Indri

The indri sings, and it sings in rhythm.

Researchers recording indri families in Madagascar found that their songs contain a steady, categorical beat, the kind of regular timing that underlies human music. Until that work, humans were the only mammal known to do it.

The songs carry over a mile through rainforest, families sing them together as duets and choruses, and they are loud enough that the forest can seem to be answering itself.

The indri is also the largest lemur alive, it lives only on Madagascar, and it is Critically Endangered.

The indri’s scientific name is Indri indri. It is the only species in its genus.

• Kingdom: Animalia
• Phylum: Chordata
• Class: Mammalia
• Order: Primates
• Family: Indriidae
• Species: Indri indri

It is a lemur, which makes it a primate and a distant relative of monkeys, apes and us. Lemurs are found in the wild only on Madagascar and the nearby Comoros.

The name has a nice story attached. One account holds that a French naturalist heard a Malagasy guide say indry, meaning roughly “look, there it is”, and wrote it down as the animal’s name. In Malagasy it is called babakoto, often translated as “ancestor of man” or “father’s child”, and it is traditionally treated as sacred and not hunted.

This is what the indri is famous for, and it is worth explaining properly.

A family group sings together, usually in the morning, for anything from 45 seconds to three minutes. The song starts with a roar, then settles into long wailing notes that rise and fall, and members of the group coordinate their parts.

It carries more than a mile through dense forest, and neighboring families answer, so a chorus can spread across a valley.

The songs do several jobs at once: marking territory, keeping the family in contact, and advertising a group’s presence and strength.

The rhythm finding came in 2021. Analysing hundreds of songs recorded over twelve years, researchers found the intervals between notes were not random but clustered around particular values, producing a regular beat, and in places a rhythm where one note lasts twice as long as another. That is categorical rhythm, a building block of music, and no other mammal besides humans had been shown to have it. Some songbirds do.

Males and females also sing at different tempos, and there is even something resembling a ritardando, the slowing at the end of a piece that human musicians use.

Indris measure 2 to 2.5 feet in body length and weigh 13 to 21 pounds, making them the largest living lemur.

• Tail. A stub of only an inch or two. The indri is the only lemur without a long tail, which is unusual for a tree-dwelling primate and makes it immediately recognizable.
• Coat. Black and white in a pattern that varies between individuals and between regions, with bare black skin on the face.
• Hind legs. Long and powerful, built for leaping between vertical trunks rather than running along branches.
• Hands and feet. Long fingers and a strongly opposed big toe, for gripping trunks.
• Eyes. Green-yellow and forward-facing, giving the depth perception needed to judge a jump.

It moves by vertical clinging and leaping, holding upright against a trunk and launching itself up to 30 feet to the next one, feet first. On the ground, which it rarely visits, it hops upright with its arms held out.

Indris live only in the eastern rainforests of Madagascar, in a strip along the island’s eastern side.

They need tall, continuous, undisturbed rainforest, with a mix of tree species to feed on and the vertical trunks they travel between.

They do not cross open ground, which matters enormously. A road or a cleared field is a wall to an indri, so forest fragmentation does not just shrink their habitat, it cuts the remaining pieces off from each other.

Family groups hold territories of around 40 to 100 acres and defend them with song.

Indris are herbivores, eating mainly young leaves, along with flowers, fruit, seeds and bark.

They are selective, preferring tender new growth, and they feed from a wide range of tree species rather than specializing in one, which is part of why they need intact forest.

A leaf diet is poor fuel, so like other leaf-eaters they rest a great deal, often more than half the day, and have a long gut with a large caecum where bacteria ferment the leaves.

They also eat soil occasionally, which probably supplies minerals and helps neutralize the tannins in young leaves.

Indris live in small family groups of two to six: a breeding pair and their young. They are usually monogamous, and pairs may stay together for life.

Unusually among primates, the female is dominant. She leads the group and gets first choice of the best feeding spots, which is the normal arrangement in many lemur species and rare elsewhere in the primate world.

They are active by day, singing in the morning, feeding and resting through the middle of the day, and sleeping high in the trees at night.

They sunbathe, sitting upright on a branch with arms spread and face to the sun, which is a well-known lemur habit and looks oddly meditative.

Pregnancy lasts four to five months and produces a single infant, and this is where the indri’s trouble begins.

Females breed only once every two to three years, so a pair produces very few young in a lifetime.

The infant clings to its mother’s belly for the first months, then rides on her back, and is not independent until around eight months. It stays with the family for two years or more and does not breed until seven to nine years old.

Indris live 15 to 20 years in the wild, perhaps longer.

One infant every two or three years, maturing at eight, means a population that loses adults cannot replace them for a decade.

The fossa, Madagascar’s largest native carnivore, is the main natural predator, along with birds of prey taking young.

Habitat loss is the overwhelming human threat. Madagascar’s eastern rainforest has been heavily cleared for slash-and-burn farming, logging, charcoal production and mining, and an animal that will not cross open ground is left stranded in shrinking islands of trees.

Hunting has become a serious problem too. The traditional taboo that protected the indri has weakened in places, particularly where people have moved into new areas or where poverty leaves few options, and indris are large, loud, slow and easy to locate.

They also cannot be kept in captivity. No indri has ever survived long in a zoo, and attempts have consistently failed, probably because of their specialized diet of fresh young leaves from many tree species. That removes the usual safety net entirely: there is no captive population to fall back on.

The indri is Critically Endangered, with numbers falling steeply, and it is considered one of the most threatened primates in the world.

Its situation combines every bad factor at once: a single island, a narrow habitat strip, forest being cleared fast, an animal that cannot cross gaps, a breeding rate of one infant every few years, weakening cultural protection, and no captive population.

What works is protecting and reconnecting forest. Reserves such as Andasibe-Mantadia hold indris and draw tourists who pay to hear the song, which gives surrounding communities a direct financial reason to keep the forest standing. Community forest management and replanted corridors are the other main tools.

The honest position is that the indri can only be saved where it already lives, because it cannot be moved and cannot be bred elsewhere.

Indris are seed dispersers, carrying seeds from the fruit they eat through the canopy and depositing them away from the parent tree, and their leaf-eating prunes new growth across many tree species.

As a large-bodied leaf specialist they occupy a niche that nothing else in Madagascar’s eastern forest fills in the same way.

They are also the best possible flagship for Madagascar. An indri chorus is one of the most extraordinary sounds in nature, and it gives a forest a voice that visitors remember and will pay to hear.

And scientifically they have changed a question. Rhythm was thought to be a human peculiarity among mammals, perhaps tied to language. Finding categorical rhythm in a singing lemur suggests it evolved more than once, which makes the indri a direct source of evidence about where music might come from.