Humans can tell a lot about one another just by listening to a voice. Age and gender are among the first pieces of information we pick up on when listening to a stranger speak, and studies have even shown that humans can determine a person's physical strength by listening to a recording of their voice. This ability is incredibly useful, because it can help us determine whether someone poses a threat without ever having to see them. Other animals can do this too, and have also been known to analyze sounds made by their predators. But African elephants may have taken this ability to the next level, as a new study suggests that elephants might be even better at decoding human voices than we are.

"Look, Look over there, a group of elephants is coming."

The study, published today in Proceedings of the National Academy of Sciences, was initially designed to find out whether the wild elephants of Amboseli National Park, in Kenya, could use the acoustic information contained in human language to distinguish the threat posed by the local Maasai tribes — an ethnic group known for its hunting practices — from the lesser threat posed by the agricultural Kamba ethnic group. In both cases, the recordings featured men saying "Look, look over there, a group of elephants is coming," in their respective languages.

It didn't take long for the scientists to realize that these elephants had no trouble figuring out which male voices were cause for alarm and which weren't, so they upped the ante by having elephants listen to recordings of Maasai women, who rarely hunt, as well as men. Once again, the elephants had no trouble figuring out who posed a danger to their calves. When they heard the male voices, the elephants would bunch together defensively or retreat, but they barely budged when they heard a woman speak. A third test revealed that the elephants didn't feel threatened by the voices of Maasai boys either. "Elephants have this amazing ability to discern predators on a fine scale," says Graeme Shannon, lead co-author of the study and a wildlife ecologist at Colorado State University. "They can ascribe different levels of threats to certain groups."

"We thought we were being quite clever when we changed the voices."

That's when things got tricky. Thinking they could outsmart the elephants, researchers decided to digitally alter the voices to make the Maasai women sound male and the Maasai men sound female. But the elephants weren't fooled. They retreated just as much after hearing the feminized male voices as they had before the voice manipulation — something that humans themselves aren't capable of doing, Shannon says. "That was really intriguing because the recordings sounded good to us," he says. "We thought we were being quite clever when we changed the voices."

"It's not so much that they can tell male from female voices, but that they tell the two languages apart," says Frans de Waal, an animal behaviorist at Emory University who did not participate in the study, "and are not fooled by digital manipulation of the voice, which suggests that they use different gender cues than we do — or probably do." Joshua Plotnik, an elephant researcher at the University of Cambridge, agrees. "This suggests that the complexity of elephant communication may rival that of most other mammals on the planet."

They would even mob the remotely activated speakers

Humans aren't the only animals that elephants can "decode" by voice alone. A 2011 study, for example, demonstrated that elephants can detect the sex of a lion based solely on its roar. But elephants react very differently to lions than they do to humans, Shannon explains. "With the lions, the elephants were vocal and made themselves known." Sometimes they would even mob the remotely activated speakers that played the recordings. This was not the case with the Maasai voices "because that would escalate the risk," he says. "They tailored their response to the predatory threat by making sure they never really encountered the speaker — they moved away."

The ability to distinguish whether a predator is worth fussing over is extremely important for wild populations competing with other animals over food and space. "If they responded to every single stimuli that actually has a fitness cost," Shannon says, "because they would stop feeding and run every time."

Today, the wild elephants of Amboseli National Park are doing well. Despite a 2009 drought that killed off many of its older females, the park continues to harbor over 58 families of elephants. And officials have, thus far, been able to keep poaching to a minimum. Yet as this study demonstrates, the humans and elephants who frequent the park don't always coexist peacefully. "Perhaps this information can be used to develop more comprehensive conflict-mitigation techniques that take the elephants' perspective and decision-making process into account." Plotnik says. "I find it very sad that humans have driven elephants to the point that they now need to learn to adapt to humans as a threat."