Campbell’s mona monkeys, while relying on a wide variety of communication types, are well known for their highly developed vocalization skills. Females, which make up the majority of adults in a group, communicate with each other extensively, both physically and vocally. They have a much wider array of vocalizations than males, including call types to indicate distress, threats, and contact with another group. Males vocalize much less often, and usually only to maintain spacing within the group and to warn of predators.

Studies show that Campbell’s mona monkeys may even use a rudimentary form of syntax, with different vocalizations having very specific meanings. For example, one study found that the call described as “krak” was only ever made after contact with a leopard and “hok” after contact with a crowned eagle (or, occasionally, a flying squirrel, whose silhouette resembles that of a crowned eagle). Meanwhile, “krak-oo” and “hok-oo” were used as general alarm types, usually associated with a predator. “Boom” calls were only given in non-predatory contexts, such as a falling tree or during fights with neighboring groups. This type of vocalization is significant, because it may indicate that Campbell’s mona monkeys add a suffix (“-oo”) to the end of a “word” to change its meaning, something that has not been shown in any species besides humans.

More research adds another level of complexity to the monkey’s vocalizations. For example, two consecutive “boom” calls are used to gather other monkeys nearby, while “boom boom krak-oo krak-oo” is used when a branch is about to fall—like “Timber!” for a human. When a “krak” is followed by multiple “krak-oo”s, it means that not only is there a leopard in the vicinity, but that it poses an immediate threat.

There is an ongoing debate as to whether a vocalizing Campbell’s mona monkey intends to inform its groupmates about a predator or event or whether the vocal response is purely instinctive. Evidence for the latter is the fact that males vocalize in response to falling trees even when all groupmates are close enough to witness it. This may suggest that the vocalization is not intended to inform, but is an instinctive response. However, the vocalization may also serve to let groupmates know that the vocalizer is safe, and to gather the group to a central area after they may have scattered in response to the falling tree. What everyone can agree on, however, is that Campbell’s mona monkeys present an exciting and novel opportunity for researchers to learn more about complex vocalization in non-human species.