Campbell's monkeys appear to combine the same calls in different ways, using rules of grammar that turn sound into language.

Whether their rudimentary syntax echoes the speech of humanity's evolutionary ancestors, or represents an emergence of language unrelated to our own, is unclear. Either way, they're far more sophisticated than we thought.

"This is the first evidence we have in animal communication that they can combine, in a semantic way, different calls to create a new message," said Alban Lemasson, a primatologist at the University of Rennes in France. "I'm not sure it has strong parallels with humans, in the way that we will find a subject and object and verb. But they have meaningful units combined into other meaningful sequences, with rules imposed on how they're combined."

Lemasson's team previously described the monkeys' use of calls with specific meanings in a paper published in November. It detailed the monkeys' basic sound structures and their uses: "Hok" for eagle, "krak" for leopard, "krak-oo" for general disturbance, "hok-oo" and "wak-oo" for general disturbance in forest canopies. A sixth call, "boom," was used in non-predatory contexts, such as when calling a group together for travel or arguing with neighboring groups.

Impressive as that was, however, it was still relatively one-dimensional, not much different from verbalizations heard in many animal species, from other non-human primates to songbirds. The team's latest findings, published Monday in the Proceedings of the National Academy of Sciences, describe something far more complicated: syntax, or principles of word sequence and sentence structure.

Though some researchers have ascribed syntax to animals, it's never been formally demonstrated – until now.

"People have criticized the use of 'syntax' to describe animals just because they produce sequences of sound. They say that each unit has no meaning, that no rules explain how they're combined," said Lemasson. "Here we have rules of combination."

For example, male monkeys called "boom boom" to gather other monkeys to them, but "boom boom krak-oo krak-oo" meant that a tree or branch was about to fall. Adding a "hak-oo" to that sequence turned it into a territorial warning against stray monkeys from neigboring groups. Multiple "krak-oo" calls added to an original "krak" meant not only that a leopard was in the area, but that it posed an immediate threat.

The research raises the question of whether early humans or our primate ancestors combined calls in a similar way, turning a small set of sounds into a rich verbal reportoire.

According to Lemasson and to Jared Taglialatela, a chimpanzee communication researcher at Clayton State University, it's too soon to say whether the monkey talk is proto-human.

"I'd shy away from that. But this is certainly syntax," said Taglialatela, who was not involved in the study. But he described the proto-human question as secondary to a far more intriguing possibility: that the potential for language is widespread in the animal kingdom.

"People like to draw lines and make boxes and put animals inside them. I don't like to do that. There are differences and shades of grey. And when you take the time to collect data in a way that allows you to recognize complexity and patterns, than you find evidence of them," said Taglialatela.

Lemasson's analysis was based on a vast set of recordings, gathered from 10 monkey groups observed for two full years in their African rain forest homes.

Lemasson, who is further investigating Campbell's monkey talk by measuring their reactions to recorded calls, suspects that a dense jungle environment drove the evolution of syntax. Since the monkeys had trouble seeing each other, they compensated by talking.

The same compensatory dynamic could operate in other species, such as whales that live in mostly sunless waters, he said.

"We can imagine that this ability has evolved in other lineages," said Lemasson.

Image: Florian Möllers

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Citation: "Generating meaning with finite means in Campbell’s monkeys." By Karim Ouattara, Alban Lemasson, and Klaus Zuberbuhler. Proceedings of the National Academy of Sciences, Vol. 106 No. 48, December 7, 2009.

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