Silent Crickets: What Does the Future Sound Like?

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Take a minute and try to imagine how your physical environment might change within the next few decades. Not just the sights, but the sounds.

If you're a city-dweller, this is pretty easy. I think of sidewalks that are relatively quiet even at rush hour, since most of the cars emit no more than the light hum of an electric motor -- possibly augmented by artificial warning sounds -- and many of the massive, roaring HVAC systems that heat and cool large buildings today are replaced by passive solar architecture. Even if these particular revolutions don't take hold, urban environments can be counted on to change very quickly, and as machines and power sources evolve, the sonic landscape will evolve with them.

On the other hand, if you step out to the countryside, it seems like the world around you is less sensitive to the passing years. Barring major environmental devastation caused by human industry, a forest probably looks and smells and sounds a lot like it did a hundred years ago. However, sometimes even the natural environment can change noticeably within a lifetime.

One example of this can be found in some parts of Hawaii, where since around the turn of the 21st century, a very different sonic landscape has emerged. On the Hawaiian islands of Oahu and Kauai, the crickets have gone silent.

When a male field cricket (Teleogryllus oceanicus) wants to attract a female, he makes a calling song by producing a mechanical scraping action between his wings. On one wing is a "scraper," and on the other wing is a series of ridges. Together, they work kind of like a bluegrass musician playing the washboard. You've probably heard versions of this sound from the crickets in your own locale -- the semi-rhythmic chirping that pervades the woodlands and fields at night. In normal circumstances, the quality of the male calling song is a major factor in female mate selection. If the male doesn't have a good chirp, he gets less mating opportunities. Bad for him.

But more than a decade ago, a new strain of field crickets began to appear on the Hawaiian islands of Oahu and Kauai. Scientists observed that some of the male crickets were no longer making sound. This was because they had acquired a genetic trait now known as the "flatwing" mutation, which changed the shape of their sound-producing wing surfaces and made the crickets silent. This mutation succeeded because it helped them avoid a really nasty enemy: Ormia ochracea, a parasitic yellow fly that locates hosts for its young by following sound. When this parasite hears a chirping male cricket, it hones in on the source of the chirp and deposits a brood of maggots into the cricket's back. As the maggots grow, they gradually consume the body of the male cricket and eventually kill the host.

Obviously, it's in the cricket's best interest to avoid being located by a killer maggot delivery service, so in one sense, the crickets with the silent-wing mutation are lucky. But they also suffered for their rare genes. The inability to produce a calling song limited their access to females, making it harder to mate and pass the flatwing gene along.

So when the need for survival and the need for mating came into conflict, which won out? Based on what I've already written, you can probably guess. Despite reduced sex appeal, the non-chirping crickets became more and more numerous, and through the power of natural selection, what started as a rare mutation became the norm. By 2006, more than 90 percent of the males on Kauai bore the silent, mutated wings.

The silent crickets have been known about for years, but I just became aware of them because of a new study in Current Biology, which piled on a fascinating observation: Apparently, the mutation that caused the male crickets to become silent on both islands occurred separately. In other words, instead of the flatwing trait occurring on one island and migrating to the gene pool of the other, the same adaptation occurred on two different islands in two different ways.

What struck me most about this story was the rapidity of the evolutionary process, and how noticeable this kind of change could be to human observers. If you've lived in a natural landscape filled with insect calls and one of those distinct calls were to go silent within just a few years, I think the world would begin to feel unfamiliar, even uncomfortable. Sound is just as much a part of the character of a place as any visual cue.

One thing I wonder is whether the sonic landscape of a natural environment tends toward equilibrium. In other words, if a noise-making animal disappears from a region or stops making noise because of selection pressure, will another species soon evolve to fill the auditory bandwidth niche evacuated by the crickets? The same way some animals might compete for the same food resources, in some cases animals might compete for sonic real estate -- the ability to be heard.

Another thing I wonder is how we can keep track of changes in sonic landscapes, especially if they happen gradually. I really like the idea suggested by Joanne McNeil in a recent article for Medium: "a product as simple as Instagram for our field recordings." If we could use a social app to crowdsource environmental audio recordings and arrange them by metadata and geotags, I can see even using computer algorithms to "average" the sonic profiles of physical locations for different time periods, so we could actually listen to how the birds and the traffic and the insects and the human chatter in one place all evolve over the decades of our lives.