How do orangutans swing through the trees? Carefully.

That awful joke takes us smoothly into the first of two studies today on the science of swinging.

First up: in a paper published in PNAS three researchers describe their work on how Sumatran orangutans move on spindly branches that appear incapable of holding their hefty weight. It seems that the animals carefully avoid setting up resonance effects of the kind that causes bridges to wobble when people march in step across them.

“We found that certain locomotor behaviours clearly are associated with the most compliant supports; these behaviours appear to lack regular limb sequences, which serves to avoid the risk of resonance in branch sway caused by high-frequency, patterned gait,” write the authors.

“Balance and increased stability are achieved through long contact times between multiple limbs and supports and a combination of pronograde (horizontal) and orthograde (vertical) body postures, used both above branches and in suspension underneath them.”

While the fact that orangs move through trees by gripping multiple branches and not using regular movements may seem obvious, the animals do differ from other primates, which often suspend themselves below branches rather than carefully balancing their way through, say the authors. Smaller animals also have to worry less about those resonance effects.

This knowledge could actually benefit conservation efforts, says study author Susannah Thorpe, of the University of Birmingham: “Now that we know more about how they move through the trees and the unique way that they adapt to challenges in their environment, we can better understand their needs. This could help with reintroducing rescued animals to the forests and efforts to conserve their environment.” (press release.)

Finally: A hairy beast swinging between trees in the jungle. Does this sound familiar? It does to Thorpe: “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.” (Time.)

Organutans then: lazy, red-head, Tarzans. Now on to humans.

In another finding that seems obvious but isn’t, a different team of researchers have found that humans swing their arms when walking because it uses less energy than holding them still.

“Humans tend to swing their arms when they walk, a curious behaviour since the arms play no obvious role

in bipedal gait. It might be costly to use muscles to swing the arms, and it is unclear whether potential

benefits elsewhere in the body would justify such costs,” they write in Proceedings of the Royal Society B (paper not online yet).

To solve this conundrum they first modelled walking with free-swinging arms and found that arm swinging ensued even with no torques driving the arms. Then they took measurements from ten human subjects and found that holding arms still required 12% more energy than normal arm swinging. Opposite-to-normal swinging increased subjects metabolic rate 26%.

As the researchers write, in a quote much favoured by the news coverage of this finding:

Rather than a facultative relic of the locomotion needs of our quadrupedal ancestors, arm swinging is an integral part of the energy economy of human gait.

Pun-watch

AFP and the Guardian both run with the headline “Out on a limb”.

AFP also give us: “Arm-swinging turned out to be a plus, rather than a negative, the investigators found. For one thing, it is surprisingly, er, “‘armless” in energy costs.”

Groan. That’s worse than my orangutan joke.

Image: Susannah Thorpe.