Orangutans Swing Through Trees Like Acrobats

July 27, 2009 -- Hefty orangutans wield their up to 180-pound bodies on flimsy treetop branches using special acrobatic maneuvers, according to a new study that could have implications for habitat conservation and reintroduction of the endangered species.

The findings, published in the latest Proceedings of the National Academy of Sciences, come at a critical time, since Sumatran orangutans are now on the verge of becoming the first great ape to go extinct in modern history.

At first, the researchers were astounded watching the circus star-like movements of wild Sumatran orangutans in the Gunung Leuser Ecosystem, which comprises pristine rainforest in Sumatra.

Project leader Susannah Thorpe told Discovery News that, in terms of navigating trees, orangutans "can basically do anything: hang from branches or stand on top of them with their limbs in any direction and each limb can be doing something completely different to the others."

Thorpe, a lecturer in locomotor ecology and biomechanics at the University of Birmingham, and colleagues Roger Holder and Robin Crompton noted the smooth moves of orangutans after conducting field studies on these primates, which are the world's largest habitually arboreal mammal.

Two orangutan treetop maneuvers really stood out. The first, which Thorpe nicknamed "the quadrupedal scramble," involved orangutans crossing gaps between trees by "scrambling on all fours between the branches at the fringes of the tree crowns."

WATCH VIDEO: The orangutans of Orangutan Island walk upright and on the ground more than their wild counterparts.

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The second, which she called the "tree sway," "is where orangutans rock flexible tree trunks from side to side with increasing magnitude until they can cross gaps in the canopy."

Thorpe added, "They can also sway tree vines hanging from above, so that 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!"

Wondering how the orangutans managed these techniques on such flexible, seemingly unstable supports, the researchers calculated a "stiffness score" for the branches. These calculations revealed orangutans handle single branch supports not much smaller than about two inches in diameter. If the orangutans can grab onto multiple branches with their hand-like feet, in addition to their actual hands, they can deal with supports measuring just over an inch in diameter.

If these limits aren't met, or if other threats are present, orangutans can come tumbling down.

"I have seen, for example, a large adult male that was feeling off color, which affected his balance, so he was really shaky and slow as he moved through the trees," Thorpe said.

"I have also seen an infant fall about 20 meters (over 65 feet) and hit the ground," she added. "His mum raced down to the ground and picked him up and fortunately he seemed unhurt, just a bit shaken. But orangutans do fall sometimes."