Orangutans spend their lives swinging in trees and eating fruit. Neither of those things is all that surprising for small animals that don't need tons of energy — but it's distinctly weird for such large primates to live that way.
It's all a question of energy balance — a fruit-based diet like the one orangutans prefer won't provide much raw energy, while choosing to live up in the trees instead of padding about on the ground really should require a lot of it. To solve this apparent paradox, Dr. Lewis Halsey and his team at the University of Roehampton enlisted some expert practitioners of parkour to simulate orangutan movements in a controlled environment, on the grounds that a bunch of dudes who are really, really into parkour are still slightly easier to work with than the orangutans themselves.
You can see the results in the BBC News report up top. According to the researchers, the orangutans' secret is to use the natural moment of the trees to keep their own energy costs down. Here the primates' extra mass is actually a benefit, allowing them to make their tree sway back and forth until it's close enough to the next tree to move on. It's not necessarily going to be the quickest way to move about, but it is energy-efficient, saving about 90% of the energy it would take to climb down one tree and up the next one.
It's also considerably safer than going to the ground — as the researchers point out, the orangutans of Sumatra share their environment with tigers, which means any time spent out of the trees is time spent courting death. But this does leave the orangutans dependent on trees being close enough to sway from one to the next — the orangutans can jump, but they can't do it often because of what Dr. Halsey calls their "energetic knife-edge."
That's troublesome, as Dr. Halsey told BBC News, because human expansion into their ecosystem has meant many trees are being cut down:
"They're very large animals and their food intake is quite poor, so everything they do is geared towards being energy efficient. As their environment is affected by humans cutting down trees, they are coming across more gaps and those gaps are bigger and more expensive."
Indeed, fellow researcher Dr. Sam Coward of the University of Birmingham suggests that, if patches of forest do become too far apart, the orangutans might become stranded in their own little islands of contiguous trees, unable to sway or jump to other parts of the forest.