title: Why do orangutans leave the trees? Terrestrial behavior among wild Bornean orangutans (Pongo pygmaeus wurmbii) at Tuanan, Central Kalimantan.
abstract: Orangutans (genus Pongo) are the largest arboreal mammals, but Bornean orangutans (P. pygmaeus spp.) also spend time on the ground. Here, we investigate ground use among orangutans using 32,000 hr of direct focal animal observations from a well-habituated wild population of Bornean orangutans (P. p. wurmbii) living in a closed-canopy swamp forest at Tuanan, Central Kalimantan, Indonesia. Ground use did not change with increasing observation time of well-habituated individuals, suggesting it was not an artifact of observer presence. Flanged males spent the most time on the ground (ca. 5% of active time), weaned immatures the least (around 1%). Females and immatures descended mainly to feed, especially on termites, whereas flanged males traveled more while on the ground. Flanged males may travel more inconspicuously, and perhaps also faster, when moving on the ground. In addition, orangutans engaged in ground-specific behavior, including drinking from and bathing in swamp pools. Supplementary records from 20 ground-level camera traps, totaling 3986 trap days, confirmed the observed age-sex biases in ground use at Tuanan. We conclude that ground use is a natural part of the Bornean orangutan behavioral repertoire, however it remains unclear to what extent food scarcity and canopy structure explain population differences in ground use.
authors: Anthropological Institute and Museum, University of Zurich, Zurich, Switzerland.
authors: Alison M
authors: AM
authors: Ashbury
authors: Department of Biological Sciences, National University of Singapore, Singapore.
authors: Mary Rose C
authors: MR
authors: Posa
authors: Anthropological Institute and Museum, University of Zurich, Zurich, Switzerland.
authors: Lynda P
authors: LP
authors: Dunkel
authors: Anthropological Institute and Museum, University of Zurich, Zurich, Switzerland.
authors: Brigitte
authors: B
authors: Spillmann
authors: Fakultas Biologi, Universitas Nasional, Jakarta, Indonesia.
authors: S Suci Utami
authors: SS
authors: Atmoko
authors: Anthropological Institute and Museum, University of Zurich, Zurich, Switzerland.
authors: Carel P
authors: CP
authors: van Schaik
authors: Anthropological Institute and Museum, University of Zurich, Zurich, Switzerland.
authors: Maria A
authors: MA
authors: van Noordwijk
mesh_terms: D000367:Age Factors; D000818:Animals; D001522:Behavior, Animal; D004326:Drinking; D017753:Ecosystem; D005247:Feeding Behavior; D005260:Female; D007214:Indonesia; D008124:Locomotion; D008297:Male; D011148:Pongo pygmaeus / Q000502:physiology*; D012737:Sex Factors
keywords: Borneo; Pongo pygmaeus wurmbii; camera trapping; direct observation; ground travel; orangutan terrestriality