Interactions between background matching and motion during visual detection can explain why cryptic animals keep still.

A widespread example of coevolution between behaviour and morphology is that crypsis is associated with motionlessness. Surprisingly, however, the adaptive function of this association has never been investigated experimentally. We tested whether the risk of being detected by a predator is affected by crypsis, movement or an interaction between these two traits. We show that, to avoid being detected and attacked by three-spined sticklebacks (Gasterosteus aculeatus L.), chironomid larvae need both to match their background and to keep still. Additionally, relatively more active individuals were targeted from cryptic prey groups, but not from conspicuous ones. Therefore, rather than crypsis and keeping still both contributing to reducing detection, the two traits are synergistic in reducing detection rates. This interdependence has implications for the coevolution of background matching and motion, and also between anti-predator traits and predators' visual systems.