Geckos run across water at up to almost a meter a second using a unique mix of surface tension and slapping, say researchers reporting December 6 in the journal Current Biology. They found that the mouse-sized lizards are too big to float on water using only surface tension, like insects, but too small to use only foot slapping, like basilisks.
"The gecko's size places them in an intermediate regime--a middle ground," says first author Jasmine Nirody (@jasnir_), a biophysicist at the University of Oxford and Rockefeller University. "They can't generate enough force to run along the surface without sinking, so the fact they can run across water is really surprising."
Flat-tailed house geckos (Hemidactylus platyurus) are known for their agility. They can climb on walls, swing under leaves, and run at high speeds. Scientists have studied the biomechanics of these physical feats, but how geckos cross water surfaces was, up until now, a mystery.
"Animals move in such weird and different ways, and geckos are a good example of that," says Nirody, who started the project as a postdoc in the University of California, Berkeley lab of Robert J. Full. "Geckos make use of several locomotive modes when running across water, which makes it more difficult to characterize."
Co-author Ardian Jusufi, a biophysicist at Max Planck Institute for Intelligent Systems, was working in the jungles of Cambodia when he noticed that the geckos there could skid across puddles to escape predators. That observation inspired Nirody, who is interested in all sorts of critter locomotion--like how bacteria swim, slugs slide, and snakes slither--to study how geckos dart across the intersection of air and water.
Nirody and her colleagues collected some Asian house geckos and set up an experiment in the lab. They built a long water tank, attached a plank across the top, and varied the water surface tension by adding soap. Then they placed the geckos on the plank and startled them by touching their tails. They shot high-speed video of the geckos darting across the water's surface and studied it closely.
"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," says Nirody. "The way that they combine several modalities to perform this feat is really remarkable."
The researchers saw that the geckos' legs slapped and stroked the water's surface, creating air pockets that kept most of their bodies afloat. They trotted across the water similarly to how they run on land. Their water-repellant skin also helped them skim over the surface, and their swishing tails stabilized and propelled them forward.
"Our new understanding of gecko locomotion could inspire design," says Nirody, who suggests the geckos' multimodal locomotive strategy could lead to rapid swimming robots for search and rescue in flooded areas. "Nature has so much to teach us. It's built all these amazing machines to look at and learn from."
This research was funded by the National Science Foundation (USA) and the Swiss National Science Foundation.
Current Biology, Nirody & Jinn et al.: "Geckos Race Across the Water's Surface Using Multiple Mechanisms" http://www.cell.com/current-biology/fulltext/S0960-9822(18)31469-6
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