The study of laughter provides a novel approach to the mechanisms and evolution of vocal production, perception and social behavior
There is a common misperception that laughter is exclusive to human beings. From at least the time of Darwin, however, it has been known that chimpanzees and other great apes perform a laugh-like vocalization when tickled or during play. To pursue the details of this primate laughter, I
teamed up with Kim Bard, who is nursery director and caregiver for young chimpanzees at the Yerkes Regional Primate Center in Atlanta. It is a pleasure to be able to play with young chimpanzees in the pursuit of one's
Chimpanzee (Pan troglodytes) laughter differs in many ways from its human counterpart. The vowel-like notes of human laughter are performed by chopping a single expiration, whereas chimpanzee laughter is a breathy
panting vocalization that is produced during each brief expiration and inspiration. Unlike human laughter, the laughter of a chimpanzee lacks discrete, vowel-like notes that have sharp leading and trailing edges on sound spectra. Chimpanzee laughter has the sound and cadence of a handsaw cutting wood. The sounds of chimpanzee and human laughter are sufficiently different that without viewing the characteristic "play face" and source of stimulation (such as play and tickle), naive human beings may be unable to identify the chimpanzee vocalization as laughter. You can experience the difference in production between the two forms of laughter by placing a hand on your abdomen and comparing the abdominal pulsations of chimpanzee-like panting with the smoother act of speaking "ha-ha-ha" during
a single expiration.
People laugh as we speak. If chimpanzees laugh as they speak, by producing one laugh sound per expiration and inspiration, we have identified an important and previously unrecognized constraint on the evolution of speech and language in chimpanzees and presumably other great
apes. The close coupling of laughter to breathing in chimpanzees may be evidence of a more general limitation on these animals to speak. (In contrast to the success of teaching hundreds of signs to chimpanzees, efforts to teach them to speak English have produced meager results.)
Indeed, the inability to modulate expiratory airflow may be at least as limiting to speech as the structure of the vocal tracts of nonhuman primates.
Breathy, panting laughter is probably the primal form that dates back to the common ancestor of all great apes and people. Human beings evolved their characteristic laughter after branching from an ancestor in common with chimpanzees (estimated to be around six million years ago, according
to DNA hybridization data).
It is noteworthy that chimpanzee laughter occurs almost exclusively during physical contact, or during the threat of such contact, during chasing games, wrestling or tickling. (The individual being chased laughs the most.) Although people laugh when tickled, most adult human laughter occurs during conversation, typically in the absence of physical contact.