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Even rats like a good giggle

Turns out rats may have a sense of humor--and not just for knock-knock jokes about the Orkin man.

According to a Thursday report on ABC News' Web site, the rodents get a kick out of being tickled. And while pet rat owners probably don't want to test the theory at home, it may shed light on the evolution of human laughter.

rat photoABC News says researchers at Ohio's Bowling Green State University found that rats emit chirps of glee when tickled, though the sounds are made at ultrasonic tones five times higher than the human ear can hear.

What's more, neuroscientist Jaak Panksepp reports in this week's issue of the journal Science that the critters enjoy the tickling so much that a simple wave of a playful researcher's hand gets them giggling like a bunch of, well, gerbils.

"The animals became so bonded to you and came back for more," he said. "Every possible measure of whether they like it shows yes, they love it."

The fact that the rats have a common form of laughter suggests that the act has been around for a long time, the ABC News report says, possibly reaching back 75 million years ago to the common ancestor of rats and humans.

"Clearly, laughter harks back to much deeper emotional recesses of our animalian past," Panksepp said.

Human laughter, like crying, has long been a source of fascination. Some scientists posit that it originated as a shared expression of relief at the passing of danger and also as a critical method of flirtation and human bonding.