June 17, 1999
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Chimpanzee Behaviors Surprise Scientists

By William J. Cromie

Gazette Staff

Chimpanzees show behaviors thought to be much like those of our early human ancestors. Photo by Richard Wrangham

The more humans study chimpanzees, the more similarities they find between the behaviors of apes and people.

The seven longest, largest studies of chimpanzees in the wild reveal far more variations in their behavior than expected. Researchers believe these variations are transmitted not through genes, but culturally, the way human infants learn from watching adults. In fact, such behaviors may be similar to those of our distant human ancestors.

The cultural differences from one chimpanzee group to another "presumably show us the platform of behavioral skill from which human culture evolved," says Richard Wrangham, a professor of anthropology at Harvard University. "The behavior of our distant ancestors probably varied in much the same way as chimpanzee behavior does today."

Wrangham studied chimpanzees for many years in East Africa. He combined his observations with researchers from various countries who have participated in the longest studies of chimpanzees all across Africa, including Jane Goodall who pioneered chimp-watching in Gombe, Tanzania. Together, these researchers accumulated 151 years of observations involving courtship, grooming, obtaining food, and even tickling each other (the chimpanzees, that is).

"This comprehensive analysis reveals patterns of [behavioral] variation far more extensive than have previously been documented for any animal species except humans," nine of the researchers report in the June 17 issue of Nature. "Moreover," the report continues, "the combined repertoire of these behavior patterns is itself highly distinctive, a phenomenon characteristic of human cultures, but hitherto unrecognized among nonhuman species." Andrew Whiten of the University of St. Andrews in Scotland is the lead author of the report.

A Show Of Inventiveness

Birds in different geographical areas sing songs in different dialects. Macaque monkeys in Japan wash sweet potatoes before eating them. Bottlenose dolphins in Australia protect their noses with natural sponges when searching for food in the crevices of coral reefs. These are variations in only a single behavior, but the scientific chimp-watchers counted 65 different categories of behavior. These "represent a unique record of the inventiveness of wild chimpanzees," they comment.

Some of the activities are common to all communities, such as inviting others to play by holding a stem in the mouth. Other behaviors depend strictly on local conditions, including fishing with a stick for edible algae that's only present in specific locations. Eliminating these two types of activities brings the count to 38 different behaviors the researchers catalog as cultural variants.

Chimpanzees in certain parts of West Africa crack open nuts with a piece of wood. Others use a stone, or place the nuts on a wood or stone anvil first. Chimps in other areas don't use any such tools, although there's no lack of wood, stones, or the same kinds of nuts.

On the west side of the Sassandra River in the Ivory Coast, nut-cracking is popular. No chimpanzees do it on the east side of the river, although the two groups are closely related genetically. Researchers use such criteria to rule out the possibility that nut-cracking behavior is inheritable

The apes also show wide variations in the way they fish ants or termites out of holes with sticks or leaf ribs. Different groups employ leaves differently, too as napkins or wash cloths, dabbing them on wounds, or squashing ticks and other parasites with them.

These arbitrary variations compare to differences in human behavior, such as the way people prepare barbecue sauce, or eat hard-boiled eggs from a cup instead of a bowl. Most New Englanders love fried clams; many people in Texas wouldn't think of using clams for anything but bait.

To attract attention, humans shout or wave; chimpanzees may slap a branch, bend a sapling then release it, or pull leaves along a stem with their fingers.

In Uganda, Wrangham watched chimps run their fingers over leaves and touch them to their lips. "That signal often leads to grooming," he notes. "A male without much social status may use it to let more dominant males know that he would like to groom. The signal avoids social blunders that might draw the wrath of others in their group." (That would be a useful type of behavior for humans at cocktail parties.)

 
Richard Wrangham has spent many years studying chimpanzees in Uganda. Photo by Karl Ammann

Teaching, Learning, Or Imitating

How do chimpanzees learn these behaviors? Some scientists suggest that they are taught by adults. Wrangham and others doubt it. "At present, there's vanishingly little evidence of teaching," he says.

Other observers believe that young apes have their attention drawn to a behavior, then they learn it for themselves. The youngster sees an adult extract termites from a mound with a thin stick. That gives it the idea that there's food inside the mound, but the young ape has to work out the details for itself. For example, it might choose a leaf stem rather than a stick to take out termites.

Supporting evidence for such behavior comes from captive apes. In one example, young chimpanzees watch an adult who has been taught to drag out-of-reach food up to his cage with a rake. The trained chimp immediately positions the rake with the flat side down. The watchers get the idea quickly, but have to learn for themselves that the flat side does a better job than dragging the rake tines-down.

Wrangham and others favor the simplest explanation, that chimpanzees learn by imitation. He cites strong evidence from other experiments and observations. An old zoo chimp with a limp lurched from side to side as she walked. Three juveniles who followed her around all adopted the same swaying gait.

"We're getting increasingly comfortable with the idea that behavioral variations come from copying others in the group," Wrangham says. Other authors of the report agree that imitation is the simplest explanation. However, they write, "this is not to suggest that imitation is the only mechanism at work."

Wrangham believes that chimpanzee behaviors, like human traditions, are constantly being invented and going extinct.

"Over a millions years or more, you can't have new behaviors being invented and acquired without some processes for losing them," he comments. Extinctions would explain why the same behaviors have not spread to all the chimpanzee populations in Africa. Otherwise, groups who come into contact with each other would exchange behaviors, giving them a more uniform distribution.

There are still lots of questions to answer about chimpanzee behavior, but this new knowledge reveals a great deal about the animals who are closest to us in an evolutionary sense

"Forty years ago, we knew very little of the behavior of our closest sister-species," notes the Nature report. "The results of the scientific collaboration presented here show a richness undreamt of at the time."

"Every chimpanzee population studied so far has a unique culture," Wrangham adds. "However, these apes are fast going extinct, often because they are being hunted for food. It will be a great loss if they became extinct before we can fully describe the rich details of their behavior."

 


Copyright 1999 President and Fellows of Harvard College