The indri ( /ˈɪndri/; Indri indri), also called the babakoto, is one of the largest living lemurs. It is a diurnal tree-dweller related to the sifakas and, like all lemuroids, it is native to Madagascar.

The indri is well known for its loud, distinctive songs, which can last from 45 seconds to more than 3 minutes. Song duration and structure varies among and even within groups, but most songs have the following three-phase pattern.
Usually, a "roar sequence" lasting for several seconds will precede the more characteristic vocalizations. All members of the group (except the very young) participate in this roar, but the song proper is dominated by the adult pair. They follow the roar with a "long note sequence", characterized by notes of up to 5 seconds in duration. After this is the "descending phrase sequence". The wails begin on a high note and become progressively lower-pitched. It is common here for two or more indri to coordinate the timing of their descending notes to form a duet.

The songs may communicate territorial defense and boundaries, environmental conditions, reproductive potential of the group members, and warning signals. The indri may sing after disturbances such as thunder, airplanes, bird calls, and other lemur calls. A group will sing almost every day, up to seven times daily. The peak singing hours are between 7 and 11 AM. Daily frequency of song is highest in November and December (near breeding season), when the indri are even heard during the night.

Several other indri vocalizations have been identified. The "roar" is also used as a warning signal for aerial predators such as hawks. The indri emit a "hoot" or "honk" to warn of terrestrial predators such as the fossa. Other vocal categories include the "grunt", "kiss", "wheeze", and "hum". The purpose of these is still not entirely clear.



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