But some species, like the greater sage-grouse, have taken things to the next level.
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Nature is full of interesting sounds, from dogs barking to birds chirping. But there are a whole host of sounds that our human ears can’t pick up. Thanks to advances in audio recording, we’re now 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.
One of those sounds is the popping noise coming from tomato and tobacco plants. Scientists discovered that when the plants are stressed, due to being cut or left without water, they create a popping noise due to a process called cavitation, where tiny bubbles trapped inside the plants burst and release audible shock waves. The pops can be up to 60 decibels and can be heard up to five meters away.
Interestingly, the pitch of these plant cries peaks at between 49 and 58 kilohertz, which is outside our hearing range of up to 20 kilohertz. This means that we may not be the ones meant to hear these sounds. Researchers think that the plants’ pops might be used to signal to other plants, and there’s evidence that plants are listening to sounds.
In addition to plants, other creatures like moths and mice might also be listening in to these sounds. The world of plant communication is still quite new, so there are still lots of unknowns. But scientists are certainly keeping an ear out for answers. 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 rather, song spiders. These arachnids try to entice far-away females by making a kind of purring noise, which can be heard up to a meter away. 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. Then they drop the beat by making a kind of drumming sound by striking their abdomen against the ground. 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.
Grey snapper larvae also make noises to one another, mostly at night. These thumps and grumbling, growl kind of sounds might be their way of snuggling up close to one another. Scientists think the fish might be making these sounds by moving a muscle connected to their swim bladder, kind of like beating a drum inside their bodies. Researchers think the sounds might be to help them stay close to each other when it’s dark and they can’t see each other. Sound is an incredibly useful form of communication underwater, since sound waves travel faster and farther than other signals, such as light or smell. This is particularly useful for smaller organisms. In 2022, researchers discovered that E. coli can make a rolling, rumbling sound when placed on an ultrathin sheet of graphene, which is created by their movement across the surface. Scientists have also used this sound to measure the activity of bacteria and even test for antibiotic resistance. Furthermore, some bacteria can even sing. Finally, sand dunes have also been found to make mysterious noises, such as booms and burps, which have been studied by researchers since Charles Darwin. As of 2015, the mystery of why some sand dunes make booming and burping noises had been solved due to a clever array of sound recording devices and several years of making recordings. It turns out that it all comes down to the physics of how the sand slides down the slope. Steep and long dune slopes consist of different layers, each moving at a different speed. The sand in the surface layer tumbles, creating 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.
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.
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, hey, 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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