Apes Use Leaves as Language, Researcher Says
Researchers studying apes in the wild have found that African bonobos use complex trail markers to silently communicate in the dense tropical forests where they live along the Congo River.
The discovery is contrary to the belief of many scientists that apes lack the brain structure for symbolic language in complex communications, said E. Sue Savage-Rumbaugh of Georgia State University.
“The evidence is there,” she said at the recent national meeting of the American Assn. for the Advancement of Science. “We only have to look at it.”
Bonobos, apes that closely resemble chimpanzees, inhabit dense forest with only faint trails. They live in bands of more than 100 and each night rest together in trees.
During the day, the apes separate into small groups and forage for food, often traveling for miles and moving silently to avoid predators. But at day’s end, members of the band find their way back together at a new resting place.
Savage-Rumbaugh said this behavior shows that the animals must communicate. Just how they do it has been a mystery until now.
She said that in following the animals through the forest, she noticed that whenever a trail crossed another trail, the lead group would stamp down vegetation or rip off large leaves and place them carefully.
“What they are doing is leaving little notes in the vegetation,” she said. “Those notes are signals about where they are going to go.”
Savage-Rumbaugh said the plants were disturbed only at the junctions of trails and it was clear that the lead group was leaving markers for those that followed.
Frequently, said Savage-Rumbaugh, a path intersection would have a single smashed plant, and two smashed plants would mark the selected trail. Sometimes, she said, intersections would be marked by large leaves pointing in the direction of travel.
In muddy areas where footprints were obvious, no plants were disturbed. When a tree trunk crossed the path, there were smashed plants in front and behind. If plants were disturbed only in front of the trunk, the animals then walked on top of the trunk, following it to another trail, she said.
“These cues are not left at arbitrary points, but rather at locations where the trails split or cross and where an individual following might be confused as to the correct direction to take,” she said.
When all the members of the band travel together, the trail markings are absent, said Savage-Rumbaugh.
To prove her discovery, Savage-Rumbaugh said she twice followed the trail signs far behind groups of the apes. At the end of each day, she found her way to the reassembled band’s new nesting trees.
Savage-Rumbaugh said it is impossible to study verbal communications between bonobos in the wild because they only vocalize when they are together in the trees. But captive bonobos, she said, have been easily trained to respond to verbal language and to point to symbols that have specific meanings.
