What's the science behind being ticklish

When chimps are tickled, they laugh like Ernie from Sesame Street. Gorillas respond with chirping that sounds almost like the chirping of birds. Bonobos cheeky and orangutans grunt. Charles Darwin, the founder of the theory of evolution, has already observed that not only humans are ticklish, but also animals. But to this day, the strange behavior of tickle victims - regardless of whether they are humans or animals - is largely inexplicable to scientists.

What is the point of the uncontrolled laughter in which all the body muscles wobble and lose their tension? Why is tickling beautiful and terrible at the same time? Strangely enough, people who are tickled make violent defensive movements while they laugh and often have a pained expression on their faces. And why does tickling only work when everyone is in a good mood? "The mind must be in a pleasurable condition", wrote Darwin in his book published in 1872 "The expression of the emotions in humans and animals".

Rats laugh in the ultrasound range

This does not only apply to humans, but also to rats, as two biologists from the Humboldt University in Berlin recently discovered. Your experiments published in the journal Science have been published show that animals only laugh when they are relaxed and not afraid. At the beginning, Michael Brecht and Shimpei Ishiyama repeated an experiment by the American psychologist Jaak Panksepp, who was the first to discover that rats almost laugh themselves to death when they are tickled - but in an ultrasound range of 50 kilohertz that is inaudible to humans.

Brecht and Ishiyama tickled their laboratory rats on various parts of their bodies and recorded the animals' ultrasound calls. "Most of the rats had to laugh when they were tickled on the stomach," says Michael Brecht. The animals were also ticklish on the back. But not at all on the tail. The rats liked being tickled so much that they kept jumping for joy, accompanied by loud 50-kilohertz laughter. As soon as Ishiyama stopped, the animals chased after his hand. It looked like an invitation to carry on.

The rats, on the other hand, found the tickling game not at all funny when they - illuminated by two powerful lamps - sat on a platform almost 30 centimeters high: an extremely unpleasant situation for rats that scares them. Ishiyama could tickle as much as he wanted - on the stomach, on the back - the rats made no sound. "Apparently rats, just like humans, have to be in a good mood for the tickling to work," says Brecht.

The scientist has discovered other parallels. Both rats and humans are particularly ticklish on the stomach. And as with humans, there are also individuals with animals who spurt out at the slightest touch, while others can hardly make a sound despite extensive tickling efforts. "We stopped working with some animals because they just weren't ticklish," says Brecht. Basically, both rat and human children are more ticklish than adults. The many parallels are an indication that ticklish arose early in evolution, "says Michael Brecht.