The male chorus starts in darkness, often between three and five in the morning, with neighbouring males answering across the forest so that the sound moves in waves. Females sing later, into the post-dawn hours, giving long trilling great calls that other females answer in turn. The pattern is a strict division by sex rather than by pair.
Singing in darkness has a practical logic. Cool, still pre-dawn air carries sound further and with less distortion than the turbulent air of a warm morning, so a call made at that hour reaches more listeners for the same effort. Predation risk while stationary and singing may also be lower before light.
Kloss's gibbon and the Javan silvery gibbon are the two hylobatids that do not duet, and they are not each other's closest relatives. That suggests duetting was lost twice, independently, on two different Indonesian islands.