In a typical gibbon duet the male contributes short phrases that interlock with the female's rising great call, and pairs improve their coordination over years together. Silvery gibbons and one other species have lost this. The female's great call is a long ascending series of notes given in the morning, often answered by other females rather than by her own mate.
Males do sing, but their songs are given at different times, frequently before dawn, and are not synchronised with the female's. The result is a soundscape structured by sex rather than by pair.
Because duetting is widespread across the family, the Javan pattern is best read as a loss rather than an ancestral state. Why it was lost is unresolved. Reduced pressure from neighbouring groups and differences in how territories are defended have both been suggested, but neither has been tested against island-specific history.