Imagine a contest that pits humans against lions to see which is more fearsome. It may sound like a Colosseum fight card, but last year, a team led by Liana Zanette, an ecologist at the University of Western Ontario, arranged exactly this matchup—for science. The point was not to settle some grade-school debate about which animal would survive a vicious fight to the death, but rather to see how much each species is feared by other animals.
It’s not a trivial question. Fear shapes animal behavior, and animal behavior shapes our world in profound ways. Scientists are only beginning to understand fear’s effects, but already, evidence suggests that a terrified animal will eat less and reproduce less than an unterrified one. In addition to everything else we’re doing to endanger wildlife, we might be scaring them into smaller population sizes. The better we can understand the fear we inspire, the more we can mitigate its harms—and maybe even try to use it for good.
To that end, Zanette’s team outfitted 21 watering holes in South Africa’s Greater Kruger National Park with automated speaker systems. When thirsty animals approached, the speakers played one of several sounds: the snarls and growls of a lion pride, the placid chatter of human conversation, gunshots, dog barks, or birdsong. Cameras recorded the animals’ reactions, including the time it took each to run away. The results, published in Current Biology, were “very dramatic,” Zanette told me. According to a data set comprising more than 4,000 interactions, animals were twice as likely to flee when they heard a human voice than when they heard lions or even gunshots, and they left the watering hole 40 percent faster. Most striking of all, Zanette said, this effect was observed across 95 percent of species.
The videos have the comic feel of TikTok pranks. In one, a herd of giraffes mills about a watering hole until the speaker flips on, broadcasting a man describing his childhood. He speaks in a calm, public-radio-style voice, but even so, his abrupt and unsolicited oversharing makes the giraffes scatter, each one breaking into a gangly sprint. Another video opens on a leopard dragging an antelope by the neck across a dry patch of dirt. When a woman starts speaking in Afrikaans, the cat drops its fresh kill and bolts off. I learned a lot from these videos. Warthogs, it turns out, have serious wheels. Rhinos seem unusually curious: After hearing a human voice, they would pause, as though identifying the language being spoken, and then bounce away on their tree-stump legs.
The conditions of the study aren’t universal to animal-human interactions. To my ear, there is something uncanny about the disembodied human voices in the videos. At watering holes that were otherwise free of technology, they sounded like the voice of God, or a visiting extraterrestrial. The animals seemed existentially startled, and who could blame them? If I were out for a hike and suddenly heard an electronically modulated voice speaking a foreign language, I would be unsettled too. I might even be tempted to run.
The animals may have already been tense for environmental reasons. Oswald Schmitz, an ecology professor at Yale, told me that watering holes are particularly risky places. Stepping softly toward the water’s edge, an animal might feel vulnerable and spook easily. William Ripple, an ecology professor at Oregon State, said that high rates of poaching in Kruger National Park could encourage skittishness. “In the National Parks of North America, where there is little or no poaching, large herbivore prey typically move close to humans to avoid large carnivores,” he told me, a behavior that is called “human shielding.” But at sites where large herbivores are actively hunted, they show a strong fear of humans. Animals, in other words, are intelligent. Their behavior depends on context.
And yet, in a growing number of contexts, animals do appear to fear humans. A few years ago, one of Zanette’s former graduate students, Justin Suraci, led a study of predator anxiety in England’s badgers. At the time, Zanette wasn’t sure how fearful the badgers would be. On the one hand, people have killed or otherwise terrorized them for ages. On the other hand, after all those ages, you could imagine “they’ve gotten used to us,” Zanette said. Alas, no: Badgers reacted much more fearfully to audio recordings of humans than to clips of wolves or bears.
Suraci, who is now a wildlife biologist at the nonprofit Conservation Science Partners, observed a similar behavior among California’s mountain lions. His team set up speakers where mountain lions had left their carcasses, knowing they’d soon return. When they played frog noises, the lions were unbothered. When they played human noises, the lions almost always fled.
Some of these results aren’t terribly surprising. (Has a frog ever killed a mountain lion?) But they’re part of a larger body of work that suggests that animals are deliberately avoiding us. Tigers in Nepal, elephants in Mozambique, and boars in Poland have all shifted to a more nocturnal lifestyle that likely entails fewer human encounters. Changes like these have untold effects on an animal’s home ecosystem, a sobering thought given humanity’s extraordinary geographic range.
Zanette isn’t sure whether these fears are learned across the experiences of a single animal’s lifetime or if they’re the product of generations’ worth of experiences, compressed into genes. Again, much depends on context. On the African Savannah, where the wildlife have co-evolved with crafty humans for longer than anywhere else, fears may be wired especially deep into animal minds and bodies. If so, evolution may need a long time to deprogram their terror. In the interim, there might be some way to use those anxieties for good, by drawing on the very skill that makes us such devastating hunters: our ability to repurpose pieces of our environment into tools. These fears that we’ve instilled in animals are now part of our environment, and we might be able to repurpose them to benefit animals or ecosystems as a whole.
Zanette is using Florida’s shorebirds as a test case. They lay their eggs on thin strips of sand in the shallows along the coast, and in recent years, coyotes and raccoons have been gorging on the hatchlings, reducing the birds’ numbers. Zanette and her team ran experiments on these predators and discovered that humans are the animals they fear most. She told me that the Florida Fish and Wildlife Conservation Commission has since asked the team to rig up speakers that play human sounds at the crossing points where coyotes and raccoons tiptoe onto the little sand strips.
Zanette told me that this isn’t the only possible use case. She hasn’t quite worked out the details, but she wants to try as many applications as possible. Even at Kruger, she imagines using the speakers, not just to scare animals, but also to protect them: By playing recordings of human voices as an audio fence, rangers could reroute white rhinos away from the park’s most poaching-intensive regions. After all, chattering humans may be unnerving, but they aren’t as dangerous as the silent ones who lie in wait.