Many fish swim in close proximity to one another in large schools, causing scientists to wonder if this behavior is motivated primarily by defense against predators or whether fish derive some hydrodynamic advantages from schooling. Examining the fluid dynamics of an entire school of fish is rather impractical, so researchers approximate two neighboring swimmers using flapping hydrofoils. The images above show flow visualizations of the wakes of these two mechanical swimmers. When the two hydrofoils flap in-phase with one another (top image), one oscillation period produces a complicated pattern of many vortices zig-zagging behind the foils. This configuration produces more efficient propulsion than a single hydrofoil, meaning that more of the energy in the wake is used to produce thrust. The cost, however, is reduced thrust overall. The bottom image shows the wake pattern for hydrofoils flapping out-of-phase. This behavior enhanced thrust without reducing propulsive efficiency. The results suggest that schooling fish might choose different swimming strategies depending on the situation. (Image credits: P. Dewey et al.)