Monkey lipsmacking develops like the human speech rhythm

Abstract

Across all languages studied to date, audiovisual speech exhibits a consistent rhythmic structure. This rhythm is critical to speech perception. Some have suggested that the speech rhythm evolved de novo in humans. An alternative account—the one we explored here—is that the rhythm of speech evolved through the modification of rhythmic facial expressions. We tested this idea by investigating the structure and development of macaque monkey lipsmacks and found that their developmental trajectory is strikingly similar to the one that leads from human infant babbling to adult speech. Specifically, we show that: 1) younger monkeys produce slower, more variable mouth movements and as they get older, these movements become faster and less variable; and 2) this developmental pattern does not occur for another cyclical mouth movement—chewing. These patterns parallel human developmental patterns for speech and chewing. They suggest that, in both species, the two types of rhythmic mouth movements use different underlying neural circuits that develop in different ways. Ultimately, both lipsmacking and speech converge on a ~5 Hz rhythm that represents the frequency that characterizes the speech rhythm of human adults. We conclude that monkey lipsmacking and human speech share a homologous developmental mechanism, lending strong empirical support for the idea that the human speech rhythm evolved from the rhythmic facial expressions of our primate ancestors.

Monkeys are perceptually tuned to facial expressions that exhibit a theta-like speech rhythm

“…Remarkably, the 5 of the 11 subjects produced their own lip-smacking gestures in response to the avatar faces. This proportion (∼45%) is roughly the same as that seen in the wild (29). Fig.…”

“…Our data show that monkeys are perceptually tuned to lipsmacking expressions within the species-typical range around 6 Hz (24,28,29). This is analogous, and likely homologous (see below), to the tuning of human perception to natural speech rhythms.…”

“…Of the different potential outcomes, our results provide clear evidence for a perceptual preference toward theta-and speech-like natural frequency of lip-smacking displays. The magnitude of the effect was smaller in the 6-vs. 3-Hz condition perhaps because the 3-Hz rhythm still falls within the species-typical range of lip-smacking rhythms, but 10 Hz does not (24,28,29).…”

“…We used a preferential-looking procedure to test whether monkeys were differentially sensitive to lip-smacking produced with a rhythmic frequency in the species-typical range (mean: 4-6 Hz) (24,28,29). Because we cannot ask monkeys to produce faster and slower versions of their facial expressions, we used computer-generated monkey avatars to produce stimuli varying in lip-smacking frequency but with otherwise identical features (Fig.…”

“…For example, in contrast to chewing and other rhythmic orofacial movements, lip-smacking exhibits a speech/theta-like 3-to 8-Hz rhythm (24,28,29), and its developmental trajectory is the same as the trajectory leading from human babbling to adult consonant-vowel utterances (29). Moreover, an X-ray cineradiographic study of the dynamics of vocal tract elements (lips, tongue, and hyoid bone) during lip-smacking versus chewing showed that the differential functional coordination of these effectors during these behaviors parallels that of human speech and chewing (24).…”