The idea that laughter is a unique human expression has long been a common assumption, but scientific inquiry suggests that the roots of mirth run much deeper across the animal kingdom. While no other species produces the distinct, rhythmic “ha-ha-ha” sound of human laughter, many animals make specific vocalizations during joyful interactions that serve a similar social and emotional purpose. Researchers examine these sounds to trace the evolutionary history of positive vocal signaling, revealing a shared biological mechanism for communicating pleasure and play intent. This demonstrates that the capacity for a laugh-like signal is far more widespread than previously thought.
Defining Play Vocalizations
Scientists refer to the sounds animals make during playful interactions as Play Vocalizations (PVs). These patterned sounds occur almost exclusively during games like wrestling, chasing, or tickling. PVs differ significantly from distress, alarm, or aggressive calls in their acoustic structure, tone, and function. They are rarely loud, often characterized by low-amplitude, high-frequency characteristics that make them non-aggressive and appropriate for close-range communication.
The key distinction is that these vocalizations are not simply a result of physical exertion, but a deliberate signal for social partners. Human laughter is acoustically unique because it is primarily produced on the exhale, a rare form of breathing control among mammals. However, the PVs of many species share a foundational structure of brief, patterned bursts of sound, supporting the theory that human laughter evolved from a general acoustic marker of labored breathing during rough-and-tumble play.
Laughter Among Our Primate Relatives
Our closest evolutionary cousins, the great apes, provide the clearest evidence for the ancient origin of laughter, producing sounds that are direct forerunners of the human chuckle. Chimpanzee laughter is often described as a “pant-hoot,” a breathy, rapid series of alternating inhalations and exhalations that sounds more like a rasping pant than a clear vocalized tone. This sound occurs frequently during rough play and tickling, and its social function is identical to human laughter.
Bonobos also laugh when tickled, but their PVs are higher-pitched and more melodic than the chimpanzee pant-hoot. Acoustic analysis shows that the bonobo laugh follows the same sonographic pattern as the laughter of human infants, suggesting a common neurological pathway for positive affect. This shared pattern indicates that the capacity for the sound emerged in a common ancestor approximately 10 to 16 million years ago.
Other great apes also exhibit these playful vocal signals. Gorillas produce a distinctive “chuckle” during play, often paired with a relaxed, open-mouthed “play face” that displays no teeth, clearly signaling non-aggression. Orangutans similarly respond to tickling and wrestling with laughter-like vocalizations, and their ability to extend exhalations during these sounds points toward the early development of breath control necessary for human speech and voiced laughter.
Play Sounds in Other Mammals and Birds
The phenomenon of play vocalizations extends far beyond primates, with fascinating examples found in non-ape species. Rats emit high-frequency, ultrasonic vocalizations (USVs) at around 50 kilohertz (kHz) when engaged in rough-and-tumble play or tickled by researchers. These rapid 50-kHz “chirps” are inaudible to the human ear without specialized equipment, but they function as a clear indicator of a positive emotional state, often causing the rats to seek out more interaction.
Dogs express their playful intent with a distinct “play-pant,” an unvoiced, breathy sound separate from the regular panting used to regulate body temperature. Research shows this sound has a wider frequency range than a normal pant. Recordings of the play-pant can be played back to other dogs to reduce anxiety and encourage play, confirming its dedicated social role.
Even certain species of birds, like the Australian magpie, use specific complex warbling and chattering vocalizations during playful interactions. This suggests that the evolutionary pressure to signal playful intent is not limited to mammals. Across all these diverse groups, the underlying mechanism involves a unique, patterned sound that deviates from a species’ typical vocal repertoire to mark an interaction as non-serious.
Signaling Non-Aggression
The fundamental biological purpose uniting all these diverse play vocalizations is signaling non-aggression to a play partner. Activities like wrestling, chasing, and mock-biting borrow movements from serious conflict, risking escalation into a real fight. The PV acts as an acoustic qualifier, informing the recipient that the rough behavior is merely a game.
This function is important in species that engage in intense, physical play, as the sound maintains the playful state and prevents misinterpretation. By producing a specific “chuckle,” “pant-hoot,” or “play-pant,” the animal communicates that the actions are not aggressive. This shared understanding, facilitated by the vocal signal, allows social partners to maintain trust, continue the interaction, and strengthen social bonds.