By studying the mechanics that one-millimeter copepods employ to jump out of water, scientists could build robots that use similar jumping techniques for practical purposes.

A new study, published March 6 in the Journal of the Royal Society Interface, describes the patterns and parameters required for animals as varied as copepods (a kind of crustacean), frogs, fish, dolphins and whales to leap from water to air – a challenging task given that water is 1,000 times denser than air.

Credit: Sunghwan Jung/Provided

Research that breaks down exactly how aquatic animals get air time has been scarce – until now.

To experiment with such leaps, the researchers created a simple water-jumping robot. But the robot also has practical applications: If fitted with a sensor that detects polluted water, for example, an immersed device could jump when triggered, and transmit a signal to notify water managers of polluted areas. Compared to sending signals through water, transmitting signals through air is more efficient, cheaper and doesn’t require large equipment to transcend the air-water interface.

“The big picture is to try to understand how animals behave in nature,” said Sunghwan Jung, Cornell associate professor of biological and environmental engineering, and the paper’s corresponding author. “Animals have evolved millions of years to optimize their behavior and maximize their performance. From there, we can learn how to design a product or engineering system to perform better.”

Jung and colleagues identified a number of patterns when analyzing the jumping behaviors of species across five taxonomic groups. Small copepods can jump up to 20 times their body length, while larger animals, like whales, are lucky to get their whole bodies out of the water. “In small animals, jumping is strongly related to survival, so that’s why their performance is way better,” Jung said. “If they cannot jump high enough, then they will get killed.”

Mid-sized animals, such as frogs or archer fish, may jump five to close to 10 times their body length, mostly to catch prey. Large animals may jump to play, communicate or attract a mate, but no one is sure why whales and dolphins jump.

The researchers also created an equation to mathematically represent these jumping dynamics, which included an animal’s maximum jumping height, body length and the role of gravity as it exits the water. The equation quantifies the balance between how much power an animal has versus how much power it must expend to raise itself out of the water, which is affected by gravity and water drag.

While designing the robot, the researchers mimicked the quick clapping motion that frogs and copepods use to jump; frogs clap their hind webbed feet together and copepods clap two special antennae. Using a simple design, the robot includes a 3D-printed hinge with two flaps, like a door hinge, a rubber band and a tiny wire. When tripped, the wire frees the hinge to snap around in a clapping motion that propels the robot from the water.

The rudimentary biomimicking robot lacked the streamlined qualities of living animals, creating a lot of water drag. On the other hand, copepods, frogs and archer fish, for example, have streamlined bodies and exit the water without carrying much fluid with them. “Our robot cannot jump as high as an animal due to the entrained water or drag,” Jung said. In the future, the researchers will streamline the robot to make it more effective.

While animals are hard to control in such experiments, the robotic system gave the researchers control so they could experiment with wing lengths and tension, to better understand the forces at play.

Brian Chang, a former graduate student at Virginia Tech and now a researcher at Temple University, is the paper’s first author. Co-authors include researchers from the Seoul National University in Korea, Ecole Polytechnique Federale de Lausanne in Switzerland, Harvard University, and the Ecole Polytechnique in France.

The study was funded by the National Science Foundation and the National Research Foundation of Korea.