Why sustained, high-intensity fat-powered endurance rarely evolves in animals

Most animals rely on flexible fuel-use strategies in which carbohydrates support short, high-power bursts of activity and fats fuel prolonged, low-intensity locomotion. A notable exception occurs in a small subset of taxa - primarily migratory birds, bats and some insects - that sustain vigorous, high-intensity aerobic locomotion while relying largely on lipid oxidation. In this Review, we examine why this extreme, fat-powered endurance phenotype has evolved so rarely. Synthesizing evidence across locomotor modes (swimming, running, flying), body sizes and patterns of endurance specialization, we argue that selection for endurance alone is insufficient to drive the evolution of this phenotype. Sustained fat-powered endurance emerges only where scaling constraints on power, heat dissipation and oxygen delivery are not prohibitive, the ecological demand for prolonged activity aligns with mechanical conditions that permit continuous high aerobic output, and the benefits of specialization outweigh the evolutionary costs of reduced metabolic flexibility. In most animals, endurance demands are intermittent or context dependent, favoring partial specialization that enhances aerobic capacity and lipid use while preserving the ability to shift fuels and performance modes. Crossing into this extreme phenotype requires coordinated, reversible co-expression of structural and regulatory changes that together commit organisms to a narrow performance phenotype and impose trade-offs in force production, metabolic versatility and resource allocation. Consequently, evolution often favors trajectories that approach, but do not fully cross into, this exceptional performance state. Together, these patterns show that sustained, high-intensity fat-powered endurance is a rare and conditional evolutionary outcome, arising only where ecological necessity, biomechanical permissibility and physiological feasibility align.