Watch a Frog Walk on Water with High-Speed Belly Flops

If you flick a flat stone toward a pond at just the right angle, it skips across in a series of smooth jumps. Inch-long cricket frogs seem to skitter over the surface of water with similar physics-defying grace. But when Talia Weiss, then an engineering graduate student at Virginia Tech, filmed the frogs with a high-speed camera, she saw a very different picture.

“The motion is so fast that if you look at it with the naked eye, you really can’t tell the difference,” Weiss says.

For a study published recently in the Journal of Experimental Biology, Weiss and her co-authors filmed cricket frogs at up to 500 frames per second, level with the water’s surface, as the frogs moved across. Playing the footage in slow motion, the researchers found that the frogs were not hopping with just their feet breaking the surface, as older studies had described anecdotally, but were actually doing a series of belly flops—sinking for a fraction of a second and then kicking themselves upward with each jump.

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Rather than skittering across water like basilisk lizards do, the frogs were rapidly “porpoising”—leaping from the water as they swam. Weiss says their legs may be too slow for true surface hopping. “To jump on the water’s surface, you have to have your legs retracted and ready to push down again by the time you’re approaching the water in every jump,” she explains. “And these frogs don’t prepare for their landing at all; they sort of just belly flop. They don’t retract their legs fast enough to immediately jump again” from the surface.

According to Jasmine Nirody, an organismal biophysicist at the University of Chicago, who was not involved in the study, “fast animal movements can be really deceiving,” and the new camerawork reveals what the frogs are actually doing. By carefully analyzing such motions, “we can think about how we might be able to use [the frog’s] strategy in various bioinspired robots,” she adds. “Now we know what to look for.”