"rigid, precisely timed, and complex vocal interactions"

Gibbons produce loud and long song bouts that are mostly exhibited by mated pairs. Typically, mates combine their partly sex-specific repertoire in relatively rigid, precisely timed, and complex vocal interactions to produce well-patterned duets. A cross-species comparison reveals that singing behavior evolved several times independently in the order of primates. Most likely, loud calls were the substrate from which singing evolved in each line. Structural and behavioral similarities suggest that, of all vocalizations produced by nonhuman primates, loud calls of Old World monkeys and apes are the most likely candidates for models of a precursor of human singing and, thus, human music. Although a few other mammals are known to produce songlike vocalizations, gibbons are among the few mammals whose vocalizations elicit an emotional response from human listeners.

…from:
Music and Primates
by Oiane Ruiz Alonsoù