Scientists listened to animals playing and they noticed something that appeared to them to be laughter. They studied this behavior for couple of years before actually realizing that it is laughter. Soon they got a device which transformed higher frequency sounds to frequencies our auditory system can hear. So they developed a way to ask these rats if they liked being tickled and the answer was definite yes!Rats Laugh When You Tickle Them - Free Science Videos and Lectures
Doesn't surprise me at all, they are damn funny animals. People keep them as pets because they can have full blown relationships with them. duh?