Bizarre Image of Man Tickling Chimp via Shutterstock
In 1897, psychologists established that there are two kinds of tickles. Knismesis is caused by very light movement across the skin. It doesn’t make you laugh, and is sometimes accompanied by an itching sensation. Knismesis is often caused by animals and insects crawling across the skin, and may have evolved in a wide range of species as adaptive defense from things like ticks, fleas, mosquitoes, scorpions and spiders.
Gargalesis is the other tickle, the laugh-producing one caused by applying pressure to "ticklish" areas. Gargalesis laughter gets you into a very small club that includes humans, chimps, gorillas, orangutans and, surprisingly, rats.
Biologists think gargalesis evolved in primates as a means of social bonding and self-defense development (since it provides lots of practice protecting your neck, ribs and belly). Primates’ tickling laughs all share some sonic similarities, too, leading researchers to think that the reaction began with one of our common ancestors.
Rat laughter, meanwhile doesn’t really sound like anything we’d recognize among apes, and consists of pulsating, high-frequency ultrasonic “chirps,” starting with a vocalized inhalation. Washington State University researcher Jaak Panksepp, who discovered rats’ tickle laughter in 2007, says the rodents are particularly ticklish in their nape area, which is where juveniles often target play activities such as pinning each other down. He also found that tickling rats to the point of laughter seemed to promote bonding and tickled rats would seek out specific lab workers’ hands that had tickled them before.
More animals may laugh when tickled, but researchers haven’t found them yet. They’re looking, though, in the same place the rest of us go to see animals being tickled: YouTube. Marina Davila-Ross from the University of Portsmouth has been poring over the large amount of animal footage on YouTube for animals reacting noisily to tickling. She's collecting examples to assess whether the noises count as laughter, or are “positive vocalizations” signaling happiness.