The idea that human linguistic sounds are in every way a manifestation of superiority over the communicative abilities of other species is now considered obsolete. The study of the vocal repertoires of other primates can show how species, even phylogenetically close, display some similar and strongly divergent traits. This is particularly true when we consider Asian species, such as gibbons and snub-nosed monkeys or lemur species, such as Lemuridae and Indriidae. Studies of the vocal repertoire of these primates have shown, on the one hand, that there are many similarities between phylogenetically close species, but, on the other, that there are considerable differences that can be traced back to peculiar adaptations to the environments in which these species live or to the behavioural and social characteristics that distinguish them. In the case of the gibbons and indri, one of the unique features is undoubtedly the songs that the main family groups of these species exchange in the forest. Detailed analyses of these songs have shown the presence of unexpected rhythmic characteristics. In particular, the white-cheeked and yellow-cheeked gibbons and the Madagascar Indri have shown their songs display isochronous intervals. In the case of the indri, isochrony is not the only rhythmic category that occurs significantly; in fact, the 1:2 category is also typical of the songs of this species. The scenario that emerges from these studies outlines multiple perspectives for future studies. Will we be able to trace a phylogeny of rhythm that can elucidate the selective pressures that led to human rhythmic capacities? Will the study of vocalization types from a comparative perspective explain how social organization influenced the size of vocal repertoires?

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