When the bat wing flaps downward, the motion produces a tiny cyclone of air called a “leading edge vortex” that pulls the animal upward. Insects are known to use these vortices while flying, but researchers have wondered whether this mechanism works for larger, heavier animals like bats, especially during slow flight or hovering. Researchers in Sweden and the United States studied small, nectar-feeding bats that were flying in a wind tunnel. With help from a fog machine, the researchers captured the movement of fog particles in the bats' wake as they flew through the tunnel. The results suggested that the vortices provided as much as 40 percent of the lift force that was helping the animals to stay in the air. This research appears in the 29 February 2008 issue of the journal Science.