Disruptive coloration provides camouflage independent of background matching

Abstract

Natural selection shapes the evolution of anti-predator defences, such as camouflage. It is currently contentious whether crypsis and disruptive coloration are alternative mechanisms of camouflage or whether they are interrelated anti-predator defences. Disruptively coloured prey is characterized by highly contrasting patterns to conceal the body shape, whereas cryptic prey minimizes the contrasts to background. Determining bird predation of artificial moths, we found that moths which were dissimilar from the background but sported disruptive patterns on the edge of their wings survived better in heterogeneous habitats than did moths with the same patterns inside of the wings and better than cryptic moths. Despite lower contrasts to background, crypsis did not provide fitness benefits over disruptive coloration on the body outline. We conclude that disruptive coloration on the edge camouflages its bearer independent of background matching. We suggest that this result is explainable because disruptive coloration is effective by exploiting predators' cognitive mechanisms of prey recognition and not their sensory mechanisms of signal detection. Relative to disruptive patterns on the body outline, disruptive markings on the body interior are less effective. Camouflage owing to disruptive coloration on the body interior is background-specific and is as effective as crypsis in heterogeneous habitats. Hence, we hypothesize that two proximate mechanisms explain the diversity of visual anti-predator defences. First, disruptive coloration on the body outline provides camouflage independent of the background. Second, background matching and disruptive coloration on the body interior provide camouflage, but their protection is background-specific.

Empirical tests of the role of disruptive coloration in reducing detectability

“…In our case, the lower survival of the high-contrast (disruptive black) edge treatment compared with the monocolour average clearly indicates that a reasonable degree of background matching of both component colours must also be involved if disruptive coloration is to provide a camouflage effect. Although both Schaefer & Stobbe (2006) and proposed on the basis of their results that disruptive patterns are still effective even when some of the pattern elements do not match the background, the current results indicate that it is important to consider the specific degree to which pattern elements differ from their background. To take an extreme, if disruptive markings are highly conspicuous, then subtle differences in where they are placed is likely to have little effect on the survivorship of the prey.…”

“…Third, since mealworms were placed on top of the artificial moths, there is a possibility that differences in relative contrast between the mealworm and the interior of each moth type played some role in affecting their detectability. Both Cuthill et al (2006) and Schaefer & Stobbe (2006) recognized this problem, and while both considered the effect to be small when compared with the edge effect (Schaefer & Stobbe (2006) had halfconcealed their mealworm for this very reason), it remains a potentially confounding factor. Finally, as a field experiment, there was no direct control over the predators and therefore no direct recording of instances in which prey could have potentially been encountered is yet overlooked owing to the difficulty of detecting them.…”

“…For example, in generating disruptively patterned nonbackground-matching moths, varied the lighter colour to create a non-matching treatment, but not the colour of the darker disruptive marks themselves. Moreover, only five different patterns were presented by Schaefer & Stobbe (2006), hence (as the authors note) their results strictly pertain to the specific colour patterns that were evaluated. Second, more importantly, in generating the 'inside' treatments in Cuthill et al (2005) (see also , the density of the interior black patterns was concomitantly increased, raising the possibility that the edge treatments survived better than the inside treatments not because edges were obscured, but because they more closely matched their backgrounds in terms of pattern density.…”

“…First, Cuthill et al used colours that effectively matched their backgrounds, so the adaptive value of disruptive coloration is hard to evaluate completely independently of background matching. This limitation has recently been addressed by Merilaita & Lind (2005), Schaefer & Stobbe (2006) and , but, nevertheless, more work needs to be done (Endler 2006). For example, in generating disruptively patterned nonbackground-matching moths, varied the lighter colour to create a non-matching treatment, but not the colour of the darker disruptive marks themselves.…”

“…Yet despite its widespread application, it is only recently that ecologists and evolutionary biologists have begun to test the effectiveness of disruptive coloration in the field, and to design experiments to distinguish between the benefits of background matching and disruptive coloration per se (Cuthill et al 2005;Merilaita & Lind 2005;Schaefer & Stobbe 2006;. In an influential recent paper, Cuthill et al (2005) pinned artificial mothlike prey, baited with mealworms, onto oak trees and evaluated their 'survivorship' over 24 h. In their main experiment, five different types of artificial moth were pinned out.…”