Rats are ticklish. So are other animals, and of course, humans. Researchers who tickled the rodents in the name of science discovered that getting a laugh from a good tickling is quite universal, the Los Angeles Times reports.
When the researchers put on gloves and tickled lab rats for 10 seconds, the animals responded with their version of laughter. In fact, they seemed to enjoy it so much that they chased the gloved hands around and ran towards it. The rats even exhibited a behavior called “Freudensprünge,” a kind of joyful jumping, that other mammals have been known to display.
Shimpei Ishiyama, a neurobiologist at Humboldt University in Berlin says this means tickling induces a “primitive form of joy” in animals. Ishiyama was joined by Michael Brecht in the experiments.
In their study, the researchers wanted to answer the questions of why tickling makes people laugh, and why body parts differ in levels of ticklishness.
To find out, they tickled rats in different manners. They tried tickling the animals on the back and on the stomach, then gently touched them on the same areas. They also tried tickling the rats on their tails, and played the hand-chasing game.
Ishiyama and Brecht recorded the rats’ responses, noting that they all reacted with ultrasonic sounds in a 50 kilohertz range. This translates to a “positive emotional valence,” they reported. The frequencies were too high for humans to hear, so the researchers translated them to lower frequencies.
In addition, the rats also performed Freudensprünge jumps – kind of like bunny hops – that reflected their happy moods.
Upon further examination with electrodes, the scientists found that tickling triggered a specific activity pattern in the somatosensory cortex, or the part of the rat brain that processes the sense of touch. They also discovered that the hand-chasing game activated the same part of the brain.
To determine if the rats would laugh no matter the condition, or only when they felt receptive, the scientists made the rats anxious. They placed them on an elevated platform and pointed harsh, bright lights at them. This “significantly suppressed” the rats’ vocalizations, and brain activity decreased compared to normal conditions, the study reports.
The scientists say that more work is needed to test whether the somatosensory cortex can help process emotions as well as touch, and that tickling just may serve a useful social purpose in the future.
“Tickling might be a trick of the brain to make animals or humans, respectively, interact and play with each other,” Brecht says.
The researchers also tried tickling the rats without touching them, by stimulating the somatosensory cortex. Remarkably, the rats laughed.
The study was published in the journal Science.
