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.
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