Researchers in Senegal recently flew drones in the vicinity of green monkeys to see how the primates might respond. Incredibly, the monkeys produced an instinctual alarm call consistent with an eagle sighting. Kinda makes sense—except for the fact that green monkeys aren’t threatened by eagles.
New research published yesterday in Nature Ecology and Evolution is offering a fresh perspective on how monkeys are able to adapt hard-wired responses to threats posed by new, but seemingly familiar, hazards. In this case, West African green monkeys were observed producing an eagle alarm call when confronted by “novel flying objects,” otherwise known as aerial drones. The new paper, led by Julia Fischer from the German Primate Center in Göttingen, also speaks to the potential early origin of language among primates.
Various species of monkeys produce alarm calls to signal the presence of a threat to their troop. These alarm calls are hard-wired behaviors, meaning they’re genetically programmed, as opposed to culturally learned responses. Of relevance to the new study, East African vervet monkeys have specific and distinctive alarm calls in response to snakes, leopards, and eagles. West African green monkeys, a species closely related to vervet monkeys, only have warning calls for leopards and snakes; these monkeys have never been observed to produce alarm calls in response to birds of prey.
With this in mind, Fischer and her colleagues wanted to see how the green monkeys would respond to an unfamiliar aerial threat: a drone. The researchers were not trying to terrorize the monkeys. Rather, they wanted to analyze the range and flexibility of vocal noises produced by the monkeys, and assess their ability to make sense of and react to warning calls.
Working in Senegal’s Niokolo-Koba National Park, the researchers exposed 80 individuals across three different troops to aerial drones from February to July 2016 and then again from June to July 2017. Exposure to the drones ranged from one to three occurrences. Recording devices were used to capture the sounds produced by the green monkeys as a drone approached.
It didn’t take long for the monkeys to identify an incoming drone by its sound alone. When a drone neared, the green monkeys emitted a vocalization the researchers had never heard before in the species. Analysis of the recordings, however, showed that the sounds were practically identical to the eagle warning calls produced by their cousins, the vervet monkeys. This similarity suggests green monkeys have retained this hard-wired warning message in their DNA over evolutionary time. Accordingly, it can be considered a vestigial trait, in that the eagle alarm call was acquired and used by a shared ancestor common to green and vervet monkeys.
In a second phase of the experiment, the researchers played back recordings of the eagle/drone alarm call made by the green monkeys a few days earlier. The monkeys “immediately scanned the sky and ran for cover” even though no drone was flying in the area, the authors wrote in the new study.
For Fischer, these observations “tell us that vocal production is even more strongly [genetically] conserved that we thought it would be, and that learning on the other hand is more rapid and flexible than we thought,” she told Gizmodo in an email. “While the former sets these monkeys’ and human communication clearly apart, the latter stresses the cognitive continuities,” said Fischer.
Indeed, these findings point to a potential origin of human language. Some of the first “words” used by our distant ancestors were likely alarm calls, which then became more sophisticated and nuanced over time and as cognitive skills allowed, leading to culturally shared terms that were passed down from one generation to another. In the new study, the three troops of green monkeys went from not using the eagle alarm call, to using it and quickly learning the message behind the signal, thus adding it to their repertoire of alarm signals. These capacities and behaviors, therefore, can be construed as precursors to culturally produced language.
Looking ahead, Fischer would like to conduct the experiment with a larger number of monkeys of representing more species.