Quanta Magazine

Why didn’t biological creatures evolve some ability to ratchet their muscles or otherwise move themselves higher, farther and faster?

Muscles are evolutionarily very old; they don’t differ that much between insects and humans. “We got muscles from our great-great-great-great-great-great-great backboneless ancestors,” Sutton said. “Changing fundamental properties of bits is really hard for evolution.”

Had there been more evolutionary pressure to jump really high, “I guess we would have evolved really high jumpers,” said Charlie Xiao, a doctoral student and co-author with Keeley and others on the new robot study. But frogs, grasshoppers and humans need to be built not only for jumping, but for reproducing, finding food, escaping predators and doing everything else that life requires.

Richard Essner, a professor of biological sciences at Southern Illinois University Edwardsville, explained how those trade-offs can work. There aren’t many situations where you’d want to jump straight up, he said. Most often, when frogs and other small creatures need jumping power, it’s because they are trying to escape a predator behind them. Then the frog wants to quickly place as much distance between itself and the predator as possible. The frog will likely decrease its takeoff angle, flattening its trajectory to jump farther rather than higher — but probably not the farthest it can, because hopping to safety usually involves a series of hops. Most frogs fold their legs under their body in midair so that upon the instant of landing, they’re ready to jump again.

Surprisingly, there isn’t always natural selection pressure to land properly after a big jump. Recently in Science Advances, Essner and his team reported that amphibians called pumpkin toadlets, some of which are smaller than the tip of a sharpened pencil, almost always crash-land when they jump. Their tiny size is at the root of their problem: Like other animals, the frogs get their sense of balance from the vestibular system in their inner ear. But because their vestibular system is small, it is relatively insensitive to angular acceleration, leaving the frogs ill-equipped to adjust for tumbling during a jump.

They’re not alone in landing badly: Grasshoppers are “just terrible at it” too, Sutton said.

In a project led by graduate student Chloe Goode, Sutton’s group is currently studying why grasshoppers spin uncontrollably during their jumps. In their experiments, they outfitted the insects with tiny weighted top hats to shift their center of gravity. The researchers found that this was enough to stop the grasshoppers from spinning in the air, which in theory might give the grasshoppers more control over their landing. Sutton and his team have no idea why the insects didn’t evolve with a little more weight in their head for that stability.

But while a crash landing sounds perilous to us as relatively massive creatures at risk of breaking bones, it’s less problematic for smaller creatures. “It’s a scaling phenomenon,” Essner said. With increasing size, body mass increases more quickly than the cross-sectional area of the supporting bones, which determines their strength, he said. Compared to an elephant, a mouse has a lot of bone shoring up its minimal mass.

Small creatures “just don’t experience any damage from falls,” Essner said. There may not have been strong enough selection pressure to oblige grasshoppers and pumpkin toadlets to evolve the ability to land properly, which freed them to evolve other abilities more important for their survival, Essner added.

Rethinking the Limits

The Hawkes team robot is undergoing an evolution of its own. The researchers are working with NASA to develop their device into a fully functioning robot that could collect samples on other worlds, using controlled jumps to quickly traverse long distances. On the moon, where there is no atmosphere, no air drag and only one-sixth of Earth’s gravity, the robot could theoretically jump more than 400 meters, Xiao said. Their hope is to launch it to the moon in the next five years or so.

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