Chimps: On the one hand ...

Researchers conclude righties and lefties are found in the wild

Animal Behavior

August 19, 2005|By Frank D. Roylance | Frank D. Roylance,Sun Staff

When chimpanzees in the famed Gombe National Park in Tanzania pick up sticks and poke them into termite mounds in pursuit of a tasty snack, most use their left hands.

In fact, when researchers worked out the numbers, they found that -- at least when they're fishing for termites -- the Gombe chimps are left-handed by a better than 2-to-1 margin.

By combing through prior chimp studies, researchers also found evidence that about twice as many chimps use their right hands to hammer nuts open with rocks and to sop up water with crumpled leaves.

That would be merely an interesting footnote, except the findings cast doubt on a long-held assumption about how humans evolved.

The findings also raise questions about the notion that, as a group, only primates with language -- namely us -- can display right- or left-handedness.

Humans are overwhelmingly right-handed, by a ratio of at least 8-to-1. And, because both human language and right-handedness are controlled by the left side of the brain, experts have long argued that the two traits evolved together.

Scientists believed that handedness in other primates was the result of random, individual influence, with no significant pattern within large groups.

"The argument is that other animals don't have language, and so shouldn't show any handedness. And that was predominantly what the data showed for many years," said William D. Hopkins, co-author of the groundbreaking report that appeared last week in the Proceedings of the National Academy of Science.

The study was conducted by scientists at the Yerkes National Primate Research Center in Atlanta and at Lincoln Park Zoo in Chicago.

The discovery that chimps show clear evidence of handedness "raises questions about that evolutionary assumption," said Hopkins, a Yerkes researcher and psychology professor at Berry College in Mount Berry, Ga.

It also suggests this left-right specialization in the brain probably evolved before the common ancestors of chimps and humans split some 5 million years ago.

Hopkins' study, co-written with Elizabeth V. Lonsdorf, director of field conservation at the Lincoln Park Zoo, was funded in part by the National Science Foundation and the National Institutes of Health.

Great apes studied

Despite some evidence of right-left preferences among chickens, pigeons, frogs and fish, previous studies never seemed to settle the issue of handedness in man's closest relatives.

Researchers who looked at captive chimps and gorillas concluded that they were mostly right-handed. But critics argued that those animals, raised among humans, might have learned their handedness from their mostly right-handed human caretakers.

In the wild, studies of great apes were rare, Hopkins said, and "until this paper, the evidence for population-level handedness for wild chimps didn't exist."

So Lonsdorf and Gombe research assistant Kadahaa John tackled the question. In 1998, they began a four-year effort to document the behavior of 17 Gombe chimps, work that included forays to termite mounds in the reserve.

(Gombe became internationally known because of naturalist Jane Goodall, who studied its chimps for decades.)

Over 63 days during four years of Gombe's rainy seasons -- prime time for "termite-fishing" -- the scientists videotaped the chimps and later noted which hand the chimps used each time they inserted or withdrew a stick from the mound.

The data were converted into a handedness index, to determine whether the chimp was predominantly left-handed or right-handed. As it turned out, 12 were left-handed, four were right-handed and one was "ambiguously handed," that is ambidextrous.

Lonsdorf and Hopkins combined their results with those from several earlier, smaller studies of termite-fishing wild chimps. Among 57 chimps, they found that 29 were left-handed, 15 were right-handed and 10 were ambiguously handed.

Next, they compared their wild chimps with previous studies of captive chimps given sticks to extract yogurt or honey from an artificial termite mound. Once again, they found that the captive chimps were left-handed by a 2-to-1 margin.

In the face of this, it's not clear why chimps use right hands to crack nuts or scoop water -- possibly genetics or learned behavior unique to those populations, Hopkins and Lonsdorf said. They did find that handedness in Gombe's wild chimps ran in families. Offspring tended to share the handedness of their mothers and, even more often, that of their siblings.

The difference might have something to do with the mental skills demanded by different tasks. Termite dipping requires fine eye-hand coordination and muscle control, while nut-cracking does not.

Mysteries remain

Also still a mystery is why humans (Homo sapiens ) and chimps (Pan troglodytes ) demonstrate such vastly different ratios of handedness -- 2-to-1 among chimps and 8- or 9-to-1 among humans.

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