Recent global surveys show that humans kill prey at much higher rates than other predators. "We usually think about the top of the food chain being large carnivore predators," says first author Liana Y. Zanette, a conservation biologist at Western University in Canada. "But what we're interested in is the unique ecology of humans as predators in the system, because humans are super lethal."

"Normally, if you're a mammal, you're not going to die of disease or hunger. The thing that actually ends your life is going to be a predator, and the bigger you are the bigger the predator that finishes you off," says co-author Michael Clinchy, also a conservation biologist at Western University. "Lions are the biggest group-hunting land predator on the planet, and thus ought to be the scariest, and so we're comparing the fear of humans versus lions to find out if humans are scarier than the scariest non-human predator."

As part of their South African mega-experiment, Zanette, Clinchy, and colleagues observed how 19 different mammal species reacted to a series of recordings, including human voices, lion vocalizations, barking dogs, and gunshots.

The human-voice clips, which were at conversational volume levels, came from radio or television recordings of people speaking the four most used languages in the region, including Tsonga, Northern Sotho, English, and Afrikaans. The dogs and gunshots were meant to represent sounds associated with human hunting, and the lion vocalizations, curated with the help of lion expert and co-author Craig Packer of the University of Minnesota, were meant to signal the presence of the top predator in the region.