Simple SummaryFree-living orangutan infants learn from their mother what to eat and where and when to find food. Rescued orangutan orphans, who lost their mother in early childhood, are deprived of this opportunity. In our rehabilitation program at the Yayasan Jejak Pulang forest school we try to compensate for this loss by immersing our orphans into a natural forest environment and by providing them with the possibility to learn from their conspecifics as well as from caregivers who model how to consume and process forest food. We evaluated this approach by investigating whether our orphans did adapt their food plants eaten and the parts of plants chosen to their availability in the forest for an observation period of three years. We found that our orphans did choose food plants and parts of plants comparable to mature free-living orangutans and appropriate to forest productivity. Accordingly, we assume that our approach does facilitate the acquisition of feeding competence of orphaned orangutans which in turn is crucial for their later re-introduction to be successful.AbstractFor critically endangered species, restorative conservation becomes increasingly important. Successful re-introduction of rescued wild orangutan orphans requires rehabilitation mimicking maternal rearing in the wild. Feeding competence—what to eat, where and when to find food—needs to be learned before re-introduction. We observed seven orphans (2–10 years old) for a period of 3 years during their rehabilitation at the Yayasan Jejak Pulang forest school. Of the 111 plant genera eaten by the orphans, 92 percent were known orangutan food plants. Five plant genera were eaten by all orphans in over 90 percent of the months within the observation period. The Fruit Availability Index (FAI) was used to predict which parts of a plant were consumed by the orphans. We found that the orphans ate primarily fruit when the FAI was high, but consumed more young leaves, cambium, and pith when FAI was low. Thus, the orphans exhibited food choices very similar to mature wild orangutans and appropriate to forest productivity. The orphans’ acquisition of feeding competence was facilitated by their immersion into a natural forest environment in combination with possibilities for observational learning from conspecifics as well as caregivers modelling food processing and consumption.