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É. Geoffroy and
G. Cuvier, 1796
The name "indri" most likely comes from a native Malagasy name for the animal, endrina. An oft-repeated, but probably incorrect story is that the name comes from indry [ˈiɳɖʐʲ], meaning "there" or "there it is." French naturalist Pierre Sonnerat, who first described the animal, supposedly heard a Malagasy point out the animal and took the word to be its name. The Malagasy name for the animal is babakoto [bəbəˈkut]. Babakoto is most commonly translated as "ancestor" or "father", but several translations are possible. "Koto" is a Malagasy word for "little boy", and "baba" is a term for "father", so the word "babakoto" may be translated as "father of a little boy." The father-son dynamic of many of the babakoto origin myths helps to explain the Malagasy name.
The indri practices long-term monogamy, seeking a new partner only after the death of a mate. It lives in small groups consisting of the mated male and female and their maturing offspring. In the more fragmented forests of their range, the indri may live in larger groups with several generations. Habitat fragmentation limits the mobility and capacity of these large groups to break into smaller units.
It is common for groups to move 300–700 m daily, with most distance travelled midsummer in search of fruit. The indri sleeps in trees about 10–30 m above ground and typically sleep alone or in pairs. It is common for young female indris, occasionally adult females, to silently play wrestle anywhere from a few seconds up to 15 minutes. Members of a single group will urinate and defecate jointly at one of their many selected areas of defecation in their territory. 
Indris reach their sexual maturity between the ages of 7–9. Female indri bear offspring every two to three years, with a gestation period of approximately 120–150 days. The single infant is usually born in May or June. The mother is the primary caregiver, though the father assists, remaining with his mate and offspring. Infants are born mostly or completely black and begin to show white coloration (if any) between 4 to 6 months of age. The infant will cling to its mother's belly until it is four or five months old, at which time it is ready to move onto her back. The indri begins to demonstrate independence at eight months, but it will not be fully independent from its mother until it is at least two years old. The indri reaches reproductive maturity between seven and nine years of age: a relatively slow rate.
The wailing "song" of the Indri
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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.
Different indri groups typically sing sequentially, responding to one another. As well as solidifying contacts between groups, 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 during the indri's breeding season from December to March.
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.
In order to amplify their song, the indri will move to the tree top before they call which allows them to be heard up to 3–4 km away. 
Diet and feeding
The indri is herbivorous and primarily folivorous. It prefers young, tender leaves but will also eat seeds, fruits, and flowers. Female indri seem to have greater preference for immature leaves than males do and will spend more time foraging among them. A wide variety of plant species are consumed, with members of the laurel family featuring prominently in the diet. The indri consumes little non-tree vegetation.
To feed, the indri plucks off a leaf or other plant part with its teeth. It uses its hands to pull tree branches closer to its mouth.
Reproductively mature females have priority access to food sources, therefore they forage higher in the trees than males.
This lemur inhabits the lowland and montane forests along the eastern coast of Madagascar, from the Réserve Spéciale d’Anjanaharibe-Sud in the north to the Mangoro River in the south. They are absent from the Masoala Peninsula and the Marojejy National Park, even though both regions are connected to forests where indri do occur less than 40 km away.
Relationship with humans
Across Madagascar, the indri is revered and protected by fady (taboos). There are countless variations on the legend of the indri's origins, but they all treat it as a sacred animal, not to be hunted or harmed.
According to one origin myth, a boy went into the forest to collect honey, was stung by bees, and fell from a tree. An indri caught him and carried him to safety.
Most legends establish a closer relationship between the indri and humans. In some regions it is believed that there were two brothers who lived together in the forest until one of them decided to leave and cultivate the land. That brother became the first human, and the brother who stayed in the forest became the first indri. The indri cries in mourning for his brother who went astray.
One explanation for the name babakoto, is that the calls made by the indri resemble a father calling for his lost son.
Another legend tells of a man who went hunting in the forest and did not return. His absence worried his son, who went out looking for him. When the son also disappeared, the rest of the villagers ventured into the forest seeking the two but discovered only two large lemurs sitting in the trees: the first indri. The boy and his father had transformed. In some versions it is only the son who transforms, and the wailing of the babakoto is analogous to the father’s wailing for his lost son.
In all of the babakoto origin myths, there is some connection of the lemur with humanity, usually through common ancestry. It is easy to see why the indri is so closely identified with humans. Its long legs, large upright body, lack of a prominent tail, vocalizations, and complex systems of communication are all reminiscent of human traits.
Another human-like characteristic of the indri is its behavior in the sun. Like its sifaka relatives, the indri frequently engages in what has been described as sun-bathing or sun-worshipping. As the sun rises each morning, it will sit and face it from a tree branch with its legs crossed, back straight, hands low with palms facing out or resting on its knees, and eyes half-closed. Biologists are hesitant to call this behavior sun worship, as the term may be overly anthropomorphic. However, many Malagasy people do believe that the indri worships the sun.
The indri is a critically endangered species. The primary threats to its existence are habitat destruction and fragmentation due to slash and burn agriculture, fuelwood gathering, and logging. This kind of destruction occurs even in protected areas.
The indri is also widely hunted, despite the many origin myths and traditional taboos (fady) which hold it sacred. Cultural erosion and immigration are partly to blame for the breakdown of traditional beliefs. In some cases, Malagasy people who resent the protective fady find ways to circumvent them. People whose fady forbid them from eating the indri may still hunt the lemurs and sell their flesh, and those forbidden to kill the indri may still purchase and consume them. Indri meat is prized as a delicacy in some regions.
Only one indri has lived over a year in captivity and none have bred successfully while captive.
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