"I was at a conference, and a colleague was talking about the locomotion of great apes in the trees," says Lewis Halsey, a physiologist at the University of Roehampton in London. The colleague mentioned that it's tough to measure how these animals use energy. That's when Halsey had an epiphany. "I was working with parkour athletes on another project," he says, studying how much energy the athletes used while jumping and climbing around a city. Why not use these human athletes to stand in for tree-living apes? Halsey and his coauthors were interested in apes such as orangutans that traverse the forest canopy, swinging and bounding from one branch to the next. When an ape reaches a gap between two trees, it might simply jump. If it's feeling more cautious, it might climb down to the ground and back up the next tree, either using the branches like a ladder or hauling itself straight up a hanging vine. Or the ape might use its body weight to wave a flexible trunk back and forth until it sways far enough to reach out and grab the next tree. The researchers built a kind of jungle-themed jungle gym indoors. To simulate trees and vines for climbing, they hung a ladder and a rope. They positioned jumping platforms about 4 and 6 feet apart (above, left). And they erected three vertical fiberglass poles, each 15 feet high, of different thicknesses. These would represent flexible trees for swaying on (above, right). They recruited 28 male parkour athletes in the UK. The athletes donned contraptions that measured how much oxygen they were using as they exercised. Then they leaped, climbed and swayed as if they were orangutans with someplace to be. The oxygen data showed how hard the athletes were working at each task. Climbing vertically, whether on a ladder ("tree") or rope ("vine"), took much more energy than jumping or swaying. That means tree-living apes ought to travel horizontally whenever they can, because vertical detours are expensive. Vertical climbing seems to be relatively cheaper for larger animals, though, Halsey says. Until now, he says, only one paper had tried to model the energy use of an ape crossing a gap between trees. But the parkour study found people using about 10 times as much energy as the earlier model had estimated. This is probably because of all the other things an athlete spends energy on besides moving—gripping a branch, squeezing the muscles, and so on. Of course, even a very springy human is not an orangutan. "Orangutans have longer arms and more flexible shoulder joints" than humans, Halsey says. So the parkour athletes aren't perfect stand-ins for treetop apes. But Halsey expects that the patterns he measured would apply to an orangutan too. Compared to having no data at all about how apes swing through trees, Halsey says, this is "a big step forward." No pun intended. Image: Halsey et al.
Halsey LG, Coward SR, & Thorpe SK (2016). Bridging the gap: parkour athletes provide new insights into locomotion energetics of arboreal apes. Biology letters, 12 (11) PMID: 27881766