Summary of “Motion Dazzle and Camouflage as Distinct Anti-Predator Defense”
Camouflage patterns that hinder detection and/or recognition by antagonists are widely studied in both human and animal contexts. Patterns of contrasting stripes that purportedly degrade an observer's ability to judge the speed and direction of moving prey ('motion dazzle') are, however, rarely investigated. This is despite motion dazzle having been fundamental to the appearance of warships in both world wars and often postulated as the selective agent leading to repeated patterns on many animals (such as zebra and many fish, snake, and invertebrate species). Such patterns often appear conspicuous, suggesting that protection while moving by motion dazzle might impair camouflage when stationary.
“Motion dazzle” patterns are a form of defensive coloration suggested to prevent successful capture during motion by causing predators to misjudge the direction or speed of prey movement. Several studies have found results supporting this idea but little is known about the factors that favor the evolution of these antipredator colorations. A recent experimental study has suggested that the longitudinal striped patterns on the body of lizards can redirect attacks to the tail via the motion dazzle effect. Using a virtual predation experiment with humans and a phylogenetic comparative analysis, we show that evolution of longitudinal striped coloration is associated with prey size. Experiments showed that longitudinal stripes located at the anterior reduced lethal attacks (i.e., attacks directed to the anterior and centre) but this benefit was greater for shorter prey. Our comparative analysis revealed a negative association between stripe occurrence and body length but no association between stripes and body width. Overall, our results suggest that the dazzle effect produced by stripes is more advantageous in shorter lizards than in longer ones and that the error induced by stripes might be distributed along the axis parallel to the prey trajectory. We discuss reasons why dazzle coloration could be associated with evolution of smaller body size in animals. e acknowledge that pattern classification by humans can have subjective bias but we employed multiple strategies to ensure that our pattern categorization was robust. First, pattern classification was performed by 3 volunteers all of whom were naïve to the hypotheses and were recruited opportunistically. Volunteers were first year undergraduate students from the same institute. Each volunteer was given written instructions for classifying the color patterns.
Targets were more effective at preventing capture when of low contrast. This is consistent with studies of cuttlefish markings and with human experiments indicating that low contrast can cause underestimation of speed. Dazzle markings, therefore, may be most effective when of low contrast. In addition, high-contrast patches and edges might present positional cues to detect and track motion. In our study, all stimuli were achromatic. However, future work should investigate motion dazzle in chromatic stimuli, both because many animal markings have high chromatic contrast, and because at slow speeds and with low-contrast stimuli, speed discrimination is worse for chromatic compared with achromatic stimuli, and perceived stimulus speed can show a greater dependence on chromatic contrast than on luminance (Scott-Samuel NE, 2011). In addition, our set-up only recorded whether a person missed or successfully captured a target, but not whether subjects were drawn to attack and miss the trailing edge, for example. This would be valuable to explore in future work. While our experiments show that motion dazzle can effectively prevent capture, the mechanisms underlying how motion dazzle works are unclear, although a range of possibilities exist. One possibility relates to the so-called 'aperture problem', where the motion of a line viewed through a narrow window is ambiguous for motion parallel to the line itself, and only movement perpendicular to the line is detectable. If movement is detected by local receptive fields that are combined to produce a global estimate of motion, then the true movement of a striped object may be difficult to judge. In addition, the advantage of striped patterns may stem from the repeating nature of the markings, as spatial frequency can affect speed perception, and such markings may fatigue or cause adaptation in motion-sensitive cells. In contrast, blotches and spots may provide reference points to facilitate effective tracking and may underlie the ineffectiveness of camouflage patterns in motion dazzle (Thayer GH, 1909).
As has been noted, the high contrast, conspicuous patterns seen on animals such as zebra, snakes and fishes have attracted a range of evolutionary explanations, including camouflage, thermoregulation, communication and the avoidance of biting flies. One hypothesis that has received attention in recent years is the ‘motion dazzle’ hypothesis, which proposes that these patterns may act to cause confusion when the animal is in motion, causing illusions in the visual system of the viewer that may lead to misjudgements of speed and direction.
Thayer GH: Concealing-coloration in the animal kingdom: an exposition of the laws of disguise through color and pattern: being a summary of Abbott H. Thayer's discoveries. 1909, New York: Macmillan
Stevens M, Yule DH, Ruxton GD: Dazzle coloration and prey movement. Proceedings of the Royal Society, Series B. 2008, 275: 2639-2643.
Zylinski S, Osorio D, Shohet AJ: Cuttlefish camouflage: context-dependent body pattern use during motion. Proceedings of the Royal Society of London, Series B. 2010, 276: 3963-3969.
Scott-Samuel NE, Baddeley R, Palmer CE, Cuthill IC: Dazzle camouflage affects speed perception. PLoS ONE. 2011