But other researchers disagree, pointing out that a few apes can use, at least to some extent, symbolic communications systems -- languages -- like American Sign Language. E. Sue Savage-Rumbaugh, a Georgia State University biology professor, says the accepted wisdom reflects a long bias and that modern studies are refuting it.

Savage-Rumbaugh studies bonobos -- a relative of ours that, like chimpanzees, shares 98 to 99 percent of human genes. When you spend all day with bonobos, she says, "the differences don't loom very large... They look like us, care like us, smell like us, think like us. They are like us."

Speaking at the recent American Association for the Advancement of Science meeting in Philadelphia, Savage-Rumbaugh observed that since apes don't have a vocal tract, they can't make human language. Previous researchers have tried to overcome that liability by teaching apes sign language. Savage-Rumbaugh uses a "keyboard" consisting of 400 symbols, and what she finds is controversial. "If you talk to apes and point to little symbols, they learn to understand language just as I'm talking to you."

Instead of using behaviorism -- rewarding the apes with food each time they use a word correctly -- she allows the animals to pick up words in "normal" conversation. This seems to work. "Watching Kanzi [an experimental bonobo] in casual 'conversation,' one is struck by the intense give and take," wrote journalist Stephen Hart (see "The Language of Animals" in the bibliography). Furthermore, the researchers found Kanzi's understanding of new sentences to be about equal to that of a two-and-one-half-year-old child, Hart found.

She suspects that bonobos are using language in the wild, but since they congregate in trees in groups of about 100, "it's almost impossible to study them." And on the ground, they are silent to avoid predators.

Please step on the daisies
Savage-Rumbaugh also contends that wild bonobos -- only an estimated 4,000 to 40,000 survive in Congo, formerly Zaire -- have a second communication system. This one resembles road signs built of smashed plants rather than steel.

The finding grew from the observation that troops of bonobos hang out in various locations during the day. When bonobos go foraging on the ground, the small groups must maintain "radio silence" to evade predators. Savage-Rumbaugh began wondering how one group manages to follow another to the next hangout.

In 1995, Savage-Rumbaugh spent two months studying bonobos at a research station operated by Takayoshi Kano, a Japanese primate researcher in the Congo forest. During two days of following troops with local bonobo trackers, she observed that their trails were clearly marked by smashed plants and branches planted at an angle to the direction of travel.

Although skeptics could counter that she was just seeing trampled plants, she contends they actually were road signs since they occurred only at trail intersections. "These clues are not left at arbitrary points in the vegetation but rather at locations where trails split and where an individual following might be confused as to the correct direction to take."

Thus in swamps, where plenty of footprints mark the trail, the road signs are not needed and not seen. While the finding has not been replicated in other primates, Savage-Rumbaugh suspects that it does represent the kind of symbolic communication system humans rely on. "This is the first time that anyone has tried to say that this altered vegetation is communicating anything."