Mechanisms and trade-offs of fat-powered endurance flight in migratory animals

Seasonal migration is one of the most physiologically demanding life history events in the animal kingdom, requiring tightly integrated solutions across molecular, organ and whole-animal scales. While three core metabolic 'gears' are broadly conserved across taxa to meet most locomotion demands, a rare 'fourth gear' of fat-fueled high-intensity endurance has evolved as a solution to the physiological problem of how to achieve long-distance migratory powered flight. We synthesize the current understanding of how birds, bats and insects perform such sustained high aerobic power output and how this capacity depends on seasonal phenotypic flexibility: reversible changes in lipid stores and composition, muscle biochemistry, cardiovascular capacity, and antioxidant defenses. Long-distance high-intensity endurance exercise involves increases in total fat stores and the remodeling of fatty acid composition, as well as key changes in fuel mobilization, enzyme kinetics and enhanced antioxidant capacity, and selective dietary intake of protective compounds to avoid oxidative damage. Finally, we outline future research priorities that connect mechanistic insights into lipid biochemistry, oxidative stress management and phenotypic flexibility with the ecological realities of a changing world.