Random hops always bring jumping beans to shade — eventually

This random strategy isn’t fast, but it’s guaranteed to get jumping beans out of the sun

A photo of a jumping bean moth resting on a seed pod.

A jumping bean isn’t actually a bean. It’s a seed pod that holds a young moth. Here, a moth rests on the seed pod that was once its home.

AuldAlliance/English Wikipedia

Given enough time, jumping beans will always find their way out of the sun.

Jumping beans are not actual beans. They’re seed pods with twitchy moth larvae inside. And they hop around in a way that will — if the larvae inside live long enough — eventually land them in shade.

Researchers shared that finding January 25 in Physical Review E.

Left out in the sun, a jumping bean might overheat and die. So, when a bean finds itself in a sunny spot, the moth larva inside will twitch. This makes the bean jump a short distance. But if these moth larvae can’t see where they’re going, how do they reach shady spots?

Two researchers teamed up to find out. One was physicist Pasha Tabatabai. He works at Seattle University in Washington. The other was Devon McKee. They are now a computer scientist at the University of California, Santa Cruz.

The two tracked the leaps of jumping beans placed on a warm surface. Each jump was in a random direction, they discovered. It did not depend on the direction of any previous jumps. Mathematicians call this way of moving around a “random walk.”

A random walk isn’t a quick way to travel, Tabatabai says. But a creature using it to move on a surface, like the ground near a tree, should visit every place on the surface eventually. That means a random walking bean will always end up in the shade if it keeps it up long enough.

Picking a single direction and jumping only that way would cover distance faster. “You’re certainly going to find shade fastest,” Tabatabai says — but only if you’re headed the right way. “It’s also very likely that you’ll pick the wrong direction and never find shade.” This makes motion in a single direction very risky.

Random walks are slow. And many jumping beans don’t survive to find shade in real life. But, Tabatabai says, their strategy maximizes the odds that they will eventually escape the sun.

James Riordon is a freelance science writer who covers physics, math, astronomy and occasional lifestyle stories.

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