The Laughter No Human Ear Can Catch
Tickle a rat and it giggles — at 50 kilohertz, far above human hearing. Electrodes found the laughter in the brain's touch center, and fear switches it off. Sound familiar?
The Giggle at 50 Kilohertz
Tickle a young rat on the nape or the belly and it erupts — not in silence, but in rapid ultrasonic chirps around 50 kilohertz, far beyond the ceiling of human hearing. To our ears, nothing is happening. Through an ultrasonic microphone, the room is full of giggling.
The psychobiologist Jaak Panksepp spent years documenting these calls, and the pattern looks uncomfortably familiar: the chirps pour out during play, during tickling, even in anticipation of tickling.
They Come Back for More
If the chirping were mere reflex, rats would not care who caused it. They do care. Tickled rats chase the experimenter’s hand around the cage, soliciting another round, and they learn to seek out the specific hands that made them laugh before. In Panksepp’s work, the rats that chirped most readily in human hands were also the most naturally playful individuals.
In other words: the sound tracks fun, bonds the rat to its tickler, and varies by personality. That is a lot of laughter-like behavior for a “reflex.”
Finding the Laughter in the Brain
In 2016, Shimpei Ishiyama and Michael Brecht at the Humboldt University of Berlin took the next step, publishing in Science. They implanted electrodes in the rats’ somatosensory cortex — the brain’s touch-processing center — and tickled.
The neurons there fired vigorously during tickling and during the ultrasonic giggles. The “laughter,” it turns out, has a traceable address in the brain, sitting right in the machinery that processes touch.
The Mood Switch
The most human detail is what turns the laughter off. Put the rat under bright light, or let it catch the scent of a cat, and the chirping collapses. Anxious rats do not laugh, no matter how expertly they are tickled.
Anyone who has tried to tickle a person who is stressed, angry, or simply not in the mood knows exactly how that works. Tickling only lands on a playful brain — in rats and in us. That parallel is part of why researchers treat rat tickling as a serious window into how joy is built in mammalian brains, including ours.
A World Full of Laughter We Cannot Hear
None of this required exotic animals or distant jungles. It was discovered in the most ordinary lab animal on Earth, making a sound it has presumably been making all along.
Rats laugh all around us — at a pitch we never catch.
