Sound is a powerful weapon. Flash-bang grenades can stun and disorient unruly mobs, and loud music is sometimes played to weaken detainees before interrogations. Even innocuous-seeming sounds can drive a person mad if they go on long enough.
When the United States invaded Panama in 1989, Manuel Noreiga, that country’s soon-to-be-deposed military dictator, sought sanctuary inside the Vatican’s embassy. But he couldn’t escape the ear-splitting American rock music that U.S. forces continuously played outside the building (including Van Halen’s “Panama,” of course). On January 3, 1990, after several days of sonic siege, Noriega surrendered.
Noise pollution, unfortunately, is not relegated to the world of human torture. Birds have had to change their songs to penetrate the clatter of traffic or adapt their schedules to avoid it completely. Most notably, noise pollution from sonar can impair a whale’s sense of direction and has been linked to strandings. And new research suggests noise can even mess with an animal’s sense of smell.
Meet the South African dwarf mongoose. As its name implies, this mongoose is itty bitty—Africa’s smallest carnivore, in fact. At under a foot in length and weighing less than a can of soup, dwarf mongooses have to watch out for a rogues’ gallery of predators—including (but not limited to) jackals, civets, servals, African wildcats, caracals, hawks, and even bigger mongoose cousins.
With so many mouths watering for mongoose, it’s crucial these guys keep their eyes, and noses, peeled for predators. The little squishies carve out subterranean burrows in old termite mounds and live in packs of up to 30. Each animal takes its turn scanning the landscape for snakes and the skies for aerial threats like goshawks, tawny eagles, and bateleurs. Dwarf mongooses investigate every new sound and smell to determine its threat level—and that’s where the noise pollution problem comes in.
In a recent study published in Current Biology, scientists tested dwarf mongoose vigilance levels by dropping pieces of predator poop outside their burrows while they slept. When the slumber party started to emerge, the scientists played audio of noisy vehicles passing on a nearby road. In other experiments, ambient bush noise was piped in as a control.
The results? When the traffic sounds were playing, dwarf mongooses took longer before inspecting the predator scat. They were also less likely to perform their typical behavior of approaching, sniffing, and then scent-marking the offending turd. Now, you might be wondering why it matters how long it takes a mongoose to sniff a piece of poop. And for that, we’ll turn to one of the study’s authors, Andrew Radford, a behavioral ecologist at the University of Bristol.
Radford points out that these actions—sniffing scat, scent-marking, and hanging out by the den’s entrance—are antipredator behaviors with direct consequences for survival. “If you fail to behave correctly when a predator is around, you could end up dead,” he says.
The mongooses barraged by traffic noise also did not exhibit heightened vigilance behavior after finding the predator scat and spent less time at the entrance of the burrow before going out to forage, a safety precaution usually triggered by evidence of a nearby enemy. Perhaps most interesting of all, when no abnormal noises were present, the mongooses were more likely to approach, inspect, and scent-mark carnivore scat than herbivore scat. But when the roadway noise was turned on, the mongooses treated each type of feces equally.
But wait, how does sound affect smell? Radford can’t say for sure, but he suspects the noise either distracts the mongooses and causes them to gather less information from the smells or stresses them out to a point where they don’t compute that information as efficiently. In other words, a mongoose near a road may eventually become as frazzled as Noreiga after days of listening to David Lee Roth.
“This work broadens our understanding of how human-induced environmental changes can affect animals in subtle ways,” says Hansjoerg Kunc, a biologist at Queen’s University Belfast who has studied the impacts of noise on cuttlefish, one of the sea’s more impressive camouflage artists.
Cuttlefish use colorful visual cues to communicate with each other, but Kunc found that the fish change their hues more rapidly when under the stress of human-made noise. This is important because it shows “cross modal” impacts, or one sense affecting another.
Kunc says that unlike cuttlefish, the mongooses may be displaying altered reactions to chemical cues (i.e., smells, suggesting that sound can affect at least two other senses in wildlife—sight and smell). “So it seems that the cross-modal effect of noise pollution will affect many species,” he says.
Quieter engines, sound-absorbing road materials, and noise-reduction barriers might be able to save a few mongoose lives. Nature may even be able to provide a technological solution or two. (Just look at this noise-reduction coating for wind turbine blades, inspired by owl wings.) And like those schedule-shifting songbirds, we too could change our behavior—by, for instance, establishing areas for marine mammals off-limits to ship traffic and sonar. The good news, Radford says, is that unlike air and water pollution, climate change, and ocean acidification, noise does not linger in the environment—it vanishes as soon as you silence the source.
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