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Rats actually "giggle" when they are tickled!
When rats are playing or being tickled, particularly on their backs or bellies, they make a high pitched chirping sound. The sound is in the ultrasonic range, around 50 kHz, which means it is too high for human ears to hear without special ultrasound detectors.
Scientists studying animal behavior discovered that rats not only enjoy being tickled, but they will actively chase after a researcher's hand to get tickled again!
