Chimp (黑猩猩) Show Hallmark of Human Culture, Study Finds
Researchers have discovered that chimpanzees not only teach each other new and useful behaviors, but conform to their group’s preferred techniques for performing them--a hallmark of human culture.
Observers have previously reported that wild chimps demonstrate more than three dozen different behaviors that have no apparent ecological or genetic origin. This diversity suggests that there are distinct ape cultures.
The notion assumes that chimps transmit culture--teaching and learning behaviors generation after generation. But the theory is very difficult to test and prove in a controlled experiment outside of a laboratory.
So researchers at the University of St. Andrews in Scotland and Emory University in Atlanta devised an experiment to test the proposition. The results were published online August 21 in the science journal Nature.
Learning and Teaching
The scientists constructed a box in which a desirable food was hidden behind a trap. Captive chimps could release the food by using a stick to move the trap in either of two ways. Researchers dubbed these the "poke" and "lift" methods.
Scientists then isolated a high-ranking female of one group from her companions and taught her the poke method to release food. A female of high rank from a second group was taught the lift method.
None of the other members of the groups were allowed to watch the training.
Finally, researchers used a third group as a control, presenting them with the box and sticks, but teaching them nothing about how to use them.
Scientists then let the chimp groups watch their matriarch (女家长) use the technique she had learned. To get the food, each dominant female consistently used the method she had been taught. The other chimps watched, often intensely, for over 36 hours spread over ten days.
During this period, 15 chimps in the two study groups successfully used one method or the other to get food, and they picked up the behavior quickly. Median times for learning the techniques in both groups were under a minute.
In the meantime, the six chimps in the control group were stymied. In more than four hours of manipulating the sticks, they were unable to extract a single piece of food.
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 explanations--individual 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 ( 变形) 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."
Personifying Animals
Groups of chimps at the Yerkes National Primate Research Center Field Station at Emery 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 (梳理毛发), 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 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.\
Some researchers have discovered that chimpanzees have a hallmark of human culture.