Frogs that seem to skitter across the surface of water as though it were solid land use a hilariously unexpected way to do so.

Rather than bouncing like a skipped stone, these tricky amphibians conduct a series of rapid belly flops across the water, sinking part way, but launching themselves upwards before becoming completely submerged. It’s a peculiar and very surprising way to get from A to B.

“Skittering is not actually a well-defined word for this behavior – one naturalist used it to describe a ‘jumping on water’ behavior in frogs in 1949, and since then, it’s been used for this type of locomotion in all the following literature,” says engineer Talia Weiss of Virginia Tech.

“Part of this research is not only studying this behavior in cricket frogs, but to try and give ‘skittering’ a more precise, scientific definition.”

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Skittering has been anecdotally documented for decades, whereby some frogs appear to hop across the surface of a body of water.

This is somewhat puzzling. In order to truly not sink into the water, the frogs might need some sort of specialized anatomy that allows them to skim.

One species with this ability is the northern cricket frog (Acris crepitans), common across much of North America.

Weiss and her colleagues used high-speed video at up to 500 frames per second to record northern cricket frogs jumping across both land and water to see if they could figure out the trick.

To their great interest, they found that the frog’s whole body briefly goes under the surface of the water each time it lands – akin to the way a cetacean leaps in and out of the water. The mechanism, the researchers say, is therefore known as “porpoising”.

The motion of the frog, though, is closer to a high-speed belly flop. It plops into the water, thrusting its legs out behind it to give it a push out of the water, before landing, righting itself, and thrusting again – all within the blink of an eye.

In this way, it appears to be moving across the surface of the water, all while secretly dunking itself.

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Questions, however, remain.

“It’s fascinating how easily we can be fooled by fast animal movements,” says engineer Jake Socha of Virginia Tech.

“Here, we’re fooled by a frog that appears like a skipping stone, but is actually jumping and dunking multiple times in a row. Frogs are great jumpers, but most of them don’t exhibit this porpoising behavior, and we still don’t know why. Is there something special about the frog’s leap, or is it simply a matter of small body size?”

Future research, we hope, will help answer these burning mysteries.

The research has been published in the Journal of Experimental Biology.