Orangutans in Borneo Spending More Time on the Forest Floor

As the world's largest arboreal mammal, orangutans quite logically spend much of their time in trees. But new research on Bornean orangutans reveals that the great apes spend more time on the forest floor than previously believed.

Mark Harrison of the Orangutan Tropical Peatland Project and an international team of collaborators led by Marc Ancrenaz from the HUTAN / Kinabatangan Orangutan Conservation Program in Malaysia, spent seven years tracking the movements of Bornean orangutans with 1,409 camera traps across 16 sites in the Borneo jungle.

Their project, which took place between June 2006 and March 2013, detailed the movements of 641 individual orangutans over nearly 160,000 days.

Prior to this study, the researchers report that evidence of orangutans coming down from the trees was rare and was usually associated with habitat disturbance.

"We've known for some time that orangutans use the ground to travel and search for food, but the influence of anthropogenic disturbances in driving this behavior has been unclear. This is crucial to understand in this age of rampant forest loss and fragmentation, which is slicing up the orangutan's jungle home," Harrison said.

"We found that although the degree of forest disturbance and canopy gap size influenced terrestriality, orangutans were recorded on the ground as often in heavily degraded habitats as in primary forests," Harrison continued.

The researchers found that Bornean orangutans of all ages and sexes were recorded on the ground, but flanged males - those with distinctive cheek pads and throat pouches - traveled on the ground the most.

"This suggests that terrestrial locomotion is a greater part of the Bornean orangutans' natural behavioral repertoire than previously understood and is only modified by habitat disturbance," Harrison said. "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."

The researchers found that in excess of 70 percent of Bornean orangutans live in forests that have lost their original characteristics, which they suggest has resulted in the great apes spending more time on the ground. Harrison said this increased terrestriality has both advantages and disadvantages for orangutans avoiding predation.

"Increased terrestriality is expected to increase predation risk, interactions with and persecution by humans, and exposure to novel diseases," Harrison said. "Unlike in Sumatra, where tigers are present, predation is less of a concern in Borneo, although infants might be at risk from bearded pigs and clouded leopards. In recent history, their biggest predator has been man, who is actually more likely to pick orangutans off in the trees: orangutans make a lot of noise and so are very obvious in the trees, whereas they can move with almost no noise and so more easily get away on the ground."

Harrison concluded that a better understanding of orangutan movement in Borneo will help wildlife managers there enact better conservation strategies.

The research is published in the journal Scientific Reports.

This article was edited on Feb. 17 to clairify that a numerous researchers were involved in the work.