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Monkeys Deaf to Complex Communication, Study Says

Stefan Lovgren
for National Geographic News
January 22, 2004
 
While monkeys are able to understand the most basic grammatical rules,
new research shows they can't master the more complex grammars that are
central to human language.

A new study of cotton-top tamarins (Saguius oedipus), a rare South American monkey, confirms previous findings that monkeys are able to grasp the rules of "finite state grammars"—basic grammar that, for example, controls the types of words that go next to each other in a sentence.


But the study, described in the journal Science last week, shows that monkeys don't understand more complex "phrase structure grammars," in which words in a sentence can be separated but still depend on each other. (The "if … then … " construction is a classic example of such grammar.)

The research suggests that the ability to perceive phrase structure is one of the main differences between animal communication and human language.

"Phrase structure is an important component of language," said Tecumseh Fitch, a lecturer at the school of psychology at the University of St. Andrews in Scotland, who led the study. "Sentences are not just lists of words, but they have a complex structure over and above that, which enables us to communicate causal connections, opinions about statements, and complex social ideas."

In an accompanying article, David Premack, a professor emeritus in psychology at the University of Pennsylvania in Philadelphia, explores the reasons why animals have not evolved languages of any kind. He points to more "humble" explanations than phrase structure as reasons for why animals don't have languages.

"Animals lack things like teaching, imitation, and voluntary control of sensory-motor systems," said Premack. "If you don't have those things, you are not going to evolve any kind of language."

Complex Grammar

The tamarins are a highly endangered species found only in the rain forests of Colombia. They live in trees and are quite vocal. When a tamarin is separated from the group, he makes a whistling contact call that sounds like a songbird.

For their research, Fitch and his colleague Marc Hauser, a Harvard University psychology professor, studied a colony of tamarins housed at Harvard in Cambridge, Massachusetts.

The researchers first exposed the monkeys to examples of grammar known as finite state. A recorded sequence repeated the letters A and B like this: A-B-A-B-A-B. When the researchers violated the grammar by playing, for example, A-B-B-A, the monkeys would look toward the loudspeaker.

"That shows that without training, they've been able to figure out a rule at the finite state grammar level—they've been able to sense that pattern," said Fitch.

The researchers then played examples of phrase structure grammar, a recorded sequence that clumped A and B together, such as A-A-A-B-B-B. But when the researchers violated the more complex grammar rule, the monkeys would not respond by looking at the speaker.

"It's quite clear that these animals are able to pick up sequential regularities in a sound," said Fitch. "But they're not able to pick up a rule at this phrase structure grammar level."

No Magic Bullet

Fitch says animals and humans share mechanisms related to language, including auditory systems, vocal production systems, and aspects of the conceptual system. But while animals have complex thoughts—for example, they may know their way through a jungle—they're unable to express them vocally.

Still, there is plenty of evidence, Fitch says, that many non-human primates, as well as birds, actually have structures and rules that govern their vocalization.

"Birds don't stick things together willy-nilly," he said. "They have particular orders in which they string together calls to make more complex songs. The question then is, What is the level of complexity in these rules?"

Fitch says a fundamental aspect of human language is the ability to use phrase structure, but he stresses that there is no one "magic bullet" that gives us language.

"I don't believe that such magic bullets exist," he said. "Language is a complex mosaic including many important abilities, and any attempt to reduce it to just one will be simplistic and unsatisfactory."

Humble Factors

In his article, Premack suggests that other reasons, apart from an animal's inability to understand complex grammar, explain why they have not evolved languages. He says recursive language, which is one way to achieve more complex phrase structure, is not the key factor to consider.

"There has been too much tendency to think that because animals don't have recursion they don't have language," Premack said. "But the reason why they don't have non-recursive language or any other language is because they lack a whole bunch of simpler things."

He says one reason that animals don't have language is because they don't have voluntary control of sensory-motor systems, specifically voice and face, which are essential for speech and sign.

Another reason, he says, is that animals don't teach the way humans do.

"Although human mothers do not teach children grammar, they definitely teach them words," said Premack. "Humans are the only species that teach. Evolution, being endlessly clever, might produce words that don't require teaching, but until it does, it is not clear how any species other than humans could evolve language."

Animals are also not as flexible as humans. While bees may be able to send messages through dance, humans have dozens of ways of sending messages.

Imitation may be yet another factor. While many species can copy a role model's choice of object or location, they can't copy the motor action. This second-level of imitation, Premack maintains, is needed for the evolution of language." "Recursive language is very powerful and it enables us to talk in the fancy way we do," he said. "But suppose we only had non-recursive language. You could still ask questions, use descriptions, and make requests, only it would not be half as wonderful as the system we have."
 

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