Characteristic of gibbons’ arboreal bipedal walking.

Abstract

Abstract Recent fossils of human bipedal ancestors, Ardipithecus ramidus, differed from any extant African ape. About the other relatives, living Asian gibbons, their habitats, forests, however, hid their lifestyles, especially locomotion by leaves in canopies. This study aimed to eliminate this obstacle and create a new comparison target for human bipedal evolution using a photogrammetry Software, 3DF zephyr which makes reliable measurements for absolute coordinates of bare deciduous trees in locomotion by gibbons. Here I show that white handed gibbons Hylobates lar, used not only suspension (brachiation) but also arboreal bipedal walking besides quadrupedal moving. The vertical angles of moving varied among their arboreal moving three styles. Less than 50 degrees of vertical angles, the gibbons progressed by brachiation or by bipedal walking. More than 50 degrees of vertical angles, the gibbons progressed by quadrupedal moving. The gibbons did not progress horizontally in quadrupedal moving like knuckle-walking by African chimpanzees and gorillas. The gibbons appeared to keep their torsos erect in all the three moving styles. The gibbons are not the specialists of brachiation, but use three arboreal locomotion patterns including arboreal bipedal walking. This is similar to our bipedal ancestor, Ardipithecus ramidus which lacks any characters typical of suspension, vertical climbing, or knuckle-walking.