The Disturbing Sound of a Human Voice

Hearing people talk can terrify even top predators such as mountain lions, with consequences that ripple through entire ecosystems.

A mountain lion
Mountain lions found the sound of human poetry disturbing. (Howard Yanes / Reuters)

In the summer of 2017, the mountain lions, bobcats, and other residents of the Santa Cruz Mountains were treated to the dulcet tones of the ecologist Justin Suraci and his friends, reading poetry. Some of the animals became jittery. Others stopped eating. A few fled in fear.

Suraci, who’s based at the University of California at Santa Cruz, wasn’t there to see their reactions. He and his colleagues had strung up a set of speakers that would regularly play recordings of human speech in an area where people seldom venture. And they found that, the quality of the poetry aside, even the gentlest of human speech can make wild animals—even top predators—unnerved and watchful, in ways that shake entire food webs. It’s the clearest demonstration yet that we are among the scariest of animals—a super-predator that terrifies even the carnivores that themselves incite terror.

Even when predators aren’t killing anything, their tracks, smells, and sounds can instill a state of simmering unease in their prey. This creates what ecologists call a “landscape of fear”—a mental map of risk that affects how hunted animals move over physical terrain. For example, in 2016, Suraci and his adviser, Liana Zanette, from Ontario’s Western University, showed that raccoons in the Gulf Islands spent less time foraging on local beaches if they heard recordings of dogs. And because the raccoons skedaddled, the rock pools filled with more fish, worms, and crabs. Fear reshaped the entire beach.

Similar studies have shown that animals react very strongly to the perceived presence of spiders, hawks, sharks, and wolves. But what about humans? We kill many animals at much higher rates than their other natural predators, and we’re unusual in taking out those predators too. “We might expect animals to fear us, as any prey fears its predators,” says Suraci.

His team has shown that they certainly do. In an English forest, the researchers played the sounds of various carnivores to local badgers. The badgers ignored the sounds of wolves entirely and were mildly concerned by the growls of wolves and bears. But they were profoundly disturbed by human speech, even the genteel tones of some BBC documentaries and a reading of The Wind in the Willows.

Next, the team wanted to see whether a larger carnivore would behave similarly. In the Santa Cruz Mountains, they placed speakers at sites where mountain lions had killed large prey and were regularly returning to feed. When the cats approached, the team played either talking humans or croaking frogs. The frogs didn’t faze them. The human voices—including those of Rachel Maddow and Rush Limbaugh—made them flee more than 80 percent of the time.

“We thought it would be funny to play political commentators,” says Suraci. “But when we had to score the videos, and listen to Rush Limbaugh all the time, it wasn’t very enjoyable.” For their next experiment, he and his colleagues decided to use calmer fare, including poetry and nature writing that they themselves read aloud. They played these recordings through 25 speakers, covering a square-kilometer grid, and programmed them to play 40 percent of the time.

By tracking seven mountain lions that had been fitted with GPS collars, the team showed that the animals kept their distance from the grids, and moved more cautiously, when humans could be heard. Using camera traps, the team saw that medium-size carnivores were also perturbed: Bobcats became more nocturnal, skunks became less active, and opossums spent less time foraging. The only animals that benefited were mice and rats, which took advantage of the predators’ absence to expand their range and forage more intensely. To the rodents, the speakers provided a human shield.

“This study suggests that a conversation between two hikers can have a butterfly effect—a mountain lion moves more quickly, an opossum changes its feeding habits, deer-mouse activity increases,” says Kaitlyn Gaynor from UC Berkeley, who was not involved in the study. “People often fear large carnivores like mountain lions, but in reality, they are far more scared of us. And as this study suggests, their fear can reshape ecosystems.”

Crucially, these effects are separate from all the other things that humans do to animals, from destroying habitats to hunting them directly. Suraci’s studies show that through our mere presence, we can affect wildlife by changing the contours of their landscapes of fear. “We’re a very loud and big species,” says Suraci. “Much of what we do is potentially terrifying to wildlife, like industrial activity and vehicle traffic. We tried to get past all of that and isolate the perceived presence of humans, separate from all the other disturbing things we do. And the implication is that we don’t need to cut down the forest to have an impact on wildlife.”

That’s not to say that people must abandon all wild areas. If anything, it’s more important than ever for us to be connected to the natural world. “But there’s potential, in areas that are key wildlife habitats, to restrict activities during the day, or limit the numbers or timing of human access,” Suraci says.

Experiments like his give “an incredibly comprehensive picture of the reverberations of the human super-predator across the entire community,” says Meredith Palmer from Princeton University. “The next frontier is demonstrating if these changes are detrimental to the animals.” If they’re stressed, could they grow more slowly? If they spend more time hiding, could they miss out on mating opportunities? If they’re not foraging enough, would they die younger?

“Human-induced behavioral changes may be ultimately harmful to species and ecosystems if they make it harder for animals to survive and reproduce,” says Gaynor, “but a more optimistic takeaway is that they could actually enable large mammals to coexist with humans on an increasingly crowded planet.”