The gibbons, also fruit-eaters, have followed a very different line of development. lncreasing size may have been the stimulus that made apes start to swing beneath branches but the ancestral gibbons subsequently exploited the new style of locomotion to the full by becoming smaller again. In the end they developed into even more accomplished acrobats than any balancing, branch running monkey. A gibbon in motion in the tree tops is one of the most glorious sights the tropical forest has to offer. With a supple grace that is breath-taking, they hurl themselves nine or ten metres across space, grabbing isolated branches and swinging themselves off again in another dazzling swoop through the air. The arms that enable them to be acrobats in the air are as long as their legs and torso combined, and if they do come to the ground, they have to be held above its head out of the way. Its versatile grasping primate hands have also become specialised at the cost of some of their manipulative abilities. Swinging at gibbon speed requires that the hands be used as hooks that can be latched swiftly on to a branch and then detached almost instantaneously. Thumbs get in the way, so they have moved down towards the wrist and become much reduced.