Since I haven't been able to find any Bonobo examples, and since their hooted "duetting" has been described as "gibbon-like," this video will have to do for now. For some examples of interactive human vocalizing of a somewhat similar type, see Post 22.
As far as function is concerned, Hohmann and Fruth state that their study
supports the general, assumption that high-hoots are part of a system of signals that facilitate communication between members of different parties. The small number of observations available on locomotion and vocal activity of different parties suggests that the calls affect movements and, thus, may regulate proximity between single individuals, groups, or parties. . . [Thus] high-hoots may be the major device to regulate and to maintain the social network of the community. (p. 780).If this is the case, and if primate duetting-chorusing is in fact "proto-musical," as the striking similarities with the "shouted hocket" of so many indigenous peoples suggests (as per the comparisons on Post 22), then the close cooperation associated with this type of vocal interaction might well have conferred an adaptational advantage on both early humans and their pre-human ancestors by enhancing social integration.
I must confess, however, that I'm not completely convinced. While interaction of this sort might well promote social stability and enhance the ability of a group to act in close coordination, I see no reason why either social stability or coordination would require the relatively precise synchronization so characteristic of both Bonobo or Gibbon vocalizations and human music-making. While primates and humans are capable of varying degrees of cooperative activity, none of these species appear to gain any sort of competitive advantage from acting in strictly synchronized concert. Aside from certain types of military drill, which are almost certainly a relatively late development, human "entrainment" of this sort appears to be limited exclusively to certain types of musical performance and dance.
Thus while the interactive element of Bonobo and human "proto-musical" and musical behavior might have conferred an adaptational advantage related to cooperation, it's much harder to see any such advantage accrueing from the precisely synchronized "entrainment" associated with it. Loosely coordinated cooperation would seem to have been equally effective as far as the survival of any of these species is concerned. It's also very difficult to see what adaptational advantage the more or less precise tuning of specific pitches, an essential element in almost all human music, might confer, since the sort of close cooperation required in deploying such pitches in either polyphony or unison appears to have no correlate in any other aspect of human behavior associated with cooperation per se.
There is one other possibility we have not yet discussed however, and this will be the principal topic of my next post.