For instance, why can't we tickle ourselves? Why do some people laugh when they're being tickled even though they call it torture? And is tickling a uniquely human trait?

The answer to the last question is no. "when you tickle the great apes they do their analog of laughter," said Provine, who in his research quest, has tickled our close relatives the chimps. "It's a panting sound."

Even baby rats squeal with pleasure when tickled, according to research presented this month at an annual conference of neuroscientists. "They're just like kids on a playground. They're shrieking and laughing," said Jaak Panksepp, a professor of psychology at Bowling Green State University in Ohio who conducted the studies and researches how the brain processes emotions, including joy.

Rat "laughter" wouldn't sound like laughter to you or me. In fact it wouldn't sound like anything at all, because most rat calls are at frequencies too high for human ears to detect.

Using a bat detector to listen in, Panksepp interprets the sounds as joy because they differ markedly from distress calls. The sounds rats make when being tickled by human hands or by the playful nips of litter mates are the same sounds a male makes while courting females.

Panksepp is trying to determine the brain chemicals that play a role in the pleasure and joy he sees in his rats. "If we found the chemistry of laughter, we might have new ideas on how to treat depression and sadness," he said.

Because tickling behavior appears to be universal among mammals, it may lie at the root of the complex social behavior of our species. Tickling is clearly social, said Provine, because it usually occurs only between people who know each other well.

"If a stranger did it, it would be a horrifying thing," Provine said. "It would be repellent." Tickling also appears to be social because we can't tickle ourselves.

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