Tummy-tickling causes rats to break out in giggles and jump with joy - but only when they are happy, a study has shown.

Scientists confirmed the ticklishness of rats in experiments that involved researchers laying the animals on a platform and tickling them mercilessly with a gloved hand.

The rats responded by approaching the tickler, jumping joyfully, and emitting ultrasonic squeaks beyond the range of human hearing that bore a striking resemblance to laughter.

By monitoring activity in the rat's brains, the scientists confirmed that the animals were behaving in much the same way people do when tickled.

Like humans, they were only able to enjoy being tickled when they were in a good mood.

Tickling-evoked laughter was greatly reduced in rats placed on a high platform, which made them anxious. This was mirrored in their brain activity.

While the rodents responded to several parts of their body being tickled, tummy-tickling set them off the most.

The German team, led by Dr Michael Brecht from the Humboldt University of Berlin, wrote in the journal Science: " Rats vocalise during tickling in a mood-dependent fashion.

"The increase of vocalisations after initial tickling and anxiogenic (anxiety-inducing) suppression of tickling-evoked calls support Darwin's idea that 'the mind must be in a pleasurable condition' for ticklish laughter ..."

They added: "The numerous similarities between rat and human ticklishness, such as tickling-evoked vocalisations and anxiogenic modulation, suggest that tickling is a very old and conserved form of social physicality."