When a threat is unfamiliar, how are animals — specifically, West African green monkeys — supposed to convey the nature of the threat to their comrades?

In an attempt to find an answer to this question, scientists sent drones to fly near a group of West African green monkeys and observed how the animals responded to the new "threat."

To their surprise, these West African green monkeys used alarm calls that are not naturally part of their language. Instead, these calls are very similar to the calls their cousins the East African vervet monkeys use to warn each other of the presence of eagles, according to the researchers from the German Primate Center.

In new research published in the journal Nature Ecology and Evolution, the team shared how these responses reveal interesting insights about monkey learning and unlock new knowledge about the evolution of language in primates.

Monkey Language

Three predators are a threat to the East African vervet monkeys, namely leopards, snakes, and eagles. The species have developed alarm calls for each predator, so that they could respond specifically to the threat. A leopard alarm would spur the vervet monkeys to climb a tree, while a snake alarm would make them stand very still on two legs. Then, when a monkey sounded an eagle alarm, the rest of the group would scan the sky for the threat.

West African green monkeys are closely related to the East African vervet monkeys, but while they have alarm calls for leopards and snakes, they lack one for aerial threats like eagles.

To see how they would respond to an aerial threat without having a specific warning call for it, lead author and behavioral scientist Julia Fischer of the German Primate Center and her team flew a drone over West African green monkeys. Later, they played recordings of the drone sounds to the animals to see how long it takes for the green monkeys to learn what these sounds meant.

Incredibly, the monkeys responded to the playback experiment by making alarm calls, searching the sky, and hiding. The alarm sounds were new to the West African green monkeys, being very different from their calls linked to leopard or snake presence. Instead, the alarm calls to the drone resembled the calls their cousin species made when an eagle is around.

What It Means

Findings showed that while the green monkeys may not have a specific alarm call for birds, they quickly identified unfamiliar drones as a threat, remembered them when the sounds were played again to them, and went on to warn the other monkeys in their group.

"This shows their ability for auditory learning," Fisher explained.

She also told New Scientist how surprised she was to see how quickly the monkeys were able to learn, adding how difficult this was to do in a laboratory setting.

Additionally, the study findings suggest that the specific calls and responses to the aerial threats are hard-wired into the animals.

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