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.
"Clearly, laughter harks back to much deeper emotional recesses of our animalian past," Panksepp wrote.
Rats aren't the only animal in which researchers have detected a form of laughing. Studies of chimpanzees and dogs have shown both animals emit a form of heavy panting when at play that scientists have likened to human laughter.
Robert Provine, a neuroscientist at the University of Maryland and author of "Laughter: A Scientific Investigation," has linked the panting of chimps at play to the standard "ha-ha" sound of human laughter. The differences in the sounds, he says, shed light on the differences between human and chimp speaking abilities.
"By contrasting chimp and human laughter we can understand why we can talk and they can't," he said. "The key is breath control. Chimps can't chop an outward breath to make complicated sounds, even a sound like 'ha-ha.' They can only make one sound with each breath. This is how a simple act like laughter can be a tool to understand a more complex thing like speech."
Of course, human laughter has evolved to include many more varieties than the basic kind that people let loose when tickled. Jo-Anne Bachorowski, a psychologist at Vanderbilt University in Nashville, Tenn., has studied laughter in men and women and found the two sexes vary the acoustics of their laughter widely, depending on the circumstance.