Studies Show Rats Enjoy Tickling

March 31, 2005 — -- We may know what kinds of things make us laugh -- slapstick, a clever pun, an inside joke with an old friend, an April Fools' gag -- but just what are those strange sounds we make when laughing?

Scientists are finding there is a long evolutionary trail to our odd noises of amusement, and the latest proof comes from ticklish rats.

You've probably never heard a rat laugh, and there's a good reason.

Jaak Panksepp, of Bowling Green State University in Ohio, and his students found that the rodents emit gleeful "chirps" when playing, but only at ultrasonic tones five times higher than the human ear can hear. Once Panksepp hooked up an ultrasonic detector to listen in on rats in his lab and started tickling the animals, he realized the effect on them was dramatic.

"We used our hands as if they were playmates and pounced and tickled the rats with our fingers. The chirping sounds were out of sight, just out of sight," said Panksepp, who wrote about the studies in this week's issue of the journal Science. "The animals became bonded to you and came back for more. Every possible measure of whether they like it shows yes, they love it."

Not only did the rats respond instantly to the tickling, after awhile, they reacted the way a child often does before a tickling hand even reaches them.

"After a couple of trials, we could just wave our fingers in front of their noses and they would chirp," said Panksepp.

The rats likely keep their chuckles to supersonic levels to avoid detection by potential predators such as hawks, he explains. Sounds of such short wavelengths won't travel far and can be deflected off something as flimsy as a blade of grass. That means the rodents can play, tickle and chirp without fear.

But what do chirping, ticklish rats have to do with human laughter? The fact that rats have a form of laughter suggests it has been around for a very long time. Scientists have estimated that the common ancestor of rats and humans lived some 75 million years ago.