Orang-utans on the ground

Orang-utan with infant. Photo: Andrew Hearn & Joanna Ross
Orang-utan with infant. Photo: Andrew Hearn & Joanna Ross

An international team of scientists studied how Bornean orang-utans cope with habitat modifications caused by logging in rainforests

Orang-utans more often come down from the trees to go on terrestrial excursions than previously thought. This is the most important result published today in the scientific journal “Scientific Reports”. In primary rainforests the orang-utans’ activity on the ground is even higher than in sustainably managed forests –and thus at a level comparable with heavily degraded commercial rainforests.

The study was undertaken by an international team of scientists. One of the main authors of the study is Dr Andreas Wilting from the German Leibniz Institute for Zoo and Wildlife Research (IZW). “We found that both males and females – and these often with their offspring – leave their trees to go on terrestrial excursions. Most often we photographed the heavier flanged males”, says Andreas Wilting. For the scientists it was very surprising to record even female orang-utans leaving the treetops to move around on the ground so frequently. Previously, most information about behaviour of orang-utans was based on direct observations which suggested that orang-utans very rarely leave their trees. Such findings appear now to be biased since this shy ape apparently avoids coming down to the ground in the presence of observers. For the current study the scientists collected a massive dataset of photos and videos taken by over 1,400 remotely operated camera-traps from 16 different rainforests across Malaysian and Indonesian Borneo.

What are the consequences for an arboreal mammal venturing frequently to the ground? Such terrestrial behaviour can facilitate movements of orang-utans especially in degraded or fragmented landscapes and provides the opportunity of accessing and exploiting ground food sources. Terrestrial excursions may also pose additional risks to orang-utans: Orang-utans could be more easily hunted and the closer contact with people could be dangerous if it increases the chance of orang-utans to become infected with human pathogens.

Orang-utans are the largest mammals living in trees. In Borneo, about 70 percent of the remaining orang-utans occupy rainforests already modified in some way by logging. The newly discovered aptitude of orang-utans to move around on the ground may help them cope with small-scale habitat fragmentation. The findings of this study emphasises the importance of commercially used forests to conservation efforts for this highly threatened species.


Ancrenaz M, Sollmann R, Meijaard E, Hearn AJ, Ross J, Samejima H, Loken B, Cheyne SM, Stark DJ, Gardner PC, Goossens B, Mohamed A, Bohm T, Matsuda I, NakabayasiM, Lee SK, Bernard H, Brodie J, Wich S, FredrikssonG, Hanya G, Harrison ME, Kanamori T, Kretzschmar P, Macdonald DW, Riger P, Spehar S,  Ambu LN, Wilting A (2014): Coming down from the trees: Is terrestrial activity in Bornean orangutans natural or disturbance driven? SCI REP-UK 4, 4024; Doi:10.1038/srep04024.



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