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Indonesia’s peat swamp forests have experienced an increase in fire frequency and intensity over the last century, producing dense smoke that alters ecosystems; yet little is known about its effects on wild primates. We investigated the relationship between peatland smoke and the energetic status of orangutans (Pongo pygmaeus wurmbii) using urinary nitrogen isotopes (δ¹⁵N) as a marker of muscle catabolism. We also evaluated variation across age-sex classes and habitat fruit availability (FAI), factors known to affect orangutan energetics. Data were collected at the Tuanan Research Station, Central Kalimantan, Indonesia, from 2012-2015 (n=456 samples from 38 individuals) and analyzed using Generalized Additive Mixed Models to examine the effects of multiple variables. Preliminary results show that δ15N was significantly higher during smoke relative to pre-smoke periods (+0.41‰, p=0.014), suggesting a temporary shift toward endogenous protein catabolism and/or greater intake of higher δ15N foods. Post-smoke values were not significantly different from pre-smoke (+0.27‰, p=0.104), and smoke versus post-smoke was also non-significant (+0.15‰, p=0.68). The relationship between δ15N and FAI exhibited a U-shaped pattern, with δ15N at its lowest values when FAI was approximately at the mean for the habitat (i.e., 4.24%). Adult females with dependents had higher δ15N than females without dependents (+0.81‰, p=0.002), likely reflecting added protein demands of lactation, while other age-sex classes showed no significant differences (p>0.05). These results suggest that multiple variables impact orangutan muscle catabolism, but future analyses must disentangle the relative contributions of dietary change versus muscle catabolism during smoke events. 

Over the past few decades, Bornean peatland fires, caused by anthropogenic activities and natural droughts, have decimated millions of hectares of forest, impacting local vegetation and wildlife ecology. Previous research indicates that smoke from these fires influences wild orangutan social behavior and activity budgets. Yet we know little about how these fires affect this critically endangered species' nutritional and energetic status. We used stable nitrogen (δ15N) isotopes in non-invasively collected urine to explore how persistent smoke from fires influences the ecology of critically-endangered Bornean orangutans in a peatland habitat at the Tuanan Orangutan Research Station. We measured δ15N values in urine collected from 44 orangutans before, during, and after two fire seasons and associated smoke periods (months with >15 days of detected smoke) in 2014 and 2015. We used Generalized Additive Mixed Models (GAMMs) to examine the effects of smoke. Preliminary analyses indicate δ15N levels were significantly higher during and after smoke exposure compared to before the fires (Estimated Marginal Means: pre-smoke= 1.26 ± 0.043%, smoke=1.42 ± 0.050%, post-smoke=1.50 ± 0.053%). Sex-age category differences were also observed, with adult females with dependents having higher δ¹⁵N than those without (1.63 ± 0.08%, 1.45 ± 0.10%). Unflanged males showed the lowest mean δ15N (1.13 ± 0.12%), and immature orangutans had mid-to-high δ15N values, with independent immatures higher on average than dependents (1.50 ± 0.08%, 1.35 ± 0.17%). These findings suggest that fire smoke impacts orangutan dietary ecology and energetic status, likely due to changes in food availability and dietary behaviors.