Human noise is a rising global pollutant. Urbanization, road networks, and energy extraction infrastructure are all widespread and expanding sources of acoustic waste. In the contiguous 48 states today, to take just one illustration, nearly 4 million miles of road cover the country; as a result, no area is more than 21 miles from a potential vehicle rumbling down one.
All this noise doesn’t go unnoticed by our non-human brethren. Some birds and frogs, for example, have (perhaps grudgingly) resorted to adjusting the pitch of their calls to avoid overlap with human-generated sounds; males of several bird species have found that they need to sing louder, less complex songs in noisy urban environments, lest their mating calls be muffled; and still others have been forced to change the time at which they sing to avoid the noisiest times of day. As a pair of researchers from the Netherlands wrote, back in 2006, “We found consistently higher minimum frequencies in ten out of ten city-forest comparisons from London to Prague and from Amsterdam to Paris.” Thanks to this flexibility, animals have been able to cope, to some degree, with human ruckuses.
But what about animals whose livelihood fundamentally depends on sound? Many bats, for instance, are echolocating predators—they screech to find food, like flying insects, in the 20 hertz-200 kilohertz range, triangulating their prey’s location via the differential speed of ricocheting sound waves. Researchers have only just begun to look at how our noise disrupts this process. Bats could, as a result, avoid foraging in noisy areas altogether. Or, the noise could be distracting—just think about what happens to us when a car, just for a second, shines its high beams right in our eyes. For some bats, noise pollution could be as annoying and disorienting as a high beam aimed constantly at your face. Noise could also mask the sounds bats rely on if they have similar properties, such as pitch.
“Most bats produce very high-pitched calls and have hearing ranges way above ours,” says Walter Metzner, a biologist at the University of California, Los Angeles. While bats can hear sounds up to 200 kilohertz, human hearing tops out at around 20 kilohertz. “Most of the noise we produce on land, such as traffic noise, is relatively low frequency,” says Metzner, “but it does extend into the ultrasonic range, up to at least 50 kilohertz.”
In a study last year at the second largest natural gas extraction field in the United States, Jesse Barber, a biologist at Boise State University, and colleagues, investigated how bats living nearby were faring. Such compressor stations, which help transport natural gas from one location to another, never stop producing noise. The researchers found fewer bats on site that echolocate at low frequencies (<35 kilohertz) near noisy gas compressor stations, while bats with higher echolocation calls were common at both loud and quiet sites. The fact that the noise affected the activity levels of only low-frequency bats, whose calls are more likely to overlap with the noise of the compressor station, suggests masking.
“The natural world is loud, but anthropogenic noise has changed the temporal and spatial footprint of that noise many, many orders of magnitude.”
The masking hypothesis is supported by a study from Metzner and colleagues with greater horseshoe bats (Rhinolophus ferrumequinum). They found that, in the laboratory, broadband noise—noise containing many different frequencies at once—affected bats’ echolocation behavior. “Our study,” says Metzner, “shows they are not distracted or annoyed by noise, but that it is a specific masking effect.” The effects on bats’ call amplitude and pitch, they found, differed depending on which frequency within the noise contained the most energy. Bats made louder calls, for instance, when the noise was centered on the dominant frequency of their echolocation calls, and raised the pitch of their calls for many more masking conditions.
“Based on the fact that we had such specific effects for different frequencies, I don’t think it is an attention issue, at least in these bats,” says Metzner. Such changes in echolocation call structure might affect bats’ ability to measure distance. “It’s conceivable that a shift in frequency makes the echo harder to hear,” he says. “Potentially, any change in the spectrotemporal characteristics of echolocation calls might have an effect on how well bats detect returning echoes.”
For specialized gleaning bats—bats that use echolocation to navigate but find prey passively, by listening for their movements—noise might be especially irritating. The rustling sounds of their prey, mostly various sorts of insects, produce a series of broadband clicks with peak energy around 12 kilohertz. Traffic noise also contains considerable energy at this frequency, so masking is a likely explanation for the bats’ performances.
Two recent studies showed that in one gleaning bat, the greater mouse-eared bat (Myotis myotis), traffic noise significantly decreased foraging efficiency and, when given the option, these bats avoided hunting in noise. Another gleaning bat, the pallid bat (Antrozous pallidus), also shows prey-catching deficits when exposed to human-generated noise in the laboratory. Bats experienced a twofold to threefold reduction in foraging efficiency when exposed to noise replicating conditions as far away as 640 meters from a road and 320 meters from a natural gas compressor station.
The idea that bats spurn noisy areas also has some support. Playback of traffic noise reduced foraging efficiency in Daubenton’s bats (Myotis daubentonii) by inducing avoidance behavior. In noisy situations, these bats went on fewer search flights to look for prey, which resulted, naturally, in less prey caught. Masking was ruled out as a cause in this 2015 study because the playback noise did not have to overlap in frequency with the echoes bouncing off prey in order to disturb the bats. Instead, it was just noise in general that sent the bats packing.
Barber doesn’t pretend that, before humans came along, bats lived in quiet tranquility. “The natural world is loud,” he says, “but anthropogenic noise has changed the temporal and spatial footprint of that noise many, many orders of magnitude.” The work his lab and others have done in the last five to 10 years, he says, “has really brought this to the attention of policy makers. There’s a rising use of noise pollution in environmental impact statements of all sorts”—for example, the use of noise-attenuating walls around natural gas extraction sites could reduce the area exposed to elevated noise levels—in this case, a bird habitat—by as much as 70 percent. The acoustic pollution, by drowning out various forms of communication, hinders the avian community from effectively managing their nests.
The hope is that, once we thoroughly characterize how our noise interferes with the lives of animals, says Barber, we can heartily commit to keeping it down a little.
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The lead photograph is courtesy of Klaus Stiefel via Flickr.