During experiments carried out in the 1990s, neuroscientists noticed that their lab rats would make ultrasonic noises of a specific frequency when they were allowed to play together.
To see if they made these noises whenever they were happy, scientists tickled the rats and found that they would squeak at the same frequency. This meant that the particular chirps they were recording were in fact rat laughter.
This was the first evidence that animals unrelated to humans were able to laugh. Up until this point, only other primates like chimpanzees and gorillas were known to do so.