Tickling rats and giggling dolphins: do animals have a sense of humour?

Laughter has many functions, and whilst language is a key part of human humour, psychologists think certain animals show signs of being in on the joke

There's a video on YouTube (above) with over three and half million views, in which a girl appears to make a dolphin giggle by doing repeated cartwheels and handstands in front of its tank at a sea world centre.

We still understand relatively little about the extent to which emotions are present in animals, but could it be that the dolphin in this clip is experiencing one of the most distinctly human forms of expression - humour?