There are four hypotheses for the adaptive function of the duet: (1) advertisement of pair-bond as a territorial defense, (2) advertisement of pair-bond as a means of mate-defense, (3) a way of strengthening the pair-bond connection, and (4) a way of maintaining social cohesion in a community of pair-bonded gibbons. The general consensus is that the duet serves multiple purposes and there is not a sole correct hypothesis (Geissman 1986).
Evidence in Support of Territory Defense
Territory defense is the most common and generally accepted view of the function of duets. Gibbon data show that an aggressive encounter or chase is much more likely to occur if stimulated by a visual rather than auditory cue. In other words, the duet acts as a preliminary signal to diffuse any conflict before it can occur. Avoiding conflict is exactly what pairbonded gibbons hope to achieve, as conflict could only lead to negative consequences for them. Established pairbonds do not expand territory, except in urban areas with high population density and low resources (Cowlishaw 1992). Because the duet is so ridiculously loud, it can easily be categorized as a ‘conspicuous display’ rather than a ‘conspiratorial whisper’. The gibbons are making themselves and their territory known.
Evidence in Support of Mate Defense
In looking at mate defense in comparison to defense of feeding territory, Raemaekers points out that it can be hard to determine the reason the gibbons are defending the territory through observation of territorial behavior. A study was conducted in which playbacks (recordings) of calls of different gibbons were played in the canopy and the response of the target gibbons was recorded. It was found that a gibbon would be more likely to warn and approach if the playback recordings were of gibbons of the same sex, which strongly indicates that mate defense is a motivating factor in territory protection for gibbons. Infanticide by loner males is fairly common, so protecting one’s mate and territory is extremely important. Widowed mothers are especially at risk, and will often sing ‘pseudoduets’ where they will sing their mate’s part along with their own (Raemaekers & Raemaekers, 1984).
Clearly the gibbons have reason to be territorial. But do the men do it for different reasons than females? Cowlishaw concludes that the purpose of the male's song is primarily mate defense, where as the purpose of the female's song is territorial. The reason for this is that the male stands much to gain from extra-pair copulations with his female neighbors, and much to lose by losing his mate or letting another male mate with his mate. Females, on the other hand, do not gain anything by mating with their male neighbors, but have strong reason to defend her territory from intruders of either sex. This explains why there are high rates of solo male songs not within a duet, but hardly any solo female songs. Females have no need to advertise themselves as individuals, only as part of a united pairbond capable of territory defense. Furthermore, a positive correlation was found between male floater (loner) density and number of solo male calls (Cowlishaw 1992).
Evidence in Support of Pair-Bond Strengthening
If someone were to wander into the beautiful rainforests of Malaysia not knowing anything about gibbons, it would be easy to romanticize the unique duet the gibbons sing to each other high in the canopy. However, the theory of pair-bond strengthening as a primary function of the duet is likely the weakest of the four. The strongest and most obvious evidence against this is the fact that some species of gibbons, for example the Kloss gibbon (H. klossi), do not duet but still exhibit close pair-bonds (Cowlishaw 1992). However, though it may be true that duetting is not necessary for pair-bonding does not necessarily mean that duetting does not play an important role in the pair-bond of duetting species.
The basic premise of the pair-bond function hypothesis is that because new partners need to spend time learning their new song, during which time they are vulnerable to invasion of their territory and lone gibbons seeking a mating opportunity, the effect would be that duetting provided a strong incentive to maintain current pair-bonds. Indeed, newly pair-bonded gibbons spent a great deal of time as quickly as possible developing and practicing their duet, adding length, complexity, consistency, and cohesiveness. Perfection of a new duet typically takes a few weeks (Geissman 1986, 1999). It was also found that each duet is truly unique to the pair. When gibbons were forced to switch mates in a zoo study, the new songs were completely different from the songs of their previous relationships, as in it's not the case that each gibbon has an individual song and the duet is simply the result of combining two individual songs. Furthermore, Geissman found in the zoo study that copulation occurred only after the gibbons had fully perfected their duet, which would indicate that the duet is essential to the pairbond. He relates this finding to the general pair-bond theory of Dawkins and Maynard Smith: "pair-bonding could be strengthened by an individual demanding a preliminary effort of each new partner prior to copulation" (Geissman 1999).
Evidence in Support of Social Cohesion
A new function of the duet has emerged more recently than the others, that the duet is used as a way of coordinating movement and boundaries in a gibbon community. There is strong evidence for this theory: one duet is very likely to trigger another duet by a different pair of gibbons to occur immediately afterwards. Also, movement of gibbons is often preceded or followed by duets, indicating the location of the gibbons to the community. This position communication is important in reducing conflict in a community and establishing slowly evolving boundaries. Additionally, it has been found in situations where there is more than one newly pair-bonded groups first, the gibbons will develop their duets so that the song is coordinated not only with each other but also with the neighbors, forming "supra-duets". The supra-duet may be a more efficient song in assisting social cohesion of the group (Geissman 1999).