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Dogs bark, birds chirp, and foxes…well, the jury’s still out on that one. Nature is full of interesting sounds, but there are a whole host of them that our human ears can’t pick up. But now, thanks to advances in audio recording, we’re hearing more of the natural world than ever before. And some of those sounds are giving scientists a whole new insight into what living and nonliving things are capable of.

So here are five sounds of nature that come from stuff we never thought could talk.

[Intro song]

Generally, we think of plants as pretty silent. Sure, we may chill out to the sounds of swaying branches or rustling leaves, but that’s all wind. The plants themselves aren’t really making any sound. At least that’s what we thought. But in early 2023, scientists set up microphones next to some potted plants and discovered that they’re noisier than we gave them credit for.

Specifically, they make a kind of popping noise, that sounds like this.

[Popping Noise]

The popping that you’re hearing comes from tomato and tobacco plants, and they’re not happy. Specifically, they’ve either been cut or left without water, so the plants are stressed. The researchers who recorded the sounds think that the sounds come from a process called cavitation. That’s where tiny bubbles trapped inside the plants’ water transport system, or xylem, burst and release audible shock waves through the stem and into the air.

At their loudest, the pops can be around 60 decibels. That could be loud enough to be heard three to five meters away. In their experiment, researchers noticed that stressed out plants, like those that were dry or had been pruned, let off way more pops than plants that were just sitting around in their pots–up to 35 times more. Plus the dried-out plant pings were slightly different from the chopped-plant pings. So much so that a computer program could tell them apart 70% of the time.

Now, you’re probably wondering why we don’t hear plants popping all over the place. Well, the pitch,

or frequency, of these plant cries peaks at between 49 and 58 kilohertz, which is way above our hearing range of up to 20 kilohertz. That recording you heard has been processed into something we can hear, because, well we all wanna know what a plant sounds like! But we’re probably not the ones meant to hear these sounds.

Researchers think that the plants’ pops might be used to signal to other plants. That’s because there’s evidence that plants are listening to sounds. In some studies, plants increase their drought tolerance after listening to particular, artificially-made sounds. And in others, plants exposed to the sounds of birds chirping or bees buzzing germinated more quickly. So it's possible that the neighboring plants hear other plants’ sounds of stress and change how they grow as a result.

Now, we don’t know for sure if plants can hear, but researchers have proposed that mechanical receptors on the plant or on the surface of plant cells might be picking up the vibrations. If plants are listening and talking, whatever that means in the plant world, there might be others listening in. The frequencies of the sounds that plants make also fall right in the listening sweet spot of insects like moths or mammals like mice. So whether plants mean to or not, those creatures might be listening in, too. 

The world of plant communication is pretty new so there are still lots of unknowns, like exactly why plants might want to talk to their neighbors or who might be listening. But scientists are certainly keeping an ear out for answers.

Animals have come up with all sorts of sexy ways to find a mate. Some make smelly perfumes, others build elaborate structures, and many have some kind of mating call. It turns out that a particular species of wolf spider, sometimes known as the drumming sword wolf spider, is one of these song birds. Or … song spiders? These arachnids try to entice far-away females by making a kind of purring noise that sounds like this.

[Spider brushing sound]

Now, it might not sound all that weird for a spider to purr. After all, insects like crickets and cicadas make rasping-like noises too. But here’s the rub. Unlike some insects or other animals that make sound, spiders don’t have ears, or any structures

for hearing sound! In fact, most spiders are totally silent. But the cool thing about sound, though. It doesn’t just have to be heard, it can also be felt.

See, lots of spiders create vibrations that can travel through substrate or along leaves, but these signals can only travel so far before they encounter a gap in whatever material they’re traveling through, and have to stop. Unlike those kinds of vibrations, airborne signals like sound are less likely to hit one of these physical roadblocks, which is what lets male wolf spiders cast their mating net just that little bit farther. 

Their purring can be heard up to a meter away. And since the ladies can’t hear, they rely on feeling those vibrations when they resonate with what’s under their feet. The whole thing only works if males are on a surface that jiggles, like leaves, and a female is on another jiggly surface so they can feel the sound.

Scientists have analyzed the wolf spider’s song and found that it actually has two to three distinct parts. The first verse, the kind of purring, comes from males rubbing together a pair of extra appendages called pedipalps to create anywhere from six to more than 20 short, rapid pulses. And then they drop the beat by making a kind of drumming sound, striking their abdomen against the ground. And some males might include a third verse, which sounds like quick, low frequency tics. This last sound is a bit of mystery and arachnologists haven’t yet figured out exactly where it’s coming from. What’s neat is that it seems only females respond to these acoustic signals, so the songs might be a way for males to talk only to them.

Since it’s such an unusual example in the spider world, researchers want to study the wolf spider and use it as a kind of model for understanding how sound signaling might have evolved in different groups of animals.

Now, if you were lying in bed and heard this sound in the middle of the night:

[Growling Sound]

Well, you’d probably be pretty unnerved. But not so for grey snapper larvae. These thumps they make in the dark might just be their way of snuggling up close to one another. Grey snapper spend about the first month of their lives floating around in the ocean as tiny larvae, about the size of a pencil-top eraser.

And scientists recently recorded the larvae making low frequency noises to each other, both in the lab and out in the wild. Most of the time, these little larvae were knocking sounds like this.

[Knocking sounds]

But sometimes, they’d make these grumbling, growl kind of sounds too.

[Growling sound]

Researchers think the fish might be making these sounds by moving a muscle connected to their swim bladder. Kind of like beating a drum, just, inside their bodies. And while we knew the adult fishes make these sounds too, these recordings were the first time researchers have heard such young fish making noise. Plus, the adults only make these noises when prompted with a jolt of electricity, so these spontaneous noises are especially intriguing.

The larvae were beating their tiny drums mostly at night, so researchers think that the sounds might be to help them stay close to each other when it’s dark and they can’t see each other. Because sticking together when you’re pint-sized is a pretty good strategy if you want to make it in the big wide ocean. Plus, generally, sound is a really useful form of communication underwater, since sound waves travel faster in water than in air and can travel farther than other signals, like light or smell.

Now, speaking of drumming, there are even smaller organisms that have percussive talents. Drum roll please, for… bacteria. In 2022, researchers discovered that E. coli can make a rolling, rumbling sound when they’re placed on an ultrathin sheet of graphene. It sounds like this.

[Rolling Sound]

These double-layered graphene sheets were circular and taut and pretty much acted the same way drums do to make sound–by vibrating that taut surface–except on a really, really tiny scale. Instead of a drumstick, it’s their movement across the surface that makes the noise.

For some of the bacteria, that came from flicking their tiny whip-like flagella to scoot along, but bacteria that didn’t have flagella still made noise as they moved, even without a built-in drumstick. Now, obviously E. coli are microscopic, so they don’t exactly make a big ruckus. The way researchers

were able to hear this drumming is by measuring how much the drum moved using a laser and then converting that vibrational signal into an audible one. How much and how loud those bacteria drummed could tell scientists something about how active they are.

See, the more they move around, the more noise they make, so how loud the drumming is can tell scientists how active the bacteria are. Scientists used this fact to test how susceptible the bacteria were to antibiotics. When they exposed different groups of bacteria to several types of antibiotics, those bacteria that were resistant kept on drumming, while those that weren’t were killed off.

So scientists think they could use the drum method to check for antibiotic resistance at the level of a single bacterium. But some bacteria don’t just make noise by accident, they actually sing. In an experiment from 1998, scientists recorded what they call sonic emissions coming from plates of Bacillus subtilis bacteria. They then played tones that were the same frequency as the sounds made by B. subtilis over a speaker to a group of bacteria from another species, Bacillus carboniphilus. And they saw that B. carboniphilus was able to grow in inhospitable conditions when played the sound, but not when it was silent. The songs were spurring on their growth.

Several other experiments since then have shown that bacteria grow better when they’re played different sounds, which makes them think that sound somehow regulates the growth between cells. But they’re not really sure how that regulation happens, why or even how bacteria sense the sound in the first place. Like, they’re probably picking up each other’s songs from feeling the vibrations caused by that jiggling, but other than that, these organisms’ musical talents are a bit of a mystery. So between the percussion and vocals, we’re about halfway to starting an all-bacterial band!

Unlike the other noise-makers on our list, the last one isn’t alive. But the noise it makes is so bizarre and mystifying to researchers that we had to include it in this list. These sand dunes….. burp. Sand dunes, like these in Death Valley National Park in the US or these in the Gobi desert in Asia, regularly let off booms and burps

that sound like this.

[Low frequency boom]

It happens when the dunes avalanche, either naturally or when disturbed by others, like, say, a bunch of researchers shuffling along the slope. Scientists have recorded and classified both types of sound and have found that booming is generally a low, long note somewhere around 85 hertz. The burps on the other hand are short and at a different frequency, although dunes can belch a whole range of notes. But how the dunes make their booms and belches has been a bit of a mystery that’s plagued researchers dating all the way back to Charles Darwin.

As of 2015, the mystery has been solved thanks to a clever array of sound recording devices and several years of making recordings. It turns out, it all comes down to the physics of how the sand slides down the slope. See, steep and long dune slopes consist of different layers, each moving at a different speed. As the sand in the surface layer tumbles, it creates a P-wave, where some sand squeezes close together before spreading apart again. As these waves travel down the slope, they overlap or interfere, which amplifies them. The sand in the layer about a meter and a half below the surface moves more slowly than the rest, and acts as a kind of guide for sound waves to travel along. 

Now, not all sand dunes are capable of making these booming noises. Shorter, less steep dunes don’t boom because there isn’t enough sand for these different layers to build up. On the other hand, those quicker burps happen for a different reason. Dunes belch because of a slow-moving, rolling wave of sand called a Rayleigh wave, which moves along the surface. Imagine an ocean wave moving towards the shore, where each crest has swirling particles inside it. In the burping sand dune, those particles are tiny grains of sand. And although they’re swirling in a kind of backwards ellipse pattern, that swirling is a little chaotic. 

It’s the interaction of those swirling grains and the movement of the wave itself that creates the different burping notes of the sand dune. So put it all together and the symphony of booming and belching depends on different types of waves, together with how fast the grains move and how thick the guiding layer is.

And, I mean, if something as seemingly still as a sand dune can create sound, who knows what other tones bioacoustic researchers will uncover in nature in the future. The world contains a symphony of sounds from all kinds of unlikely sources, and science is only scratching the surface at listening to them!

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