Rats enjoy being tickled but, just like humans, only if they are in the mood

The rats 'seemed to warm up to tickling', scientists report as they probe the 'mysterious sensation'

The father of evolution, Charles Darwin, once wrote “of laughter from being tickled, the mind must be in a pleasurable condition”.

Now researchers have discovered the same is true for rats in a finding that could help shed light on the “mysterious sensation” and also on the basic wiring of the mammal brain.

In a series of experiments, the rats found themselves being repeatedly tickled by the hand of a scientist.

In a normal situation, they appeared to find this intensely enjoyable.

“We tickled and gently touched rats on different body parts and observed a variety of ultrasonic vocalizations [USVs],” the researchers wrote in the journal Science.

“Rats seemed to warm up to tickling and vocalized less before the initial interaction than during breaks between interaction episodes.

“Play behaviour (rat chasing experimenter’s hand) also evoked USVs.”

A video of one such encounter also showed the rat making “joy jumps” during the tickling. There was also a large amount of squeaking – the USVs – which the researchers altered so it could be heard by humans.

However when the rats were put on a raised platform illuminated by bright lights, they appeared much less keen.

“Tickling-evoked USVs were significantly suppressed,” reported the scientists, Dr Shimpei Ishiyama and Dr Michael Brecht, of the Bernstein Centre for Computational Neuroscience in Berlin.

They speculated the tickle-response must have evolved early in the development of mammals.

“The numerous similarities between rat and human ticklishness, such as tickling-evoked vocalizations and anxiogenic modulation, suggest that tickling is a very old and conserved form of social physicality,” they wrote.

The mechanisms involved in tickling are poorly understood. For example, it remains a mystery why no-one can tickle themselves.

The researchers found that tickling caused neurons in the rats' somatosensory cortex – an area of the brain mainly associated with the sense of touch – to fire.

They were also able to get the rats to ‘laugh’ when they artificially stimulated the same neurons.

The discovery suggests this area of the brain may also play a role in mood.

“The observation that the somatosensory cortex is involved in the generation of tickling responses suggests that this area might be more closely involved in emotional processing than previously thought,” the researchers wrote.

“Identification of the neural correlates of ticklishness will allow us to frame questions about tickling in neural terms and thus help us to understand this mysterious sensation.”