If you speak French, you might be able to make out what an Italian is saying — but if you're a particular species of monkey, could you pick up on warnings from a different monkey species? The answer is yes, based on research into the vocabularies of Campbell's monkeys and Diana monkeys in a West African rainforest.
Previous studies have shown that male Campbell's monkeys use a variety of calls to sound alarms. "Hok" means there's an eagle in the area, while "Hok-oo" is a more general alert about something going on up in the trees. "Krak" means a leopard has been sighted, while "Krak-oo" can apply to any disturbance — for example, a falling tree. "Wak-oo" is roughly the same as Hok-oo, and "Boom" lets the other monkeys know there's a hubbub that doesn't involve predators.
For the latest study, published in the Proceedings of the Royal Society B, the research team recorded natural and artifically modified Krak and Krak-oo calls. Then they played the sounds in the wild for 42 groups of Diana monkeys — a closely related species that tends to hang out with the Campbell's monkeys in the Ivory Coast's Tai National Park. The researchers tallied up how the groups responded with their own alarm calls.
The result? Statistically speaking, the Diana monkeys responded to the Krak calls more vigorously than to the Krak-oo calls. That's what you would expect, if the listeners understood the difference between a leopard alarm (Krak!) and a less specific alert call, as indicated by the "oo" suffix. "This study indicates that suffixation is an evolved function in primate communication in contexts where adaptive responses are particularly important," the researchers write.
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The lead author of "Suffixation Influences Receiver's Behaviour in Non-Human Primates" is Camille Coye of the University of Rennes. Other authors include Karim Ouattara, Klaus Zuberbühler and Alban Lemasson.