By Megan McGrath
Even before human babies can speak, they will giggle when tickled. Jaak Panksepp, a neuroscientist of emotion at Washington State University, noted “if one wants to become friends with a young child, there is no easier way to negotiate the social terrain than by gently escalating tickle games” [1]. Who could disagree with that?
Dr. Panksepp’s subjects, however, are not human babies: They’re rats. In 1998 he discovered that young rats, while playfully wrestling, emit a curious chirping sound above the range of human hearing [2]. For years, his team studied the sound, trying to divine its purpose—until one morning, struck by a crazy idea in the middle of the night, Dr. Panksepp walked into his laboratory, turned to a student, and said, “Let’s go tickle some rats.”
This is as charming a story as you get in science—like Archimedes shouting “Eureka!” as he stepped into the bath. Because when Dr. Panksepp reached his hand into a young rat’s cage and tickled it, it chirped uproariously—as if it were laughing.
Dr. Panksepp now had a real conundrum, as there is a potential scientific problem with this conclusion. When you’re tickling a young child, you know they are laughing. You know this because you know laughter: you have laughed yourself. You know that laughter is a human phenomenon, and that when you and others laugh, you feel something: happiness and joy.
But when a baby rat emits a cacophony of chirps as it wrestles with a littermate, or chuckles its head off when a human experimenter flips it on its back and tickles its belly…is that baby rat laughing? Does it feel joy?
The question of animals’ inner emotional lives has been a scientific challenge as long as science has existed. Scientific findings rely on direct observations as empirical proof—and you can’t observe an animal’s mental processes. So if a baby rat can’t tell us, “I’m having a great time right now!” as it chirps, how do we know the chirps are like laughter? Indeed, how can we know if they experience any emotions at all?
Panksepp’s team tested the rats’ propensity for chirping in various situations [3]. Here are the many ways that their play-chirps resemble human laughter. You can be the judge of whether the rats are laughing or not.
- Young rats chirp predominately when they are play-wrestling, or when they are tickled by human experimenters.
- Rats that chirp when tickled actively approach the experimenter’s hand. In fact, they will chase it around the cage, as if soliciting tickles. This suggests that tickling is a positive experience for them—like playing.
- Like a baby, who will erupt in laughter even if you just say “Coochie coo!” and threaten to tickle them, a baby rat that is used to being tickled will chirp in response to a wiggling hand alone.
- The baby rats chirp more when tickled on certain parts of their bodies—especially their bellies.
- When the tickling stops, the rats don’t just wander away: they actually behaved more socially, rather than less, increase their chirping playfully nip at the experimenter’s hand—as if to say “come back and play with me!”, a strong indication that the chirping is a social cue.
- Baby rats who were socially isolated, were far more likely to seek out, and chirp about, the experimenters’ tickling hands – more evidence that this is a social vocalization: Rats are highly social animals, and given no other social contact, they sought out the human touch for play.
What do you think? When the rats chirp, are they laughing?
Dr. Panksepp certainly believes that this is a rat equivalent of laughter—rats are mammals, their brains are similar to ours, and, like us, they are very social creatures. Laughter was historically thought to be limited to our species. We now believe that great apes demonstrate laughter. Is rat “laughter” really so far-fetched?
Though rats cannot tell us that they are enjoying themselves, from the results of these studies it sure does look like they are. So the next time you see rats wrestling each other on the subway tracks, don’t turn away in disgust. Realize that above your range of hearing, they’re emitting a burble of trilling chirps that sound a lot like giggles. Those rats—your playful evolutionary cousins—just might be laughing.
References
1. Panksepp, J. (2000). The riddle of laughter: neural and psychoevolutionary underpinnings of joy. Current Directions in Psychological Science, 9(6),183-186.
2. Knutson, B., Burgdorf, J., & Panksepp, J. (1998). Anticipation of play elicits high-frequency ultrasonic vocalizations in young rats. Journal of Comparative Psychology, 112(1), 65-73.
3. Panksepp, J., & Burgdorf, J. (2000). 50-kHz chirping (laughter?) in response to conditioned and unconditioned tickle-induced reward in rats: effects of social housing and genetic variables. Behavioural Brain Research, 115, 25-38.