So what’s the reason humans are so ticklish? Could rats help provide the answer?

Here’s a light-hearted one. Are you ticklish? And if so, are you only ticklish in certain places? Can you tickle yourself? (No). Have you ever wondered why we’re ticklish? Does it serve a purpose?

The answer to that last one is yes, it does. As the result of some recent research, experts say tickling and being tickled has evolved so we interact, laugh, have fun and bond closely.

Did you know rats jump for joy when tickled? Furthermore, they seek out the researchers’ hands to get a tickle. And, even stranger, they make ultrasonic calls (which we can’t pick up) that the researchers say are the equivalent of human laughter.

So why tickle rats? Well, for starters they’re ticklish in the same places as humans, such as the soles of their hind paws (feet). Berlin researchers Michael Brecht and Shimpei Ishiyama point out in their report that tickling raises many questions. We don’t know why it evolved, what purpose it might serve and why only certain body parts are ticklish. And what about that universal puzzle – you can’t tickle yourself.

The researchers were also inspired by earlier studies on rats published in 2003. This was when they discovered that rats would emit ultrasonic calls when tickled. Ultrasound is too high for human ears to pick up. We can hear soundwaves up to a frequency of about 20 kilohertz. Rats giggle at about 50 kilohertz.

Since then we’ve found a way of recording brain activity while playing with the rats. So, researchers can track what’s going on in their brains while they’re being tickled and “laughing”.

The researchers first accustomed young rats to play and tickling which they would invite. “They are very eager to be tickled,” said Dr Brecht.

The scientists used electrodes implanted in the rats’ brains for deeper investigation of the tickling and ­concentrated on an area where we
(and rats) perceive touch called the ­somatosensory cortex.

Dr Brecht explains that we still don’t understand how moods affect behaviour and learning more about them would be very important in psychology.

The link to play and emotions in the somatosensory cortex is also intriguing, while the similarity of tickling in rats and humans is, Dr Brecht said, “amazing”.

That similarity he says, suggests that tickling, in evolutionarily terms, is very ancient, going back to the roots of touch as a way to form social bonds in the ancestors of rats and humans.

“Maybe,” Dr Brecht speculated, ­“ticklishness is a trick of the brain to make animals or humans play or interact in a fun way.”