed cameras were placed above and to the side at right angles to capture the movement. The geckos would be placed on the entry ramp, and team members would lightly touch their tails to startle them into swimming away.

Geckos have developed a set of complicated mechanisms all their own for walking on water.

It turns out that geckos have developed a set of complicated mechanisms all their own for walking on water. There are two objectives when skimming across water: keeping one's body above the surface, and forward propulsion. For the first, the gecko combines surface slapping and surface tension, aided by their unique hydrophobic skin that repels water, according to Nirody. A water droplet will just sit on top of a gecko's skin.

Lifting their bodies above the water reduces drag, making it easier for geckos to propel themselves forward than if they were fully immersed. They also employ a wriggling motion with their bodies and tails, much like swimming. "If you look at them from the top, it almost looks like they're just swimming really fast," says Nirody. "And then you look at them from the side and you realize their upper body and their legs are completely out of the water, even though they're still doing the swimming motion that helps propel them forward."

To verify that surface tension did indeed play a role in the gecko's surface skimming ability, the researchers added a surfactant (dish soap) to the water. Surface tension occurs because water molecules tend to stick to each other (molecular adhesion), forming a kind of supportive film to keep very light creatures afloat. Adding soap causes the molecules to lose that stickiness. Put a water strider into soapy water and it will sink because it relies entirely on surface tension. But a basilisk lizard is unaffected since it relies entirely on the surface slapping.

Nirody et al.

Nirody et al.

Once again, the geckos fall somewhere in between. They didn't sink, but Nirody et al. found that adding soap to the water reduced the geckos' speed by half, most likely because their bodies were much lower in the water because of the decreased surface tension. "We knew they couldn't maintain their entire body weight by slapping alone from the theoretical calculations," said Nirody. This test proved it.

The geckos did exhibit some interesting reactions to the soapy water. Roughly half would redouble their efforts to swim as fast as possible, even though their speed was severely limited. The other half, after the first few strokes, simply gave up and planked, sinking to the bottom. Geckos can hold their breath for several minutes, so they weren't in any immediate danger, although the team members rescued them after about 30 seconds. "We hypothesized that if they can't dart away in time, rather than slowly skimming from a predator, it's best for them to just hide under the water and hold their breath" says Nirody.

One of the goals of this research is to improve the design of bio-inspired robots. The authors note that modeling a robot on the basilisk lizard would work, but it would require a lot of energy and some kind of active stabilization for it to really be functional. An undulating tail similar to the gecko's might help with the latter issue and strengthen forward propulsion, while coating the robot with a hydrophobic material similar to the gecko's skin structure could significantly reduce drag. "Nature has so much to teach us," says Nirody. "It's built all these amazing machines to look at and learn from."

DOI: Current Biology, 2018. 10.1016/j.cub.2018.10.064 (About DOIs).