Monkeys use alarm calls to warn family of predators, with some calls identifying specific threats. When exposed to drones, West African green monkeys developed a call immediately. Despite being unafraid of birds of prey, their call is very similar to the one used by East African vervet monkeys for eagles. Rather than an odd coincidence, the primatologists who discovered this think that it indicates something deep about how aspects of language are hard-wired in the monkey brain, and probably our own.

Vervet monkeys, the model animal for the study of monkey language have three main predators; eagles, leopards, and snakes. They've developed distinctively different calls for each, which inspire differing evasive responses from members of the troop. Past experiments have shown that playing one of these calls to a troop of monkeys will produce the appropriate defensive action, even in the absence of the actual threat.

Vervet monkeys encountering a drone might be expected to mistake it for an eagle, or at the very least use a similar call. However, Professor Julia Fischer of the German Primate Center chose to investigate something different. Green monkeys are closely related to vervets, but live on the other side of the continent. When shown models of leopards and snakes they have their own calls, and react in a similar fashion to their counterparts. However, models of eagles have produced no reaction in past studies, nor any living birds, presumably because raptors pose no threat in the green monkeys’ habitat.

Fischer wanted to know if, when presented with something obviously terrifying, the green monkeys would develop a new call, and if so, how quickly this occurred.

She flew a drone over a green monkey troop. The monkeys called in alarm and some ran for cover. Indeed, even before the drone was visible some searched for the source of the sound. The calls were quite different from those used for leopards or snakes, although not quite as distinct as those vervet monkeys use to distinguish eagles from other threats.

When recordings of these calls were played back to green monkeys they looked upwards for the source of the threat, proving they had quickly learned to associate this sound with danger from the skies. “Green monkeys can instantly attach meaning to a novel sound and retain this knowledge,” Fischer writes in Nature Ecology and Evolution. 

The most intriguing part of Fischer's work is the “striking similarity” she describes between the calls adopted by green monkeys for drones and the one vervets have been using, probably for millions of years, for eagles. It seems even when neglected for thousands of generations, certain calls are hardwired in the monkey brain to be associated with particular types of threats.