Speech-like vocalized lip-smacking in geladas

SummaryRecently, we have seen a surge of interest in identifying possible evolutionary links between primate facial communication and human speech (for example [1]). One suggestion is that primate ‘lip-smacking’ — a non-vocal, rhythmic movement of lips usually given in conjunction with affiliative behavior — may have been a precursor to speech [1]. This idea arose because lip-smacking shares several production features with human speech that the vocalizations of non-human primates lack, most notably a 3–8 Hz rhythm [1]. Evidence that non-human primates are indeed able to vocalize while simultaneously producing rhythmic facial movements would lend initial, but important, support to the notion that lip-smacking is a plausible evolutionary step towards speech. Here, I report that a wild primate, the gelada (Theropithecus gelada), makes a derived vocalization (the vocalization is absent in their close relatives, the Papio baboons) that is produced while lip-smacking, called a ‘wobble’. The rhythm of wobbles (6–9 Hz) closely matches that of human speech, indicating that a vocalized lip-smack produces sounds that are structurally similar to speech. Geladas are highly gregarious primates with a relatively large vocal repertoire. Their independent evolution of a speech-like vocalization involving complex facial movements provides initial support for the hypothesis that lip-smacking was a precursor to the emergence of human speech