Photogrammetry Disclosed Gibbons’ Arboreal Bipedal Walking and Its Characteristic in Canopies

Abstract

Before our terrestrial bipedal ancestors, our ancestors lived in canopies. The behavior of these arboreal primates was unknown, because leaves in canopies hide the arboreal primates, especially Asian gibbons, our related species. Differing from tropical rain forests, in temperate zones deciduous trees spontaneously drops leaves in autumn and winter, then the primates in canopies become visible. Here I show that white handed gibbons Hylobates lar, used not only suspension (brachiation) but also arboreal bipedal walking besides quadrupedal moving in canopies. Using a photogrammetry Software, 3DF zephyr which makes reliable measurements, 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 arboreal moving styles. In Asian rainforests, tigers, Panthera tigris, the large terrestrial predators might prevent gibbons from terrestrial bipedal walking.