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Bornean orangutans (Pongo pygmaeus wurmbii), with their long lifespans and inter-birth intervals, and late age at first birth, have the most extended life history of all primates. According to the Ecological Risk Aversion Hypothesis, slow growth is a strategy to avoid starvation in unpredictable environments. When fruits are unavailable, bark and leaves, high in indigestible fiber, dominate orangutan diets. Orangutan hindgut fermentation is critical in the extraction of nutrients from fibrous fallback foods. However, body size and gut passage times are predicted to influence digestive efficiency. We investigated age-sex class differences in fiber digestibility in response to fluctuations in fiber intake. Researchers at Gunung Palung National Park, Indonesia, collected fecal samples during focal follows from 2016-2019 which were analyzed at Boston University’s Primate Ecology Lab. Samples (N=143) were weighed and dried overnight before grinding in a Wiley Mill. Seeds separated from the fecal matter were counted, measured, photographed and identified using our reference database. Dry matter and percent organic matter were determined through drying, ashing, and hot weighing. We used an ANKOM 200 Fiber Analyzer to determine NDF and compared the percent fiber excreted by mother/offspring pairs with feeding data. While adult female and juvenile diets showed similar percentages of fiber intake, juveniles excreted 50% more fiber than their mothers. Our results highlight the influence of body size, with smaller individuals demonstrating a lower capacity for fiber digestion. Juvenile digestive efficiency likely constrains growth, development, and survivorship during periods of low food availability when orangutan diets are high in fiber.