We’re all pretty familiar with how, for example, an “-ing” at the end of a word means something different than an “-ed.” As it turns out, monkeys have their own versions of suffixes too. In a new Proceedings of the Royal Society B study, researchers conducting playback experiments in a tropical rainforest show that monkey listeners can tell the difference between danger calls with and without suffixes, and that they’ll alter their behavior accordingly. What’s more, even monkeys of different species seem to grasp the distinction.

Adult male Campbell’s monkeys (Cercopithecus campbelli) have a repertoire of three basic alarm calls: “Krak,” “Hok,” and “Wak.” And each of these can occur with the suffix “oo.” In particular, they give one call type to warn others about leopards (“Krak”), and they use the suffixed version of the same call for unspecific danger, like a falling tree (“Krak-oo”). The root “Hok” seems to indicate birds of prey (like crowned eagles), Discovery News explains, and we haven’t figured out what “Wak” means just yet.

To see if this suffixation is meaningful to receivers, a team led by Camille Coye from the University of Rennes 1 conducted a field experiment in Taï National Park in Ivory Coast, West Africa. They broadcasted natural and digitally edited suffixed and unsuffixed “Krak” calls made by local male Campbell’s monkeys. Altered calls include “Krak-oo” with the suffix deleted or “Krak” calls with the suffix added. Then they analyzed the reaction of 42 wild groups of Diana monkeys (Cercopithecus diana diana). These two monkeys often hang out in trees with each other, and they even coordinate their travel directions and attend to each other’s alarms. (The team didn’t pick other Campbell’s monkeys as listeners in order to avoid potentially hostile territorial behavior.)

The team found stronger responses to unsuffixed calls (for leopards) than suffixed calls (for non-predator threats). The Diana monkeys remained on alert for longer when they heard both natural and artificial “Krak” calls: Males and females gave more alarm calls afterwards, and females also gave fewer social calls. Their reactions were mainly determined by the presence or absence of the “oo,” and not by the intonation of the “Krak.”

"Several aspects of communication in Campbell's monkeys allow us to draw parallels with human language," Coye tells Discovery News. Suffixation, it seems, is an evolved function in primate communication.