Gibbons really look assured as they swoop from handhold to handhold through the forest canopy. But the risks are high; miss a handhold and at best, a gibbon is in for a bone-crunching tumble. A brachiating gibbon has two distinct gaits as it negotiates a path through its jumbled environment. The first, `continuous contact' brachiation, is used when branches are closely spaced and one of the gibbon's hands is always in contact with a handhold. But during `ricochetal' brachiation, the ape releases hold of one branch and flings itself through the air before reaching the next. John Bertram is fascinated by these `extreme brachiators', and has built a series of models that describe both gaits. By thinking of the gibbons as simple swinging pendulums, he realised that in theory, they could swing continually between handholds with the minimum effort by avoiding `colliding' with their next handhold. He also realised that gibbons can avoid a collision in two ways;either by just reaching the top of their swing as their free hand grasps the next branch, or, cleverly matching the trajectory of the next swing with the final moments of their previous swing. But when Jim Usherwood and Bertram began analysing the ape's trajectory as they swung between handholds, they realised that gibbons always swung more enthusiastically than they needed to;they overshot (p. 1631). But surely they would waste energy by colliding with the next handhold. Intrigued by the energetic paradox, Bertram and Jim Usherwood began investigating a gibbon's complex gyrations as it swung through a less challenging environment;the lab.

Visiting Susan Larson's lab at Stony Brook University, Bertram began working with a female gibbon called Georgia. As Georgia brachiated between a series of evenly spaced handholds, he videoed her movements. And by varying the distance between the handholds, Bertram convinced her to move using both gaits.

Back in Florida, the team noticed that when Georgia retained contact with at least one handhold she twisted her body and raised her legs just before reaching the next handhold, like a child moving its legs on a swing. Bertram and Usherwood realised that Georgia had altered her trajectory at the last moment by moving her centre of mass, and matched the trajectories to avoid a collision. And by simply dropping her body when she carried on into the next swing, she retained almost all of the energy that would otherwise have been lost.

However, when Usherwood and Bertram analysed Georgia's movements when she geared up for ricochetal brachiation, they realised that she was overshooting for another reason: safety. By aiming beyond the handhold, she gave herself an error margin, in case she misjudged the distance. After all `overshooting for safety costs, but in this case, lack of safety costs more,' says Bertram.

But they also think that the gibbon's caution could be one reason why they have such long arms. Usherwood explains that when Georgia overshot her handhold, there was a sudden yank on her arm as her flight path jolted into a swing. But the longer the arm, the smaller the jolt; saving Georgia from paying too high an energetic price for her safety margin.

References

Usherwood, JX. R. and Bertram, J. E. A. (
2003
). Understanding brachiation: insight from a collisional perspective.
J. Exp. Biol.
206
,
1631
-1642.