Rats have been observed giggling when they're tickled - but only when they're in a good mood.

Researchers at Germany's Humboldt University, who tickled young male rats for a new study published in Science, say they have shown “for the first time that laughter can be elicited by stimulation".

Neurobiologist Shimpei Ishiyama told the LA Times that tickling induces a “primitive form of joy” in animals.

Ishiyama and researcher leader Dr Michael Brecht tickled the rats on their back, stomachs and tails and observed the animals' responses, which included high-pitched vocalisations - laughter, essentially - which, although generally inaudible to the human ear, can be heard in slowed-down form in the YouTube video below: