Many populations of consumers consist of relatively specialized individuals that eat only a subset of the foods consumed by the population at large. Although the ecological significance of individual specialization is recognized, it is difficult to document and its underlying mechanisms are poorly understood. Optimal foraging theory provides a useful framework for predicting how individuals might select different diets, positing that animals balance the ‘opportunity cost’ of stopping to eat an available food item against the cost of searching for something more nutritious; diet composition should be contingent on the distribution of food, and foragers should be more specialized when individuals have high energy reserves to spend searching for high-quality foods. We tested these predicted mechanisms of individual niche differences by quantifying environmental (resource heterogeneity) and organismal (nutritional condition) determinants of diet in a widespread browsing antelope (bushbuck, Tragelaphus sylvaticus) in an African floodplain-savanna ecosystem. We quantified individual diet breadth and composition using DNA metabarcoding of fecal samples collected repeatedly from 15 GPS-collared animals (6–14 samples per individual, median 12). Bushbuck diets were structured by spatial heterogeneity and constrained by individual conditions. We observed significant individual-level partitioning of food plants by bushbuck both within and between two adjacent habitat types (floodplain and woodland). Individuals with home ranges that were closer together and/or had similar vegetation structures (measured using LiDAR) ate more similar diets, supporting the prediction that heterogeneous resource distribution promotes individual differentiation. Individuals in good nutritional condition had significantly narrower diets (fewer plant taxa), searched their home ranges more intensively (intensity-of-use index), and had higher-quality diets (percent digestible protein) than those in poor condition, supporting the prediction that animals with greater endogenous reserves are more specialized because they can invest time in searching for nutritious foods. Our results support predictions from optimal foraging theory about the energetic basis of individual specialization and provide a potentially generalizable framework for understanding how individual niche width is governed by animal behavior and physiology in heterogeneous landscapes.