Nature’s secret soundscape: listen with me to the world’s quietest creatures

The most obvious answer is that we need them. Without fish, food webs would collapse, and billions of people would go hungry. Tuning in to the sounds of fish is one way of keeping track of how many there are, where they live, and whether their populations are growing or shrinking.

But fish can also serve as emissaries from parts of the planet that are hard for us to access – turbulent rivers, ice-covered lakes, mysterious deep-sea realms. “So much of the ocean is still unexplored,” says Lauren Hawkins, a researcher at Curtin University in Perth, Australia. “We have to find ways to monitor the health of our marine environments, and fish are very good indicators for that.”

Hawkins says fish sounds can be used to infer important information about changes in water temperature, currents, salinity, pollution and more. They can also help researchers assess whether conservation projects are actually working. “Using acoustics is a way of looking at measuring the changes that are happening. And the more tools we have, the better.”

But it all begins with identifying who’s talking. Or unk-ing. Or … splooshing? That’s the best word I can come up with to describe this sound:

This is the voice of a very cold Swedish fish, most likely a grayling, Arctic char, perch or trout. While I was off skiing around that lake, it swam up to my hydrophone and introduced itself. It may not have the haunting allure of the cuckoo’s call, or the ferocious power of a lion’s roar, but when I listened back to my recording and heard this sound, I found myself unexpectedly moved. Suddenly that lake didn’t just have some fish in it, it had this fish. One particular animal that had survived under the ice, all winter long.

It was a moment of contact, not just with this creature, but with a whole world that in many ways feels alien to me, even though it’s right there, just below the surface.

I had a similar feeling of dropping into an alternate realm when I clipped a small microphone on to the stem of a plant in a park in Iowa City, Iowa, and heard this sound:

The sound was quite loud in my headphones, but when I pulled them off, I couldn’t hear it all. I also couldn’t see what was making it.

Intrigued, I sent the recording to Rex Cocroft, a biologist at the University of Missouri, and he confirmed my suspicions: I had managed to capture the sounds of a small insect, likely some sort of leafhopper, communicating in a hidden world of sound called the vibroscape. Even more exciting, he thought the odds were good that no one had ever heard or recorded this sound before.

That’s because leafhoppers and their cousins, treehoppers, don’t transmit their sounds through the air, like we do, or through water, like fish. These little creatures shake their abdomens, causing acoustic waves to move through their legs and out into the stems and leaves of plants. Other hoppers pick up on these vibrations with their legs, and often respond in kind. “It’s like they take a different transect through acoustic space and put together sound in ways that we never thought to do,” Cocroft says.

These calls are not subtle at all – they are big, bold and sometimes even a little scary. But without the right kind of microphone, a person standing right next to two conversing treehoppers will not hear a thing. It’s almost like they turn the plants, and their own bodies, into magical musical instruments that only they can hear.

The key to tapping the lines of communication between treehoppers was Cocroft’s curiosity, not technological wizardry. He was doing research for his PhD in animal communication when he decided to find out if treehoppers had anything to say. This was at Cornell University in the 1990s.

“I just walked out onto a meadow near where I lived,” he says. “I had a cassette tape recorder and headphones.” He found a goldenrod plant with some treehoppers on it, leaned a sensitive microphone up against it, “and immediately I heard these wonderful sounds I’d never heard before. Just this tiny insect, this beautiful song. And then I was hooked. I never looked back.”

Before Cocroft started documenting their sounds, treehoppers had attracted quite a bit of attention for their ornate and sometimes downright whacky body shapes. The ones Cocroft studies the most, thorn bugs, look like rose thorns with legs. Others appear to have sand castles on their heads, “or a little Starship Enterprise on their backs”, he says.

These minute rockstars of the insect world, pushing the boundaries of sound and fashion, live on every continent but Antarctica. If it happens to be warm outside where you are today, odds are good they’re calling on some plants nearby. That means hundreds of millions of us probably walk right by treehoppers and leafhoppers every day, unaware of the messages they’re sending to each other.

But what about plants? What, if anything, are they doing with all of this acoustic information, coursing through their plant-y veins?

In 2007, Cocroft stumbled upon this question in a casual conversation with another scientist, Heidi Appel. “We both immediately thought, ‘No, that’s very far-fetched and unlikely,’” Cocroft says. The idea of plants being able to detect sound waves produced by insects – in a sense “listen” to the vibroscape – just seemed too out there. But they decided to test the idea anyway, almost as a lark.

I tell the full story of what happened next in this episode of Threshold, but (spoiler alert) what they discovered ended up kicking off a whole new era of plant bioacoustics.

Talking fish. Bellowing insects. Listening plants. Again and again, organisms have evolved ways to use sound to help them survive. That includes us.

“I think for humans, language and cooperation are some of the features that really helped move us forward and helps us be successful,” says neuroscientist Alison Barker.

I met Barker at the Max Planck Institute for Brain Research in Frankfurt, Germany, where she studies connections between cooperation and acoustic communication in naked mole rats – small mammals that spend their whole lives in underground colonies in east Africa.

Naked mole rats are not moles, or rats, but they are mostly naked, other than the hundred or so highly sensitive hairs on their finger-sized bodies. And they are chatterboxes.

“I wish I knew what they were saying,” said Barker, as we stood over the colony in her lab, listening to them. She leads a team of researchers who are working on figuring that out. They’ve installed an array of microphones which allows them to eavesdrop on naked mole rat conversations, and they’ve cataloged more than 25 sound types, including the scream, the hiss and the toilet call.