Biophysicists measure how geckos glide across water

Dec. 7 (UPI) -- Walking on water was a skill most scientists thought was reserved for small creatures, like spiders and various water bugs, as well as a handful of larger ones, like the basilisk lizard.

It was assumed medium-sized creatures, like geckos, weren't cut out for water gliding. But a new study proved that assumption incorrect. Geckos can walk on water, and thanks to the work of two physicists, scientists know how.

Small animals use surface tension to move across the surface or water, while the basilisk lizard uses sheer force to hurl itself above the water's surface with each stride. Geckos, meanwhile, are too big to rely on surface tension, yet too small to use sheer force.

To better understand how geckos perform the impressive feat, researchers filmed geckos running on water in the lab using high-speed cameras. The footage allowed scientists to measure the forces generated by the gecko's legs and tail.

"We were able to extract various quantities from the videos -- for example, velocity, speed at which the front leg hits the water, head height, length of a step -- as well as taking several morphological measurements from the gecko -- foot radius, body plus tail length -- and used these to estimate the surface tension and surface slapping forces," Jasmine Nirody, a biophysicist at the University of Oxford and Rockefeller University, told UPI in an email.

The high-speed videos showed the gecko pushes itself up off the water with its slapping limbs, while its rounded belly creates a planing action, like speed boat, helping keep the body above water. The lizard's tail also moves like an alligator's, pushing the body upwards and forwards.

But when Nirody and her research partners totaled the forces generated by the gecko's body parts, they didn't add up. The slapping of the lizard's legs and swishing of the tail weren't sufficient to explain its water-gliding abilities.

To see surface tension aided the gecko's cause, researchers tested the lizard's ability to run across soapy water. Soap reduces water tension, and sure enough, the geckos sunk farther into the water, forcing them to rely on a swimming action to make it out of the suds.

Yet, when Nirody and her colleagues did the math, they found the combination of surface tension and slapping forces still weren't enough. Something else was providing the gecko a small boost, keeping the lizard afloat.

Researchers decided to take a closer look at the gecko's skin. It turns out, the lizard's skin repels water.

"We used high-resolution cameras to photograph water droplets on the skin of the gecko to measure the contact angle and confirm that the gecko's skin is superhydrophobic," Nirody said.

Nirody and her research partners recounted their mystery-solving process in a science paper published this week in the journal Current Biology.

It makes sense that geckos would evolve the ability to scurry across the water. Scurrying is a faster, more efficient movement than swimming, and speed is everything when trying to escape predators.

Though researchers only studied one type of gecko, they think it's likely many gecko species can walk on water.

"It's reasonable to assume that geckos that live in similar regions -- for example, that get flooded easily during the rainy seasons -- would have adapted similar behavior for escape and prey capture," Nirody said.

Geckos are capable of a variety of impressive feats. They can scurry up walls and hang upside down. Scientists have previously translated some of the lizard's biomechanics for use in robots, and researchers think their latest work could help engineers develop robots capable of gliding across water.

"Nothing is underway, but we're always very excited to talk and collaborate with engineers to get projects like this going," Nirody said.