Supplementary information accompanies this paper at http://www.nature.com/scientificreports ; The orangutan is the world's largest arboreal mammal, and images of the red ape moving through the tropical forest canopy symbolise its typical arboreal behaviour. Records of terrestrial behaviour are scarce and often associated with habitat disturbance. We conducted a large-scale species-level analysis of ground-based camera-trapping data to evaluate the extent to which Bornean orangutans Pongo pygmaeus come down from the trees to travel terrestrially, and whether they are indeed forced to the ground primarily by anthropogenic forest disturbances. Although the degree of forest disturbance and canopy gap size influenced terrestriality, orangutans were recorded on the ground as frequently in heavily degraded habitats as in primary forests. Furthermore, all age-sex classes were recorded on the ground (flanged males more often). This suggests that terrestrial locomotion is part of the Bornean orangutan's natural behavioural repertoire to a much greater extent than previously thought, and is only modified by habitat disturbance. The capacity of orangutans to come down from the trees may increase their ability to cope with at least smaller-scale forest fragmentation, and to cross moderately open spaces in mosaic landscapes, although the extent of this versatility remains to be investigated. ; Financial support was given to: MA: Foundation Abraham, Arcus, Care for the Wild, CGMK, Elephant Family, Ensemble, Margot Marsh, Mohamed bin Zhayed, Shining Hope, Stichting Bring the Elephants, WCN, Wood Tiger, World Land Trust, Zoos of Apenheul, Basel, Beauval, Boise, Cleveland, Chester, Columbus, Houston, la Palmyre, Oregon, Philadelphia, Phoenix, Saint Louis, Toronto, Victoria, Woodland, US Fish and Wildlife, EAZA, Great Ape TAG Conservation Initiative, the Orangutan Project; EM, Arcus; AJH, JR, DWM: Darwin Initiative, U.K. Department for Environment Food and Rural Affairs, Panthera, Kaplan family, Clouded Leopard Project/Point Defiance Zoo and Aquarium, Felidae Conservation Fund, HGH Wills, International Trust for Nature Conservation, Wild About Cats, Houston Zoo; HS: Japan Society for the Promotion of Sciences as Grant-in-Aid for Scientific Research (22251004 and 810104300001). SMC: Recanati-Kaplan Foundation, Robertson Foundation and Clouded Leopard Project/Point Defiance Zoo and Aquarium; DJS, PCG and BG: Zoos of Cincinnati, Columbus, Houston and Phoenix, Malaysian Palm Oil Council and Sime Darby Foundation; AM and AW: Point Defiance Zoo and Aquarium, Clouded Leopard Project, WWF–Germany, WWF–Malaysia, Cleveland Metroparks, Minnesota Zoo, Houston Zoo, Nashville Zoo, Panthera Foundation, Zoological Society for the Conservation of Species and Populations, British Ecological Society, Chester Zoo – The North England Zoological Society; Columbus Zoo, Shared Earth Foundation, The Usitawi Network, Wild Cat Club and Leibniz Institute for Zoo and Wildlife Research; TB and PK: German Ministry of Science and Education (FKZ033L045) and Zoo Leipzig; IM: Human Evolution Project of KUPRI, Grant-in-Aid for challenging Exploratory Research (24657170); HB: Ministry of Higher Education, Malaysia (Research Grant No. FRG0098-NSH-1/2007), Nagao Natural Environment Foundation Japan, PETRONAS, Universiti Malaysia Sabah and Pro Natura Foundation Japan 2012; GH: Grant-in-Aid for Young Scientists (No. 22687002). ; Peer-reviewed ; Publisher Version