The basis for laughter may have originated in an ancient primate ancestral to both humans and modern apes, a study suggests.
Scientists found that orang-utans had a sense of empathy and mimicry which forms an essential part of laughter.
Facial expressions, such as the open, gaping mouth resembling laughter, were picked up and copied by orang-utans.
The speed with which they were mimicked suggests these expressions were involuntary, Biology Letters reports.
In other words, the "laughter" was contagious.
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.
When one of the orang-utans displayed an open, gaping mouth, its playmate would often display the same expression less than half a second later.
Dr Davila Ross commented: "In humans, mimicking behaviour can be voluntary and involuntary. Until our discovery there had been no evidence that animals had similar responses.
"What is clear now is the building blocks of positive emotional contagion and empathy that refer to rapid involuntary facial mimicry in humans evolved prior to humankind."
She added that the findings shed a new light on empathy and its importance for animals which live in groups such as orang-utans.