Rats laugh when you tickle them

Humboldt University : Berlin researchers make rats laugh

The similarity is amazing. If a young rat is tickled on the stomach, it kicks its legs in the air, squeaks in inaudible high tones and is obviously very happy. Children react little differently to tickling, only their giggles differ from the ultrasound calls of the rodents. Shimpei Ishiyama and Michael Brecht from the Bernstein Center for Computational Neuroscience at the Humboldt University in Berlin describe in the journal "Science" what happens in the rats' thinking organ.

However, not every animal likes to be tickled. "Restrained rats had to get used to me for a few days before they lost their shyness," says Ishiyama. But then the rat acknowledges the killing with a happy ultrasonic squeak. The animals behave similarly to people who only like to be tickled when they are in good spirits. Incidentally, the father of evolutionary theory, Charles Darwin, had already observed this. And you should be young, bipeds and rodents also agree on this: Tickling is only popular with kids, but rather frowned upon by the elderly.

The "tickling center" discovered in the brain

The sensations during the touch lead nerves from the surface of the body to a region of the brain called the "somatosensory cortex". There the researchers measure an activity when they tickle the rat. If they lead a weak current through a probe into these "tickling regions" - in the order of magnitude of the nerve signals - the rats promptly begin to squeak. But only if they are in a good mood. After this stimulation, the microphones do not pick up any calls of joy from fearful animals. "Obviously we found the tickling center there in the somatosensory cortex," says Ishiyama.

Elke Zimmermann from the University of Veterinary Medicine Hannover found it very interesting that the Berlin scientists can trigger the typical reactions to tickling directly in the brain of rats without touching the animals. The behavioral scientist herself studies how different species of monkeys, shrews and bats communicate with one another. In 2009 she and Marina Davila Ross analyzed the playing behavior of great apes - and also observed “outbursts of amusement” when tickling these relatively close relatives of us humans.

Laughter as an invitation to play

In the case of the great apes, too, the offspring are particularly enthusiastic about playing. Orangutan babies, for example, like to challenge their companions with a "playful face" to frolic. In doing so, they pull back the corners of their mouths and expose part of their otherwise invisible teeth. The whole thing looks like a big laugh and is similarly contagious. In any case, the playmates usually join in with this “laughter”.

This laugh seems to be an invitation to play not only with great apes. The rats in the laboratory also chase after the hand of the scientists in order to be tickled further and thus show typical play behavior. On top of that, the scientists observed similar activities in the somatosensory cortex of the rat brain while playing as when tickling.

Tickling friends is one of the most popular games among young chimpanzees. The “victim” obviously enjoys such an attack and begins to breathe out rhythmically. The resulting noise sounds like giggling to a person's ears.

Family tree of laughter created

Research shows that these sounds sound slightly different depending on the type of monkey. Chimpanzee and bonobo children sound similar to giggling human children. With an orangutan, giggling sounds more like panting and therefore quite strange. That fits with the family relationships: orangutans are much further away from us humans than chimpanzees or gorillas.

“We were able to set up a family tree of this smile whose branching pattern corresponds to the molecular genetic data,” says Zimmermann. It shows that our common ancestors laughed and tickled each other a few million years ago. Even with the gibbons, which are even further away in the family tree, the researchers hear giggling again and again when they are playing.

Giggling monkeys and squeaking rats obviously have the same reason for these apparently happy noises: they encourage play. For example, the more little orangutans show their panting giggles and “smiles”, the longer they play. And while playing, monkey children, just like human offspring, learn a few things that can still be important in adult life. So tickling and laughter seem to increase educational opportunities.

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