The loose skin around a cat's neck makes for a perfect hand-or-mouth grip, as evidenced by mother cats--from house cats to lionesses--carrying their cubs around in a gentle neck grip. After some impressive research, io9 discovered why that grip is so effective. The answer has to do a myth about cats called clipnosis, which io9 describes as " the phenomenon whereby a cat is rendered suddenly immobile by a gentle squeezing of the loose skin on the back of its neck." Despite the term sounding sketchy--anything invoking hypnosis is going to be a little suspect--it turns out that clipnosis is a very real effect.
Scientists have studied clipnosis by placing a few binder clips along the loose skin of cats' necks. And then, bam, the cats are immobile. Clipnosis sounds like a myth, but it's actually true.
"Led by Tony Buffington, a professor of Veterinary Clinical Sciences at Ohio State University, the study examined the effectiveness of clipnosis by placing – what else? – standard 2-inch binder clips along the neck-skin of 31 cats," io9 explains. "Much to my relief, Buffington and his colleagues referred to the phenomenon not as 'clipnosis' but as 'pinch-induced behavioral inhibition,' or 'PIBI.' "
PIBI, then, is a better way to refer to clipnosis. There's no hypnosis involved, but the pinching is creating a very real response in cats. They become passive and their tails curl up between their legs. And it's not like their bodies are frozen while their minds are freaking out. "The cats' pupils did not dilate – a physiological response often seen in fearful animals– and their heart rates did not increase," io9 writes. "Nor did their breathing quicken. The cats also remained responsive, in contrast with cats seen to exhibit what is called 'tonic immobility,' whereby the animal will freeze and become entirely unresponsive in the face of highly threatening stimuli."
Buffington's conclusion was that the response was meant to make it easier for mother cats to pick up and carry their cubs. Young cats are easier to pick up and carry when they go limp. Buffington's study didn't answer why PIBI works, but other studies have found that the response isn't just present in cats. Similar responses happen in dogs, rabbits, mice, even humans. Researchers have found a number of systems contribute to the physiological response in mammals. We have evolved to experience a calming effect from being held by our mothers.
Want to know more about PIBI? Check out this paper by zoologist Stephen C. Gammie.