It is thought that the capacity of mammals to directly supply the energetic needs of exercising muscles using recently ingested fuels is limited. Humans, for example, can only fuel about 30%, at most, of exercise metabolism with dietary sugar. Using indirect calorimetry, i.e. measurement of rates of O 2 consumption and CO2 production, in combination with carbon stable isotope techniques, we found that nectarivorous bats Glossophaga soricina use recently ingested sugars to provide ∼78% of the fuel required for oxidative metabolism during their energetically expensive hovering flight. Among vertebrate animals, only hummingbirds exceed the capacity of these nectarivorous bats to fuel exercise with dietary sucrose. Similar experiments performed on Anna's (Calypte anna) and rufous (Selasphorus rufus) hummingbirds show that they use recently ingested sugars to support ∼95% of hovering metabolism. These results support the suggestion that convergent evolution of physiological and biochemical traits has occurred among hovering nectarivorous animals, rendering them capable of a process analogous to aerial refueling in aircraft.