To find out what the pyow-hack might mean, the researchers played recorded leopard growls to 17 different putty-nosed monkey groups in Nigeria's Gashaka Gumti National Park.
In nine of the groups, the males produced at least one pyow-hack sequence. In the eight others, the males made single-call series only. In response, some of the groups moved to evade the supposed predator.
After waiting 20 minutes, the scientists played a recorded series of monkey calls indicating the presence of crowned eagles. This elicited the hack call from all 17 males.
Putty-nosed monkeys avoid moving when threatened by an eaglemovement through the canopy would cause them to be more obvious to the winged predator.
With the monkeys no longer on the move, the scientists were able to re-locate the groups using global positioning equipment.
They found that the nine groups that produced the pyow-hack sequence had moved significantly farther than those that produced other call series.
The pyow-hack series, the researchers conclude, has a different meaning from the other calls, and "move farther and faster" might be a possible translation.
Based on the finding, the scientists say that combining calls seems to increase the variety of messages that monkeys can communicate. But does this mean the monkeys are actually putting together a kind of language?
Not quite, says Edward Wasserman, professor of psychology at the University of Iowa in Iowa City.
But he called the results "interesting initial observations of differential responding by putty-nosed monkeys to two distinctive vocal calls emitted together."
He suggests that the researchers figure out whether the reverse calla "hack-pyow"has a different meaning.
"[That would] strengthen the case that these monkeys exhibit a primitive form of syntax," he said.
Michael A. Huffman, associate professor of ecology at Kyoto University in Japan, questioned whether the pyow-hack call is actually an invention that creates a new message or simply another innate call.
"Further research would help pin this down," he said. "But at present this work stands as an interesting proposition."
Zuberbühler cautions that analogies to human language are not always helpful in understanding the utterances of animals.
But he does see a parallel in the fact that, like humans, the monkeys appear to be assembling a finite number of sounds to create new and different meaningful combinations.
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