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.

Adaptation behavior of Bornean Orangutan (Pongo pygmaeus morio) reintroduction in Kehje Sewen Forest, East Kalimantan, Indonesia

“…Another parameter for determining the success of adaptation is the ability to move on the tree, in addition to their ability to climb into the center of the tree canopy and move freely (Ashbury et al 2015). The climbing height of orangutans is influenced by the presence of food sources and the ability to competently perform their activity on the tree.…”

Primate social behavior

“…Small, isolated populations are unlikely to possess the genetic variation upon which natural selection works; therefore, we are now looking increasingly at learning and innovative, novel behavior patterns as the sources for adaptions during individual lifetimes, and the cross generational transmission of these behaviors (instead of genetics) for their persistence over time. Examples of these may include the significant increases in terrestrial behavior that have been documented in some populations of muriquis (Tabacow et al, ) and orangutans (Ashbury et al, ; Russon et al, ), but whether they reflect adaptive responses to ecological or demographic stimuli remains to be seen. We can look to the next set of comparative studies of primate (and animal) innovations and traditions for a new generation of insights.…”

Being fat and smart: A comparative analysis of the fat-brain trade-off in mammals

“…Gorillas are relatively terrestrial and their daily travel distances are very short, suggesting that low locomotion costs relax the constraints on the quantity of fat stores. As wild orangutans are largely arboreal (Ashbury et al, 2015), it remains unclear whether the high percentage of body fat noted in the Pontzer et al (2016a) study is due to a captivity effect or whether the capacity of fat storage is strongly selected for in this genus, which suffers from severe and unpredictable starvation periods due to El Niño effects (Knott, 1998).…”