A New Hope: Rescue of Orangutans


Scientists report the case of a rescued orangutan and how it health state varied before rescue and after release, with important consequences for the conservation of the species.

 A research team led by Renata Mendonça and Rafaela Takeshita rescued a stranded juvenile Bornean orangutan. The team rehabilitated the ape and released it back to the wild, assessing its health state throughout the process. The group’s study presents vital insight in how rescue and rehabilitation programs affect juvenile orangutans at a time when the number of deaths by deforestation are dangerously high.

The oldest of the orangutans stranded in a durian tree.

The oldest of the orangutans stranded in a durian tree.

This study is the first to report the case of two juvenile Bornean orangutans getting stranded in a durian tree, near a palm oil plantation. The research team first observed the two females in the end of July, and understood they were stranded by the third week of August, almost a month after the tree stopped fruiting. The only connection to other threes had been broken and the orangutans were unable to escape.


With the help of the Wildlife Rescue Unit of the Sabah Wildlife Department, the investigators installed a rope to help the juveniles climb down the tree. The older orangutan was able to escape through the rope. However, after five days, the younger female was still stranded and had to be rescued. The juvenile was then brought to captivity, at the Sepilok Orangutan Rehabilitation Centre, where it stayed for 75 days before being released near the area where it was rescued.

The oldest of the orangutans exploring the installed rope, which it eventually used to escape the tree.

The oldest of the orangutans exploring the installed rope, which it eventually used to escape the tree.

During most time it spent in the tree, the orangutan didn’t have access to fruit, orangutan’s food of preference. Over that period, the juvenile only had access to low-quality food, such as bark and leaves, and spent more time feeding than wild juvenile orangutans do. The investigators argued that this abnormal behaviour reflected an increase in the stress levels of the orangutan due to being stranded. It also showed that the juvenile was compensating for its low-quality diet, evidencing its capacity to adapt its behaviour to adverse situations. 


By finding increased levels of glucocorticoids in faecal material collected when the orangutan was stranded, the investigators confirmed it was under chronic stress. The health status of orangutans is reflected both in their behaviour and hormonal levels and by analysing and monitoring both, their health condition can be assessed. Glucorticoids are a group of hormones produced in the adrenal gland that increase with increasing levels of stress. They are particularly useful as a tool to monitor an animal’s wellbeing in recuperation programs as it can be measured without the need to handle the animal. Additionally, blood samples collected when orangutan was rescued showed it was dehydrated, anaemic, malnourished, and had kidney and liver problems. 


Throughout the rehabilitation period, the orangutan increased weight and was re-hydrated. At the end of rehab, the juvenile’s liver and kidney function were also re-established. Blood tests, however, showed it was still anaemic, which the investigators suggested to be due to the long time necessary to recover from anaemia. 


After releasing it back to the wild, the investigators followed the ape for three days. Behavioural observations collected over this period suggested that the release process was stressful for the juvenile. The orangutan showed reduced feeding time and increased resting time compared to other wild juveniles. The levels of faecal glucocorticoids also increased significantly after release, corroborating the behavioural observations. After a few days, however, the orangutan’s behaviour started to resemble that of other wild juveniles. This suggests that the young female was gradually readjusting her behaviour to the new environmental conditions.


Bornean orangutans, who are amongst the most closely related species to humans, face an uncertain future as the IUCN Red List of Threatened Species lists them as critically endangered. A major threat to orangutans is deforestation due to palm oil plantation. The industry of palm oil is directly responsible for deforesting areas inhabited by orangutans, fragmenting their habitat, and this case is just an example of how it affects orangutans.


While it’s usually assumed that orangutans cannot survive in disturbed forests, this study showed that juveniles adjust their behaviour and physiology to adverse conditions. Similarly, a 2015 study led by Dr. Russon evidenced that Bornean orangutans adjusted their behaviour to survive in challenging conditions after their habitat was destroyed by wildfires. Also in 2015, Dr. Loken’s research group showed that orangutans, a mainly arboreal species, were most terrestrial in a newly logged, open area, evidencing their capacity to adapt to human disturbance. All these studies suggest that Bornean orangutans can adjust to challenging conditions, bringing hope into the orangutan’s fight against extinction.


However, it is believed that human action is still essential to ensure the orangutan’s survival. Researchers argue that we still need to increase our understanding in how changing environmental conditions affect orangutans, as well as our knowledge on their adaptability, suggesting that the orangutan’s survival is far from guaranteed.


Study and photos by Renata Mendonça and fellow research group


Studies cited in this article: 

Loken, B., Boer, C. and Kasyanto, N.. 2016. Opportunistic behavior or desperate measures: Logging may only partially explain terrestriality in the Bornean orangutan (Pongo pygmaeus). Oryx.


Mendonça, R., Takeshita, R.S.C., Kanamoria, T., Kuze, N., Hayashi, M., Kinoshita, K., Bernard, H. andMatsuzawa, T. 2016. Behavioral and physiological changes in a juvenile Bornean orangutan after a wildlife rescue. Global Ecology and Conservation , 8, 116-122.


Russon, A. E., P. Kuncoro, and A. Ferisa. 2015. Orangutan behavior in Kutai National Park after drought and fire damage: adjustments to short- and long-term natural forest regeneration. American Journal of Primatology, 77, 1276-1289.


Article by André Pereira