He seems to be onto something. While tickling isn't always pleasant — thus the term "tickle torture" — in multiple experiments Burgdorf has demonstrated the rats' 50 kilohertz chirping is only associated with positive experiences. For example, the rats only made this sound during rough and tumble play when the animals were of similar size. The vocalizations changed when one of the animals involved was much larger than the other, when it was no longer fun and games and instead looked more like bullying. And when given a choice, Burgdorf's rats would push a bar to play a recording of the 50 kilohertz chirp as opposed to other rat noises, suggesting they had a preference for the sound. Finally, when Burgdorf and his colleagues used electrodes, opiates, and other manipulations to stimulate the reward centers of rats' brains, the rats produced that same laughter-like noise.
Whether you call it laughter or not, Burgdorf is convinced these rats' ultrasonic noises signal they're experiencing happiness. Hence the "laughing pill" experiment: He and his colleagues are testing a new antidepressant medication on rats, to see if it makes them "laugh," or chirp happily. If all goes well, Burgdorf believes the resulting medication could eventually be approved for humans. Rats, so often seen as a malicious pest, could end up making the world a happier place.
This series is adapted from "The Humor Code: A Global Search for What Makes Things Funny": http://amzn.to/1h0IJvq. McGraw is a professor at the University of Colorado, Boulder. Warner is a former Westword staff writer.