Motion camouflage

Motion camouflage is camouflage which provides a degree of concealment for a moving object, given that motion makes objects easy to detect however well their coloration matches their background or breaks up their outlines. The principal form of motion camouflage, and the type generally meant by the term, involves an attacker's mimicking the optic flow of the background as seen by its target. This enables the attacker to approach the target while appearing to remain stationary from the target's perspective, unlike in classical pursuit. The attacker chooses its flight path so as to remain on the line between the target and some landmark point. The target therefore does not see the attacker move from the landmark point. The only visible evidence that the attacker is moving is its looming, the change in size as the attacker approaches. Motion is also used in a variety of other camouflage strategies, including swaying to mimic plant movements in the wind or ocean currents. First discovered in hoverflies in 1995, motion camouflage by minimising optic flow has been demonstrated in another insect order, dragonflies, as well as in two groups of vertebrates, falcons and echolocating bats. Since bats hunting at night cannot be using the strategy for camouflage, it has been named, describing its mechanism, as constant absolute target direction. This is an efficient homing strategy, and it has been suggested that anti-aircraft missiles could benefit from similar techniques. Camouflage is sometimes facilitated by motion, as in the leafy sea dragon and some stick insects. These animals complement their passive camouflage by swaying like plants, either preventing predators from detecting them, or appearing as something other than prey. Many animals are highly sensitive to motion; for example, frogs readily detect small moving dark spots but ignore stationary ones. Therefore, motion signals can be used to defeat camouflage. Moving objects with disruptive camouflage patterns remain harder to identify than uncamouflaged objects, especially if other similar objects are nearby, even though they are detected, so motion does not completely 'break' camouflage.All the same, the conspicuousness of motion raises the question of whether and how motion itself could be camouflaged. Several mechanisms are possible. One strategy is to minimise actual motion, as when predators such as tigers stalk prey by moving very slowly and stealthily. This strategy effectively avoids the need to camouflage motion. When movement is required, one strategy is to minimise the motion signal, for example by avoiding waving limbs about and by choosing patterns that do not cause flicker when seen by the prey from straight ahead. Cuttlefish may be doing this with their active camouflage by choosing to form stripes at right angles to their front-back axis, minimising motion signals that would be given by occluding and displaying the pattern as they swim. Disrupting the attacker's perception of the target's motion was the main intended purpose of dazzle camouflage as used on ships in the First World War, though its effectiveness is disputed. This type of dazzle does not appear to be used by animals.