October 24, 2005Link
Humans may be the great communicators of the natural world, but we're hardly the only ones. Plenty of animals trade signals with one another--calls to love, calls to war. Flowers even woo bees with scents and colors, while bacteria can decide when enough of them have gathered in our guts to start making us sick. Looking at these simpler communication systems is offering scientists some clues to how our own gifts of language evolved. And so it should be no surprise that they've focused much of their efforts on our closest living relatives, the chimpanzees. Primatologists have spent years observing them, either in African forests or at zoos. And they've seen and heard a lot. Chimpanzees hoot, blow Bronx cheers, wave sticks, clap their hands and drum on trees. The tough part comes in figuring out what all the ruckus means.
For some time now, primatologists have paid more attention to the hands of chimps than their mouths. The sounds made by chimpanzees seemed to be little more than emotional outbursts without much meaning or intention. But scientists were struck by the rich vocabulary of gestures chimpanzees use. Particularly provocative was the fact that chimpanzees used certain gestures only in certain situations--trying to get another chimp's attention, for example, or playing, or picking a fight. The fact that the gestures don't come randomly suggests they carry some meaning.
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This research helped give rise to a theory that human language has its roots not in speech, but in hand gestures. Scientists who favor this theory point to the fact that monkeys have special neurons that track the motion of hands. These so-called "mirror neurons" might have given apes the mental power necessary to recognize subtle differences in hand gestures. This communication system might have grown more complex after our own ancestors split from other apes 6 million years ago. Only later, this argument goes, did the sophisticated brain circuitry for reading gestures get rewired in humans to process speech.
But don't write off those grunts and hoots just yet, at least according to a new study that appears in the Oct. 15 issue of the journal Current Biology. Katie Slocombe and Kaus Zuberbuhler, two primatologists at the University of St. Andrews in Scotland, investigated a particular noise chimpanzees make when they find food, called a "rough grunt." At the Edinburgh Zoo, the scientists fed the chimpanzees two different foods--apples and bread--and recorded the sounds they made. Chimpanzees prefer bread to apples, and Slocombe and Zuberbuhler discovered a corresponding difference in the rough grunts they made for each food. They hit a distinctively high note when they came across the bread, and but made lower and noisier grunts for apples.
Slocombe and Zuberbuhler then ran an experiment to see if the chimps could tell the difference too. They nailed a pair of hollow tubes on the walls of the chimpanzee enclosure and loaded them with four containers. Each day, they'd open the tubes with the tug of a string, letting the containers tumble into a concrete gully. The chimpanzees would then be allowed to come out of their indoor room. Invariably at the front of the line was an eager 5-year-old male named LB. LB would race outside and climb down a ladder into the gully, followed by his fellow chimpanzees. The chimpanzee would then discover that only one of the eight containers contained food--either apples or slices of bread. What's more, one tube only delivered apples, the other only bread.
After giving the chimpanzees six weeks to learn this pattern, Slocombe and Zuberbuhler added a twist to the experiment. Now they played recorded rough grunts when they dropped containers into the gully. On some days, they played the apple grunts, on others the bread. They then filmed LB to see if the sounds made a difference to how he behaved.
It turned out that they did. First, LB would stop for a few seconds on the ladder to listen to the grunts. If he heard the bread grunt, he would spend more time looking at the containers that fell from the bread tube. He would spend more time under the apple tube when he heard an apple grunt. Slocombe and Zuberbuhler's study is the first experiment to show that vocal sound can carry meaning for chimpanzees.
That's not to say that a high-pitched rough grunt means, "Hey, everyone, check out the bread!" It could just be a spontaneous outburst. But LB was tuned in to rough grunts well enough to figure out which food to look for. Slocombe and Zuberbuhler argue that their results call into question the need to look to gestures for the origins of language. Perhaps language got its start with rough grunts, rather than hand-waving.
Even if Slocombe and Zuberbuhler are correct, mirror neurons may still have played an important part in the evolution of communication. They might have provided some of the mental firepower necessary to give crude sounds meaning. For one thing, mirror neurons are good for representing actions--which, in an abstract way, is what sentences do. And it turns out that some of the regions where monkeys have mirror neurons correspond to areas of the human brain that handle language. What's more, mirror neurons have also been linked to our ability to get inside other people's heads and understand their intentions--something that chimpanzees can barely do, if at all. It's one thing to figure out why someone else is making a rough grunt. But when you know that other people are trying to figure out what your rough grunt means, you've got a conversation.
Copyright 2005 Carl Zimmer