"One of the biggest mistakes people still make is assuming that laughter is uniquely human; it's not," says Robert Provine, a neuroscientist at the University of Maryland in Baltimore who's studying laughter to gain insight into the biology of social behavior. ...If you tickle a rat, it laughs; we just can't hear it."from "I laugh, therefore I am" by Blaine GretemanOde Magazine, August 2009
If this doesn't make you laugh, I don't know what will. This video explains more of the process of how they record the laughter. The rats really like it and develop a relationship with their ticklers.
Laughter really is the glue that holds relationships together.