Why Did The Mexican Jumping Bean Jump? - KQED

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Sponsor MessageBecome a KQED sponsorupper waypoint Deep LookWhy Did the Mexican Jumping Bean Jump?Mike SeelyMar 23, 2021Updated Feb 6, 2025Save ArticleSave ArticleFailed to save article

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To find its place in the shade! Each hollowed-out seed is home to a head-banging moth larva, just trying to survive the harsh Sonoran Desert sun.

TRANSCRIPT

What if you spent most of your life in near darkness, surrounded by the same walls, eating the same food, all alone?

That’s life inside a Mexican jumping bean… and that’s just how these creatures like it.

They’re the same jumping beans you find in markets all over Mexico and exported worldwide. But they’re not something you’d want to eat. They aren’t beans at all.

They’re seeds of this scraggly shrub. It primarily grows in Mexico, in the mountains of the Sonoran Desert. There are three of them that make up this fruit.

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Some of these sections have a stow-away – a tiny moth larva that burrowed into the seed while it was still on the plant.

The larva devours the inside of the seed, hollowing out its new pad to make room for its growing body.

Over the next 8-10 months our squirmy friend lines the walls with a comfy layer of silk. Just enough air and moisture sneak in through tiny holes in the seed walls. It’s a cozy life. Except for the sweltering desert sun.

That heat can dry out and kill our sweet little larva. So, of course, it starts jumping. A few hops out of the sun can mean the difference between life and death.

ADDITIONAL RESOURCES

For further information about Mexican jumping bean ecology and the life cycle of the Mexican jumping bean moth, check out this factsheet or this informative site with some frequently asked jumping bean questions.

David Hu, a researcher at Georgia Tech (whose lab footage we used in our episode), studied the locomotion of Mexican jumping beans, suggesting we may be able to use their ways of getting around to help us design small spherical robots.

Luckily, it’s got wheels – well sort of. The seed’s shape – with two flat sides and one curved – even allows the jumping beans to travel uphill.

Inside, the larva is working hard. With its back legs, it grabs onto the silk lining and thrashes its head against the seed wall. The force topples the seed.

Researchers think these headbangers aim themselves in the right direction using a finely-tuned sense of temperature.

Check out this experiment. One side of the pan is warmed by a heat lamp, the other cooled by an ice pack. Over time, the larvae move away from the heat.

It’s not always a smooth trip. If the seed gets damaged, the larva springs into action, repairing holes with a dense patch of silk.

But the larva can’t stay in its comfort zone forever.

With sharp mandibles, it cuts a circular door in the seed wall. But it doesn’t open it. It’s doing its future self a favor.

When it’s done it turns into a pupa. And then transforms into an adult moth. It simply pushes itself through the pre-cut exit door – which is handy, because now its mandibles are gone.

The liberated moth has mere days, maybe a few weeks, to quickly mate and lay eggs before it dies.

But after all, freedom isn’t really what this animal lives for.

For most of life, it’s totally fine being a young larva trapped inside a seed, just hoping – and hopping – for a place in the shade.

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