"Although no one has investigated the possibility of rat humor, if it exists, it is likely to be heavily laced with slapstick. Even if adult rodents have no well-developed cognitive sense of humor, young rats have a marvelous sense of fun. We have already bred rats that exhibit playful chirping, and thereby hope to track down some of the genes for joy. Perhaps we will even stumble on new molecules to alleviate depression as well as some excessive-exuberance disorders."
For more info & a video, check out this link: What happens when you tickle a rat?