Move over Planet of the Apes. The rhythmic vocal sounds made by lip smacking in wild gelada monkeys have similarities to human speech, a new U-M study shows.

Lip smacking, a common primate facial gesture used in friendly interactions, involves rapid opening and closing of mouth parts in a speech-like fashion. However, geladas are unique because they simultaneously vocalize while lip smacking to produce a sound that has been called a "wobble."

The gelada wobbles have a rhythm that closely matches the pacing of syllables spoken by humans, says Thore Bergman, U-M assistant professor in the departments of Psychology and Ecology and Evolutionary Biology.

Researchers tracked geladas in the Simien Mountains National Park in Ethiopia where they recorded the unusual sounds of th is species. Bergman describes geladas as sociable creatures with a large vocal repertoire. The wobble is used primarily by males and always in a friendly context.

Bergman analyzed the recordings of geladas' vocal sounds, tracking their duration and frequency. Using the digitized waveform of wobbles, he measured the peaks (loud parts) and valleys (quiet parts) that occurred several times a second. The analysis shows a close match between the intervals in gelada sounds and human speech -- something that no other primate vocalization has been shown to do.

As Bergman and other U-M researchers continue their work analyzing lip smacks, they want to learn if these sounds have a special function for the geladas. The paper was published in Current Biology April 8, 2013.

U-M News Service press release

Caption: Gelada monkeys in conversation. Image credit: Clay Wilton