“Tickle is one of the broadest and deepest subjects in science.”
So says Robert Provine, a neuroscientist at the University of Maryland, Baltimore. His career has included exploration of various “curious behaviours”, including hiccupping, yawning and farting, so why does this one stand out?
In short, there is far more to tickling that you would think. “It concerns everything from body defence and the neurological program for play to the generation of the sense of self and other,” says Provine.
As with so many other complex human behaviours, our animal cousins can help us understand what tickling is all about.
There are two types of tickling, and they both have great names: knismesis and gargalesis.
It’s the only joke you can tell to both human babies and chimpanzees
Knismesis is a primitive response, a slightly irritating sensation triggered by a light movement across the skin, and it is widespread. “I would think that lizards, insects and virtually all beings have some sort of behaviour that has to do with defence of the body’s surface,” says Provine. Animals need to defend their bodies against biting insects and parasites, whether that means a quick scratch or a flicked ear, and knismesis describes such a response.
Gargalesis, on the other hand, is a singularly mammalian phenomenon. It is a harder tickle that results in laughter and is linked with play – a distinguishing feature of mammals.
At a basic level, tickle is a sensation involving nerve fibres associated with both touch and pain. But there is more to it than that. “Laughter-associated tickle might best be considered a social behaviour rather than a reflex,” writes dermatologist Samuel T Selden in a 2004 review of the subject. Somewhere in evolutionary history, tickling became funny.
“Tickle is the primal stimulus for laughter,” says Provine. “In fact, ‘feign tickle’ is my candidate for the world’s most ancient joke. The ‘I’m gonna get you…’, threatening-to-tickle behaviour. It’s the only joke you can tell to both human babies and chimpanzees.”