Bornean orangutans (Pongo pygmaeus) are the largest arboreal, or tree-habitating, animals, some of the most intelligent primates—and close relatives of humans. They’re also endangered. Their homes in the trees of Borneo are consistently threatened with deforestation.

In addition, their behavior in the wild is poorly understood. Although the orangutans appear to spend most of their time in trees, a group of international researchers wondered how often the apes use the ground for travel—a method they may have to adopt more and more as trees are lost.

Led by Marc Ancrenaz, the researchers set up camera traps across 16 different sites in Borneo to study how terrestrial the orangutans are, and whether anthropogenic disturbances (such as deforestation) influence this behavior.

In the study, published last week in Scientific Reports, the scientists described the detriments and benefits of ground living for the primates:

We can suppose that increased terrestriality would increase predation risk, interactions with and persecution by humans, and exposure to novel pathogens. On the other hand, terrestrial behavior could also facilitate movement and, therefore, dispersal, especially in degraded or fragmented landscapes as a result of natural or man-made processes. It could also create new opportunities to access different food sources.

Remarkably, the camera traps frequently recorded orangutans on the ground, regardless of age or gender of the animal. Additionally, the primates visited the ground in primary forests as well as in heavily degraded habitats, suggesting that anthropogenic canopy disruptions may influence terrestrial activity, but are not the sole driver of this behavior. The findings suggest that terrestrial locomotion may have a larger than expected role in the Bornean orangutan’s natural behavioral repertoire.

In fact, the researchers note that this ground activity is supported evolutionarily. The fossil record displays evidence that potential orangutan ancestors were more terrestrial than the present-day primates. “…Ancestral orangutans may have been able to cover larger distances on the ground,” according to the study.

These findings suggest that as Bornean forests degrade further, terrestriality will allow orangutans to be more resilient to drastic habitat change than previously thought, and thus help to save these animals from extinction.

Image: Andrew Hearn and Joanna Ross

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