A baby orangutan called Naru laughs with delight as he is tickled by a keeper.

Credit: Miriam Wessels

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SYDNEY: Tickle a baby chimpanzee and it will giggle just like a human infant. This is because laughter evolved millions of years ago in one of our common ancestors, say scientists.

Published today in the journal Current Biology, a new study shows that laughter is not a unique human trait, but a behaviour shared by all great apes.

"Throughout evolution, gradual changes occurred, which accelerated approximately over the past five million years, making the acoustic laugh characteristics of humans quite distinct from those of great apes," said Marina Davila-Ross, an evolutionary psychologist from the University of Portsmouth in England and lead author of the study.

Mistaken identity

Despite this, scientists have long suspected that the physical expressions of human emotions, like laughter, arose in our primate ancestors. If true, this implies that apes have similar experiences of emotion to us.

Accurately identifying emotions in animals is difficult, however, because they express themselves very differently to humans. If you tickle an orangutan, for example, it makes a series of loud panting hoots; it would be easy to mistake these sounds for pain or distress, rather than joy.

Davila-Ross and her team have found the best evidence yet that the grunting sounds are the ape equivalent of laughter, and have also shown that the more closely related a primate is to us, the more human-like its laughter appears.

For the study, they recorded the sounds made when they tickled three human babies and 22 ape babies, including orangutans, gorillas, chimpanzees and bonobos. They then compared and contrasted the sounds, analysing features such as the duration and frequency.

Harmonically rich

Clear similarities and differences emerged. All the babies were able to laugh continuously for at least 3 seconds. Humans, however, produced a harmonically richer, more regular voice-type sound, than the noisy hollering of the apes.

The characteristics of the different laughs were used to create an evolutionary tree showing the relationships between the species. This was then superimposed on top of a tree created using genetic similarities, which included estimated dates for when different species split, based on the rate of mutation of genes.

Using this ingenious method, the researchers estimate that laughter has its roots at least 10 to 16 million years into our past.

"Our main results indicate that great apes can produce laughter, a clearly important positive expression in humans," said Davila-Ross. "This study is not only important for emotion research in humans and animals but also for the management of primates in captivity and in the wild."