Orangutans might be the king of the swingers, but primatologists in Borneo have found that the great apes spend a surprising amount of time walking on the ground. The research, published in the American Journal of Primatology found that it is common for orangutans to come down from the trees to forage or to travel, a discovery which may have implications for conservation efforts.
An expedition led by Brent Loken from Simon Fraser University and Dr. Stephanie Spehar from the University of Wisconsin Oshkosh, travelled to the East Kalimantan region of Borneo. The region's Wehea Forest is a known biodiversity hotspot for primates, including the Bornean orangutan subspecies, Pongo pygmaeus morio, the least studied of orangutan subspecies.
"Orangutans are elusive and one reason why recorded evidence of orangutans on the ground is so rare is that the presence of observers inhibits this behaviour," said Loken. "However, with camera traps we are offered a behind the scenes glimpse at orangutan behaviour."
The team positioned ground-based cameras across a 38-square-kilometre region of the forest and succeeded in capturing the first evidence of orangutans regularly coming down from the trees. The amount of time orangutans spent on the forest floor was found to be comparable to the ground-dwelling pig-tailed macaque, Macaca nemestrina, which is equally abundant in Wehea Forest. Over 8-months orangutans were photographed 110 times, while the macaques were photographed 113 times.
The reason orangutans come down from the trees remains a mystery. However, while the absence of large predators may make it safer to walk on the forest floor, a more pressing influence is the rapid and unprecedented loss of Borneo's orangutan habitat.
"Borneo is a network of timber plantations, agro-forestry areas and mines, with patches of natural forest," said Loken. "The transformation of the landscape could be forcing orangutans to change their habitat and their behaviour."
This research helps to reveal how orangutans can adapt to their changing landscape; however, this does not suggest they can just walk to new territory if their habitat is destroyed. The orangutan subspecies P. p. morio may be adapted to life in more resource scarce forests, having evolved larger jaws which allow them to consume more tree bark and less fruit but they are still dependent on natural forests for their long term survival.
"While we're learning that orangutans may be more behaviourally flexible than we thought and that some populations may frequently come to the ground to travel, they still need forests to survive," said Dr. Spehar. "Even in forest plantation landscapes they rely heavily on patches of natural forest for food resources and nesting sites."
Wehea Forest is one of the only places in Borneo where ten primates species, including five species found only in Borneo, overlap in their ranges. Since Wehea Forest is a biodiversity hotspot, paperwork have been submitted to legally change the status of Wehea Forest from "production forest" to "protected forest." However, given that 78% of wild orangutans live outside of protected areas, it is critical that all of Borneo's remaining forests are either protected or sustainably managed.
"We do not know how long this may take, but protecting Wehea Forest and Borneo's remaining forests is vital to the long term survival of the orangutans," concluded Loken. "Fortunately 60% of Wehea Forest falls under Indonesia's logging moratorium, which helps give legal protection to a large part of the forest for a few more years."
- Brent Loken, Stephanie Spehar, Yaya Rayadin. Terrestriality in the bornean orangutan (Pongo pygmaeus morio) and implications for their ecology and conservation. American Journal of Primatology, 2013; DOI: 10.1002/ajp.22174
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