National Geographic : 2016 Dec
70 national geographic • December 2016 as 400 individuals, they’re threatened by a pro- posed 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, vocaliza- tions, and adaptations to the environment. The diversity of orangutans extends even further— into differences whose origins continue to resist scientific understanding. From hiS perch high in the rain forest canopy of Sumatra, a big male orangutan known as Sito- gos jumps to the trunk of a dead tree and, using all his 200 pounds, rocks it back and forth until it snaps at the base. At the last moment Sitogos leaps to a nearby limb, while the tree falls toward me with an enormous crash. Orangutans do this a lot when they’re mad, and they’re very good at it. The tree couldn’t have been aimed any more accurately if it had been laser guided. Sitogos means “the strong one” in the Batak language of northwestern Sumatra. True to his name, the big male stares down at me, shakes the branch he’s holding, and gives a guttural, bub- bling call. There may be Sumatran tigers and sun bears roaming the forest floor, he seems to say, but up here in the treetops, I’m the king. Stretching his arms to their full span of seven feet, Sitogos moves through the canopy by us- ing his long-fingered hands and dexterous feet to clamber from branch to branch. A young fe- male, Tiur (“optimistic”), follows his every move, approaching closely whenever he pauses. Much smaller and more delicately built, she persists in her pursuit even though he seems indifferent. They sprawl on a branch together, eating flowers and breaking off cuplike fern fronds to drink the water inside. When he leans forward against a limb, Tiur grooms his back. Sometime in the recent past, Sitogos had un- dergone an astounding transformation. He’d spent years hardly larger than Tiur. Then, with testosterone flooding his body, he’d grown pow- erful muscles, longer hair, fleshy pads called flanges on the sides of his face, and a massive throat sac to amplify his calls. The sybaritic scene in the forest canopy—the devoted attention of Tiur and access to her 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 an- other male orangutan. Sitogos stands up, trans- fixed, 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 in- dividuals. 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 in- evitably 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 devel- op 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. Preserving old-growth rain forest is crucial, but the human-altered landscape is also vital to orangutan survival.