Tim Bransford

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. Discussion: Despite 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.

The spatial and temporal variation in food abundance has strong effects on wildlife feeding and nutrition. This variation is exemplified by the peatland forests of Central Kalimantan, which are characterized by unpredictable fruiting fluctuations, relatively low levels of fruit availability, and low fruit periods (<3% of trees fruiting) that can last nearly a year. Challenged by these environments, large, arboreal frugivores like orangutans must periodically rely on non-preferred, lower-quality foods to meet their nutritional needs. We examined variation in nutrient intake among age-sex classes and seasons over a 7-year period at the Tuanan Orangutan Research Station in Central Kalimantan. We conducted 2,316 full-day focal follows on 62 habituated orangutans (Pongo pygmaeus wurmbii). We found differences in total energy and macronutrient intake across age-sex classes, controlling for metabolic body mass. Intake of both total energy and macronutrients varied with fruit availability, and preference of dietary items increased with their nutritional quality. Foraging-related variables, such as day journey length, travel time, and feeding time, also varied among age-sex classes and with fruit availability. Our results add to the growing body of literature suggesting that great variation in foraging strategies exists among species, populations, and age-sex classes and in response to periods of resource scarcity.

Bottom-up regulatory factors have been proposed to exert a strong influence on mammalian population density. Studies relating habitat quality to population density have typically made comparisons among distant species or communities without considering variation in food quality among localities. We compared dietary nutritional quality of two Bornean orangutan populations with differing population densities in peatland habitats, Tuanan and Sabangau, separated by 63 km. We hypothesized that because Tuanan is alluvial, the plant species included in the orangutan diet would be of higher nutritional quality compared to Sabangau, resulting in higher daily caloric intake in Tuanan. We also predicted that forest productivity would be greater in Tuanan compared to Sabangau. In support of these hypotheses, the overall quality of the diet and the quality of matched dietary items were higher in Tuanan, resulting in higher daily caloric intake compared to Sabangau. These differences in dietary nutritional quality may provide insights into why orangutan population density is almost two times greater in Tuanan compared to Sabangau, in agreement with a potentially important influence of diet quality on primate population density.