Wild Things

What Makes a Jumping Bean So Jumpy?

Mexican jumping beans.
Mexican jumping beans.


A few months ago, I had the chance to take a bush walk with one of the most respected trail guides in South Africa, Stefan Winterboer. After showing us where a hippo had marked its territory by power-washing a small tree with its dung and then navigating us between two breeding herds of elephants on either side of a dry creek bed, Winterboer knelt down to consider a pile of hardened fruits beneath a gray-barked tree.

This was a tamboti tree, he said, though some call it the jumping bean tree. In a few weeks, this quiet grove would be crackling with the sound of seeds leaping off the ground like popping corn, the result of moth larvae trying to break out of their makeshift incubators.

We could still hear the rustling menace of elephants a drainage over, but all I wanted to do was camp out and wait for the jumping beans.

The African jumping bean is not nearly as well known, of course, as its Mexican counterpart. The seedpods or “beans” of the shrub Sebastiania pavoniana, which grows in the deserts of northwest Mexico, are also infected by moth larvae. Holding one in the palm of your hand raises the temperature within the seedpod and causes the larva to fidget. But these beans aren’t quivering for our amusement. Too much heat, like that delivered from direct Mexican sunlight, will kill a baby moth. So instead of being scorched alive, the larva hurls itself against the inside of the bean, which makes the whole capsule jump. If it’s lucky, the larva will hop until it finds a nice place in the shade.

So the moth larvae in Mexican jumping beans jump to change their climate, and the ones in tamboti fruit jump to escape from their seedpods. These two behaviors, similar in form but different in their goals, appear to have evolved separately: The two species of moth are found in different families. But by a weird coincidence, the plants these species parasitize are pretty close to each other. Both belong to something called the spurge family, or Euphorbiaceae.

Cut into the bark of either plant and you’ll release a milky latex that has been used as a poison. For hundreds of years, people living near tamboti trees have put the stuff on darts for hunting large game. Even a sprinkle of crushed-up tamboti bark can make a stream full of fish go belly-up—like a chemical grenade.

In Mexico, traditional hunters and fishermen employed the same strategies with the sap of Sebastiania bilocularis, a lesser-known species of tree that also produces jumping beans. According to Adrian Burton, who wrote about jumping beans for an article last year in Frontiers in Ecology and the Environment, the trees can be so toxic that locals avoid using them for firewood. The smoke taints food and causes eye irritation.

If you’re really in a pinch, the liquid can be used for medical procedures otherwise impossible when you’re deep in the bush. As Winterboer tells it, a friend of his once cracked a tooth while he was on an expedition. The pain was so great, and the friend so desperate, that he allowed the local guides to apply a bit of tamboti sap to the exposed cavity in his tooth. Like liquid flame, the sap burned the nerve out, effectively achieving the same result as a root canal.

How exactly moth larvae cope with such a seriously lethal habitat has not been studied. Perhaps setting up in a tree with poison blood affords the little guys some sort of protection from predators.

If that’s the case, we do know that the plan is not foolproof.

According to Peter Oboyski, collections manager for the Essig Museum of Entomology at the University of California–Berkeley, there are species of parasitic wasps that descend upon moth larvae in their seedpod homes, inject their spawn into the beans with alien-like ovipositors, and fly away. The wasp eggs eventually hatch and start doing what parasitic wasp larvae do—eating their hosts alive. (Oh, parasitic wasps, I love you so.)

Other types of parasitic wasps have also been known to do a little bebopping of their own. The California jumping gall wasp lays its eggs on the underside of oak tree leaves. It’s not completely understood how the oak knows the egg is there, but it tries to protect itself from the invader by sealing the egg inside a hard shell of tissue. Botanists call this growth a gall, but to the wasp, it’s every bit as good as a cocoon. Eventually, the galls detach and fall to the ground, and then they, too, skitter around. In this case, it seems the wasps are looking to move their capsules into leaf litter or cracks in the soil where they will ride out the winter.

We know a little more about one species of hopping wasp in particular, Bathyplectes anurus, thanks to a study published in December in a journal called Science of Nature. These wasps attack alfalfa weevils, injecting their spawn into the cocoon the weevils spin and then eating them alive. Once the weevils are consumed, the wasp larvae then form another cocoon within the ones they’ve just invaded and settle in for about 10 months of pupation.

The thing is, a lot can happen in 10 months. Branches can grow or break and expose the wasps to excess sunlight or too much rain. Whatever the case, the larvae seem to give themselves a better chance at survival by moving their hibernation chambers around a bit—using the same larval lurch we’ve come to know and love. According to the recent paper, B. anurus larvae that were placed in the shade survived at higher rates than those left in the sun. Similarly, when presented with the chance to hop into the shade, many of the larvae did exactly that.

Japanese giant ants are also known to make the larvae shiver. When the scientists introduced some ants into the same controlled environment as the double cocoons, the larvae started jumping at frequencies 83 percent higher than those in an ant-free environment. The ants didn’t even have to touch the cocoons for the wasps to know they were there. It’s unclear exactly how wasp larvae enshrined within two layers of cocoon would know there were ants nearby, but the scientists suspect the larvae may be able to pick up on chemical cues or vibrations.

Once the ants made contact with the cocoon—feeling it out with their antennae or clutching it with their mandibles—the larvae began to chill out. It could be that once they know they’ve been spotted, the larvae play possum—a widespread behavior in the animal kingdom.

But all this larval leaping comes at a cost. The researchers found that larvae that had been induced to jump lost a lot of weight compared with larvae that were allowed to veg out—and for insects especially, body size can be the difference between life and death.

Both moths in jumping beans and wasps evolved strategies to make the best of a bad situation. Jumping affords them some semblance of control over their environment, despite lacking legs and wings and being sealed up in the insect equivalent of a body bag. And the fact that this behavior has evolved independently multiple times is super interesting.

So if you ever come across a little pod shimmying its way across your path, take pity on the creature and nudge it into the shade. Inside, there’s a living thing desperately trying to change its own fate.