Noise made by humans can be bad news for animals

I keep looking over my shoulder at the dark wall of roadside trees that passing headlights make slightly less black. Muggers are less of a worry than some suburban samaritan materializing out of the winter gloom to ask if everything’s OK with a reporter down on her hands and knees in front of a parked car, caressing the pavement.

Explanation would not be easy. This is not an obvious place to pull over. The shadowed shoulder of a roaring commuter parkway looks as if it might pothole itself in shock at the footstep of a strolling pedestrian. But it’s a pilgrimage destination for the acoustically curious, and it’s not a bad place to contemplate looking over one’s shoulder.

This 3.5-kilometer stretch of Fairfax County Parkway near Herndon, Va., is one of only three places in the state’s northern sprawl where a driver can try to discern the subtle effects of two unusually textured road surfacings called “quiet pavement,” or sometimes less ambitiously, “quieter pavement.” These experimental road surfaces suggest that worries about the effects of human-made, or anthropogenic, sounds have reached the point to compel action.

In reality, the Virginia legislature probably approved these roads more out of concern that noise harms the health and voting behavior of humans than out of concern for local wildlife. But a surge of evidence is pushing the National Park Service, at least, to take steps to restore and preserve the natural soundscapes of the nation for all its residents.

The wildlife side of that burgeoning research has come a long way since studying anthropogenic noise meant exploring what Navy sonar might be doing to marine mammals or how city birds’ songs differ from their country cousins’. Recent experiments have tackled the basic question of whether noise alone can cause ecological effects, separate from the lights or motion from whizzing cars. And researchers have expanded the study of human noise to consider its effects on what might be called the nonconversational ways that animals use sounds.

To people, “noise pollution” often brings to mind the vexations of words drowned out, of trying to have a soul-satisfying conversation in a clatteringly loud restaurant. Of course, wild creatures also need to convey the full nuances of their chirps, hums and howls. But creatures draw information from all sorts of other noises, many of them accidental, barely audible and of life-or-death importance.

Some examples related to food: Tiny tip-taps of beetle feet in the dark can mean a decent meal for a hungry bat. And the merest whisper of a cat’s paw over a dry leaf can give a chipmunk one last chance to dodge. Milliseconds matter.

If road roar keeps a person from hearing footsteps in time to jump up from pavement-gazing, the main risk is embarrassment. But for the hunters and the hunted, the stakes are much higher.

Phantom evidence

To explore a basic question about wildlife and noise, Jesse Barber and his colleagues built what they call the phantom road.

Earlier studies of noise effects often compared animals near roads or other clamorous human-made features with animals in rural landscapes. This approach left questions about how much of the difference came from noise instead of from artificial lights, exhaust fumes or other non-noisy aspects. Other research teams have turned to, of all things, gas wells to try to sort out the problem — by monitoring wildlife near wells equipped with thundering compressor motors versus otherwise similar wells without the noisy equipment. In Canada’s boreal forest, songbirds didn’t settle as densely near the monster motors, and in a New Mexico gas field, there weren’t as many bird species at the loud sites. The impact rippled onward: Because the animals found in the neighborhood changed, plants’ exchange of pollen and spread of seeds would change. Noise seemed to be the cause.

“Challenging” is his restrained word for the travails of the experiment. It took a month just to position the speakers and get the broadcast to sound realistic. And once the spectral road was running, lab members spent hours each day hiking out and back to replace batteries and take data because, of course, there was no real road to the site. But the effort was worthwhile, as revealed in the team’s 2013 report in the Proceedings of the Royal Society B.

The ghostly road ran beside a major rest stop for migratory birds. Just beyond the ridge, the great Douglas fir forest of central Idaho frays into clusters of bitter cherry and chokecherry, and then the landscape opens into what small migratory birds would have every right to call challenging. They must cross miles of low-growing steppe vegetation with little cover but plenty of bird-eating raptors. Typical migrants “stop at the edge for a few days and fatten up and get ready for this dangerous and exhausting nocturnal journey,” Barber says.

When the researchers turned on the speakers for four days of faux traffic, the numbers of birds stopping to rest dropped by more than a quarter on average. And during the alternating four-day stretches of silence, bird numbers bounced back. Noise matters, Barber and colleagues concluded. It can change animals’ most basic stay-or-go assessments of habitat. It can prompt more than the usual number of birds on thousand-mile marathons to skip a chance to rest and refuel.

Wild sounds

Not that natural soundscapes are always quiet. Nature makes and uses noise in an intricate information network rich in content and as varied as caterpillar clicks and elephant rumbles, snow hush and thunder.

To go back to food, barn owls and gray mouse lemurs can locate prey by listening for rustlings in fallen leaves or shrubbery. The sound of worming through soil can tip off robins to their dinner. And from the opposite side of the eat-or-be-eaten struggle, female túngara frogs shy away from recordings of seductive males if researchers add wingbeat sounds of a frog-eating bat. Recordings of footsteps of birds called pied currawongs shush the nestlings of one of their prey species, the white-browed scrub wren. And maybe it’s not strictly an approaching predator, but the crackling noises of fire will send African reed frogs rushing away.

What’s worrisome about anthropogenic noise is its sudden arrival, evolutionarily speaking, and its confusing properties. Navy sonar, for instance, shares acoustic qualities with the calls of killer whales, as noted by Peter Tyack at Woods Hole Oceanographic Institution in Massachusetts. The accidental similarity may help explain why beaked whales, one of the killer whales’ prey species, apparently stop feeding and hide at the sound of sonar signals or even flee so frantically that they sometimes die stranded in shallow water.

And vehicles rolling by the burrows of Stephens’ kangaroo rats may tap into the animals’ communication system. The small, largely solitary animals drum their feet to others in neighboring burrows and also, it seems, in response to the noise of passing cars. Playing recordings of the low-frequency vibes of traffic caused alert postures and drumming bouts similar to those that followed broadcasts of the low-frequency foot drummings, Debra Shier of the San Diego Zoo Institute for Conservation Research and her colleagues reported in 2012.

Mixing unfamiliar, artificial noises into the natural soundscape isn’t just confusing the wildlife. These evolutionarily novel human noises may just drown out what animals need to hear. “Drowning out” and “hearing,” however, can mean a variety of things depending on the species of the animal and its needs, cautions avian acoustics researcher Robert Dooling of the University of Maryland in College Park. For too loud, “there is no one number,” he says. A person can identify sounds with about 50 percent accuracy despite a buzz of junk noise five decibels louder than the important tones. In one sense that’s hearing, and it might do for noticing smartphone ring tones, but it’s hardly enough for conversation.

Will listen for food

And it may not be enough for hunting, at least for some predators.

Greater mouse-eared bats of Europe, like other gleaning species, listen for their supper. Gleaners pick up the faint rustles of beetles, spiders or centipedes moving along the ground at night and then swoop down for the attack. To mimic that kind of hunting in the lab, the late Björn Siemers of the Max Planck Institute for Ornithology in Seewiesen, Germany, and colleague Andrea Schaub filled a room with 64 little platforms, each with its own speaker. When a speaker broadcast the faint patter of beetle feet, bats needed only about five seconds to dart to the right platform and pick up a mealworm treat. During the same experiment against a backdrop of noise approximating a nearby highway 7.5 meters away, the bats took almost five times as long to find the right platform, the researchers reported in 2011.

Bats that catch their prey in the air hunt by ear in a different way. And some of them might also have trouble in human-loud spots, Boise State’s Jessie Bunkley, Barber and colleagues suggest in the January Global Ecology and Conservation. These bats attack insects in midair, locating prey by pinging into the night and listening to the subtleties of the echoes. Bunkley took bat detectors, which record ultrasonic bat squeaks, to gas wells in New Mexico to eavesdrop on aerial sonar. Echolocation happens at frequencies higher than a lot of motor noise. Yet at the noisy wells with big compressor motors, she found 40 percent less activity among Mexican free-tailed bats (Tadarida brasiliensis).

It’s not clear yet whether the noise bothered the free-tailed bats directly or just chased away many of the insects that bats eat. “Either way,” says Barber, “it means less bat habitat.”

Human noise doesn’t always mean bad news for predators, though. Humming wind turbines in the seas have stirred debate on whether the noises of their construction and operation affect animals. Yet 11 harbor seals wearing tracking devices routinely foraged among drilling platforms and wind farms in the North Sea, Deborah Russell of the University of St. Andrews in Scotland and her colleagues reported last July in Current Biology (SN Online: 7/21/14). Three of the seals swam from turbine to turbine, presumably feasting on fish living around the brand new reefs that the utility infrastructure created.

By filtering out sensitive species and inviting tolerant ones, noise may be reshaping ecosystems. The resulting mix-and-match food webs and species combinations may not occur in the wild and may never have. “We’re not studying noise,” Barber says. “We’re studying ecology.”

Danger, danger

That filtering and jumbling effect, with noise-phobes and noise opportunists, appears among prey species, too.

Young European eels (Anguilla anguilla) didn’t do so well in tests of vigilance when researchers played recordings of ships chugging through a harbor, Stephen Simpson of the University of Exeter in England and his colleagues report in the February Global Change Biology. Vigilance is especially important for these youngsters as they leave their oceanic nursery and confront the strange new menaces of harbors, rivers and lakes.

In a laboratory enclosure with a clear window, youngsters sometimes saw a looming, scary fish shape as they swam by the window. Little eels serenaded by ship noise were only half as likely as eels in quiet water to startle and twitch when they spotted the scary fake fish in the window. Even eels that startled took an unusually long time to do so. And a simulated predator that pursued them, actually a researcher with a net, swept up young eels more than twice as quickly if the ship noise was humming.

The noise effects probably didn’t come from ship engines drowning out approach cues, because there are no audible approach cues; the fish shape stayed on the opposite side of the glass. Instead, Simpson says, tests on the young eels suggested they were stressed, which can dull performance. Or maybe the noise just distracted them.

Prairie dogs, in contrast, do pretty well on vigilance despite traffic noise. But vigilance around human-made noise may become a burden in itself.

Black-tailed prairie dogs (Cynomys ludovicianus) in free-ranging colonies in Colorado grew much more watchful when researchers broadcast recordings of highway noise. Such heightened awareness meant less time for other important activities, says Graeme Shannon of Liverpool University. He and his colleagues created a little bit of phantom highway by setting up speakers near prairie dog colonies and broadcasting bouts of traffic noise as if a highway were 100 meters from the colony center.

“It wasn’t that they heard the noise and ran,” he says. But with road noises in the air, over the course of watching the colony, the researchers found that the number of prairie dogs foraging — presumably an important thing to do — declined 18 percent during the traffic broadcasts. Social interactions and resting declined by half, Shannon and colleagues reported last August in Animal Behaviour. Even though he revisited the same colonies twice a week for three months and played some noise, he saw no sign that the animals were getting used to it.

It’s tempting to assume that animals staying near a highway aren’t bothered by it, Shannon says. Just knowing that animals stay, however, doesn’t reveal what their persistence costs them.

Settling in a loud neighborhood can have measurable benefits for tolerant prey species just as it does for certain predators. Nests of black-chinned hummingbirds and house finches were far more common near the very loud gas wells in New Mexico than near wells without any roaring compressors. And — possibly not a coincidence — scrub jays weren’t raiding many nests in the noise zones, Clinton Francis at California Polytechnic State University in San Luis Obispo and his colleagues reported in 2009. Whatever the downsides of all that racket, it brought some protection.

Where rubber meets the road

Noise “is about the most manageable pollutant we have,” Exeter’s Simpson says. And various projects are already under way to create a quieter world (see “Quiet design,” below).

Experiments with quieter pavement address what may seem counterintuitive to the driver. At speeds over about 35 miles an hour, the majority of highway noise typically comes not from engines but from tires hitting the road, says Kevin McGhee at the Virginia Center for Transportation Innovation and Research in Charlottesville.

To coax the rubber and the road to meet less noisily, engineers are making asphalt smoother with abundant pores. The dimples and holes are for noise to get lost in “like acoustic ceiling tile,” McGhee says. But during mid-Atlantic winters, sand and deicers plus general grit clog the pores, and the road gets louder.

At experimental site 6, surfaced in 2012, on the Fairfax County Parkway, rush hour is tapering off. Four lanes of cars and light trucks whoosh, whoosh, whoosh by in irregular crowds, then leave the road darker and almost empty for maybe a minute before the next cluster rushes by.

I’ve stood by the side of regular pavement for several chilly, awkward minutes in preparation for appreciating the experimental stretch of rubberized asphalt mixed with coarse surface rock. It’s the most promising of the pavements in the current test. But McGhee is right. After several winters, it’s hard for the unaided ear to tell whether there’s a real difference in noise or just some touristic wish fulfillment. It’s still four-lane highway noise, just (maybe) muted a bit.

Down on my hands and knees at the quieter pavement, I can see and feel the difference from pavement just down the road. Here are pores galore, big, little and irregular. McGhee has described them as looking “like popcorn that has been painted black.”

And in spite of the spookiness, no one walked up behind me. Or if somebody did, I didn’t hear. 

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Quiet design

As the human-made world gets noisier, in some ways it’s also — thanks to clever design — getting a bit quieter.

Aircraft design over the last three decades, for example, has taken some 20 decibels out of typical plane noise, says the 2010 report Technology for a Quieter America from the National Academy of Engineering. The roar comes not just from the engines but from the airflows, and several extreme initiatives have worked on concepts for drastic reductions. Through his Powering Imagination organization, Erik Lindbergh (grandson of Charles) is championing students at Embry-Riddle Aeronautical University in Daytona Beach, Fla., who are working on an all-electric airplane. He imagines it replacing the intrusive growls of tourism overflights with soft whirs soaring over splendid places like the Grand Canyon.

To reduce its workplace noise, NASA has initiated a “Buy Quiet” program. Improving the soundscape inside homes may be more difficult, cautions the engineering academy report. On the plus side, consumers are likely to attribute more overall quality to quieter versions of products such as appliances. Yet consumers aren’t showing signs of paying a premium for that noise respite.

Sometimes a cost-effective measure for restoring some serenity can be simple. Visitors to California’s Muir Woods National Monument made less racket on special quiet days and in designated quiet zones. All it took was a few posted signs. — Susan Milius

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This article appears in the February 21, 2015 issue with the headline: “Hush, humans — we’re trying to survive here.”