BECAUSE of a number of behavioural, ecological and physiological factors, African honeybees (Apis mellifera scutellata) are better adapted to tropical environments than European bees. African bees achieve higher rates of reproduction and colony growth partly by collecting more pollen relative to nectar1 and by allocating more nutrients to brood rearing2. African colonies generate more swarms3,4 which travel far greater distances5. These adaptive traits have been dramatically manifested in the neotropics: African bees have formed large expanding feral populations, whereas few European bees have survived outside apiaries6. The feral populations consist of African matrilines7–9 that have not hybridized extensively with European drones from apiaries10. Perhaps because of their European heritage, hybrids are poorly adapted to the tropics and thus do not survive. European matrilines, however, do not become part of the feral population, even after repeated backcrossing with feral African drones7–10. Conceivably, negative heterosis could further reduce hybrid survival, where European maternal factors are particularly detrimental7. We report here physiological findings consistent with these possibilities. Mass-specific metabolic capacities were higher in African bees than in European bees and were low and nonintermediate in hybrids. Thus, higher metabolic and flight capacities may contribute to African bees' competitive advantages in the tropics.