Dr Marina Davila Ross, from the University of Portsmouth and Professor Elke Zimmermann at the University of Veterinary Medicine in Hanover, Germany, studied the play behaviour of 25 orang-utans aged between two and 12 at four primate centres around the world.

BBC: RELATED BBC SITES

"When humans laugh, they voice stable sounds: that means the vocal folds are moving in a very regular synchronised way, " Dr Davila-Ross explained.

BBC: Tickled apes yield laughter clue

To put the idea on a firmer footing, Dr Davila-Ross made more than 800 recordings of the tickle-induced laughter of the apes and infants.

BBC: Tickled apes yield laughter clue