In the wild, monkeys need to keep their eyes peeled for all sorts of dangers, like leopards and eagles and snakes. But the green monkeys studied by Julia Fischer of the German Primate Center have an additional challenge: they also have to scan the skies for drones.

“Why did we fly a drone over green monkeys, one may ask.”

One may indeed. The answer is that Fischer and her colleagues are interested in how primates communicate.

In a classic study back in the 1980s, scientists showed that East African vervet monkeys produce alarm calls that are specific for the predators they encounter. So for example, vervet monkeys hearing a leopard alarm [clip] might scurry up a tree, whereas the eagle call [clip] sends them running for cover under the closest shrub.

Now, the green monkeys that live in Senegal share a similar system to warn of leopards and snakes. But they aren’t known to raise a ruckus in response to birds of prey.

“And so therefore we decided to fly a drone over them.”

The researchers treated 80 green monkeys to a show of drones. How did the animals react to this unfamiliar aerial intruder?

“The monkeys did respond. They responded with alarm calls [clip], and they responded by running away.”

Here’s where things get really interesting: the calls the green monkeys made after spotting the drones were different from the ones they use to signal leopards [clip] or snakes [clip]. But even more intriguing:

“And when we did an acoustic analysis, these alarm calls [clip] were almost eerily similar to the ones of the east African vervets. [clip]”

The findings are described in the journal Nature Ecology & Evolution. [Franziska Wegdell, Kurt Hammerschmidt and Julia Fischer, Conserved alarm calls but rapid auditory learning in monkey responses to novel flying objects]

The fact that the two monkey species seem to speak the same language, if you will, even though they diverged from their last common ancestor some 3 million years ago, suggests that this vocal warning system is hardwired.

So if you hear a monkey go [clip], watch out for a hungry bird. Or check to see if you got a package delivered.

—Karen Hopkin

(The above text is a transcript of this podcast)