Language is one thing we do not share with other primates. While we humans have the ability to form words, our close relatives lack such finally tuned vocal control. Instead, like the majority of other animals, primates have evolved complex methods of conveying information, which range from grunts to body language to smell.

Now it seems that some species of monkey not only adjust the meanings of their calls using a simple grammatical trick, but other species know how to "translate" those calls to hack their neighbors' predator warning system. The finding hints at a universal system of communication among some monkeys that includes some of the basic tools of human language.

Several years ago, researchers discovered that wild Campbell’s monkeys can alter the meaning of their “krak,” “hok” and “boom” calls by adding suffixes. Just as adding the suffix "-dom" to the word "king" creates "kingdom," the monkey's suffixes help indicate specific threats. “Krak” means a leopard is present, for example, while “krak-oo” indicates unspecified danger, such as a falling branch or another troop of monkeys encroaching on the caller’s territory. “This is the first time that we can demonstrate that these sequences convey something about the environment or an event the monkey has witnessed,” Klaus Zuberbuehler, a professor at the University of St Andrews, commented when the findings were announced.

Now, Zuberbuehler and several international colleagues have delved deeper into that discovery. As they report this week in Proceedings of the Royal Society B, they found that another species, the Diana monkey, has tapped into the suffixed system of communication. Diana monkeys not only recognize the Campbell’s monkeys’ danger calls, but they know which call corresponds to which type of danger.

To reveal these secret monkey ways, the researchers traveled to Ivory Coast and conducted field experiments in Tai National Park, the largest tropical forest in West Africa. They sought out 42 wild groups of Diana monkeys and then played one-minute clips of Campbell’s monkeys making “krak” or “krak-oo” alarm calls. Some of the calls were natural, while others had been digitally edited, either taking off the “oo” suffix or adding it on.

From their previous work, the team knew that Campbell’s monkeys respond more strongly to “krak” calls than to “krak-oo” calls. This makes sense, given that leopards are the monkeys’ natural predator, while a falling branch or neighboring troop may pose less of a threat. The Diana monkeys likewise responded to the “krak” calls more intensely. When they thought a leopard was nearby, they gave significantly more of their own alarm calls then when they heard a general “krak-oo” call. They also remained on high alert for longer and made fewer social calls following the “krak” alarm. These findings held true for both natural and edited clips, meaning it’s most likely the “oo” suffix—not some special intonation on the “krak”—that marks the distinction between a leopard and a general disturbance.

To the best of the researchers’ knowledge, this is the first time that scientists have demonstrated experimentally that wild, untrained animals use suffixation to communicate with one another in the natural world. The result suggests “that basic features of human speech … can evolve independently in species that are not so closely related to humans,” the authors write. The researchers plan to investigate whether other animals have developed similarly refined species-to-species communication hacks, which they strongly suspect is the case.