Rats communicate using sounds that are too low for humans to even hear, but their vocal behaviour changes with age. Rats use these low-pitched sounds when they are happy and are most used when they tickle each other, so it’s similar to when humans laugh.
Baby rats giggle at frequencies of around 50 kHz in all of their social interactions and play behaviour, but that all changes when they grow into adults. Older rats still tickle and play, but only bring out these ultrasonic sounds when they need to. This rat ‘laughter’ helps adults from getting aggressive, so they use it as a tactic to let others know they are only playing and don’t want to fight.
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Image from www.flickr.com/photos/jean-jacquesboujot