Tickling rats can tell us a lot about the ability of animals to laugh and joke

Right now, in a high-security research lab at Northwestern University’s Falk Center for Molecular Therapeutics in Illinois, scientists are tickling rats. Their goal? To develop a happiness pill. But their efforts might also produce some of the best evidence yet that humour isn’t something experienced exclusively by humans.

Scientists believe human laughter evolved from the distinctive panting emitted by our great-ape relatives during rough and tumble play. That panting functions as a signal that the play is all in good fun and nobody is about to tear anybody else’s throat out.

In a clever bit of scientific detective work, psychologist Marina Davila-Ross of the University of Portsmouth in the UK analysed digital recordings of tickle-induced panting from chimps, bonobos, gorillas and orangutans, as well as human laughter. She found that the vocal similarities between the species matched their evolutionary relationships.

Chimps and bonobos, our closest relatives, boast the most laughter-like kind of panting, while the noises of gorillas, further down our family tree, sound less like laughing. And orangutans, our truly distant cousins, pant in a most primitive way.


Cheeky monkey

Non-human primates don’t just laugh – there is evidence they can crack their own jokes. Koko, a gorilla in Woodside, California, who has learned more than 2000 words and 1000 American Sign Language signs, has been known to play with different meanings of the same word. When she was asked: “What can you think of that is hard?” the gorilla signed “rock” and “work”. She also once tied her trainer’s shoelaces together and signed “chase”.

But what about other members of the animal kingdom – do they have funny bones? Marc Bekoff at the University of Colorado, Boulder, professor of ecology and evolutionary biology and author of The Emotional Lives of Animals, believes they do. In fact, he thinks we are on the cusp of discovering that many animals have a sense of humour, maybe even all mammals.

The idea that animals can appreciate comedy isn’t as far-fetched as it sounds, considering some of the other groundbreaking discoveries scientists like Bekoff are making about animal behaviour. They have found dogs that understand unfairness, spiders that display different temperaments and bees that can be trained to be pessimistic.

As Bekoff points out, Darwin argued that the difference between human and animal intelligence is a matter of degree, not kind. Or as Bekoff put it: “If we have a sense of humour, then non-human animals should have a sense of humour, too.”

Let’s go tickle some rats

A similar sentiment inspired psychologist Jaak Panksepp to enter his lab at Bowling Green State University in Ohio one day in 1997 and tell undergraduate Jeffrey Burgdorf: “Let’s go tickle some rats.” The lab had already discovered that its rats would emit unique ultrasonic chirps in the 50 kilohertz range when they were chasing one another and engaging in play fighting.

Now the researchers wondered if they could prompt this chirping through tickling. Sure enough, when the researchers began poking at the bellies of the rats in their lab, their ultrasonic recording devices picked up the same 50 kilohertz sounds. The rats eagerly chased their fingers for more. Soon, as the media trumpeted the existence of rat laughter, people the world over were opening up their rat cages and engaging Pinky and Mr Pickles in full-scale tickle wars.

We met Burgdorf at his office at Northwestern’s Falk Center, where as a biomedical engineering professor he has continued his rat-tickling efforts. He was cautious, however, about overselling what is happening with his rodents. “I don’t necessarily call it laughter, I call it a signal of positive affect,” Burgdorf told us.

His careful choice of words makes sense. Not everyone was convinced he and Panksepp had uncovered real rat laughter when their rodent-tickling activities first went public. But whatever you want to call it, Burgdorf, a quick-witted man with a boyish face and a sign on his office door that reads “Know It All,” has been obsessed with that strange rat noise he first heard in 1997.

Laughing pill

He seems to be on to something. While tickling isn’t always pleasant – thus the term “tickle torture” – in multiple experiments Burgdorf has demonstrated the rats’ 50 kilohertz chirping is only associated with positive experiences. For example, the rats only made this sound during rough and tumble play when the animals were of similar size. The vocalisations changed when one of the animals involved was much larger than the other, when it was no longer fun and games and instead looked more like bullying. And when given a choice, Burgdorf’s rats would push a bar to play a recording of the 50 kilohertz chirp as opposed to other rat noises, suggesting they had a preference for the sound.

Finally, when Burgdorf and his colleagues used electrodes, opiates and other manipulations to stimulate the reward centres of rats’ brains, the rats produced that same noise.

Whether you call it laughter or not, Burgdorf is convinced the ultrasonic noises signal the rats are experiencing happiness. Hence the “laughing pill” experiment: he and his colleagues are testing a new antidepressant medication on rats, to see if it makes them “laugh”, or chirp happily. If all goes well, Burgdorf believes the resulting medication could eventually be approved for humans. Rats, so often seen as a malicious pest, could end up making the world a happier place.

This article first appeared in Slate