West African green monkeys produced a different hard-wired alarm call when faced with a new aerial threat and instantly learned what the new sound represented, according to a study published online this week in Nature Ecology & Evolution. This research sheds light on the evolution of complex communication systems in the animal kingdom.

East African vervet monkeys produce alarm calls signalling different predators - such as leopards, snakes and eagles - and respond to each alarm call in a different way. The closely related green monkeys living in Senegal show similar behaviours, but were never observed to produce an alarm call to eagles.

Julia Fischer and colleagues flew drones past 80 green monkeys to study how they might respond to a new aerial threat. Once the green monkeys could see the drone, they made alarm calls that were different from calls given in response to snakes or leopards. The authors observed that this new alarm call was strikingly similar to the sound that vervet monkeys make when they spot eagles.

The authors suggest that the similarity of the calls signifies that the alarm call structure is hard-wired and deeply rooted in the evolutionary history of the animals. When the authors played back the sound of the drone to 16 of the monkeys, they scanned the sky or ran away, suggesting that the monkeys had immediately learned what the sound indicated.