Admit it: You love being tickled. There’s something about that “pleasurable agony,” the strange combination of discomfort and pleasure that elicits such explosive fits of shrieks and laughter. And it turns out you aren’t the only one: Rats, too, break down in supersonic “giggles” and “joy jumps” when you gently ruffle their fur—but only if they’re in the mood.
For a new study published today in the journal Science, a group of German scientists had the pleasure of tickling some rats to find out that—like humans—these rodents’ responses to tickles are mood-dependent. Stressful situations stifled the rats’ otherwise impulsive laughter, while a more relaxed atmosphere made for uninhibited giggles. The new research, led by animal physiologist Shimpei Ishiyama at Humboldt University in Berlin, offers a new insight into where exactly in the brain this ticklish laughter appears to come from.
There’s a reason you probably haven’t heard a rat giggle yourself. Tickled rats emit high-pitched chirping and squeaking sounds, which are only audible through a special microphone. Researchers were able to observe this laughter by using the microphones, as well as by measuring behavior and neuron activity of rats that they tickled and gently touched in various regions of the body, including the back and belly.
Their conclusion: rats are most ticklish on the belly. “It’s the weirdest job ever, tickling rats professionally,” Ishiyama says, laughing. “I tickle rats for a living.”
Previous studies have linked rats’ high-frequency chirps to rewarding situations. Research has also shown that alarm calls and other negative vocalizations occur at different frequencies, suggesting that the chirps represent positive emotions. In fact, the giddy rats in the new study often sought out more tickles by playfully chasing the scientists’ hands around the test arena, Ishiyama says.
“They were so excited,” says Ishiyama. “They were jumping around and they chased my hand. Pretty much like human kids, giggling and chasing around, playing rough and tumble.”
But the experiment wasn’t all fun and games for the rats. The researchers also tickled the animals while they were in an anxiety-provoking situation: on a platform about 10 inches, surrounded by bright lights (which are especially stressful to these nocturnal animals). Under stress, their giggles subsided.
This idea that ticklish laughter weakens under stress is not new. Charles Darwin himself observed in his 1872 book The Expressions of the Emotions in Man and Animals that “the mind must be in a pleasurable condition” for tickles to induce laughter. Even Aristotle considered stoic questions of tickles, many of which remain unanswered today. But for a large part, the role of tickling remains a neurological mystery.
To better understand the role of mood in tickling and laughter, researchers inserted a very thin wire into the brains of rats to measure neuron activity during the tickling. Surprisingly, they found that the most action occurred in a part of the brain called the somatosensory cortex—a region typically associated with direct touch—and that this region exhibited less activity during tickles administered under stressful conditions. These findings suggest that this brain region in rats—and potentially humans—may be more involved in mood than previously thought.
“Traditionally the somatosensory cortex is known to represent just the tactile information on the body surface,” says Ishiyama. “Mood is thought to be handled somewhere else in the brain. But the somatosensory cortex is actually doing more.”
To determine whether this area of the brain required direct touch to become stimulated, the researchers sent a current down the metal wires into the brains of the rats. The rats reacted in a similar way as they did when they were actually touched: They giggled, and neurons fired. “They don’t hear anything, they don’t feel anything, they don’t see anything,” says Ishiyama. “But they do vocalize in response to the stimulation. This was very surprising.”
The rats giggled even when they were chased but not touched by the scientists’ hands—similar to when a child chases a sibling without touching, sending their sibling into a fit of anticipatory laughter. This increased neuron firing during periods of anticipation strengthens the idea that this region of the brain may be more involved in emotion than previously thought, says Carolyn McGettigan, a psychology researcher at Royal Holloway University of London who studies emotional vocalizations in humans.
“You get increased firing in a region where the animal is not being stimulated physically, they are anticipating the stimulation,” says McGettigan, who was not involved in the study. “That’s really intriguing in terms of trying to link this as a behavior that is mood dependent.”
Primates have been scientifically shown to be ticklish, as demonstrated by one study which reported tickle-induced laughter across species of orangutans, gorillas, bonobos, and chimpanzees. But anecdotal evidence suggests that other animals, such as porcupines and platypuses, are also susceptible to the tickles, says Ishiyama. Sharks and trout react as well—but instead of showing signs of joy, they become paralyzed and float belly up, he adds. Still other animals such as mice show no outward signs of ticklishness.
The reason some animals seem to enjoy tickles but others don’t likely has something to do with the playfulness and social nature of the animal, says Jeffrey Burgdorf, a researcher at Bowling Green State University in Ohio who studies laughter in rats and was a peer reviewer on the new study. Tickling helps strengthen social bonds, which can create a positive opportunity for group bonding that can increase an individual’s fitness.
“It’s a pro-social behavior, that’s the key,” says Burgdorf, who has worked to develop antidepressant drugs based on his own research on rat laughter.
These new findings are a great achievement in the study of emotion, Burgdorf adds. In the future, Ishiyama hopes to tackle other enigmas of tickling to better understand the brain and what types of things stimulate joy. He’d especially like to answer Aristotle’s riddle of why we can’t tickle ourselves. One way, he says, might be to set up a system in which a rat could press a button to stimulate the ticklish region of their brain.
If an image of self-tickling rats doesn’t give you joy, we don’t know what will.