What's your rat laughing at?

Animal play is nothing new, however. Scientists have long suspected that some animals smile or laugh.

Rats, for example, have been shown to chirp delightedly above the range of human hearing when wrestling with one another.

But be careful not to trip and fall the next time you're taking around your rat. If your rat starts chirping, it might be at your expense!

...could these be the next stand up comedians? :o)