When green monkeys spy a drone, they use their cousins’ cry for ‘eagle’

Humans have long looked for the origins of language in our primate cousins. But now, researchers recording the calls of West African green monkeys have underscored how different monkey communication can be from human language. A never-before-heard call the monkeys made when researchers flew a drone overhead is nearly identical to another monkey species’s cry for “eagle.” That similarity bolsters the idea that the alarms are part of a fixed repertoire of hardwired calls, very different from the creative, open-ended vocalizations of humans.

East African vervet monkeys (Chlorocebus pygerythrus) have three distinct alarm calls—one for snakes, one for leopards, and one for eagles. The West African Green monkey (C. sabaeus), the vervet’s evolutionary cousin, has alarm calls for leopards and snakes that sound nearly the same as the vervet calls. But researchers had never heard green monkeys raising the eagle alarm.

To test the green monkey’s response to a new aerial threat, researchers flew a drone over a troop of the monkeys in Niokolo-Koba National Park in Senegal. Once the drone came into view, some of the monkeys produced an alarm call that the researchers had not heard in their prior 8 years of study in the park. The team recorded the calls and repeated the experiment with two other troops of green monkeys. They all produced the same unique alarm call. Days after the drone flights, the researchers played an audio recording of the drone, which caused the monkeys to sound the alarm and scan the sky.