Coping with feast and famine: Integrated behavioral and metabolically flexible responses of wild orangutans to ecologically driven dietary variation

Diet and nutrition are critical factors influencing energetics and health.
Laboratory studies show that organisms adjust to changes in nutrient
intake through flexible metabolic responses. While the physiological
effects of nutrient balance in humans have been studied, data from closely
related species living in nature are lacking. We integrate macronutrient
regulation and metabolic flexibility to elucidate how wild Bornean
orangutans are buffered against natural fluctuations in nutrient intakes.
We found that orangutans regulate protein and regularly switch between
exogenous and endogenous nutritional substrates as preferred food
availability declines. When total caloric, lipid, and carbohydrate intakes
decline, orangutans drew on fat and endogenous amino acids for energy.
This strategy is beneficial only in the context of alternating periods of
fruit scarcity and abundance. Our findings provide a direct analog for the
current global obesity pandemic, which has arisen in parallel with
transitions in human diets towards energy-dense, protein-dilute foods.