According to the scientific journal, Bioacoustics, laughter is more common among animal species than people initially supposed. Besides humans, primates, dogs, seals, parakeets, magpies, and even rats all have the capacity to laugh. At the Humboldt University of Berlin, scientists took a particular delight in tickling rats as part of their research in social behavior. They found rats laugh at a frequency of 50 kHz, which is inaudible to human hearing. The rats soon identified the human hand as its playmate and would pursue it when not being tickled. These so-called "play vocalizations" act to signal to the rat is having fun.