I've reported on this before, but there's a new article about rats laughing
(including a video). Dr Panksepp has been gathering more evidence
outlining the parallels to human laughter and there's even speculation about rats' sense of humour: He thinks they favour slapstick. Funny. I've always had a weakness for toilet humour myself. And anyone who's kept rats as pets know they delight in being mischievous.