Monkeys not only have language, but distinct local dialects. Research into the variations could throw light on both our own linguistics and the way our fellow primates think.
Like other social species, Campbell's monkeys (Cercopithecus campbelli) have warning calls to alert fellow members of their troupe to danger. Recordings in the Tai Forest of the Ivory Coast show that the monkeys there use a sound we might write as krak to warn of the presence of a leopard, while hok referred to an eagle. They also use the suffix oo to indicate a more minor threat, so krak-oo mean something to keep an eye on below, while hok-oo warns of minor danger above, such as falling branches.
However, when the monkeys of Tiwai Island, Sierra Leone, were studied it was found that the same sounds had different meanings. “On the island, in an eagle situation, you did find a lot of hok but you also found a lot of krak,” Phillipe Schlenker of the French National Center for Scientific Research told Scientific American. “That was surprising because krak is supposed to be a leopard alarm call.”
The clue to this difference is that Tiwai has no leopards. Publishing in Linguistics and Philosophy Schlenker and colleagues conclude that without the threat from below, krak “functions...as a general alarm call on Tiwai”
It shouldn't surprise us that monkeys use calls differently depending on geography. Britain and America are “two countries separated by a common language.” Even in one country the same word can have localised meanings.
Another aspect of human communication Schlenker put to use to explain the mystery is the way we prefer the specific to the general. “Words compete with each other,” Schlenker says. “And you use the more informative one.”
Seeing monkeys as unsophisticated creatures can blind us to the fact that they are very well adapted to their environment, including in finding language that maximizes survival. “The important thing is that in this situation, both krak-oo and hok are more informative than krak,” Schlenker says. “By logic, if you hear krak you can infer there was a reason krak-oo and hok were not uttered, so you infer the negation.”
Faced with multiple threats it is important for the monkeys of the Tai to be specific, using krak only when it refers to leopards. But when there is only one major predator to worry about it doesn't matter whether the Tawai monkeys say krak or hok, as either will warn of an eagle attack.
Reportedly, this is the first paper published by a professional linguist on wild monkey vocalizations, and Schlenker hopes for a “primate linguistics”.
To ensure they knew what threat the monkeys were referring to, primatologists collaborating with Schlenker placed realistic models of leopards where the monkeys would see them, and played eagle calls before recording the monkeys' responses. “If you yourself are the trigger,” Schlenker says, “You have much better control over what causes each calling sequence in the first place.”