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 (where the attacker moves straight towards the target at all times, and often appears to the target to move sideways). 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. 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 in the wind or ocean currents, delaying their recognition by predators. First discovered in hoverflies in 1995, motion camouflage by minimizing optic flow has been demonstrated in another insect order, dragonflies, as well as in two groups of vertebrates, falcons and echolocating bats. Since bats hunt at night, they cannot use camouflage. Instead they use an efficient homing strategy called constant absolute target direction. It has been suggested that anti-aircraft missiles could benefit from similar techniques.

Camouflage of approach motion

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.

Stealthy movements 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.

Minimising motion signal 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 perception of motion

Disrupting the attacker's perception of the target's motion was one of the intended purposes of dazzle camouflage as used on ships in the First World War, though its effectiveness is disputed. Broadback cuttlefish, Ascarosepion latimanus, hunt prey such as shore crabs, Carcinus maenas, by swimming directly towards them, making a "passing-stripe" display on their front. The cuttlefish colours its head white and forms six out of eight of its arms into a forward-pointing cone. The other two arms are stretched out sideways, their broad sides facing the prey. The display consists of moving dark stripes downwards over the forward-facing parts of the head and arms. The crab sees the image of the predator looming larger as it approaches; on its own, this elicits a strong reaction from the crab. Crabs presented with a combination of looming with a passing-stripe display reacted less strongly. The cuttlefish's posture helps to mask movements of its mantle (behind the outstretched arms), perhaps further reducing motion cues to the crab. If the crab is using radial motion from looming to detect attack, then the passing-stripe display deceives the crab by reducing that cue. Instead, it offers a wide horizontal body shape with vertical stripe movements.

Mimicking optic flow of background

Some animals mimic the optic flow of the background, so that the attacker does not appear to move when seen by the target. This is the main focus of work on motion camouflage, and is often treated as synonymous with it.

Pursuit strategies An attacker can mimic the background's optic flow by choosing its flight path so as to remain on the line between the target and either some real landmark point, or a point at infinite distance (giving different pursuit algorithms). It therefore does not move from the landmark point as seen by the target, though it inevitably looms larger as it approaches. This is not the same as moving straight towards the target (classical pursuit): that results in visible sideways motion with a readily detectable difference in optic flow from the background. The strategy works whether the background is plain or textured. This motion camouflage strategy was discovered and modelled as algorithms in 1995 by M. V. Srinivasan and M. Davey while they were studying mating behaviour in hoverflies. The male hoverfly appeared to be using the tracking technique to approach prospective mates. Motion camouflage has been observed in high-speed territorial battles between dragonflies, where males of the Australian emperor dragonfly, Hemianax papuensis were seen to choose their flight paths to appear stationary to their rivals in 6 of 15 encounters. They made use of both real-point and infinity-point strategies.

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