Glossophagine nectar-feeding bats exploit flowers while hovering in front of them. Aerodynamic theory predicts that power output for hovering flight in Glossophaga soricina is 2.6 times higher than during horizontal flight. We tested this prediction by measuring rates of gas exchange during hover-feeding. Five individuals of Glossophaga soricina (mean mass 11.7 g) were trained to feed from a nectar dispenser designed as a flow-through respirometry mask. Single hover-feeding events lasted for up to 4.5 s. Measured rates of gas exchange varied as a function of hovering duration. O2 and CO2 during short hovering events (up to 1 s) were 20.5+/-6.7 ml g-1 h-1 (N=55) and 21.6+/-5.6 ml g-1 h-1 (N=39) (means +/- S.D.), respectively. These values are in the range of a previous estimate of the metabolic power input for level forward flight (23.8 ml O2 g-1 h-1). However, during hovering events lasting longer than 3 s, oxygen uptake was only 16.7+/-3.5 ml g-1 h-1 (N=73), which is only 70 % of the value expected for forward flight. Thus, bats reduced their rate of oxygen uptake during longer periods of hovering compared with level forward flight. This result is in contrast to the predicted hovering cost derived from aerodynamic theory. The exact metabolic power input during hovering remains uncertain. During longer hovering events, bats were probably not in respiratory steady state, as indicated by the deviation of the respiratory exchange ratio from the expected value of 1 (oxidization of nectar sugar) to the measured value of 0.8.

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