Although the possible animal origin of human languages is still very controversial, many field studies have been able to prove that each species of monkeys has evolved a language. In particular, studies have shown that male monkeys produce different warning sounds to alert the group of an incoming danger.
Scientists being curious by nature, a research team tried to find out whether monkeys of a particular species could understand the language of another species (their study is published in the Proceedings of the Royal Society B).
The study involved Campbell’s monkeys and Diana monkeys in Ivory Coast, because members of these species like to hang out with each other. The team first recorded various calls produced by Campbell’s monkeys, according to the following dictionary:
Hok: there’s an eagle in the area
Hok-oo: something is going on up in the trees
Krak: there’s a leopard nearby
Krak-oo: there’s trouble nearby
Wak-oo: synonym of Hok-oo
Boom: something is going on, but it’s not dangerous
So it looks like Diana monkeys do at least to a certain extend understand Campbell’s monkeys… but they probably wouldn’t make very good interpreters!