Acoustic biologists who have learned to tune their ears to the sounds of life know there's a lot more to animal communication than just, "Hey, here I am!" or "I need a mate."
From insects to elephants to people, we animals all use sound to function and converse in social groups — especially when the environment is dark, or underwater or heavily forested.
"We think that we really know what's going on out there," says Dartmouth College biologist Laurel Symes, who studies crickets. But there's a cacophony all around us, she says, that's full of information still to be deciphered. "We're getting this tiny slice of all of the sound in the world."
Recently scientists have pushed the field of bioacoustics even further, to record whole environments, not just the animals that live there. Some call this "acoustic ecology" — listening to the rain, streams, wind through the trees. A deciduous forest sounds different from a pine forest, for example, and that soundscape changes seasonally.
Neuroscientist Seth Horowitz, author of the book The Universal Sense: How Hearing Shapes the Mind, is especially interested in the ways all these sounds, which are essentially vibrations, have shaped the evolution of the human brain.
"Vibration sensitivity is found in even the most primitive life forms," Horowitz says — even bacteria. "It's so critical to your environment, knowing that something else is moving near you, whether it's a predator or it's food. Everywhere you go, there is vibration and it tells you something."
And hearing is special among senses, Horowitz says. Sound can travel a long way. It will propagate through anything — the ground, water. It works at night, goes around corners. "Sounds give you sensory input that is not limited by field of vision."
Given how well sound reflects what's going on around us, the brains of vertebrates — including humans — evolved to be exquisitely sensitive to it.
"You hear anywhere from 20 to 100 times faster than you see," Horowitz says, "so that everything that you perceive with your ears is coloring every other perception you have, and every conscious thought you have." Sound, he says, "gets in so fast that it modifies all the other input and sets the stage for it."
It can do that because the brain's auditory circuitry is less widely distributed than the visual system. The circuitry for vision "makes the map of the New York subway look simple," says Horowitz, whereas sound signals don't have as far to travel in the brain.
And sound gets routed quickly to parts of the brain that deal with very basic functions — "precortical areas," Horowitz says — that are not part of the wiring for conscious thinking. These are places where emotions are generated.
"We're emotional creatures," Horowitz says, "and emotions are evolutionary 'fast responses' — things you don't have to think about."
That speediness pays dividends in the survival department: "You hear a loud sound?" he says. "Get ready to run from it." Emotions are rapid delivery systems in the brain, and sound drives emotions.
So sound hits you in the gut. But sound is also rich with patterns that carry information.
"The brain is really a wet, sloppy drum machine," Horowitz says. "It's desperately seeking rhythms." Not only rhythm, but patterns in pitch too, that have a mathematical regularity that captures the brain's attention.
The sound of a familiar voice, for example, has its own set of rhythms and pitches. So do particular sounds in nature: birds, insects, rain. The Bayaka people, who live in the rain forest of Central Africa, incorporate the syncopation of falling rain into their music.
Sounds that alarm us don't have those patterns. Consider what Horowitz calls "the sound everybody hates."
In the screech of a fingernail scraping a blackboard, the familiar rhythmic and tonal patterns there are broken — the sound is ragged, as in a scream.
Sound gets in your head and stays there. When the brain processes sound, it actually resonates with it, like a tuning fork that's been struck. You can hear the brain's resonance if you have the right equipment.
"If you play a sound to a frog, [and] drop an electrode into their auditory nerve, you will hear the sound that the frog is hearing," Horowitz says, "because it is so absolutely represented — a change in frequency or pitch will be represented in how the nerves fire."
And even without an introduced sound, the working brain makes its own sound continuously, Horowitz says. He calls it a "neuronal symphony."
"It sort of sounds like a well-tuned, old-school radio noise or crackling sound," he says. "You start to hear tonality; and you start hearing little songs."
Horowitz can sometimes tell what part of a frog's brain he's tapping into with his electrode by its sound (a process that doesn't harm the frog, he says).
He's listened to just about anything you can hear on Earth, and has started thinking about sound that's unearthly.
Space is full of electromagnetic radiation — which is essentially another form of vibration. Scientists have taken to turning electromagnetic radiation into sound to study it. You can "hear" the sound of black holes that way — or of microwave radiation from the Big Bang, when the universe was created.
Horowitz wonders if there might be intentional sound out beyond our solar system as well. "If we find life on other planets — if it's more complex than microbes or viruses — they'll have vibrational sensitivity," he says. And maybe they'll make noise we recognize. As long as we're listening.
This is the seventh and final piece in Morning Edition's summer series Close Listening: Decoding Nature Through Sound. The radio series was edited by NPR's senior editor for science, Alison Richards. Thanks also for online help from NPR science editor Deborah Franklin and visual producer Meredith Rizzo; for guidance from Greg Budney at the Cornell Lab of Ornithology and from audio engineer Chris Nelson; and to Morning Edition's executive producer Tracy Wahl.