Why did the orangutan come down from the trees?

Dr Mark Harrison, based in the Department of Geography at the University of Leicester and Managing Director of the Orangutan Tropical Peatland Project (OuTrop) has, along with international colleagues, published results of a seven year study of Orangutans in Borneo in the journal Scientific Reports.

The research, conducted between June 2006 and March 2013, is based on a large-scale analysis of Orangutan terrestriality using comprehensive camera-trapping data from 16 sites across Borneo. In total there were 641 independent Orangutan records taken at 1,409 camera trap stations over 159,152 trap days.

The Bornean Orangutan (Pongo pygmaeus) is the world's largest arboreal (tree-dwelling) mammal. Records of terrestrial behaviour are rare and tend to be associated with habitat disturbance.

Marc Ancrenaz, from the HUTAN / Kinabatangan Orangutan Conservation Programme in Malaysia, and colleagues conducted the study. Dr Harrison, said: "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 behaviour 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.

"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.

"All age-sex classes were recorded on the ground, but flanged males -- those with distinctive cheek pads and throat pouches -- travel on the ground more. This suggests that terrestrial locomotion is a greater part of the Bornean Orangutan's natural behavioural repertoire than previously understood and is only modified by habitat disturbance."

Dr Harrison added: "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 authors report that more than 70% of Orangutans occur in fragmented multiple-use and human-modified forests that have lost many of their original ecological characteristics. Modified Orangutan behaviour which sees them increasingly spending time on the ground therefore has its pros and cons:

Dr Harrison explains that, "Increased terrestriality is expected to increase predation risk, interactions with and persecution by humans, and exposure to novel diseases. 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."

The scientists report that terrestrial behaviour therefore could also facilitate movement and dispersal, especially in degraded or fragmented landscapes as a result of natural or human-made processes. This could also create new opportunities to access different food sources."

Dr Harrison concludes: "Ultimately, a better understanding of what drives Orangutan terrestriality, how this influences their dispersal, movement and survival in a human-modified landscapes is important for designing effective management strategies for conservation of this endangered species in Borneo."