The proboscis monkey is classified as Endangered by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species, and populations are currently in decline.

Proboscis monkeys face a host of threats in the wild. In particular, habitat loss is a major contributor to this species’ decline. All across Borneo, forests are being degraded and destroyed, cleared for timber or to make space for farms and human settlements. Palm oil and rubber plantations have replaced swaths of what were once some of the world’s most biologically diverse forests with carefully managed monocultures. The island’s mangrove forests are being cut down to make way for shrimp farms. Without these habitats, healthy and intact, proboscis monkeys cannot survive.

As arboreal primates, proboscis monkeys are vulnerable to any and all changes in the canopy. This means that forests don’t have to be completely cleared in order to negatively affect them. When patches of forest are left, they are often cut off from each other. This phenomenon, called forest fragmentation, limits an arboreal primate’s ability to roam freely and widely. Even a small gap in the canopy made by a road, can become an impassable obstacle for them. In this way, groups become isolated, limiting their access to food as well as mates and increasing the likelihood of inbreeding.

With inbreeding come a host of other threats. When gene flow slows or halts altogether, new generations become more vulnerable to diseases and parasites, and individuals may be born with debilitating birth defects. All of these changes severely decrease the chances that new generations live long enough to ever procreate. For proboscis monkeys, who never stray far from their natal rivers, the problems that forest fragmentation creates become even more detrimental. By greatly reducing their area of possible distribution, the clearing of riverbanks and mangrove forests has highly adverse impacts on proboscis monkey populations throughout Borneo.

As a biodiversity hotspot, Borneo represents some of the most unique and fragile ecosystems in the world. The extinction of one species initiates a domino effect, ensuring that others begin to die out in turn. As seed dispersers, the loss of proboscis monkeys brings their forest ecosystems to a screeching halt. A lack of new trees renders the few remaining forests unhealthy, limiting the diversity of species capable of thriving in them. The effects of global climate change are already, and will continue to, exacerbate these problems, not to mention create their own. Borneo is not just fragile, it’s irreplaceable. Once Borneo’s habitats become degraded, rejuvenation is unlikely; once species are destroyed, there is no way to bring them back. In this way, Borneo’s identity and fate are both tied to that of the proboscis monkey, not to mention all of its countless endemic species, both discovered and undiscovered.

Borneo’s habitats are also threatened by forest fires. Southern Borneo’s rainforests grow in an accumulation of partially decayed vegetation or organic matter known as peat. Peat builds up in wetlands and, because it holds water well, causes conditions in the area to gradually become wetter and wetter. As a result, the wetlands grow. During the last several decades, draining Borneo’s peatlands has become a common practice for farmers and loggers. After drainage, the peat dries out, becoming highly flammable. Farmers have been known to purposely set drained peatlands on fire in order to quickly clear the area. Once it breaks out, a fire is nearly impossible to extinguish as it will continue to burn invisibly underground. The 1997–98 forest fires inflicted extensive damage on the habitat of proboscis monkeys and are believed to have caused the most significant loss of habitat among all primate species in Kalimantan (Indonesia).

On top of these threats, proboscis monkeys are frequently hunted in Borneo. Because they spend hours at rest while they digest their leafy meals, they are easy targets for poachers. Once found, entire communities can be wiped out in little time and with little effort. While they are mostly hunted for meat, they are frequently killed in pursuit of bezoar stones. Bezoar stones are small masses that form in a creature’s intestine, which practitioners of ancient Chinese medicine claim (without scientific evidence) can cure ailments and neutralize poisons. The practice of hunting proboscis monkeys seems to have begun inland, but it is now spreading towards Borneo’s coasts.

While rehabilitation and release programs have worked to return rescued proboscis monkeys to the wild, their complex diets, impossible to replicate in captivity, make breeding programs unfeasible. Once they are gone from the wild, they are gone.