Word: trees
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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. (See pictures of a bonobo Eden...
...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...
...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...
...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...
...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...