You know those times when you just can’t stop laughing? You try to keep it down, but you can’t help it and that juice you were drinking comes out your nose? Or you keep laughing so long your abdominal muscles hurt? And what about times when those giggles come from being tickled? Laughter may seem like a trait unique to humans, but we are not the only animals that laugh.

Apparently, rats are among those animals that giggle. Scientists have found this out by tickling them. At Humboldt University of Berlin, scientists discovered more about this thanks to new research. These scientists wanted to find out how of touch affects social bonds and how mood affects behavior. They found that rats have to be in the right mood to laugh, just like humans. When the rats are stressed, they don’t giggle while tickled—but when they’re relaxed, they do. This is more important than it sounds, as it reveals a deeper connection between emotion and sensing the physical touch of the tickling. We sense tickling in the somatosensory cortex of our brain, the area associated with direct touch. However, when tickling the rats, researchers found that this part of their brains was less active when the animals were stressed, and therefore the neurological reaction must be affected by mood.

Further evidence of the connection between emotion and touch comes from scientists finding that rats liked being tickled enough to follow the hand that tickled them. The rats giggled when they were about to be tickled but not yet physically touched, which further supports their similarities to humans. Tickling studies may seem unscientific, but they’re important enough to have their own unique vocabulary. For example, gargalesis is a touch that makes you laugh, and knismesis is a light touch that does not make you laugh.

And it turns out there is a lot we don’t know about tickling. We’re not sure why it evolved, what its purpose is or why certain parts of the body are more ticklish than others. Humans have been wondering about this connection between mood and how easily we laugh for a long time. Darwin and Aristotle considered it important enough to write down questions about it hundreds of years ago.

There are actually several animals that respond to being tickled, though they have different reactions. There is anecdotal evidence of platypuses and porcupines giggling, but sharks go belly-up and become paralyzed. These reactions are related to how social and playful the animal is, with the more social and playful enjoying it more. One example is the reaction of apes. Their laugh sounds a lot like a human’s, and scientists suggest humans got the ability to laugh from a primate ancestor who lived 10 to 16 million years ago.

Of all the similarities humans share with other animals, this is a great one. Picturing a platypus chuckling certainly gives me the giggles.

Did You Know?

We are 30 times more likely to laugh at something if we are with other people. This means that laughter is not just an indicator of when we think a joke is funny, but shows that we understand how our friends are feeling.