At the end of September, 22 animals and one plant officially joined the growing list of extinct species – with most extinctions due to human interference in their lives. And as a biologist specializing in sound and hearing, I wondered if the noise that we humans add to the planet plays a role in the extinctions.
We’ve known for a long time that animals change their vocalizations in response to human sounds. Birds, frogs and even whales – like humans – increase the volume of their voice as the environment becomes louder. Animals also change their call rates or pitchesor do other qualitative changesjust to sound off on us.
But sometimes species just give up. Ship sonar can silence whales. It also interferes with the echolocation they depend on for navigation and is considered a due to some failures. Man-made noise forced a variety of animal species around the world change their behaviorwhich has consequences for mating and migration and affects their continued existence.
We glimpsed what could happen with less human noise. While the human world temporarily went silent during the first weeks of the COVID-19 pandemic, the natural world seemingly turned up the volume, to levels not heard in decades. Many of us who opened a window or stepped out cautiously in the spring of 2020 suddenly heard birdsong everywhere.
The birds increased the complexity of their songs, perhaps realizing that the hard work they put into their compositions would not be lost, buried under the usual man-made din. They also shifted the pitch of their songs down to fill the vacant lot space once filled with traffic noise.
But here’s a little surprise about the birdsong during the quiet period: the volume knob hasn’t been turned up – birds actually reduced the volume of their songs. Yet their quirky, sophisticated, and now softer songs traveled twice the distancestanding out with bold relief against the newly calm backdrop of our shutdown of the unrest caused by the pandemic.
Communication between birds is crucial for their survival. It is used in the defense of their territory and especially in mating. A well-trained song by a male bird signals a fit, well-fed, and healthy potential mate for the female to choose from among potential suitors. This general song and dance also applies to insects, helping to ensure the continued survival of these species.
Of course, sound connects species to each other in a way that is far from any human intervention. Squirrels communicate by tapping their tails on the wood of trees. The ears of some flea larvae are closely adapted to the precise chirping frequency of the katydid on which they are feeding.
Interspecific use of sound, even with humans involved, can be beneficial. The sounds can stimulate plant growth and promote their pest resistance. Dog stress is reduced during kennel stays when the music is playing. Outside the realm of human intervention, some flowers release their pollen only when the favorite species of buzzing bees in his characteristic tone. Evolution has “taught” tomatoes, peppers, eggplants, kiwis and other plants that the pollen from their flowers spreads best when carried on the bodies of species of bees that buzz at around 200-400 hertz.
Any plumber can tell you that plant roots like to make their way into underground water pipes – often using sound to find their way there. When peas are grown in forked pots, their roots can grow either to the left or to the right. Playing an audio recording of water in a fork branch reliably induces the plant to sends its roots in the direction of the sound even if no water is actually present.
How might other species be affected by the recently reported disappearance of Bachman’s Warbler, Ivory-billed Woodpecker and Little Fruit Bat? What sounds have been lost to Earth listeners? What other organisms are worse for their absence?
The power of sound is often underestimated. We rally around causes that reduce visual pollutants in our cities or protest the loss of forests, which are easy to see. But too little is known about the acoustic network that helps many species thrive. Among all the evidence we have of the harm our noise can bring, we should take pride in the ways in which sound can be used for the good of the environment, such as promote crop growth and reduce plant-destroying and disease-carrying pests.
We’ve heard a little of what we’ve been missing in the spring of 2020. As man-made noise continues to return to its pre-pandemic roar, we need to keep it from stopping the chirps, buzzes, and echoes of the world natural to contribute to the well-being — and survival — of other species.
Nina Kraus is a neuroscientist and professor of neurobiology, communication sciences, and otolaryngology at Northwestern University. She is the author of the new book “Of Sound Mind: How Our Brain Builds a Meaningful World of Sound.”