Felicity had the pleasure of joining colleagues to speak about wild orang utan behaviour at the latest Orang utan Veterinary Advisory Group (OVAG) meeting in Bali (23 July 2023). During the discussion, there was a question about “bark” (kulit) feeding. The video below illustrates that this term is misleading and why we prefer “cambium” feeding as a more accurate way to describe this component of the plant part repertoire of wild orang utan diets
Orang utans do not eat the mostly dead plant tissue that makes up the outer bark of trees. Instead, they strip off the vascular cambium layer that lies underneath the outer bark, wad it up in their mouth and extract the plant sugars and minerals by pressing the fibres against the back of their teeth with their tongue. Once it has lost its sweetness, they spit out the fibres like a used wad of gum (see photos below). These “wadges” are often found on the ground.
In an 11-year analysis (2005 – 2015) of the feeding activity of 78 individuals in the long-term HUTAN KOCP study site, we found cambium feeding accounted for a mean of 6 % of all feeding activity. The percentages ranged from 1.3% to 21.7%, with unflanged adult males (transient occupants of a fixed border study site ) being the age-sex-class with the highest rates of use of this plant food part. If non-residents were removed from the dataset, the mean rate of feeding activity on cambium drops to 3 % (Oram 2018). Therefore, cambium is likely a vital component of the diets of male orang utans, who, as the dispersing sex, migrate across broad areas they may not know as well as they range between the places resident females live and raise their offspring.
The vascular cambium lies underneath the outer protective bark of trees. It comprises the xylem and phloem, the pipeline that transports water and nutrients from tree roots to crown. Orang utans mostly use vines or limbs of trees for this extractive activity. In 15 years of personal observation, they have NOT been observed to girdle (entirely disrupt the nutrient pathway of the tree) that could cause tree death by this activity.
Oram, F., 2018. Abundance, behavioural and feeding ecology of wild orangutans (Pongo pygmaeus morio) in the fragmented forests of the Kinabatangan floodplain. Ph.D. Dissertation, Universiti Malaysia Sabah.
