An analysis of white-handed gibbon male song reveals speech-like phrases

Our goal was to document song phrases of the white-handed gibbon (Hylobates lar), an Asian ape that produces elaborate songs, often in well-coordinated male/female duets. We focused on the male coda, which is produced during vocal turn-taking with one's mate, and particularly its phrases containing rapid spectral and temporal variation, to investigate if modulation rates resemble those of lip-smacking in other nonhuman primates and human speech rhythm.

We produced recordings from a large population of wild gibbons. Using terminology consistent with that used to describe vocalizations in other singing species, we analyzed coda phrases, overall coda properties, coda distinctiveness across individuals, and flexibility of phrase production within song bouts.

Our song phrase-level analysis showed that male codas differed between individuals and increase in complexity within song bouts by the addition of the only two male-specific phrases of the species' repertoire. These phrases differ from all others of the species and from vocalizations typical of the larger, nonhuman great apes, in that they contain rapid within-phrase modulation. Their modulation rates (6.82 and 7.34 Hz) are similar to that of lip-smacking in other nonhuman primates and speech in humans and, like human speech, are produced exclusively during exhalation. One phrase type (trills) contains multiple notes per exhalation, another characteristic similar to speech but not most primate vocalizations.

Our data highlight the complexity and flexibility of gibbon song, and show that particular phrase features likely arose from sexual selection pressures and possess similarities to human speech rhythm.