Your Plants Are Trying To Tell You They’re Thirsty

12:02 minutes

Photo of plants in plastic bottle chaptured in the morning right after the last night rain
Credit: Shutterstock

Spring is in the air, with flowers blooming and gardens starting. Most people with a green thumb will know a droopy plant is a signal that it needs water. But new research has found another way that plants will signal that they’re thirsty: emitting staccato popping sounds, too high pitched for the human ears.

Elsewhere in the world of science journalism, an argument has been made that elephants have self-domesticated. If true, this would make these gentle giants only the third creature to have done this, alongside humans and bonobos. 

Joining Ira to talk about these stories and other science stories of the week is Rachel Feltman, host of the podcast “The Weirdest Thing I Learned This Week” and editor at large of Popular Science based in Jersey City, New Jersey.

Segment Guests

Rachel Feltman

Rachel Feltman is author of Been There, Done That: A Rousing History of Sex, and is the host of “The Weirdest Thing I Learned This Week.”

Segment Transcript

IRA FLATOW: Spring is in the air, flowers are blooming, and I’m starting to get my garden together. I hope you are, too. So it’s good to know that when you forget to water plants for a couple of days and they start to droop, it turns out they do something else when they need water. They cry for help. Well, not literally. But new research finds that plants make noise when they’re stressed.

Joining me to talk about this and other science news of the week is Rachel Feltman, editor-at-large at Popular Science, and host of the podcast, The Weirdest Thing I Learned this Week, based in Jersey City, New Jersey.

Rachel, this may be the weirdest thing I learned this week.

RACHEL FELTMAN: Yeah, it’s a fun one and kind of a disturbing one in a roundabout way I guess. Yes, it turns out that when plants are stressed, most frequently when they’re deprived of water, they make sounds. You could say they scream.

IRA FLATOW: Oh, goodness. So even though the sounds are normally not something we can hear, the researchers, they down-sampled the sound to a range we can hear. And let’s take a brief listen.


Wow. It sounds like either bad Morse Code or somebody hunt-and-peck typing there.

RACHEL FELTMAN: Yeah. Yeah, or popping bubble wrap really furiously.


RACHEL FELTMAN: Yeah, researchers had known for a while that plants produce some amount of vibration when they’re really drought stressed from this process called cavitation. It’s basically when air bubbles form and collapse in the plant’s vascular tissue. It’s the same phenomenon that makes a noise when you crack your knuckles, actually.

But they had only picked it up by placing sensors directly on the plant. And this is the first time that scientists have shown that it projects sound waves that something can hear it. Not humans, but probably insects and maybe even mice and bats.

IRA FLATOW: Really? So what plants are we talking about here?

RACHEL FELTMAN: So the main study was on tomato plants and tobacco plants. But that’s really just because they’re very good cultivars to study in a lab. The researchers did some preliminary work on wheat, corn, cacti, and grapevines, and those all also emitted sounds. So it seems like this is probably a pretty universal phenomenon.

IRA FLATOW: And we don’t know why this happens. It’s not really a cry for help, is it– I mean, as we would normally think of it?

RACHEL FELTMAN: No. Fortunately, there is no reason to think that this is actually a cry of anguish. It’s probably an accident of a physical phenomenon that happens when plants are lacking water or otherwise experiencing distress. But the researchers do point out that just because the plants aren’t doing it consciously doesn’t mean it’s not an important signal to the animals that can hear it. And it’s also a signal that we could learn to listen to.

They actually trained an AI to pretty reliably detect how stressed out the plants were from a drought perspective by listening for these pops.

IRA FLATOW: Yeah. Our next story brings us to a creature that a lot of people don’t like to talk about. I mean, cockroaches, right?


IRA FLATOW: Talk about weird. It turns out that cockroaches are changing their sex lives and we are the reason why.

RACHEL FELTMAN: Yes. And it’s not the first time we’ve interfered with cockroach romance either. Basically, when a male roach targets a female roach, he offers a nuptial gift, which is a great euphemistic phrase for basically what is like a cluster of proteins, fats, and sugars. So it’s kind of like giving a date some chocolates.

And while the female is enjoying that gift, the mating starts to occur. And actually, the poisoned roach baits that so many of us in the New York City area rely on use sweetened bait to hijack the system. Roaches are primed to look for sweet stuff because it’s part of their reproduction process. And now we use that to poison them.

And what happened is that some roaches started having this aversion to glucose. More roaches were surviving and mating if they didn’t want to seek out that sweetened bait. So now we’re seeing roaches that don’t like the sweet stuff.

And at least in this one lab– we don’t know if this has happened in the wild– by which I guess I mean like my kitchen– but in the lab, some roaches have now developed a mutation that allows the males to produce this other sweeter, slower metabolizing sugar. And the female roaches actually seem to prefer it, including the ones that have this glucose aversion.

So they’re just getting sweeter to get around our poison bait traps.

IRA FLATOW: Wow. I know you’ve written a book called, Been There, Done That, about the history of sex.


IRA FLATOW: Do you have to upgrade your book now or write something else?

RACHEL FELTMAN: I mean, listen, I wish this had come out sooner. It would have been a great addition. I do think it’s a great reminder that sex is not just a biological process. It’s also very much influenced by our environment and by environmental pressures and cultural pressures.

Obviously, we are very different from roaches, but I think seeing the way that roach mating rituals change in response to environmental pressures is a good reminder that sex hasn’t always been the way it is and it might not always be the way it is now.

IRA FLATOW: Yeah. And we’re still giving out boxes of chocolate. So that’s still going to happen.


IRA FLATOW: Let’s go from a very small creature to a very large one. And I’m talking here about elephants. We know that elephants are smart, right, but new research says not only are they smart, they have potentially self-domesticated. What does that mean?

RACHEL FELTMAN: Yeah. So it’s an evolutionary process– really a hypothesis of a process– that leads to less aggressive and more prosocial individuals. Which is just the technical term for basically making nice. Like, having culture and community instead of fighting.

And so far, while scientists have discussed it in lots of animals, it’s only really been demonstrated in humans and our very close relatives, bonobos.

IRA FLATOW: And these are the only two species that have self-domesticated, humans and bonobos. What makes elephants fit into this mold? What did they do that we’re doing and the bonobos are doing?

RACHEL FELTMAN: Yeah. So on the one hand, there’s the behaviors, right? They show these really advanced traits. They mourn their dead. They’ll help out sick and injured other elephants. They can recognize themselves in mirrors.

And also, culturally, they seem very empathetic. They’re not aggressive. They have a very long juvenile period and they play a lot. They’re very curious. They’ll even babysit each other. It’s a lot of very community-based behaviors that are not super common in the animal kingdom.

And researchers are basically suggesting that this developed through a selective reproductive process, where you were more likely to successfully reproduce if you exhibited these kinds of behaviors. And they did actually show that elephants have a couple of genetic markers that are associated with domestication. But it’s kind of an open question.

Because the whole idea of self-domestication is almost as much of a philosophical question as it is a biological one. So it’s sort of about, how actively did cultural shifts help shape the changes that we see in elephants versus other big awesome animals?

IRA FLATOW: Yeah. Well, when they start playing mahjong or poker, we’ll know they’ve totally moved into it. Hopefully, nobody has fallen asleep in our talk this morning because this next story is about possible problems that arise from sleep apnea. Tell us about this. This is pretty important.

RACHEL FELTMAN: Yeah. So, basically, sleep apnea, there are a couple of types. There’s obstructive, which is where your throat muscles relax while you sleep and it blocks your airways. Then there’s central sleep apnea, which is less common. Which is where your brain literally doesn’t send the right signals to keep you breathing while you sleep.

Either way, it’s definitely an issue. You’re not getting enough oxygen in the night. And a lot of people either don’t know they have it or don’t realize that it’s a big deal.

And traditionally, the thought has been that the big risk is developing heart disease because of that chronic low oxygenation through the night. And sometimes we see cognitive issues. But the idea has always been that these are a consequence of the heart problems.

And now, this very small preliminary study showed that otherwise healthy men with sleep apnea who did not have heart problems– and sometimes even had quite mild sleep apnea– were compared to men without it. They showed poor mental function in areas like judgment, impulse control, and even recognizing other people’s feelings.

IRA FLATOW: Really? Wow.


IRA FLATOW: And that’s a lot of men we’re talking about here, right?

RACHEL FELTMAN: Yeah. Because it’s estimated that as many as 26% of adults in the US age 30 to 70 have sleep apnea. And it’s actually probably higher than that. Because, again, it happens while you’re sleeping. So if you don’t have a partner, like mine, who said, I notice you’ve stopped breathing sometimes in the night, which is how I finally went to get a sleep study done and got diagnosed, you can really just go a really long time not realizing it.

So if you’re feeling sleepy during the day or you know you snore, you should really see a doctor about that.

IRA FLATOW: Well, we’ll know why you’re making bad judgments now.

RACHEL FELTMAN: Yeah, exactly.

IRA FLATOW: You gave us an excuse. Let’s go back in time for our last story. It turns out that medieval monks recorded some of history’s biggest volcanic eruptions accidentally.


IRA FLATOW: Tell us about that.

RACHEL FELTMAN: So researchers realized that when there’s a lot of volcanic activity, you see changes in the sky because you have all that particulate matter. So you can have crazy sunsets across the planet after a period of intense volcanic activity. And one of the ways that that can play out is in making eclipses look particularly dark.

And what’s cool is that in many religious traditions, both in European medieval monks and in contemporary religious and spiritual scholars in Asia, they cared a lot about eclipses because it seemed like a pretty significant cosmic event from a spiritual standpoint. And they took note of when they looked particularly freaky, when they were blood red or particularly dark.

And so researchers realized that they could cross-reference this with the ice core data we have that tells us when sediment from volcanic activity was prevalent. And they use that to confirm some periods of volcanic activity that these monks wouldn’t have had any way of knowing about because they were not sitting there, looking at a volcano as it erupted.

IRA FLATOW: Really interesting. You always bring us interesting stuff, Rachel. Thank you for taking time to be with us today.

RACHEL FELTMAN: Yeah. Thanks for having me.

IRA FLATOW: Rachel Feltman, editor-at-large at Popular Science, and host of the podcast The Weirdest Thing I Learned This Week. She’s based in Jersey City, New Jersey.

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Meet the Producers and Host

About Kathleen Davis

Kathleen Davis is a producer at Science Friday, which means she spends the week brainstorming, researching, and writing, typically in that order. She’s a big fan of stories related to strange animal facts and dystopian technology.

About Ira Flatow

Ira Flatow is the host and executive producer of Science FridayHis green thumb has revived many an office plant at death’s door.

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