Chlorocebus sabaeus
A drone flown by German researchers at the Leibniz Institute for Primate Research elicited a new alarm-call from West African green monkeys, Chlorocebus sabaeus. The meaning of this call was learned very quickly by other monkeys. This alarm call originally belonged to the closely-related East African vervet monkey which uses it to warn against aerial predators like eagles. Taken together, these findings suggest that the call structure was determined many generations ago and was conserved despite the two species diverged evolutionarily.
In a new study, Julia Fischer and colleagues at the German Primate Centre introduced West African green monkeys at a research station in Simenti, Senegal, to an aerial drone. The aim of the experiments was to see how quickly the animals learned to recognize the drones as a threat. Much to everyone’s surprise, the green monkeys made calls resembling the calls that East African vervet monkeys utter when they detect eagles.
The drone was flown once over 80 monkeys at a height of about 60 meters. As the drones circled above them, the animals produced chirping warning sounds, scanning the sky and running for cover just as vervet monkeys do when eagles are overhead.
The researchers have confronted West African green monkeys with a new potential threat from the air: a drone, which they flew over the animals at a height of 60 meters. Photo: Davide Montanari
Incredibly, after hearing a drone in the sky only one to three times, the monkeys remembered the threat almost three weeks later. When researchers played five green monkeys the whirring sound of a drone, 19 days later on average- four of them looked up at the sky to try to find the drone. Three out of the five monkeys ran away in fear.
Fischer, the head of the Cognitive Ethology Laboratory at the German Primate Centre and lead author of the study said, “The animals quickly learned what the previously unknown sounds mean and remembered this information which shows their ability for auditory learning.”
Although the green monkeys never encountered aerial threats, the structure of their alarm call for drones was almost identical to East African vervet monkeys. Fischer says that this must mean that the vocalization is deeply rooted in the evolution of vervet monkeys.
Spectrograms showing alarm calls that female West African green monkeys (above) and female East African vervet monkeys (below) emit in response to predators. It becomes clear that the “drone calls” of West African green monkeys resemble the “eagle calls” of East Afrcian vervet monkeys. Picture: Julia Fischer
The study involved a year’s worth of fieldwork by a team of eight, who flew the drone above the monkeys. The research wasn’t without incident. Fisher had to duck inside a hide made of palm leaves at one point, after a baboon– which are known to attack leopards – came running to attack the leopard model she was holding.
It was expected that the green monkey was they would either stay silent, come up with a new alarm call or come out with one like the vervet monkey eagle call. Fisher’s bet was on the latter, and she was proved right. The vocalisation was highly conserved by evolution.
The authors wrote in the journal Nature Ecology & Evolution: “Our findings support the view of a fundamental dichotomy in the degree of flexibility in vocal productions versus the comprehension of calls. Collectively, these studies indicate that the emergence of auditory learning abilities preceded the evolution of flexible vocal production.”
Citation: Conserved alarm calls but rapid auditory learning in monkey responses to novel flying objects, Franziska Wegdell, Kurt Hammerschmidt & Julia Fischer, Nature Ecology & Evolution (2019), https://doi.org/10.1038/s41559-019-0903-5 – https://www.nature.com/articles/s41559-019-0903-5