Of the world’s 25 biodiversity hotspots, two are in India: one along the Western Ghat mountains, which spread coastally close to Goa and Kerala; and one around Assam, in the region of the Eastern Himalayas. In the latter, apart from Assam’s eponymous tea, and rice and betel, only a handful of other plants have found a way to thrive. It is in the state’s protected forest reserves that what has been lost becomes truly apparent. Ringed like islands by tea plantations and rice paddies, they serve as reminders of what populated the land before. Within them are around 500 species of plants; 300 species of birds; a host of reptiles and amphibians; and 41 species of mammals, large and small, among which are the highest number of primate species in the country.

There are 24 such reserves across Assam, scattered on the state map. They glow like tiny pockets of intense green pitted against an expansive area under development. Five of them are national parks, and 19 are much smaller, scattered sanctuaries that protect particular pockets of rare and endangered species. Of Assam’s nearly 80,000 square kilometer area that spreads from Bangladesh in the south to Bhutan in the north (and nearly to China and Myanmar), its sanctuaries account for less than 2 percent.

Just east of the fertile Bhogdoi tributary of the Brahmaputra lies one of the state’s smallest sanctuaries. Its 21-square-kilometer area is enclosed on all sides by tea-growing villages; and on its south side, divided by railroad tracks built during the British Raj in the early years of the 20th century, now run by the government-owned Indian Railways. Wandering elephants have lost their lives along the tracks; as have some of the tigers, clouded leopards and five species of monkeys—four types of macaques and the capped langur—that call this forest their home.

The trees here remain green throughout the year. Significant among them is the hollong, native to Assam, which can rise above 30 meters (about 100 feet) in height. The Hoollongapar sanctuary is named after this tree, which is a favorite of one of India’s most unique animal species—the one that this forest was designated to preserve. For while India has some of the world’s greatest biodiversity, including 26 primate species from wide-eyed slow lorises to flighty macaques, it has only ever had one non-human ape. If Africa is known for its gorillas, chimpanzees and bonobos; and Indonesia for orangutans; India (and only here, in what remains of its forests across its northeast states), is home to the gibbon. Known as hoolock gibbons, they are one of 19 species of gibbon that spread from East India and into Southeast Asia and China. Two of China’s six species have become extinct in living memory; another two now comprise less than 30 animals. Over the years, India’s gibbons, like Southeast Asia’s, have become endangered, entirely intertwined, as they are, with what remains of the forests of Assam and its neighboring states.

These hoolocks are beautiful, charismatic animals, tailless, around half a meter (20 inches) in height, and weighing no more than seven kilograms (15 pounds). They are covered entirely in a soft fur, save for their faces. It is because of their faces that hoolocks look different from the other 19 gibbon species spread across Southeast Asia. Its females, a light golden-brown, have faces ringed with white fur; the males, entirely black, have somewhat comical eyebrows, thick set and entirely white. Despite the comedic eyebrows above them, both male and female gibbons have large brown eyes set in expressions that appear serious, introspective, almost sad.

They have arms that are strikingly elongated, appendages that they use skillfully to move rapidly across the forest’s uppermost canopies. In Assam, they are known as holou for their haunting houuu-houuu-houuu calls. In neighboring states, and in some tribal areas, the “holou” is suffixed with bandar, monkey. Indeed, because of their size, they are often mistaken for monkeys. But if you pay close attention—something not easy to do if gibbons do not wish to be seen—it becomes evident that these animals are entirely different in deportment and demeanor from the forest’s many troupes of rowdy monkeys.

The long-tailed capped langurs feast midway up tall trees, often in the village fringes, where they also settle down for the night as sunset draws in. Gangs of small rhesus macaques fly up and down lower, thinner trees, marauding through village gardens, rummaging on the ground where their fancies take them. But in the forest, and mostly up into the high canopy around 30 meters above, the gibbons’ grace in motion is striking. If you can find them—camouflaged as they are in the light and shade of the distant leaves—it will be because you followed their call. Even then, you may be no nearer than one kilometer from them, because their songs carry way beyond the distance their diminutive sizes would seem to allow.

Creatures of the morning, their songs are rarely heard after noon, so that tracking them requires waking before sunrise, trekking to the vicinity of their favorite feeding trees, and waiting on the sodden soil underfoot. When the first songs come, they start in ghostly, staccato notes of an overture that gently ascend the scale into a long-drawn crescendo; when the hoo, hoo hooooos from one gibbon is intersected by another’s.

If you follow their call, and crane your neck to catch sight of them, you’ll see the black and buff of the male and female hoolocks sitting close to each other, before breaking into spirited, highly energetic bursts of swinging. Sometimes, as you watch, they seem to drop off a branch, suddenly. The foliage is thick here, so they can hide. As you stand there, your neck aching as you observe them, it’s clear that they are watching you as well. Not just any human, but you. At first they will croak a guttural sound that means they are taking stock of a potential threat. But if you remain at a distance and persist in calmly watching them, they will come closer, unless you return with another person, a stranger to them, in which case, they once again recede into the high canopy.

They are wise to be cautious. India’s gibbons are hunted. For some, their meat is a delicacy; for others, they are medicine, their blood mixed with wine to enhance male virility; for still others, they are an income generator, as they are valuable exports to China, where they are believed to have many other medicinal uses. But while hunting or capture of a few gibbons here and there for zoos is undoubtedly destructive, given their small numbers, it is not their most pernicious threat: In recent years, the loss of their habitat has been significant.

In the 1980s and 1990s, nearly 70 percent of Assam’s hoolock gibbon habitat was lost. The trend has only increased since then. If forests are not being removed wholesale, there are still pressures upon them. People from local communities collect firewood, timber and other products, and bring their animals into the forest to graze. It is not necessarily the wholescale destruction of forests that has led to the demise of the region’s gibbons; the process can be far subtler than that. Small parts of forests cut into for human travel or agriculture have the effect of separating off wooded areas, isolating gibbons one from another. Today, there are no longer any hoolock gibbons in the Goalpara district, the place in which Western scientists first described them in 1830. In nearby Bangladesh, in eight localities which gibbons once frequented, they are now extinct.

A gibbon family will usually need between a quarter and half a square kilometer area of forest if they are going to be able to acquire sufficient food and find safety. With adequate territory, sexually mature males and females normally pair for life, and if all goes well, she will menstruate every 28 days. They will have a baby every three or so years throughout their lifelong partnership. With human competition for the land, however just as finding enough food and territory is becoming ever more challenging for the gibbons, maintaining the core of their family lives is also very much under threat. There have already been reports of “alternative” types of gibbon families—not the preferred one male-one female model, but a variety of different permutations—because of the inability to find a suitable, unrelated mate.

Food gathering behavior has changed as well: as the business of coming onto the forest floor and into human settlements for food has become more risky, gibbons have been seen doing unusual things in the pursuit of food. The apes normally eat fruit, flowers, leaves and insects, and have a phobia of water—but one gibbon in Thailand was filmed next to a riverbank, catching snails. Others have been seen climbing rock faces looking for sustenance.

Hoollongapar’s conservationists have been working hard to minimize the human impact on their gibbons’ territories. They have encouraged less invasive activities through which villagers might bring in an income: small tea gardens; cottage beekeeping for honey and biodiversity; the planting of Ficus tree species in which gibbons and other primates and birds eat and lodge but that are no good for firewood or building.

As for Assam’s state tree, the hollong (whose name resonates so with the hoolocks who call them home), it belongs to the Dipterocarpaceae family, which forms the most characteristic rainforests of pan-tropical Asia. It makes good plywood, and timber, and extensive logging is bringing some species of the tree to the brink of extinction.

So while protecting these animals is valuable in itself, the gibbons’ complete dependence on the trees means that stopping the apes’ demise will also protect the vast and beautiful forests of East and Southeast Asia.