Hamburg - We humans are not the only creatures who giggle, according to a team of German scientists who spent 10 years tickling baby chimpanzees to see what would happen.

What happened was that the baby chimps grinned and giggled exactly the same way that baby humans grin and giggle when grown-ups say tickle them and say "cootchie-coo".

Every zookeeper and circus animal trainer has always known that chimps smile and laugh. But this is the first time that lab coat-clad researchers have documented the phenomenon as scientific fact.

Far from being a waste of research money, the German researchers at the School of Veterinary Medicine in Hannover say that their discovery demonstrates clear evidence that laughter is a common evolutionary behaviour designed to bond grown-ups to newborns.

In other words, grown-ups (from parents and grandparents to doting aunts) simply can't resist the infectious giggle of a newborn baby - and neither can grown-ups amongst our primate cousins.

"The acoustic and phylogenetic results provide clear evidence of a common evolutionary origin for tickling-induced laughter in humans and tickling-induced vocalizations in great apes," the researchers wrote in a report published in the online edition of Current Biology.

The tests were conducted not only on baby chimps, but also on infant gorillas, orangutans and bonobos - as well as baby humans.

"We were surprised that tickling elicited giggles even from gorillas and orangutans," says Dr Elke Zimmermann of the Hannover veterinary school. The professor of zoology, who initiated the international study, has been collecting data for more than 10 years.

In all, the scientists pored over 800 ton of videotape documentation involving zoos as well as an orangutan centre in Malaysia where animal care personnel tickled 22 infant and juvenile primates on their hands, feet, neck and underarms.

Three human babies also underwent tickling experiments, albeit under the fingers of their own parents, not at the hands of scientists.

"There were not artificial situations," stresses Zimmermann. "Giggling clearly is part of the play repertory amongst primates."

The findings support the theory that laughter is not a strictly human phenomenon, but rather that it evolved from our primate family tree, according to Dr Marina Davila Ross, a Hannover alumnus currently at the University of Portsmouth, where she collaborated with the Hannover experiment project.

"It supports the idea that there is laughter in apes," Davila Ross writes in the report.

The findings above all mean that nobody will laugh at zookeepers and circus animal trainers who have always claimed that chimps giggle.

"At a minimum, one can conclude that it is appropriate to consider 'laughter' to be a cross-species phenomenon, and that it is therefore not anthropomorphic to use this term for tickling-induced vocalizations produced by the great apes," the researchers conclude. (dpa)