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Wild orangutans cope with dramatic, unpredictable fluctuations in food availability throughout development which have significant implications for energy balance for growth, development, and maintenance of body function. Foods that are especially important when preferred fruits are not available (e.g., bark/pith, termites, seeds) can require force, manual manipulation, and/or skill to access. Orangutans’ extended life history may mitigate ecological risk associated with the challenging environment of juvenile growth and development. Given the complexity of important foods, orangutans’ semi-solitary lifestyle, and the relatively brief period between weaning and independence, offspring transition to ecological independence during their extended nursing period. Here, we examine mother-offspring food transfer in the context of nutritional quality and complexity, with handling time as a proxy. Data were collected between July 2009-July 2019 in Gunung Palung National Park, Indonesia. We predict complex food items are transferred most frequently. In 245 observations of food sharing, fruit was most frequently transferred, followed by invertebrates and bark/pith. Handling time and transfer frequency were negatively correlated (Rho=-0.75, p<0.001): slower-eaten fruits were shared more frequently than rapidly-eaten fruits. Fruit size and sharing frequency were also negatively correlated (Rho=-0.73, p<0.001). There was no correlation between sharing frequency and free simple sugar concentration (Rho = 0.36, p=0.13), though sharing frequency and total nonstructural carbohydrate concentration were correlated (Rho=0.46, p=0.04). Food sharing was most common when mothers ate large, slowly-eaten fruits rich in nonstructural carbohydrates. Food sharing allows mothers to transfer more complex foods to their offspring, and may facilitate knowledge transfer as offspring become ecologically competent. 

Orangutan habitats are characterized by fluctuations in the availability of ripe fruits. During non-fruiting periods orangutans typically incorporate more lower-quality foods such as pith and bark in their diet. Condensed tannins (CT) are secondary plant compounds that bind to proteins, thus impeding the digestibility of proteins, and tending to make foods bitter or unpalatable. We analyzed condensed tannin content in 129 plant samples collected from Gunung Palung National Park in Borneo, Indonesia between 1994 and 2001. We predicted that CT concentrations would be highest in bark, and that there would be a correlation between protein and condensed tannin content. We used ANOVA with Bonferonni’s method for post-hoc comparisons to test for differences in tannin content between plant parts, and Pearson’s correlation to test for relationships between tannin concentrations and other nutrients. There were significant differences in condensed tannin content (F(4)=2.70, p=0.03) but no differences after adjusting the alpha-level for post-hoc comparisons. Whole fruit (including the skin) tended to have the highest CT concentration. However, we found no correlation between CT and concentration of nutrients including crude protein (R=0.12, p=0.19, N=127), free simple sugars (R=-0.09, p=0.40, N=100), or fiber (R=-0.38, p=0.67, N=128). This underscores that plants rich in desirable nutrients may also be rich in antifeedants, posing challenges for orangutan consumption and digestion even as they provide a source of high-quality energy. Additionally, for some food categories where high tannin content is predicted, such as bark, orangutans may be choosing to eat species that are lower in these compounds. 

Orangutan habitats are characterized by fluctuations in the availability of ripe fruits. During non-fruiting periods orangutans typically incorporate more lower-quality foods such as pith and bark in their diet. Condensed tannins (CT) are secondary plant compounds that bind to proteins, thus impeding the digestibility of proteins, and tending to make foods bitter or unpalatable. We analyzed condensed tannin content in 129 plant samples collected from Gunung Palung National Park in Borneo, Indonesia between 1994 and 2001. We predicted that CT concentrations would be highest in bark, and that there would be a correlation between protein and condensed tannin content. We used ANOVA with Bonferonni’s method for post-hoc comparisons to test for differences in 11tannin content between plant parts, and Pearson’s correlation to test for relationships between tannin concentrations and other nutrients. There were significant differences in condensed tannin content (F(¬4)=2.70, p=0.03) but no differences after adjusting the alpha-level for post-hoc comparisons. Whole fruit (including the skin) tended to have the highest CT concentration. However, we found no correlation between CT and concentration of nutrients including crude protein (R=0.12, p=0.19, N=127), free simple sugars (R=-0.09, p=0.40, N=100), or fiber (R=-0.38, p=0.67, N=128). This underscores that plants rich in desirable nutrients may also be rich in antifeedants, posing challenges for orangutan consumption and digestion even as they provide a source of high-quality energy. Additionally, for some food categories where high tannin content is predicted, such as bark, orangutans may be choosing to eat species that are lower in these compounds. Funders: Boston University Undergraduate Research Opportunities Program, NSF (BCS-1638823, BCS-0936199); National Geographic; USFish/Wildlife (F18AP00898, F15AP00812, F13AP00920, 96200-0-G249, 96200-9-G110, 9414388); Leakey; Disney Wildlife Conservation; Wenner-Gren; Nacey-Maggioncalda; Orangutan Conservancy; Conservation, Food, and Health Foundation. 

Wild Bornean orangutans experience fluctuations in the availability of their preferred food, fruit. During periods of low fruit availability, orangutans rely on fallback foods which are expected to be higher in fiber and generally lower in free simple sugars. However, it is not clear whether there is a consistent relationship between fiber content and the content of other nutrients. Here, we examine acid detergent fiber (ADF) content of 101 plant foods consumed by orangutans in Gunung Palung National Park, West Kalimantan, Indonesia, and the correlation between ADF and other important plant macronutrients. Samples were collected during full-day behavioral follows between 1994-2001. Samples were analyzed in triplicate through a reflux apparatus, which quantified ADF proportion by weight. An ANOVA revealed significant differences between ADF concentrations of different plant parts (F(5)=20.89, p < 0.001). Post-hoc analyses (α= 0.005) determined that bark had a significantly higher ADF concentration than pulp and seeds (p<0.001), leaves had a significantly higher ADF concentration than seeds (p<0.001), and whole fruit had a significantly higher ADF concentration than pulp or seeds (p<0.001). We found a negative correlation between free simple sugar concentration and ADF (R = -0.63, p<0.001). However, there was no significant correlation between ADF and protein (R=-0.14, p=0.17) or lipid (R 0.134, p=0.19) content. Our findings corroborate work showing that bark and leaves are particularly high in ADF. However, they underscore the fact that determining dietary quality is complex, and that food items that are high in fiber may still be good sources of non-carbohydrate energy. National Science Foundation (BCS-1638823, BCS-0936199, 1540360, 9414388); National Geographic Society; US Fish and Wildlife (F15AP00812, F12AP00369, 98210-8-G661); Leakey Foundation; Disney Wildlife Conservation Fund; Wenner-Gren Foundation; Nacey-Maggioncalda Foundation; Conservation, Food and Health