Some chimps in the "lift" group discovered the poke method, and some in the "poke" culture discovered lifting. But they were a small minority. When the apparatus was reintroduced two months later, the chimps reverted to their own culture's preferred method.
This, the researchers maintain, provides evidence of a "conformist bias." The animals discount their own experience and instead adopt the behavior of the group, just as humans do.
"This is a very nice experimental setup," said Diana Reiss, a research scientist with the Bronx, New York-based Wildlife Conservation Society, who was not involved in the study. "It was controlled for biases, and included a control group where there was no trained expert. The setup eliminated the problem of learning by interacting with humans."
The researchers believe they have demonstrated for the first time an ability among chimpanzees to transmit alternative technologies and alternative methods of using tools.
Monkey See, Monkey Do
"When all these different [wild chimp] behaviors were discovered in the field, there was controversy," said Frans de Waal, a professor of primate behavior at Emory University and study co-author. "Some [scientists] claimed it was social learning. Others claimed there were other possible explanationsindividual learning, genetic differences, ecological variables, and so on."
"We did the experiment to prove that you could plant a behavior by training one chimp and see it spread to other chimps by observation."
Giving the chimps two alternative methods of accomplishing the same task, the researchers say, shows that chimps are capable of adopting local variants of a technique, just as they would if the variant behaviors seen in the wild are in fact socially transmitted.
Not all experts agree with this conclusion. Rob Boyd, a professor of anthropology at the University of California, Los Angeles, said, "I have argued that any time true imitation evolves, so will a tendency to copy the majority. So I would very much like it to be true that the data supported this prediction."
But Boyd believes the study data fail to offer the necessary proof. He notes that while a few chimps dropped their group's rarer behavioral variant (using a stick to poke or lift a trap to release food), the study "does not show that they switched to the common variant, which is what I believe is necessary."
Groups of chimps at the Yerkes National Primate Research Center Field Station at Emory University, where the research was carried out, have developed cultural differences on their own, without the intervention of human teachers.
One community, for example, practices hand-clasp grooming, in which two chimps each grasp one of the other's hands over their heads, grooming with the free hand. Other groups do not engage in this behavior.
Research with animal behavior, and perhaps especially with the great apes, risks wrongly attributing human characteristics to animals. But the researchers in this experiment say they have been careful to avoid that trap.
"We aim to avoid naïve anthropomorphism," said the lead author on the paper, Andrew Whiten of the University of St. Andrews, "by developing a rigorous experimental design that can unambiguously answer the question we pose."
He adds that the results were scored objectively from videotapes viewed by other scientists to avoid bias. Whitten and his colleagues plan to do similar experiments with human children as subjects.
"If we see similar responses in the two species," Whiten said, "then a concern of interpretive anthropomorphism becomes rather contrived."
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