If I were a neurobiologist, I could get paid to tickle rats:
[Bowling Green State University in Ohio psychobiology professor Jaak] Panksepp has studied rats and found that when they "play," they often chirp-a primitive form of laughter, according to the scientist. In an article to be published tomorrow in the journal Science, he makes the argument that animal laughter is the basis for human joy.
In a 2003 study Panksepp and Bowling Green State University neurobiologist Jeff Burgdorf demonstrated that if rats are tickled in a playful way, they readily chirp. Rats that were tickled bonded with the researchers and became rapidly conditioned to seek tickles.
So much for that whole "laughter is uniquely human" thing.