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THURSDAY, Jan. 15 (HealthDayNews) -- Researchers say they've come closer than ever to figuring out why humans can string sentences together and our hairy cousins can't.

One of the major barriers to a simian Shakespeare appears to be the inability of primates to comprehend anything other than the simplest rules of grammar, according to a new study.

Monkeys who listened to alternating male and female voices couldn't pick up on complex patterns designed to mimic those of human speech, researchers found. This lack of understanding could be a "fundamental bottleneck on animal thought," says study co-author Marc D. Hauser, a professor of psychology at Harvard University.

While monkey grammar skills may seem like a pretty arcane topic harkening back to diagrammed sentences on chalkboards, it's actually at the center of a major mystery: How did human speech evolve? How does the way humans talk to each other differ from the way animals communicate?

"Grammar may be the defining feature of human language, what makes our language different from other forms of animal communication," says Keith R. Kluender, a professor of psychology who studies speech perception at the University of Wisconsin. "It's a pretty big deal."

It's no secret that animals can communicate with each other in simple ways -- think of those midnight neighborhood symphonies of howling dogs or cats in heat. And humans have managed to teach sign language to apes and dolphins.

But Hauser says the top number of words they have learned is in the 300-to-400 range, which doesn't even compare to the 60,000-word vocabulary of a typical high school graduate.

Apes "have learned signs that refer to things in their world, like food and actions," Hauser says. "They could label things and could sign 'apple' or 'pond.' But what they couldn't say is, 'My apple is in the pond' or 'on the chair.' "

But is that because they truly can't handle that kind of grammar?

To explore that issue, Hauser and a British colleague developed a way to analyze grammar abilities in cotton-top tamarins, an endangered monkey found in South America.

The results of the study appear in the Jan. 16 issue of the journal Science.

Using male and female voices, the researchers let the monkeys listen to specific orders of several syllable-type sounds. In the most simple test, the monkeys looked at the loudspeaker whenever the voices broke the patterns.

The researchers increased the complexity of the patterns, using a system similar to that used by scientists who study the language development of human infants. But the monkeys couldn't keep up.

According to Hauser, this suggests the monkeys could possibly understand a simple pattern of grammar -- that an adjective typically precedes a noun, for example -- but be baffled by a more complex pattern, like an "if-then" statement with words in between.

This limitation makes sense, he adds. "The paradox is that many animals have rich conceptual systems and ways of thinking about the world, and rich social relationships. What they lack is a system that can convey that to each other," he says.

Kluender agrees with the finding that the monkeys can't process complex language patterns, but he says he's not sure that the researchers truly replicated human grammar in their study.

Both Kluender and Hauser do agree that it's very unlikely that scientists will stumble across advanced grammar skills in any non-human animal species.

"But it's hard to argue that it can't possibly exist," Kluender says.

More information

For more on cotton-top tamarins, visit the Brevard Zoo. You can explore the evolution of language at the Science Museum.

(SOURCES: Marc D. Hauser, Ph.D., professor of psychology, Harvard University, Cambridge, Mass.; Keith R. Kluender, Ph.D., professor of psychology, University of Wisconsin, Madison; Jan. 16, 2004, Science; photo courtesy of Marc D. Hauser)

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