Indri families sing together, usually in the morning, with members overlapping and answering one another in bouts lasting a minute or more. Researchers recorded and measured song intervals from wild groups in eastern Madagascar over more than a decade, then examined the distribution of note durations.
The intervals did not spread continuously. They clustered around two ratios, one to one and one to two, which in human terms correspond to isochronous beats and to a note twice the length of its neighbour. The study also reported ritardando, a gradual slowing towards the end of a song, and differences between the sexes in tempo. Categorical rhythm had previously been described only in humans and in some songbirds.
The finding rests on a single well-studied population and one analytic approach, and the sample of singing species available for comparison is small. What it does not show is that indri song is music in a human sense, only that it shares a specific timing structure that had been thought rarer than it is.