Monkeys smack their lips during friendly face-to-face encounters, and now a new study says that this seemingly simple behavior may be tied to human speech.
Previously experts thought the evolutionary origins of human speech came from primate vocalizations, such as chimpanzee hoots or monkey coos. But now scientists suspect that rapid, controlled movements of the tongue, lips and jaw — all of which are needed for lip smacking — were more important to the emergence of speech.
For the study, published in the latest Current Biology, W. Tecumseh Fitch and colleagues used x-ray movies to investigate lip-smacking gestures in macaque monkeys. Mother monkeys do this a lot with their infants, so it seems to be kind of an endearing thing, perhaps like humans going goo-goo-goo in a baby's face while playing. (Monkeys will also vibrate their lips to make a raspberry sound.)
Monkey lip-smacking, however, makes a quiet sound, similar to "p p p p". It's not accompanied by phonation, meaning sound produced by vocal cord vibration in the larynx.
Fitch, who is head of the Department of Cognitive Biology at the University of Vienna, and his team determined that lip-smacking is a complex behavior that requires rapid, coordinated movements of the lips, jaw, tongue and the hyoid bone (which provides the supporting skeleton for the larynx and tongue).
The smacks occur at a rate of about 5 cycles per second, and that's the clincher. It's the exact same rate as for average speed human speech, and much faster than chewing movements (about 2.5 cycles per second).
Put vocalizations and these lip smacks together and voila- the basis of speech. As our ancestors began to live closer together and to cooperate, perhaps the conditions were just right to promote this skill.
It's a mystery, though, as to why monkeys never got our gift of gab. The "singing" component of speech, which requires voluntary control over the larynx, remains an evolutionary mystery too.