Some 40 years ago, scientists discovered that East African vervet monkeys (Chlorocebus pygerythrus) produce distinct alarm calls when they encounter their three main predators: leopards, snakes and eagles. Their cousins in West Africa, green monkeys (Chlorocebus sabaeus), are also known to cry out at the sight of leopard and snakes, but for some unknown reason, they don’t seem to emit a unique call for birds of prey. A team of researchers recently discovered, however, that the sight of a drone prompts green monkeys to emit an alarm call that is strikingly similar to their vervet cousins’ eagle warning—a finding that suggests such vocalizations are evolutionarily “hard-wired,” the researchers write in Nature Ecology & Evolution.
For the new study, the researchers first tried to get green monkeys in Senegal’s Niokolo-Koba National Park to respond to papermaché models of eagles, without any success.
“Perhaps our artwork was unconvincing,” writes study co-author Julia Fischer, a primatologist at the University of Goettingen in Germany. So the team decided to expose the monkeys to drones, an aerial threat that the animals had not encountered before.
Over the course of several months in 2016 and 2017, the researchers conducted drone flights over three different green monkey groups, using audio equipment to record the sounds they made. Each group was exposed to a drone between one and three times.
When they saw the strange flying object, the monkeys emitted a warning call and ran to hide. Upon conducting acoustic analyses of the drone response call, the researchers found that it was distinct from the monkeys’ leopard and snake warning signal. What’s more, the green monkeys’ drone call was remarkably similar to the vervet monkeys’ eagle alarm—a fascinating discovery, given that green monkeys and vervet monkeys diverged from a common ancestor around 3.5 million years ago. Producing the warning call, perhaps, is not a learned response, but a genetically innate one that has been conserved over a lengthy evolutionary history.
The researchers were not done there, however. They also set out to test how quickly green monkeys could learn to associate the sound of a drone with the scary device that hovered in the sky—and the answer, as it turns out, is quite quickly. When the researchers hid a loudspeaker on the ground and played back the sound of the drone, the monkeys looked up and scanned the sky, trying to locate the source of danger. Of the 14 monkeys that were tested with the drone sound, five had seen the drone only once, seven had been exposed twice and two had been exposed three times.
The study thus exposes what the researchers deem a “fundamental dichotomy”: green monkeys are quick to understand the meaning of new sounds, but are not particularly flexible when it comes to producing new ones. And this sets them apart from humans in an important way. Like green monkeys, humans are born with an “innate repertoire of pre-verbal sounds such as moaning, laughing and crying,” study co-author Kurt Hammerschmidt, a language evolution expert at the University of Goettingen, tells the Agence France-Presse. But we are able to move beyond this limited repertoire to produce a vast array of sounds with associated meanings.
Given that green monkeys seem more adaptable to learning new sounds than producing them, it is possible that auditory comprehension in primates evolved before flexible vocal production, the study authors note. But what happened over the course of human evolution that allowed us to transition from a limited set of vocalizations to flourishing speech? That, as Fischer writes, is the “million-dollar question.”