Nutritional Importance of a Liana Species for a Population of Bornean Orangutans

Objectives: Temporal variation in food availability can pose nutritional challenges to primates. Characterizing the nutritional content of the non‐preferred foods that primates switch to, termed fallback foods, is useful for identifying the nutritional challenges of lean periods, the nutritional limits of what primates can subsist on, and physiological adaptations. We explored the temporal patterning and the nutritional contribution of food items for Bornean orangutans (Pongo pygmaeus wurmbii) at Tuanan, Indonesia, with particular attention to the liana, Bowringia callicarpa. Materials and Methods: We quantified the nutritional contribution of food items to the diet of wild orangutans over 18 years. We modeled the relationship between preferred food availability and the nutritional contribution of Bowringia. Results: Bowringia played an outsize role in the feeding time and nutritional intake of orangutans. It can be characterized as a fallback food because it is increasingly consumed when preferred tree fruits are less available. Its immature leaves are particularly important as the greatest source of protein and energy. However, the nonprotein energy‐to‐protein ratio of Bowringia is extremely low, and overreliance on it would bring orangutans away from their estimated nutritional intake target.DiscussionDespite its high energy and protein content, Bowringia is a nutritionally imbalanced food. Fallback food quality should thus be evaluated based on the ability to bring an animal toward its nutritional goal rather than nutrient density. We propose that orangutans are preadapted to falling back on protein‐dense foods and the great abundance of Bowringia has contributed to the high population density of orangutans at Tuanan.