Orangutans: Barking Up the Right Tree!

Text by: BOSF PRM team from Nles Mamse Camp, Kehje Sewen Forest, East Kalimantan
Date Posted: Feb 2, 2022

As a frugivore, or fruit eater, an orangutan’s diet consists mostly of forest fruits. However, orangutans also eat young leaves, termites, honey, eggs, mushrooms, and young shoots, as well as certain varieties of tree bark.

In the forest, orangutans have been recorded consuming up to 2,000 different varieties of plants. Some of these are visited by orangutans in search of fruit, leaves, bark, flowers, or pith. There are some plants and trees with various parts that can be consumed by orangutans; for example, the ficus family. Ficus, or figs trees, are an all-in-one food stop for orangutans, with their edible fruits, leaves, and cambium.

Since fruit, an orangutan staple, is not always available in sufficient quantities throughout the year, orangutans have to forage for other food sources. One of the favorites among these sources found in the forest is tree cambium, the layer of tissue underneath the bark.

It might be impossible for humans to peel the bark of a ficus tree and chew on its cambium lining, but not for orangutans. With teeth and jaws much stronger than ours, orangutans can easily peel bark by biting it. Once peeled, they then chew off the cambium tissue and savor it.

When our Post-Release Monitoring (PRM) team from Camp Nles Mamse in the Kehje Sewen Forest, East Kalimantan, conducted a phenological survey in March last year, the team found a few trees with visibly peeled bark. Phenology is the study of the phases that occur naturally in plants: A phenological survey in the forest involves the collection of data on plants that are flowering or fruiting, with the results used to determine which areas orangutans could potentially visit to forage.

One of these trees stripped by an orangutan can be seen in the photo below. At this particular location, only a few trees were bearing fruit that was ready to eat. So, it is clear that orangutans passing through the area would have to forage for alternative food, and it seems tree cambium was the perfect choice!

Remaining cambium

Given that the tree bark was still wet upon discovery, our team suspected that the orangutan in question must have just left the location and this meant that he or she might be still roaming nearby to forage for food.

Keep exploring, dear orangutan. We hope you keep finding enough food to satisfy your appetite in the forest.

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