The mechanism of the behavior refers to both the proximate causes of that behavior as well as the way in which the behavior operates and is regulated. The causes of the behaviors were somewhat explained by their adaptive value: to keep other gibbons away from the pair's feeding grounds and mate.
The duet is only sometimes induced by a proximate cause. The clearest example of a proximate cause is that the approach of a loner gibbon will indeed induce a duet (Raemaekers & Raemaekers 1984). Another proximate cause is a duet of another couple. These direct proximate circumstances (approaching loner, another duet) were found to induce about half of all duets, which is consistent with the idea that the songs are multifunctional. The duet accomplishes this by its extreme volume; it can be heard up to 1,100 meters (about .7 miles) away! Attenuation (rate of sound decrease by absorption of sound into environmental features) was found to be extremely low for the gibbons compared to calls of other animals because of the very unique properties of the sound, as can be heard in the video below. To increase the range of the calls, the calls are greatly concentrated at dawn, even though the gibbons remain active all day. This is comparable to bird calls in the early morning. Also, the gibbons traverse all parts of the trees they are in, but they most often perform the duet in the highest part of the canopy to help the sound travel (Mitani 1985).
Different species of gibbons have different general formulas of duets that all gibbons of that species follow. In some species, the female provides a steady, rhythmic accompaniment and the male ‘riffs’ a dynamic coda mid-way through. In other species it is the male that supplies the 'beat'. In most species the female performs a long and vibrant solo, which is her unique ‘great call’. The length of call and notes per call also greatly varies, as does the type of note. Researches use words like ‘wa’, ‘hoo’, ‘oo’ and ‘waoo’ to describe the specific kinds of vocalizations (Raemaekers & Raemaekers 1984). An example of the way these calls are noted is below (Cowlishaw 1992).