Predators hide against similarly coloured species to camouflage on the move.

Many predators aim to approach prey without being detected or identified. Ambush predators often achieve this through camouflage, such as background matching. This strategy is thought to be ineffective for actively moving predators, as motion typically breaks camouflage. However, if animals could select and remain close to similarly coloured moving objects, then camouflage could be achieved on the move. Using detailed natural observations, we demonstrate that colour polymorphic trumpetfish (Aulostomus maculatus) selectively choose to 'shadow'-swim closely alongside-other fishes of similar colour to themselves. Specifically, yellow trumpetfish individuals most frequently shadow yellow Spanish hogfish (Bodianus rufus), while blue morph individuals most frequently shadow the blue terminal phase of queen parrotfish (Scarus vetula). When we experimentally presented three-dimensional models of these shadowing combinations to bicolour damselfish (Stegastes partitus) colonies in situ, these prey exhibited reduced anti-predator responses towards trumpetfish that were paired with a similarly coloured species versus trumpetfish that were dissimilar in colour to the fish they were shadowing, particularly for the yellow trumpetfish morph. Selecting a similarly coloured moving object (here a fish) represents a novel form of moving camouflage and illustrates how some predators can use strategic decision-making to improve their hunting success on the move.