Survival of the funniest.
“However, many scientists believe that humour is far more widespread amongst the animal kingdom than this…. other researchers have noted that dolphins appear to produce sounds of joy while they are play-fighting, and elephants trumpet in excitement when playing. Some parrots have been known to tease other animals for fun, for example by whistling at and confusing the family dog.
There’s even evidence that rats enjoy a good laugh. For the last decade or so, Jeffrey Burgdorf, research associate professor at Northwestern University in the US, has been tickling rats for a living. When the rats are tickled, they squeak joyfully in a high-pitched noise similar to a giggle.”