Like Tarzan, Orangutans Glide Through Trees

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DLILLC / Corbis

Husky and hairy, with the flaming orange coloring of a bad dye job, the orangutan doesn't look like the most agile of primates. But in fact these great apes, native to the jungles of Malaysia and Indonesia, climb and swing nimbly through the canopy more than 100 feet in the air, their fleet-footed acrobatics allowing them to make their home in the treetops and access remote food sources, like the fruit at the very ends of branches.

Until recently, however, scientists did not understand exactly how such a large primate — weighing up to 180 lb., orangutans are the largest living arboreal animal — can navigate the delicate branches at the top of the tallest trees. At that height, tree branches are thin and begin to wobble as animals climb on them, much as a suspension footbridge vibrates as people walk over it. Too much vibration and an orangutan can be thrown off altogether. From high in the trees, such a fall would be deadly.

Now a new study published in the July 27 Proceedings of the National Academy of Sciences (PNAS) shows that orangutans keep their footing — and their fingering — in the trees by moving with an irregular, offbeat rhythm that effectively counters the shaking caused by their considerable weight. "Orangutans rock flexible tree trunks from side to side with increasing magnitude until they can cross gaps in the [tree] canopy," says Susannah Thorpe, a bioscientist at the University of Birmingham in England and the lead author of the PNAS paper.

To understand how orangutans swing, it helps to compare them to their primate cousins, the chimpanzees. Chimpanzees pull their bodies close to the tree branch as they move, but being relatively small, they can do that without worrying about the vibrations caused by their own body weight. If orangutans behaved that way, the vibrations would build dangerously, as they do on a suspension bridge.

Instead, Thorpe and her colleagues found that orangutans move irregularly, shifting from side to side, moving backward and forward, using all four limbs at once, even walking upright on branches — all to keep disturbance to a minimum. In a way, they climb like humans might, if we were transplanted to the Sumatran jungle. "They move a bit like Tarzan in the old movies, swinging from branch to branch — only, orangutans do it like they do everything else, much more slowly," says Thorpe, whose team obtained nearly 3,000 visual observations of orangutans in motion during a yearlong study.

The orangutans' unique locomotion also helps them reduce the time and energy needed to climb. The more flexible a tree branch is, the more it will bend under an animal's weight. "That means they can lose height, and gaining height again is costly because you have to oppose gravity," points out Thorpe. When an orangutan leaps from a flexible branch it also loses motion energy — think of jumping off a pile of sand versus one of asphalt — and when they land on a flexible branch, they have to wait for the vibrations to stop before they can jump again, which costs more time.

But the PNAS study found that by swaying from one flexible tree branch to the next, orangutans actually use less energy than they would if they leaped from branch to branch, or if they climbed down trees, moved on the ground and climbed back up again. (The fact that the Sumatran tiger — before it became critically endangered — was a serious threat to the orangutan probably helped encourage tree travel.) Climbing helps the orangutan adapt neatly to its arboreal environment.

The PNAS study contains some other less than surprising facts — for instance, adult females swing conservatively when it comes to tree travel, while males and adolescents are the risk takers. But the ultimate point is that orangutans, as odd and ungainly as they look, are uniquely adapted to the jungle, to life among the trees — an existence that is being threatened by the continued logging of Southeast Asian jungles. "Orangutans can move in logged forest, but the energetic cost may be much greater, and food availability is likely to be lower, so populations become less healthy and less viable in the long term," says Thorpe. A 2007 United Nations report estimated that if current trends continue, 98% of forests in Sumatra and Borneo could be gone by 2022. If that happens, the orangutan could follow — the first of the great apes to disappear.