Male Kloss’s gibbons begin their songs in a chorus before daybreak. Their songs are complex and unique, but have in common a series of rising and falling notes in phrases, accentuated by trills. The males may “solo”  from 10 minutes to a full concert length of 2 hours! Post-dawn, the females begin to sing in a female chorus. The phrases of the female’s songs are longer, with an accelerating trill to a long crescendo, and a concluding phrase that in music is called a “coda.”

While males and females don’t generally duet, mating pairs can be an exception, and may duet a few hours after dawn.

Each of these gibbons has a one-of-a-kind distinctive voice and way of phrasing their song that allows others to know them at great distances. Their choruses, with their break-out solos, may be the way in which individual groups let each other know just how powerfully they will defend their territory against intruders.

The stirring music of the Kloss’s gibbon is much admired by their human relatives. Researchers note that the songs of Kloss’s gibbons are considered the most complex and beautiful of any gibbons. Researchers also posit that the differences between the songs of Kloss’s gibbons and other gibbons may be due to adaptations to avoid the most dangerous ape: humans. Humans, for the last 2,000 years, have been their most successful predator on the Mentawai islands. Some researchers theorize this is why singing does not occur throughout the day, or in male-female duets. Perhaps same-sex choruses reduce individual group risk. Males sing longer than females do, but while it is still relatively dark. Females sing for less time, just at the break of day. Males are present during the female chorus, but do not duet, perhaps to keep watch and protect their mate?

In addition to vocalizations, Kloss’s gibbons also communicate through smell, facial and body gestures, play, and grooming.