The belief that laughter is a uniquely human trait has long shaped our understanding of emotion and communication. Modern scientific inquiry, however, has revealed that a form of laughter exists across a wide array of the animal kingdom. Researchers now understand that these sounds are structured vocalizations used primarily during social interaction. The study of these specialized sounds explores a shared evolutionary behavior that helps regulate playful encounters.
Defining Animal Laughter
Animal laughter is scientifically categorized as “Play Vocalizations” (PVs), which are acoustically distinct from human laughter. Human laughter is characterized by voiced, rhythmic expirations. In contrast, PVs are often unvoiced, breathy, and quieter, helping them remain inconspicuous.
These vocalizations involve a rapid, rhythmic alteration of breathing that signals a positive emotional state. The sounds are context-specific, occurring almost exclusively during rough-and-tumble play, wrestling, or other non-serious interaction.
Species That Exhibit Play Vocalizations
A review of scientific literature has identified vocal play behavior in at least 65 animal species, spanning mammals and a few birds. Primates, our closest relatives, exhibit some of the most recognizable forms of PVs. Chimpanzees, gorillas, and orangutans produce a soft, panting sound, often described as a breathy “ha-ha” that occurs during chasing and tickling.
In canids, specifically domestic dogs, the play vocalization is known as the “play-pant” or “dog laugh.” This sound is a distinctive series of forced, breathy exhales that create a “hhuh-hhuh” pattern, which is acoustically different from the rapid, shallow breathing used for cooling. The play-pant is frequently paired with the visual “play bow” to communicate that the subsequent rough interaction is non-threatening.
Rodents also produce play vocalizations, though they are inaudible to the human ear. Juvenile rats emit high-frequency ultrasonic vocalizations (USVs) around 50 kilohertz when they are tickled or engaged in wrestling. These high-pitched chirps are a clear indicator of a positive affective state. Recordings of the sounds can be played back to other rats to encourage them to engage in play.
The Social Function of Laughter
Play vocalizations serve a social purpose by regulating interactions and promoting group cohesion. The primary function of PVs is to signal non-aggression, which is necessary because play often involves mock aggression like biting, chasing, and wrestling. This signaling helps prevent the interaction from escalating into genuine conflict, allowing individuals to practice fighting and social skills in a safe environment. PVs strengthen the affiliative bonds within a social unit.
Play vocalizations can also have a contagious effect, helping to spread play behavior throughout a group. When one animal begins to emit PVs, it can encourage others nearby to join the activity, leading to a coordinated burst of playful energy. This spread of positive emotion facilitates the development of social cognitive skills and helps to maintain the unity of the group.