BioEd Online

Red apes have an easy life: their preferred method of getting from one tree to the next in the jungle not only keeps them safe from harm, but also saves them a lot of hard work. A detailed investigation into how orangutans use the sway of branches to propel themselves from tree to tree shows that it is way more efficient than climbing down one tree and up the next.

Susannah Thorpe at the University of Birmingham, UK, and her colleagues studied video footage of Sumatran orangutans. These are the only primates known to live exclusively in the tree canopy, in part because of the Sumatran tiger and other predators that await them on the ground.

Crossing the gaps between trees is crucial for these animals. But the shortest gaps are also where the branches are thinnest and most flexible. "If you put an orangutan on a flexible branch it's going to sink fast," Thorpe says. Orangutans have a strategy to avoid this problem: they go to stronger vertical branches nearer the tree trunk and, by shifting their body weight, sway them until the thin branches of the next tree are within reach. They can then either move over directly or use the thin branch to pull a stronger branch of the next tree towards themselves and cross over that way.

How efficient is this? Thorpe calculates that the energy the orangutans use in their swaying manoeuvre is half as much as the energy needed to leap (although orangutans are generally too heavy to leap from tree to tree), and an order of magnitude less than the energy they would use if they climbed down, walked across to the next tree, and then climbed up again. "It's a huge difference," Thorpe says. "They've evolved a very economical way of crossing these gaps."