Inventor Nobuhiro Takahashi programmed his creation, called "SHIRI" or "butt" in Japanese, to respond with different emotions to different human touches.

Takahashi hopes to use the proto-type technology to develop responses which can be applied to other part of a robot's body, in particular the face, to help with non-verbal communication.

He decided to develop his technology with a rear end because a bottom's movements are large and make it easier to convey emotion.

"I wanted to try and use a butt to reflect emotions - fear, joy and relaxation," the 24-year-old Takahashi, a graduate student at the Tokyo University of Electro-Communication, told Reuters.

Robots are able to communicate with voices but have largely lacked the sort of non-verbal, physical responses that help power much of human communication. Takahashi hopes his technology can overcome that.

To get a robot to show fear, Takahashi would use a hit, or in the case of the buttocks, a spank.

"Fear is a very human - very living - emotion, so it's expressed with force, with a spank," he said, speaking inside a black tent designed to keep light from damaging the silicon-coated invention.

The robot bottom responds to a hit with quivering. A slow wobble is its default state, the sign for relaxation, while a gentle stroke brings clenches that Takahashi said signal pleasure.

"If we could apply this technology to conventional humanoid robots - these things which are on the dividing line between man and robot - well, they would be able to express feelings sufficient to communicate with humans properly," he said.

(Reporting by Ruairidh Villar; Editing by Elaine Lies and Robert Birsel)

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