Tree-dwelling orangutans on ground

When researchers in Borneo set up camera traps to monitor tropical mammals on the ground, they didn't expect to be photobombed by orangutans.

In the wild, humans rarely see the red apes come down from the trees, says Dr. Rahel Sollmann, an NC State postdoctoral researcher in fisheries and wildlife. Sollmann helped analyze a database of camera trap images of orangutans, the largest arboreal mammals, for an article in Science Reports.

"The surprise was that all over Borneo, researchers using camera traps were getting orangutan pictures, so the species does come to the ground," says Sollmann, whose colleague Andreas Wilting with the Leibniz Institute for Zoo and Wildlife Research in Berlin enlisted her help. Wilting and Marc Ancrenaz, as well as a number of researchers working in the area, had set up camera traps in tropical forests as a way to indirectly observe mammal behavior.

Team members analyzed their data and those of 26 other collaborators to see if orangutans spent more time on the ground in areas disturbed by development. Palm oil plantations in Borneo are fragmenting the habitat and leaving gaps in the forest.

"We were pretty surprised to see that rates of coming to the ground were fairly similar across forest types, with the exception of forests that were logged a while ago using sustainable or reduced impact logging, where this rate was much lower," Sollmann says.

If orangutans are able and willing to cross gaps in the forest, that could be good news for the species. It would allow orangutans to cope with some habitat fragmentation by crossing open spaces to move from one forest to another.

Flanged orangutans—mature males with large cheek pads—made the most trips to the ground, but Sollmann says the team was pleased to find images of male and female orangutans of all ages. See a video clip of an orangutan.

Although Sollmann joined the project in the data analysis stage, she has been to Borneo. "I was lucky enough to spot some orangutans, which in itself is a pretty remarkable experience, because they are such fascinating creatures."