Cricket frogs belly flop their way across water

Cricket frogs can’t walk (or hop) on water like once thought.

Their bodies sink below the surface between successive jumps, researchers report in the November Journal of Experimental Biology. This mode of locomotion is a form of “porpoising,” when an animal leaps in and out of the water as it travels.

Eleven species of frogs have been noted to hop atop the water’s surface. But the movement hadn’t been studied in detail.

So biomechanist Talia Weiss and colleagues collected cricket frogs (Acris crepitans) from a swamp in North Carolina. Native to the eastern half of the United States and northeastern Mexico, these critters are so tiny that one can fit on a penny, says Weiss, formerly of Virginia Tech in Blacksburg.

The team filmed five frogs as they leapt across water in a glass tank, in a total of 45 trials. High-speed videos revealed that each frog starts with its body submerged, front legs by its sides and back legs tucked. The frog then pushes off the water with its back legs, flying in an arc through the air. As it descends, its front legs push forward into an upright Superman pose, before belly flopping into the water, Weiss says. Finally, the frog pulls in its limbs to prepare for the next jump.

With each consecutive hop, the frogs traveled about 16 centimeters forward and flew nearly 4 centimeters high. Their locomotion looked like that of the leaping dolphins, seals and fish who porpoise to conserve energy.

The energy savings come from the reduced resistance animals face when traveling through air compared with the water, Weiss says. Porpoising “is a continual motion. They’re going forward the entire time, and they’re using their momentum to repeatedly jump out of the water and jump into the water.”

The cricket frogs, however, paused between jumps, differentiating their movements from traditional porpoising, Weiss says. The amphibians may be too slow or too small to take advantage of the forward momentum they generate.