Unlike gorillas and chimpanzees—fellow great apes that live in groups and can be followed and observed relatively easily—orangutans live mostly solitary lives. They spend nearly all their time in the treetops, they wander widely, and for the most part they inhabit rugged forest or swampy lowland that’s hard for humans to traverse. As a result, orangutans long remained among the least known of Earth’s large land animals. Only during the past 20 years or so has scientific evidence begun to outweigh speculation as a new generation of researchers has tracked the elusive apes across the islands of Borneo and Sumatra, the only places orangutans live. Using drones to find and follow orangutans in rugged terrain is a possibility.
Female orangutans give birth only every six to nine years. No other mammal has a longer interval between births. The juvenile will stay with its mother for up to 10 years as she passes on essential survival skills. Among them: how to find the most nutritious rain forest fruits. Humans and orangutans are so similar that researchers use standard drugstore test kits on urine from female orangutans to determine whether they’re pregnant. They collect and preserve urine from female orangutans on filter paper so that the samples could be tested for hormones later.
Typical of many forests in southeastern Asia, the trees at Indonesia’s Gunung Palung National Park produce little or no fruit in most seasons. Then, every four years or so, trees of various species simultaneously bring forth massive amounts of fruit in a process called masting. Reproductive hormones in female orangutans peak when fruit is most abundant in the forest—an adaptation to the boom-and-bust environment. The put on weight during these high-fruit periods, and then live off that during the low-fruit periods. During these high-fruit periods, females are more likely to conceive.
Males weigh as much as 200 pounds, orangutans are the world’s largest tree-dwelling animals. Although orangutans spend most of their time in the trees, some males climb down often and walk significant distances on the ground.
Orangutans display resourcefulness, cunning, and intelligence through complex behaviours: kiss squeaking and snag crashing, leaf wiping, nestbuilding and umbrella making (use leafy branches as a makeshift umbrella, a learned behaviour and an example of orangutan “culture” passed down from one generation to the next), termite feeding and thirst quenching. Orangutans learn most of the behaviours illustrated here from their mothers before reaching adolescence around age 12. Some of the behaviours are ubiquitous. Others occur only in certain areas, suggesting there could be regional cultures.
In the 1980s and ’90s, some conservationists predicted that orangutans would go extinct in the wild within 20 or 30 years. Fortunately that didn’t happen. Many thousands more orangutans are now known to exist than were recognized at the turn of the millennium. The overall population of orangutans has fallen by at least 80 percent in the past 75 years. Between 40,000 and 100,000 live on Borneo and 14,000 survive on Sumatra. Much of this loss has been driven by habitat destruction from logging and the rapid spread of vast plantations of oil palm, the fruit of which is sold to make oil used in cooking and in many food products. As many as 65,000 of the apes may have been killed on Borneo alone in recent decades. Some were killed for bush meat by people struggling to survive. Others were shot because they were raiding crops—or protecting their young. The expressive, heart-melting faces of baby orangutans make them highly valuable in the black-market pet trade, within Indonesia as well as smuggled out of Borneo or Sumatra to foreign destinations. The ferocious protectiveness of female orangutans means that the easiest way to obtain a baby is to kill the mother—a compounded tragedy that not only removes two animals from the wild but also eliminates the additional offspring the female would produce during her lifetime.
Rehabilitation centers such as International Animal Rescue near Gunung Palung have a steady influx of orphaned orangutans showing that this killing remains a serious problem. More than a thousand orangutans now live at rehab sites, and though the goal is to release as many as possible back into the forest, attempting to teach survival skills to young orangutans is challenging and unproven. Unequipped to survive in the wild after years as a pet, these orangutan will spend their lives at a rescue center. More than a thousand orangutans now live at rehab facilities.
Smoke from forest fires blanketed much of Borneo in late 2015. Fires set illegally to clear trees for oil palm and other crops burned more than six million acres across Indonesia, damaging or destroying large swaths of orangutan habitat.
A recent boom in research is revealing a surprising range in their genetic makeup, physical structure, and behaviour—including the beginnings of cultural development that could help us understand how we transitioned from ape to human. For centuries, scientists considered all orangutans to belong to one species, but in the past two decades new insights have led researchers to see Bornean and Sumatran orangutans as distinct species, both of which are critically endangered. Surprisingly, researchers have found that a recently discovered population at a site called Batang Toru in western Sumatra is actually closer genetically to Bornean orangutans than to other Sumatran populations—possibly the result of differing waves of migration to the islands from mainland Asia. The Batang Toru orangutans are believed by some researchers to diverge from others enough to constitute a third species. Numbering as few as 400 individuals, they’re threatened by a proposed hydropower project that would fragment their habitat and open the area to more human intrusion, including illegal hunting.
What’s more, several populations on Borneo are now deemed to be separate subspecies, based on factors such as differing body types, vocalizations, and adaptations to the environment. The diversity of orangutans extends even further—into differences whose origins continue to resist scientific understanding.
Stretching his arms to their full span of seven feet, Sitogos moves through the canopy by using his long-fingered hands and dexterous feet to clamber from branch to branch. A young female, Tiur (“optimistic”) is much smaller and more delicately built. Sometime in the recent past, Sitogos had undergone an astounding transformation. He’d spent years hardly larger than Tiur. Then, with testosterone flooding his body, he’d grown powerful muscles, longer hair, fleshy pads called flanges on the sides of his face, and a massive throat sac to amplify his calls. Access to Tiur and other females for mating—is Sitogos’s reward, but his physical change has a price too. From somewhere in the distance comes the call of another male orangutan. Sitogos stands up, transfixed, and begins moving toward his challenger.
The males of many species of animals undergo major physical changes as they mature, but for orangutans the process is especially intriguing. Not all males develop the massive bodies, facial flanges, and throat sacs shown by Sitogos. Many retain smaller bodies long after they reach sexual maturity, transforming years later than other individuals. Some remain undeveloped their entire lives. The mechanism behind this divergence, called bimaturism, ranks among the greatest mysteries of zoology.
In the forests of northern Sumatra, only one dominant flanged male maintains control over a local group of females. Many males in the area retain smaller bodies and don’t develop flanges, thereby avoiding the confrontations that inevitably occur when several males try to assert dominance (until they themselves can try to move into the dominant role). For the smaller males, the only chance to pass on their genes is to watch from the sidelines, out of reach of the boss, sneaking in for mating whenever possible.
In Borneo, by contrast, nearly all males develop flanges. They wander across large areas, with no one male maintaining an associated group of females. A male’s best chance at mating is to grow strong and join the competition, leading to more confrontations and injuries.
Some scientists believe the dichotomy between male orangutans arose in part because of the differing geologic histories of Sumatra and Borneo. Sumatra is more fertile than Borneo, where ancient, weathered soil lacks plant nutrients, and many forests see the boom-and-bust cycles of masting fruit trees, leading to periods of low food availability. Orangutans on Sumatra don’t have to travel far to find enough food, and female density is higher. This gives males the ability to remain in a single place and develop associations. The relatively poorer environment of Borneo has created a free-for-all in which individuals roam over large areas, finding food and mating opportunities where they can.
A long call is a complicated and thrilling medley of deep rumblings and bubbling hoots that can carry a mile through the forest. Usually males’ long calls last less than a minute, but can continue for more than five minutes. He proclaims his power to rival males and potential female mates alike.
This may explain why the development of dominant male characteristics differs between the islands. But it brings up a far more difficult question. How does a Sumatran male know that if he grows flanges and he’s not the boss, he’s not going to be successful at mating? The answer, of course, is that the male doesn’t “know,” in the human sense. It’s not something they can learn, there has to be a switch, the sensitivity of the switch has to be different for different populations, and it has to be somehow genetic. This question of how male development is triggered remains unanswered, in part because of the same challenge that faces orangutan researchers on so many fronts: Their subjects are just so difficult to study.
In addition to their physiological diversity, orangutans exhibit differences in behavior that are passed from individual to individual and generation to generation in ways that can legitimately be called cultural. Primates aren’t supposed to do vocal learning, and yet, unless you believe this is genetic, which we think we can reject, then it’s very likely that it’s cultural. What orangutans do isn’t like the human voice, but the comprehension and learning and imitating of sounds is there. A call used by mothers when they reassure their kid is the “throat scrape”. We had a female that we knew pretty well before she gave birth for the first time. The day after giving birth she already gave that call. It had never been heard before from her. It’s clearly something she learned from her mom.
Unlocking all the secrets contained in the brains and bodies of these great-ape relatives means preserving the entire spectrum of adaptations. If every group is unique, it’s not good enough to say we’ll protect them at just a few spots. The loss of any single population brings an end to any chance to learn from its unique environmental and cultural adaptations.
Marc Ancrenaz, since 1996 has directed an orangutan research and conservation project on the Kinabatangan River, in the Sabah region of northeastern Borneo. Here several hundred orangutans live in a narrow corridor of degraded habitat along the river, among villages that themselves are surrounded by a sea of oil palm. The patchy woodland is nothing like the “virgin rain forest” usually associated with orangutans. Of course they would prefer primary forest, but this is what they have. Twenty years ago science thought orangutans couldn’t survive outside primary forest and were very surprised here. How come orangutans are in a place where they are not supposed to be? Several researchers see the human-altered landscape as vital to orangutans’ survival. This is the future of biodiversity.
In western Borneo, Knott has set up an organization to work with local communities to develop sustainable alternative livelihoods, reduce illegal logging and poaching, and provide conservation education in areas surrounding Gunung Palung National Park. In the same spirit, they have established conservation education programs in Sabah schools and communities, trying to find ways that people and nature can coexist – partners with people living along the Kinabatangan, helping them make money from orangutans and other wildlife through ecotourism and related enterprises. The hope is that residents will become invested in the survival of animals. Remote villages are the front line for wildlife conservation. If we don’t incorporate local people into our plans, I think we’re going to fail.
For orangutans to survive in their present diversity, governments and conservationists must make smart choices about where to establish preserves, how to manage them, and how to use limited resources. They must find ways for the species to coexist with humans on two islands where habitat is constantly shrinking. Conservation has to be backed up with strong science. The goal of people doing research is to produce better knowledge, better understanding of orangutan ecology and genetics. The rest is actually using this knowledge to impact land use and communities. This is where conservation takes place.
In the forests of Borneo and Sumatra, orangutan behaviour determined by millions of years of evolution endures: Males challenge each other with their calls, young males wait for their chances to assert dominance, and females teach their young how to survive in the treetops. Some of the mysteries of their lives have been revealed. What else we learn will depend on the success of this teaming of science and conservation, seeking answers about the links between humans and these apes that seem so like us when we look into their eyes.