The study of camouflage has a long history in biology, and the numerous ways of concealment and disguise found in the animal kingdom provided Darwin and Wallace with important examples for illustrating and defending their ideas of natural selection and adaptation. Thus, various forms of camouflage have become classical examples of evolution. In a broader sense, camouflage has been adopted by humans, most notably by the military and hunters, but it has also influenced other parts of society, for example, arts, popular culture and design.Animals use camouflage to make detection or recognition more difficult, with most examples associated with visual camouflage involving body coloration. However, in addition to coloration, camouflage may make use of morphological structures or material found in the environment, and may even act against senses other than vision (Ruxton 2009). In nature, some of the most striking examples of adaptation can be found with respect to avoiding being detected or recognized, with the strategies employed diverse, and sometimes extraordinary. Such strategies can include using markings to match the colour and pattern of the background, as in various moths (e.g. Kettlewell 1955), and to break up the appearance of the body, as in some marine isopods (Merilaita 1998). Camouflage is a technique especially useful if the animal can change colour to match the background on which it is found, such as can some cephalopods (Hanlon & Messenger 1988) and chameleons (Stuart-Fox et al. 2008). Further remarkable examples include insects bearing an uncanny resemblance to bird droppings (Hebert 1974) or fish resembling fallen leaves on a stream bed (Sazima et al. 2006), to even making the body effectively transparent, as occurs in a range of, in particular, aquatic species (Johnsen 2001;Carvalho et al. 2006). Examples such as

Disruptive coloration breaks up the shape and destroys the outline of an object, hindering detection. The principle was first suggested approximately a century ago, but, although research has significantly increased, the field remains conceptually unstructured and no unambiguous definition exists. This has resulted in variable use of the term, making it difficult to formulate testable hypotheses that are comparable between studies, slowing down advancement in this field. Related to this, a range of studies do not effectively distinguish between disruption and other forms of camouflage. Here, we give a formal definition of disruptive coloration, reorganize a range of subprinciples involved in camouflage and argue that five in particular are specifically related to disruption: differential blending; maximum disruptive contrast; disruption of surface through false edges; disruptive marginal patterns; and coincident disruptive coloration. We discuss how disruptive coloration can be optimized, how it can relate to other forms of camouflage markings and where future work is particularly needed.

For camouflage to succeed, an individual has to pass undetected, unrecognized or untargeted, and hence it is the processing of visual information that needs to be deceived. Camouflage is therefore an adaptation to the perception and cognitive mechanisms of another animal. Although this has been acknowledged for a long time, there has been no unitary account of the link between visual perception and camouflage. Viewing camouflage as a suite of adaptations to reduce the signal-to-noise ratio provides the necessary common framework. We review the main processes in visual perception and how animal camouflage exploits these. We connect the function of established camouflage mechanisms to the analysis of primitive features, edges, surfaces, characteristic features and objects (a standard hierarchy of processing in vision science). Compared to the commonly used research approach based on established camouflage mechanisms, we argue that our approach based on perceptual processes targeted by camouflage has several important benefits: specifically, it enables the formulation of more precise hypotheses and addresses questions that cannot even be identified when investigating camouflage only through the classic approach based on the patterns themselves. It also promotes a shift from the appearance to the mechanistic function of animal coloration.This article is part of the themed issue 'Animal coloration: production, perception, function and application'.

Cryptic prey coloration typically bears a resemblance to the habitat the prey uses. It has been suggested that coloration which visually matches a random sample of the background maximizes background matching. We studied this previously untested hypothesis, as well as another, little studied principle of concealment, disruptive coloration, and whether it could, acting in addition to background matching, provide another plausible means of achieving camouflage. We presented great tits (Parus major) with artificial background-matching and disruptive prey (DP), and measured detection times. First, we studied whether any random sample of a background produces equally good crypsis. This turned out to not be the case. Next, we compared the DP and the best background-matching prey and found that they were equally cryptic. We repeated the tests using prey with all the coloration elements being whole, instead of some of them being broken by the prey outline, but this did not change the result. We conclude that resemblance of the background is an important aspect of concealment, but that coloration matching a random visual sample of the background is neither sufficient nor necessary to minimize the probability of detection. Further, our study lends empirical support to the principle of disruptive coloration.

We studied selection by predators for cryptic prey coloration in a visually heterogeneous habitat that consists of two microhabitats. It has been suggested that the probability of escaping detection in such habitats might be optimized by maximizing crypsis in one of the microhabitats. However, a recent model indicates that a coloration that compromises the requirements of di¡erent microhabitats might sometimes be the optimal solution. To experimentally study these hypotheses, we allowed great tits (Parus major L.) to search for arti¢cial prey items in two di¡erent microhabitats (background boards): small patterned and large patterned. On each board there was one prey item that was either small-patterned, large-patterned or medium-patterned and thus compromised. Search time was used as the measure of crypsis and was on average longer on the large-patterned than on the small-patterned background. On the small-patterned background, the small-patterned prey was more cryptic than the compromised prey, which was in turn more cryptic than the large-patterned prey. On the large-patterned background, the small-patterned prey was least cryptic, but the compromised prey did not di¡er signi¢cantly from the large-patterned prey. The compromised coloration had lower predation risk than the matching colorations. This indicates that in some conditions a compromised coloration might be the best strategy for the prey and has important implications for the study of animal coloration.