(a) An image sequence showing a pair of synchronized slither swimming bull sperm, adopting a shared beating frequency and flagellar waveform and (b) their overlaid trajectories. Synchronized motion of (c) four bull sperm and (d) two human sperm in the slither swimming mode. The head of each individual sperm is colour coded along its swimming path. Scale bars, 20 μm. The image was inverted and contrast-adjusted for clarity.