If you’re light enough, the surface of the water is just another place to stretch your legs. The animal kingdom abounds with water walkers: water striders perch on ponds with spindly legs held aloft with the help of water-repellent hair; Basilisk lizards can run up to five feet through water with long toes creating floating air pockets; Birds called Western Grebes have a mating ritual that involves running for up to seven seconds through the water as their wide, flat feet slap the surface. And cricket frogs were observed crossing ponds through “gliding,” which means jumping through the water without sinking.
Intrigued by the frogs’ dexterity, a group of scientists decided to study how the amphibians managed to stay on top of the water’s surface while gliding, findings they recently reported in a paper in the paper Journal of Experimental Biology. When they filmed the cricket frogs in high-speed videos, researchers learned the hard truth behind the slides. Instead of jumping across the surface like a real water walker, the cricket frogs were completely submerged in water between each jump. “This study is a testament to how even processes that seem so obvious can have a lot going on beneath the surface,” Jasmine Nirody, a biophysicist at the University of Chicago who was not involved in the new paper, wrote in an email.
Nirody grew up watching frogs gliding through water, and had found some documents describing locomotion, some dating back to the 1970s. But the images in the new newspaper surprised her. “I would never have guessed that this is what this form of locomotion looked like,” he said.
Talia Weiss, an engineer and biomechanist, worked at the newspaper when she was a doctoral student at Virginia Tech. Her advisor was working on a grant about animals that interact with the air-water interface, like the Basilisk lizard. Then Weiss read about slides. “Learning about another water walker I had never heard of before was enough of a hook to start researching,” he wrote in an email. Weiss scoured the literature to learn which frogs were known to glide, and one species stood out: northern cricket frogs, which lived nearby. According to Weiss’s research, the first researcher to explicitly define skittering was zoologist Carl Gans, in a 1976 paper. But other researchers had noted the behavior before, sometimes calling it skipping or water skipping.
Before researchers could study the slide, they needed to find some frogs. “Collecting the frogs was an adventure,” Weiss said. Researchers looked for northern cricket frogs, which can grow up to an inch long, around the edges of a state pond in North Carolina. But after dark, biologist Jon Micancin, who studies cricket frog populations, took the crew to a special spot on the lake. “There were hundreds of these frogs swimming, jumping and calling,” Weiss said. Micancin helped researchers collect 15 male northern cricket frogs, returning all females to protect the breeding population as well as any southern cricket frogs. On average, each frog weighed half a cent.
Back in the lab, the researchers needed to figure out how to make the frog boys shoot. They placed styrofoam pads and lily pads at the ends of the tank for the frogs to rest on, and ideally aimed. The researchers would then encourage the frog to jump (read: Gently jump with a finger). But the amphibians didn’t always cooperate. “Frogs often just swim below the surface of the water,” Weiss said. The researchers wanted to film them gliding straight across the center of the tank, but sometimes the frogs would jump in different directions, even crashing into the walls of the tank.
As such, “running these experiments required trapping and repositioning the frogs several times, which was probably quite bothersome for them,” Weiss said. And some more recalcitrant frogs had other tricks up their sleeves. After they filled with glide, they would swim to the bottom of the tank or even stop moving completely. “The frogs were probably faking death just so he would leave me alone,” Weiss said. When he later consulted the literature, he was assured that frogs also play dead in the wild.
The researchers used high-speed cameras that captured up to 500 frames per second. When they slowed down the footage, they discovered that the frogs weren’t going to skip across the water like a rock. “I was originally disappointed,” Weiss said. But then he realized that frog locomotion had never been described before, making the discovery exciting in a new way.
Glenna Clifton, a comparative biomechanist at the University of Portland who was not involved with the new paper, found the cricket frogs’ ability to fully submerge themselves in the water between each jump “surprising and impressive.” Although the frog’s jump out of the water seemed graceful, the return to the surface looked more like a belly flop.
Each frog’s jump went through four stages in less than a second: a submerged takeoff, airtime of a jump, re-entry into the water, and a recovery for the next jump. The researchers decided to rename this locomotion “marsopulator,” after the way a porpoise or dolphin swims fast, jumps out of the water, splashes downward, and then accelerates again for another jump. But unlike porpoises, cricket frogs stop after each jump and cannot harness the energy of their last jump. The researchers hypothesize that the frogs are too slow to move their legs into position in time to jump off the water surface. This is just one reason why a porpoise frog’s performance is less elegant than that of a dolphin.
Clifton found the tide analogy “somewhat appropriate” with some key differences, such as the frog’s inability to propel its jumps with the momentum of its last jump. The shape and size of frogs limits them from using this drive in the way that a sleek, streamlined animal like a dolphin could. Instead, frogs “start from stillness and must generate the required force within a single foot propulsive paddle,” Clifton wrote in an email. “In some ways, this could be similar to squid jet propulsion out of water.”
Clifton found the paper to be a “good first investigation” into the differences between how cricket frogs jump on water and on land. She said she was less convinced by the authors’ findings that frogs could achieve similar jumping heights from water and land, since the frogs’ land jumping behavior was much more erratic. Clifton hopes this study inspires further research with larger data sets.
But even if cricket frogs can’t stay completely out of water in the true definition of gliding, and even if they don’t look particularly graceful at what they do, their method of locomotion seems to adapt well. “The fact that several species of frogs are using it means that it is good (enough) for certain purposes,” Nirody said. Basilisks can stay on top of the water’s surface because they generate so much force, and geckos can traverse water thanks to adaptations like superhydrophobic belly skin, he added. “The frogs are doing the best they can within the limitations of their size and morphology,” Nirody said.
In fact, cricket frogs offer a wonderful example of what animals can achieve under specific limitations. This could be useful in influencing bioinspired design, Nirody said. “For example, if we need a robot that is smaller/less powerful than a basilisk and can’t use a superhydrophobic coating like geckos, then maybe we can look at what these frogs are doing for some ideas,” he added.
Additionally, the cricket frog is only one of a dozen frog species that have been described as gliding. And until those species star in their own high-speed videos, researchers can’t rule out the possibility of a true gliding frog. In fact, Weiss and his colleagues are currently working on another paper that might answer the question. “We think it’s possible for a frog to actually glide,” he said, no shade for the cricket frog, of course.