Phil Ridges and Emmie
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How to tickle a gorilla

30 December 2011 Last updated at 00:03 GMT

Scientists are studying how animals respond to being tickled in a bid to find out how laughter evolved.

Here, Phil Ridges, head keeper for the ape palace in Port Lympne Wild Animal Park in Kent, explains how Emmie the gorilla reacts when she is tickled.