Chimps warn their unaware friends of dangers

— -- Unseen by anyone else on a hike, a snake crosses your path. What would you do?

Chimps, our closest animal cousins, emit soft hoots to warn their unaware friends about the unseen danger. But field biologists report for the first time on Thursday that they don't bother warning other chimpanzees already alert to the dangers, a kind of cleverness once seen as unique to people.

The finding adds to recent scientific suggestions that the evolutionary roots of language, and altruism, may reside in awareness of others' thinking that goes back more than 6 million years, to the common ancestor of humans and chimps.

"Chimpanzees really seem to take anothers' knowledge state into account," says study lead author Catherine Crockford of Scotland's University of St. Andrews, in a statement on the study released by the Current Biology journal. "They are less likely to inform those who already know about the danger."

Wild chimps typically voice quiet "alert hoos" when sighting snakes, snares or signs of leopards and other predators. In two years of experiments in Uganda, Crockford and colleagues monitored 33 separate chimps' reactions to fake gaboon vipers and rhinoceros vipers placed in their paths, to see how they used the sounds.

The team tracked the chimps through thick rain forest undergrowth to record their calls. Overall, the 33 study chimps almost never voiced an alert to chimps that had also seen the viper models. But they were roughly twice as likely, a statistically significant effect, to repeatedly warn other chimps ignorant of the danger, compared to ones that had heard previous alerts.

"The finding itself is not completely surprising, but it is the first time it has been documented. And obviously it gets into some pretty deep questions about the evolution of language," says Jon Cohen, author of Almost Chimpanzee: Redrawing the Lines That Separate Us from Them. "These are the most sophisticated studies ever done on calls in wild chimps."

Over the past four decades, animal researchers have varied widely in their estimations of chimp language. In the 1970s, there was optimism that they could communicate as well as people (one study even raised a chimp infant named Nim Chimpsky as a human child). Later, researchers were more dubious that chimps possess a "theory of mind" able to take into account what other chimps might be thinking, a skill quickly picked up by human children.

The new study swings the pendulum back toward views that chimps do partly understand each others' intentions, a kind of awareness believed essential to language skills.

"We need to bear in mind they aren't human; they are separated from us by millions of years of evolution," Cohen says. "These aren't words they are speaking to each other. It's not that sophisticated."