Published: November 2008
Orangutans in the Wild
(Originally published in the August 1998 issue of National Geographic)

Backbreaking fieldwork and meticulous attention to scientific detail bring a deeper understanding of the elusive red apes of the Borneo rain forest.
By Cheryl Knott

In darkness, submerged in water up to my neck, I was plunging through a flooded creek, a creek that I had easily jumped over that morning. My husband, Tim, was wading along just a few feet ahead of me, while balancing a backpack full of camera gear on his head. He turned and said, "Are we in Borneo, or what?"

We were slogging our way back from a full day of following a wild orangutan as she searched for food on the rain-drenched mountain slopes of Indonesia's Gunung Palung National Park, near the west coast of Borneo. We had made a research site there our home. Tim was continuing his studies on the animal and plant life high in the rain forest canopy, and I had begun my investigation of wild orangutans. For several months we had been following an adult female we had named Beth, and she was now well past the stage of vocalizing and dropping branches on us, behavior typical of orangutans when they first encounter people. These large red apes can be surprisingly difficult to find and follow. Like fat-bellied acrobats, they seem to traverse the canopy effortlessly, leaving researchers such as me to crash through the undergrowth trying to keep a constant eye on them.

The year before, we had made the trip upriver to the research site in the middle of an unusually severe dry season. The water was too low to use a boat with a motor, so we had to drag our dugout canoes, burdened with more than a year's worth of research supplies, across the sandy river bottom. As we penetrated farther and farther into the forest, a steady stream of white and pink flowers floated down to greet us as if we were in a ticker tape parade. The flowers heralded a so-called mast fruiting—an event in Southeast Asia rain forests when a large proportion of trees bear fruit at the same time. The mast provides a boon for orangutans, like Roman, who gorge themselves on high-calorie fruits.

The sun's last rays turn mist into a flaming yellow blanket, tucking the rain forest in for the night. Lush and teeming with life, this botanical wonder leads many to think of it as a virtual Garden of Eden that produces an unceasing cornucopia of succulent fruits. In reality, though, fruit abundance varies greatly even if temperature varies little.

Periods of high fruit production happen only at odd intervals in Borneo, about once every four to seven years, although smaller fruit peaks occur every year. Being there during a mast fruiting proved ideal for studying how changes in food abundance influence orangutan reproduction and behavior.

Unlike chimpanzees, for example, orangutans do not live in groups. Adult males travel alone, and mothers are usually accompanied only by an infant and sometimes by an older juvenile.

This is partly because the fruits that the animals prefer are widely dispersed and can’t support large gatherings of the apes.

But orangutans are not wholly unsociable. During the mast we've seen as many as eight individuals feeding together in the huge dipterocarp trees that dominate the rain forest. Clearly orangutans get together when the food supply permits.

I spend countless hours sitting on soggy ground observing the animals. One day I watch Roman eat one after another of the pineapple-size durian fruits. Hearing a rustle, I turn and see Rob, a subadult male, throw together a nest in a tree behind me and dive in. Rob lacks the cheek pads and throat sac of fully mature males like Roman and doesn't announce his presence with long, bellowing calls. He eyes Roman and the durian tree. When Roman has eaten his fill, he quits the tree, ignoring Rob as the smaller ape approaches to feed. After gorging on durians, Rob pauses at a small Baccaurea tree to grab a handful of its glossy red fruit.

When he leaves, one of my assistants shinnies up the tree to take some fruit for us to weigh and dry out. I pick up some loose durians that Roman and Rob have knocked down. Later in the laboratory I'll be able to figure out how many calories they have consumed today.

I spread plastic sheets beneath animals sleeping in their tree nests to collect urine to analyze for hormones and signs of disease. In bringing such scientific techniques into the forest, my goal is to gain a deeper understanding of orangutans without intruding on their life in the wild.

When Times Are Lean

Doing a little jungle gymnastics, young Misha hangs by one arm, peeling ribbons of bark from the tree as her mother, the upside-down Marissa, does the same.

After several months of superhigh fruit production the forest now offers slim pickings, and orangutans scramble to find enough to eat. Males sometimes journey to the forest floor (though females rarely do); Jari Manis descends to the ground to suck termites from their nest.

Though they now have to turn to low-quality fruit and vegetation such as bark, leaves, or the celery-like Pandanus that Beth has broken off, they make do.

Such periods of abundance and scarcity have helped shape orangutan evolution. As with humans, orangutans store fat when food is abundant. By measuring the by-products in their urine, I've found that during periods of scarcity they produce ketones—telling me that they are burning up their fat deposits. The extra calories they stored when fruit was plentiful are now helping them survive.

Conflict Between Males

When we came upon Rocky on the forest floor one day, we saw new gashes on his right shoulder. His wounds could have been made only by another male's canine teeth, like those displayed by Rob, in the infrequent but sometimes deadly fights between males. Several weeks later when I was following Rob, I came upon Rocky again. He had become severely emaciated and was in no shape for an encounter with Rob. The two grappled and rolled on the ground until Rob finally left, leaving Rocky even further exhausted.

The next morning we found Rocky curled in a ball on his side, his matted hair giving little cover to his protruding ribs. Around midday he managed to shuffle over to a small tree, where he grabbed a handful of fruit and collapsed again, slowly chewing without lifting his head from the ground. Sadly, I knew that he didn’t have long to live.

With a last burst of energy, Rocky tried to cross a fallen log bridging a stream. Halfway across he faltered and, turning around to go back, collapsed. As an afternoon downpour thundered out a final farewell, Rocky died.

We placed Rocky's body on a crude wood-and-wire frame and hoisted him up for his last trip into the canopy. I hoped that this would keep scavengers, like Borneo's bearded pigs, from scattering his bones, which I wanted to study for signs of disease and injury. Two weeks later we were amazed to see Rocky's clean white skeleton laid out on a bed of stringy red hair. Later I finished the job that the insects had started.

So far as I know, such an orangutan death had never before been seen in the wild. Given the apes' low population densities and long lives, the probability of witnessing a death is extremely low. Even so, Rocky's remains were joined a few months later by those of another adult male, found in a streambed.

These rare finds were most likely the result of an increase in male conflicts arising from the influx of orangutans drawn to feast on the abundant fruit. We had seen as many as six large—and mutually intolerant—adult males ranging within a small region. Here was natural selection at work: Countless contests between adult males during orangutan evolution likely contributed to males being twice the size of females.

The battle between males to mate has reproductive payoffs but is not without cost. Testing urine samples, I have detected significant signs of infection resulting from wounds that can, as in Rocky's case, lead to death.

Field Data

When the apes turn in early, our open stilt house provides a retreat to review the day's data. At dawn I am back on the forest floor, collecting urine from an orangutan just awakening in her nest above. I can test the urine on site for signs of menstruation, infection, and weight loss.

I save some samples for later genetic analysis, so I can learn who is fathering offspring and how the animals are related. I'll also analyze the hormones in the urine back at the lab.

By measuring hormones for the first time in wild orangutans, I've found that estrogen levels increase when nutritional status improves. Pregnant Beth conceived her baby Bekti during the mast fruiting, when estrogen levels were high.

Growing Up Slowly

"Misha's got a rat!" I yell to Tim in my astonishment. The young ape bites off the rat's head and swings it by the tail like a stuffed toy.

Orangutans rarely eat meat, and in this case Misha seems motivated more by curiosity than by appetite. She also occupies herself with the orangutan version of playing house, making a simple nest and practicing the skills of independent living she'll need when Marissa, her mother, one day turns her attention to a newborn.

The pioneering work of Biruté Galdikas suggests that orangutans bear offspring only once every eight years on average—an extremely long interval among mammals.

After a mother gives birth, her baby will cling to her for several years, rarely venturing away from her side, and continue to nurse for about six years. A juvenile sibling may stay with its mother for a few years, as does Emy with mother Ely and her infant.

Emy is learning how to fend for herself since, unlike human mothers, orangutans normally do not provide food for their offspring beyond lactation.

One of my research goals is to try to determine why these periods of juvenile dependency last so long.