Geographic Distribution and Habitat
Like all lemurs, indris (Indri indri) are endemic to the island of Madagascar. This large island’s unique geography makes it a vibrant tapestry of diverse climates and distinct environments. Indris make their homes in the lush tropical rainforests growing along this island’s eastern coast. Here, the land slopes down from the mountainous central highlands toward the coast. Indris generally prefer forests at low-to-mid elevations, but groups are occasionally found at higher altitudes.
Today, indris range from Anjanaharibe-Sud, in the north, to Anosibe An’ala, in the south, but their distribution was likely greater in the past.
Size, Weight, and Lifespan
Indris are the largest lemurs in the world. They measure between 25 inches (64 cm) and 28 inches long and can weigh up to 21 pounds (9.5 kg). Their stubby tails—remnants of their last common ancestor with monkeys—add little to their overall size, measuring a mere 2 inches (about 5 cm). Such short tails are unique among lemurs. Other species tend to have tails closer to the length of their bodies.
In the wild, indris live between 15 and 18 years, and they do not survive for long in captivity.
There is something otherworldly seeming about indris. They have long, dog-like muzzles sticking out from their flat faces. A pair of black tufted ears jut off the top of their heads around a patch of white fur on their crown. Big, round, forward-facing eyes pivot inside their sockets as though looking out at the world through a mask.
Indris move like ghosts through the forest, leaping from tree to tree propelled by their long slender—but no less muscular—legs. They glide silently through the air as though in slow motion. Keeping their torsos vertical and arms raised as they fly, they latch onto the targeted branch with their large grasping feet first before securing themselves with their hands. Both their hands and feet have opposable digits to help with clinging, but their opposable toes are much larger than their thumbs. The rest of their toes are fused with webbing and move jointly as a single unit, almost like a vice grip.
Indris have smooth, silky fur covering their whole bodies. Their black-and-white patterned coats make it hard to tell where they begin and end amid the dark-and-light canopy. Individuals display somewhat unique color patterns, but the only significant difference is between regional populations. Indris that live in the north are almost completely black, while those in the south have more white fur. Those in the center of their range are mostly black except for the white on their backs and bellies and the gray areas in between.
For a time, it was theorized that their regional coloration differences might imply they were separate species of indris or that some regional populations might represent subspecies. Such a theory has since been disproven, however.
Indris are herbivorous and eat a variety of plants and plant parts, including fruits, seeds, flowers, and sometimes bark. Their dietary preferences vary from season to season depending on what is available. They are known to go out of their way to procure certain foods over others. Over all, leaves are their favored meal and make up the bulk of their diets.
The cellulose found in leaves is an especially difficult fiber to break down. Folivorous (leaf-eating) primates around the world evolve special adaptations related to digestion that ensure they can extract nutrients from their meals efficiently. Though not strictly leaf-eaters themselves, indris exhibit many such adaptations—the most of any lemur.
Hypertrophic salivary glands produce large volumes of saliva to start breaking down the fibers in the mouth. Big stomachs and long, looping colons provide plenty of space and time for digestion to take place. Expandable pouches between their large and small intestines—called caeca—extend the digestion process, while specialized bacteria living inside the caeca aid digestion by fermenting the food matter. Studies have found that indris’ caeca contain almost five times more protobacteria than other lemurs.
Behavior and Lifestyle
Indris spend much of their life in the trees but are not strictly arboreal. They find their meals at every level of the canopy and at times descend to the forest floor. Here, they are typically found eating clumps of soil. Researchers believe that ingesting earth—a practice called geophagy—may serve a medicinal purpose, preventing the harmful effects of toxins found in many of the leaves they eat.
Leaping is their primary mode of locomotion. Indris launch themselves from tree to tree with their large and powerful legs. Keeping their bodies vertical as they fly through the air, they grasp the target branch with their dexterous hands and feet as they land. Their leaps are calculated and graceful—more like gliding than falling. Without doubt or hesitation, indris quickly rise or fall to their desired level in the canopy. On the ground, they continue to move in leaps and bounds, keeping their arms partially raised for added balance. In rare cases, indris have been observed walking upright.
Indris’ active hours align closely with the rising and setting of the sun, making them the most strictly diurnal lemurs in all of Madagascar. Because of this, their routines fluctuate with the seasons, according to the length of the day.
Like all primates, indris are social animals. They live in small familial groups that function according to established hierarchies. Groups keep to a small territory, which they actively defend against outsiders, sometimes using physical force. Despite being so territorial, groups may range outside their territory to find food, risking an altercation with another group if it means procuring a more preferable meal.
Social animals need to establish ways of communicating with each other. Indris are a particularly vocal species of lemur, and they are one of the few species of primates known to sing. Songs unite in-groups, helping members keep in contact with each other, and warn out-groups, warning outsiders to keep their distance.
The name “indri” is technically a misnomer, and the true Malagasy word for these animals is “babakoto,” a word with various meanings but often translated as ancestor.
From October through December, indris tend to stay at lower levels of the canopy in order to avoid horseflies.
Indris live in small groups of two to six members, comprising two parents and their offspring. These family units operate according to a strict hierarchy in which females are the dominant sex. They have first dibs on the best quality food and even decide how much their male counterparts are allowed to eat. A female indri is also more likely to be groomed than to be the groomer.
Families wake when the sun first strikes the top of the canopy. They begin the day by singing together, an activity that reinforces their group bond and tells other groups to keep away. As the light penetrates into the lower levels of the canopy, groups settle down for a short feeding session.
Feeding is often followed by some downtime, during which groups can laze about while they digest their breakfasts. This gives adults time to groom each other and offspring to play together.
By midday, families grow hungry again and set off to search for more food. When they find a new spot, they spend most of the afternoon foraging before winding down for the evening. Some groups spend this time socializing, while others cease all activity by late afternoon. By nightfall, groups are settled down to sleep safely in trees high above the forest floor.
As social creatures, indris use a variety of methods to communicate. Body posturing and facial gestures act as visual cues, letting others know when it is safe to approach and when it might be better to give them space.
Scent-marking is a way for indris to mark their group’s territory. Males take on this responsibility, applying their urine and secretions from special glands near their muzzles to various substrates as they move about their ranges.
Indris have rich vocal repertoires and are the only lemurs—and one of the few primates—that communicate by song. Their loud and boisterous calls are truly musical, lifting and falling in long melodic sequences as they echo for miles through the forest. Some songs help group members keep tabs on each other while others are used to tell neighboring groups to stay away. Some songs may help facilitate the formation of new groups or communicate information about a group’s composition.
Recently, researchers discovered that indris have a sense of rhythm. In a 12-year study analyzing more than 600 vocalizations from 39 adult indris, individuals were discovered to create unique rhythmic patterns by purposely manipulating the duration of their calls and the silences between them. Might this ability for nuance mean that indris’ songs contain more complex information than we can grasp according to our current knowledge?
Indris make a variety of vocalizations in addition to singing. Grunts, kisses, wheezes and hums all tend to be made by group members just before they depart. In the presence of a threat, indris make either a honk or a roaring noise, often following it with a song.
Studying indris in the wild is not easy. The dense foliage and rough terrain of their habitats make them hard to track down and observe for long stretches of time. While we know a lot about indris in spite of this, one area of study that remains elusive is mating and reproduction.
Indris live in small family groups, headed by a mating pair. Many researchers assume that mating pairs are monogamous, but it’s unclear how true this is.
Mating is seasonal, occurring December through March—the hottest and wettest season in Madagascar. Males initiate copulation by sniffing the female’s genitals. After 120 to 150 days, sometime in May and June, the female gives birth.
Infants have black fur coats except for the splotch of white on their torsos. They cling to their mothers’ bellies for the first two to three months of their lives. They are nursed three to four times a day until weaned at approximately six months of age. At around five months, they start to ride on their mothers’ backs. By eight months, they start to gain some independence, but remain close to their mothers until they reach the age of two. By that time, their pelage has taken on a more adult patterning.
Males protect the group’s territory but do not care directly for their offspring. Taking care of young is taxing to do alone, so a female focuses on one at a time and cannot become pregnant again until her latest offspring is competent enough not to take up her attention—which is about every two to three years.
Offspring reach maturity between seven and nine years of age. Females remain with their natal group while males leave to start new families of their own. How the formation of new groups plays out in the wild is not well understood at this time.
The ecological role of indris is not well-researched at this time. As partial frugivores (fruit-eaters), indris may help disperse seeds, but it is not currently known to what extent they do or do not.
The indri is classified as Critically Endangered by the International Union for Conservation of Nature (IUCN, 2018), appearing on the IUCN Red List of Threatened Species. Though a solid estimate cannot be made at this time, their population is in certain decline. This species appears in the latest Primates in Peril iteration as one of the 25 most endangered primates in the world.
Much of Madagascar’s wildlife—90% of which is found nowhere else in the world—is threatened by habitat loss—indris among them. In the last 2,000 years, human activities have reduced the island’s forest cover by more than 90%. Of that reduction, almost half occurred between the years 1950 and 2000! Despite these harrowing figures, habitats continue to be dismantled in the wake of human development, and the poor regenerative powers of Madagascar forests makes this situation basically irreversible.
Swidden agriculture, also known as slash-and-burn, is a particularly problematic practice in Madagascar. Technically illegal, rice farmers use this technique to rapidly clear forest and provide space for their crops. Once the trees are burned away, they mix the ash into the soil to create a nutrient-rich blend perfect for raising their harvest. After only a few seasons, however, the soil becomes barren. Without roots holding it in place, the earth erodes away, and farmers have to move to another location, where they begin this unsustainable cycle anew. While there have been efforts by both government and non-government entities to force an end to slash-and-burn practices, they continue to be used. During the on-going pandemic, the desperation felt by local peoples has actually caused swidden agriculture to increase in many places across the island.
Sapphire mining is another illegal industry many locals turn to in order to combat poverty in Madagascar. Great swaths of land are ravaged in order to mine these tiny precious stones out of the ground. In October 2016, a sapphire rush brought roughly 50,000 small-scale miners to the Ankeniheny-Zahamena Corridor, a protected area in eastern Madagascar and home to indris. This corridor happens to join two of the area’s national parks, allowing indri populations to mingle at-will. Its on-going destruction speaks to another threat indris face: habitat fragmentation.
When indri habitats are fragmented, groups become isolated. This limits who can mate with whom, eventually creating genetic bottlenecks. When gene flow slows or halts altogether, it can quickly affect the viability of their offspring. Within a few generations, a lack of genetic diversity can increase vulnerability to diseases and parasites and even breed debilitating deformities. As generations become less viable, fewer individuals survive long enough to pass on their genes. If fragmentation of their habitat is not remedied, indri populations will likely die out in their isolation and the entire species could pass into extinction.
Figures now show deforestation rates of Madagascar’s tropical rainforests—prime indri habitat—have increased by more than 1% every year between 2010 and 2014. By 2070, researchers predict human-led deforestation, combined with the effects of global climate change, may reduce the island’s rainforests by a catastrophic 93%. By 2080, the indri’s total range may decrease by as much as 39% due to the effects of climate change alone. It should be noted that our inability to keep indris in captivity means that the types of captive breeding programs that have helped bring other primate species back from the brink are not an option. Once their habitats are gone, so too are indris.
Destruction of their habitat goes hand-in-hand with social unrest and political corruption in Madagascar. Locals trying to meet their own biological and financial needs find themselves at odds with conservationists. Protections that exist are seldom enforced, leaving indris in the dust.
These factors have created an unstable situation in which cynicism can flourish. As people migrate to where the money is said to be—hoping to strike it rich in sapphires, for instance—local cultures erode. With them, the local taboos about hunting indris that once protected them lose their value. Conservationists have long feared this scenario, and now it has come to fruition. Indris are increasingly hunted for meat and pelts, both of which fetch a pretty penny on the blackmarket. Current levels of hunting are already unsustainable. Some locals who still subscribe to such taboos have even found ways around them: killing indris for sale but not to eat themselves or purchasing indri-derived goods but not doing the killing.
The indri is listed in the Convention on International Trade in Endangered Species (CITES) Appendix I and occurs in multiple protected areas, including three national parks, two nature reserves, and five special reserves. The socio-political situation in Madagascar, however, typically renders these protections meaningless. Governments seldom enforce them, and citizens rarely consider the well-being of lemurs in their everyday lives. Indri conservation depends on a new strategy, one which uplifts local communities by tying conservation projects to their economies. Fortunately for indris, such projects are already underway throughout the Ankeniheny-Zahamena Corridor and its neighboring regions.
The Aspinall Foundation—a charity dedicated to protecting the world’s most vulnerable ecosystems and endangered animals—began this region’s first community-based conservation initiative. Locally employed rangers work together with the foundation’s scientifically trained professionals to perform research and monitor the situation. Initially, its project was inspired by a 2009 survey that found new occurrences of the indri’s cousin, the greater bamboo lemur (Prolemur simus), in the area. In the last decade, however, its efforts have helped more than just the greater bamboo lemur. By protecting their habitat, its work has indirectly helped other lemur species with which the greater bamboo lemur is sympatric, including the indri.
Since 2009, the Aspinall Foundation has broadened its purview to include these other endangered lemurs, using their resources to conduct more general surveys of the flora and fauna and performing much-needed research on them. It also facilitates educational programs that teach local peoples about the importance of lemur conservation. The project’s phenomenal success has already inspired a number of new community-based projects throughout the region. The greater bamboo lemur has already begun to bounce back. Hopefully indris will follow suit.
Zachary Lussier, February 2022